#import "lib/definitions.typ": *

== Htmx Patterns

Now that we’ve seen how htmx extends HTML as a hypermedia, it’s time to put it
into action. As we use htmx, we will still be using hypermedia: we will issue
HTTP requests and get back HTML. But, with the additional functionality that
htmx provides, we will have a more _powerful hypermedia_ to work with, allowing
us to accomplish much more sophisticated interfaces.

This will allow us to address user experience issues, such as long feedback
cycles or painful page refreshes, without needing to write much, if any,
JavaScript, and without creating a JSON API. Everything will be implemented in
hypermedia, using the core hypermedia concepts of the early web.

=== Installing Htmx <_installing_htmx>

#index[htmx][installing]
The first thing we need to do is install htmx in our web application. We are
going to do this by downloading the source and saving it locally in our
application, so we aren’t dependent on any external systems. This is known as "vendoring"
the library. We can grab the latest version of htmx by navigating our browser to `https://unpkg.com/htmx.org`,
which will redirect us to the source of the latest version of the library.

We can save the content from this URL into the `static/js/htmx.js` file in our
project.

You can, of course, use a more sophisticated JavaScript package manager such as
Node Package Manager (NPM) or yarn to install htmx. You do this by referring to
its package name, `htmx.org`, in the manner appropriate for your tool. However,
htmx is very small (approximately 12kb when compressed and zipped) and is
dependency free, so using it does not require an elaborate mechanism or build
tool.

With htmx downloaded locally to our applications `/static/js` directory, we can
now load it in to our application. We do this by adding the following `script` tag
to the `head` tag in our `layout.html` file, which will make htmx available and
active on every page in our application:

#figure(caption: [Installing htmx],
```html
<head>
  <script src="/js/htmx.js"></script>
  ...
</head>
```)

Recall that the `layout.html` file is a _layout_ file included in most templates
that wraps the content of those templates in common HTML, including a `head` element
that we are using here to install htmx.

Believe it or not, that’s it! This simple script tag will make htmx’s
functionality available across our entire application.

=== AJAX-ifying Our Application <_ajax_ifying_our_application>

#index[hx-boost]
#index[htmx patterns][boosting]
To get our feet wet with htmx, the first feature we are going to take advantage
of is known as "boosting." This is a bit of a "magic" feature in that we don’t
need to do much beyond adding a single attribute,
`hx-boost`, to the application.

When you put `hx-boost` on a given element with the value `true`, it will "boost"
all anchor and form elements within that element. "Boost", here, means that htmx
will convert all those anchors and forms from
"normal" hypermedia controls into AJAX-powered hypermedia controls. Rather than
issuing "normal" HTTP requests that replace the whole page, the links and forms
will issue AJAX requests. Htmx then swaps the inner content of the `<body>` tag
in the response to these requests into the existing pages `<body>` tag.

This makes navigation feel faster because the browser will not be
re-interpreting most of the tags in the response `<head>` and so forth.

==== Boosted Links <_boosted_links>
Let’s take a look at an example of a boosted link. Below is a link to a
hypothetical settings page for a web application. Because it has
`hx-boost="true"` on it, htmx will halt the normal link behavior of issuing a
request to the `/settings` path and replacing the entire page with the response.
Instead, htmx will issue an AJAX request to
`/settings`, take the result and replace the `body` element with the new
content.

#figure(caption: [A boosted link],
```html
<a href="/settings" hx-boost="true">Settings</a> <1>
```)
1. The `hx-boost` attribute makes this link AJAX-powered.

You might reasonably ask: what’s the advantage here? We are issuing an AJAX
request and simply replacing the entire body.

Is that significantly different from just issuing a normal link request?

Yes, it is in fact different: with a boosted link, the browser is able to avoid
any processing associated with the head tag. The head tag often contains many
scripts and CSS file references. In the boosted scenario, it is not necessary to
re-process those resources: the scripts and styles have already been processed
and will continue to apply to the new content. This can often be a very easy way
to speed up your hypermedia application.

A second question you might have is: does the response need to be formatted
specially to work with `hx-boost`? After all, the settings page would normally
render an `html` tag, with a `head` tag and so forth. Do you need to handle "boosted"
requests specially?

The answer is no: htmx is smart enough to pull out only the content of the `body` tag
to swap in to the new page. The `head` tag is mostly ignored: only the title
tag, if it is present, will be processed. This means you don’t need to do
anything special on the server side to render templates that `hx-boost` can
handle: just return the normal HTML for your page, and it should work fine.

Note that boosted links (and forms) will also continue to update the navigation
bar and history, just like normal links, so users will be able to use the
browser back button, will be able to copy and paste URLs (or "deep links") and
so on. Links will act pretty much like "normal", they will just be faster.

==== Boosted Forms <_boosted_forms>
Boosted form tags work in a similar way to boosted anchor tags: a boosted form
will use an AJAX request rather than the usual browser-issued request, and will
replace the entire body with the response.

Here is an example of a form that posts messages to the `/messages`
endpoint using an HTTP `POST` request. By adding `hx-boost` to it, those
requests will be done in AJAX, rather than the normal browser behavior.

#figure(caption: [A boosted form],
```html
<form action="/messages" method="post" hx-boost="true"> <1>
  <input type="text" name="message" placeholder="Enter A Message...">
  <button>Post Your Message</button>
</form>
```)
1. As with the link, `hx-boost` makes this form AJAX-powered.

#index[Flash Of Unstyled Content (FOUC)]
A big advantage of the AJAX-based request that `hx-boost` uses (and the lack of
head processing that occurs) is that it avoids what is known as a _flash of unstyled content_:

/ Flash Of Unstyled Content (FOUC): A situation where a browser renders a web page before all the styling
  information is available for the page. A FOUC causes a disconcerting momentary "flash"
  of the unstyled content, which is then restyled when all the style information
  is available. You will notice this as a flicker when you move around the
  internet: text, images and other content can "jump around" on the page as styles
  are applied to it.

With `hx-boost` the site’s styling is already loaded _before_ the new content is
retrieved, so there is no such flash of unstyled content. This can make a "boosted"
application feel both smoother and also snappier in general.

==== Attribute Inheritance <_attribute_inheritance>
Let’s expand on our previous example of a boosted link, and add a few more
boosted links alongside it. We’ll add links so that we have one to the `/contacts` page,
the `/settings` page, and the `/help` page. All these links are boosted and will
behave in the manner that we have described above.

This feels a little redundant, doesn’t it? It seems silly to annotate all three
links with the `hx-boost="true"` attribute right next to one another.

#figure(caption: [A set of boosted links],
```html
<a href="/contacts" hx-boost="true">Contacts</a>
<a href="/settings" hx-boost="true">Settings</a>
<a href="/help" hx-boost="true">Help</a>
```)

#index[htmx][attribute inheritance]
Htmx offers a feature to help reduce this redundancy: attribute inheritance.
With most attributes in htmx, if you place it on a parent, the attribute will
also apply to children elements. This is how Cascading Style Sheets work, and
that idea inspired htmx to adopt a similar "cascading htmx attributes" feature.

To avoid the redundancy in this example, let’s introduce a `div` element that
encloses all the links and then "hoist" the `hx-boost` attribute up to that
parent `div`. This will let us remove the redundant `hx-boost`
attributes but ensure all the links are still boosted, inheriting that
functionality from the parent `div`.

Note that any legal HTML element could be used here, we just use a `div`
out of habit.

#figure(caption: [Boosting links via the parent],
```html
<div hx-boost="true"> <1>
  <a href="/contacts">Contacts</a>
  <a href="/settings">Settings</a>
  <a href="/help">Help</a>
</div>
```)
1. The `hx-boost` has been moved to the parent div.

Now we don’t have to put an `hx-boost="true"` on every link and, in fact, we can
add more links alongside the existing ones, and they, too, will be boosted,
without us needing to explicitly annotate them.

That’s fine, but what if you have a link that you _don’t_ want boosted within an
element that has `hx-boost="true"` on it? A good example of this situation is
when a link is to a resource to be downloaded, such as a PDF. Downloading a file
can’t be handled well by an AJAX request, so you probably want that link to
behave "normally", issuing a full page request for the PDF, which the browser
will then offer to save as a file on the user’s local system.

To handle this situation, you simply override the parent `hx-boost`
value with `hx-boost="false"` on the anchor tag that you don’t want to boost:

#figure(caption: [Disabling boosting],
```html
<div hx-boost="true"> <1>
  <a href="/contacts">Contacts</a>
  <a href="/settings">Settings</a>
  <a href="/help">Help</a>
  <a href="/help/documentation.pdf" hx-boost="false"> <2>
    Download Docs
  </a>
</div>
```)
1. The `hx-boost` is still on the parent div.
2. The boosting behavior is overridden for this link.

#index[hx-boost][disabling]
Here we have a new link to a documentation PDF that we wish to function like a
regular link. We have added `hx-boost="false"` to the link and this declaration
will override the `hx-boost="true"` on the parent
`div`, reverting it to regular link behavior and, thus, allowing for the file
download behavior that we want.

==== Progressive Enhancement <_progressive_enhancement>

#index[progressive enhancement]
A nice aspect of `hx-boost` is that it is an example of
_progressive enhancement_:

/ Progressive Enhancement: #[
    A software design philosophy that aims to provide as much essential content and
    functionality to as many users as possible, while delivering a better experience
    to users with more advanced web browsers.
  ]

Consider the links in the example above. What would happen if someone did not
have JavaScript enabled?

No problem. The application would continue to work, but it would issue regular
HTTP requests, rather than AJAX-based HTTP requests. This means that your web
application will work for the maximum number of users; those with modern
browsers (or users who have not turned off JavaScript) can take advantage of the
benefits of the AJAX-style navigation that htmx offers, and others can still use
the app just fine.

Compare the behavior of htmx’s `hx-boost` attribute with a JavaScript heavy
Single Page Application: such an application often won’t function
_at all_ without JavaScript enabled. It is often very difficult to adopt a
progressive enhancement approach when you use an SPA framework.

This is _not_ to say that every htmx feature offers progressive enhancement. It
is certainly possible to build features that do not offer a "No JS" fallback in
htmx, and, in fact, many of the features we will build later in the book will
fall into this category. We will note when a feature is progressive enhancement
friendly and when it is not.

Ultimately, it is up to you, the developer, to decide if the trade-offs of
progressive enhancement (a more basic UX, limited improvements over plain HTML)
are worth the benefits for your application users.

==== Adding "hx-boost" to Contact.app <_adding_hx_boost_to_contact_app>
For the contact app we are building, we want this htmx "boost" behavior…​ well,
everywhere.

Right? Why not?

How could we accomplish that?

Well, it’s easy (and pretty common in htmx-powered web applications): we can
just add `hx-boost` on the `body` tag of our `layout.html` template, and we are
done.

#figure(caption: [Boosting the entire contact.app],
```html
<html>
...
<body hx-boost="true"> <1>
...
</body>
</html>
```)
1. All links and forms will be boosted now!

Now every link and form in our application will use AJAX by default, making it
feel much snappier. Consider the "New Contact" link that we created on the main
page:

#figure(caption: [A newly boosted "add contact" link],
```html
<a href="/contacts/new">Add Contact</a>
```)

Even though we haven’t touched anything on this link or on the server-side
handling of the URL it targets, it will now "just work" as a boosted link, using
AJAX for a snappier user experience, including updating history, back button
support and so on. And, if JavaScript isn’t enabled, it will fall back to the
normal link behavior.

All this with one htmx attribute.

The `hx-boost` attribute is neat, but is different than other htmx attributes in
that it is pretty "magical": by making one small change you modify the behavior
of a large number of elements on the page, turning them into AJAX-powered
elements. Most other htmx attributes are generally lower level and require more
explicit annotations in order to specify exactly what you want htmx to do. In
general, this is the design philosophy of htmx: prefer explicit over implicit
and obvious over
"magic."

However, the `hx-boost` attribute was too useful to allow dogma to override
practicality, and so it is included as a feature in the library.

=== A Second Step: Deleting Contacts With HTTP DELETE <_a_second_step_deleting_contacts_with_http_delete>
For our next step with htmx, recall that Contact.app has a small form on the
edit page of a contact that is used to delete the contact:

#figure(caption: [Plain HTML form to delete a contact],
```html
<form action="/contacts/{{ contact.id }}/delete" method="post">
  <button>Delete Contact</button>
</form>
```)

This form issued an HTTP `POST` to, for example, `/contacts/42/delete`, in order
to delete the contact with the ID 42.

#index[hx-delete]
We mentioned previously that one of the annoying things about HTML is that you
can’t issue an HTTP `DELETE` (or `PUT` or `PATCH`) request directly, even though
these are all part of HTTP and HTTP is
_obviously designed_ for transferring HTML.

Thankfully, now, with htmx, we have a chance to rectify this situation.

The "right thing," from a RESTful, resource-oriented perspective is, rather than
issuing an HTTP `POST` to `/contacts/42/delete`, to issue an HTTP `DELETE` to `/contacts/42`.
We want to delete the contact. The contact is a resource. The URL for that
resource is `/contacts/42`. So the ideal is a `DELETE` request to `/contacts/42/`.

Let’s update our application to do this by adding the htmx `hx-delete`
attribute to the "Delete Contact" button:

#figure(caption: [An htmx-powered button for deleting a contact],
```html
<button hx-delete="/contacts/{{ contact.id }}">Delete Contact</button>
```)

Now, when a user clicks this button, htmx will issue an HTTP `DELETE`
request via AJAX to the URL for the contact in question.

#index[htmx patterns][delete]
A couple of things to notice:
- We no longer need a `form` tag to wrap the button, because the button itself
  carries the hypermedia action that it performs directly on itself.
- We no longer need to use the somewhat awkward
  `"/contacts/{{ contact.id }}/delete"` route, but can simply use the
  `"/contacts/{{ contact.id }}` route, since we are issuing a `DELETE`. By using a `DELETE` we
  disambiguate between a request intended to update the contact and a request
  intended to delete it, using the native HTTP tools available for exactly this
  reason.

Note that we have done something pretty magical here: we have turned this button
into a _hypermedia control_. It is no longer necessary that this button be
placed within a larger `form` tag in order to trigger an HTTP request: it is a
stand-alone, and fully featured hypermedia control on its own. This is at the
heart of htmx, allowing any element to become a hypermedia control and fully
participate in a Hypermedia-Driven Application.

We should also note that, unlike with the `hx-boost` examples above, this
solution will _not_ degrade gracefully. To make this solution degrade
gracefully, we would need to wrap the button in a form element and handle a `POST` on
the server side as well.

In the interest of keeping our application simple, we are going to omit that
more elaborate solution.

==== Updating The Server-Side Code <_updating_the_server_side_code>
We have updated the client-side code (if HTML can be considered code) so it now
issues a `DELETE` request to the appropriate URL, but we still have some work to
do. Since we updated both the route and the HTTP method we are using, we are
going to need to update the server-side implementation as well to handle this
new HTTP request.

#figure(caption: [The original server-side code for deleting a contact],
```python
@app.route("/contacts/<contact_id>/delete", methods=["POST"])
def contacts_delete(contact_id=0):
    contact = Contact.find(contact_id)
    contact.delete()
    flash("Deleted Contact!")
    return redirect("/contacts")
```)

We’ll need to make two changes to our handler: update the route, and update the
HTTP method we are using to delete contacts.

#figure(caption: [Updated handler with new route and method],
```python
@app.route("/contacts/<contact_id>", methods=["DELETE"]) <1>
def contacts_delete(contact_id=0):
    contact = Contact.find(contact_id)
    contact.delete()
    flash("Deleted Contact!")
    return redirect("/contacts")
```)
1. An updated path and method for the handler.

Pretty simple, and much cleaner.

===== A response code gotcha <_a_response_code_gotcha>

#index[Flask][redirect]
Unfortunately, there is a problem with our updated handler: by default, in Flask
the `redirect()` method responds with a `302 Found` HTTP Response Code.

According to the Mozilla Developer Network (MDN) web docs on the
#link(
  "https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/302",
)[`302 Found`]
response, this means that the HTTP _method_ of the request
_will be unchanged_ when the redirected HTTP request is issued.

We are now issuing a `DELETE` request with htmx and then being redirected to the `/contacts` path
by flask. According to this logic, that would mean that the redirected HTTP
request would still be a
`DELETE` method. This means that, as it stands, the browser will issue a
`DELETE` request to `/contacts`.

This is definitely _not_ what we want: we would like the HTTP redirect to issue
a `GET` request, slightly modifying the Post/Redirect/Get behavior we discussed
earlier to be a Delete/Redirect/Get.

Fortunately, there is a different response code,
#link(
  "https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/303",
)[`303 See Other`], that does what we want: when a browser receives a `303 See Other`
redirect response, it will issue a `GET` to the new location.

So we want to update our code to use the `303` response code in the controller.

Thankfully, this is very easy: there is a second parameter to
`redirect()` that takes the numeric response code you wish to send.

#figure(caption: [Updated handler with `303` redirect response],
```python
@app.route("/contacts/<contact_id>", methods=["DELETE"])
def contacts_delete(contact_id=0):
    contact = Contact.find(contact_id)
    contact.delete()
    flash("Deleted Contact!")
    return redirect("/contacts", 303) <1>
```)
1. The response code is now a 303.

Now, when you want to remove a given contact, you can simply issue a
`DELETE` to the same URL as you used to access the contact in the first place.

This is a natural HTTP-based approach to deleting a resource.

==== Targeting The Right Element <_targeting_the_right_element>

#index[hx-target][example]
We aren’t quite finished with our updated delete button. Recall that, by
default, htmx "targets" the element that triggers a request, and will place the
HTML returned by the server inside that element. Right now, the "Delete Contact"
button is targeting itself.

That means that, since the redirect to the `/contacts` URL is going to re-render
the entire contact list, we will end up with that contact list placed _inside_ the "Delete
Contact" button.

Mis-targeting like this comes up from time to time when you are working with
htmx and can lead to some pretty funny situations.

The fix for this is easy: add an explicit target to the button, and target the `body` element
with the response:

#figure(caption: [A fixed htmx-powered button for deleting a contact],
```html
<button hx-delete="/contacts/{{ contact.id }}"
    hx-target="body"> <1>
  Delete Contact
</button>
```)
1. An explicit target added to the button.

Now our button behaves as expected: clicking on the button will issue an HTTP `DELETE` to
the server against the URL for the current contact, delete the contact and
redirect back to the contact list page, with a nice flash message.

Is everything working smoothly now?

==== Updating The Location Bar URL Properly <_updating_the_location_bar_url_properly>

Well, almost.

#index[htmx][location bar]
If you click on the button you will notice that, despite the redirect, the URL
in the location bar is not correct. It still points to
`/contacts/{{ contact.id }}`. That’s because we haven’t told htmx to update the
URL: it just issues the `DELETE` request and then updates the DOM with the
response.

As we mentioned, boosting via `hx-boost` will naturally update the location bar
for you, mimicking normal anchors and forms, but in this case we are building a
custom button hypermedia control to issue a
`DELETE`. We need to let htmx know that we want the resulting URL from this
request "pushed" into the location bar.

#index[hx-push-url]
We can achieve this by adding the `hx-push-url` attribute with the value
`true` to our button:

#figure(caption: [Deleting a contact, now with proper location information],
```html
<button hx-delete="/contacts/{{ contact.id }}"
  hx-target="body"
  hx-push-url="true"> <1>
  Delete Contact
</button>
```)
1. We tell htmx to push the redirected URL up into the location bar.

_Now_ we are done.

We have a button that, all by itself, is able to issue a properly formatted HTTP `DELETE` request
to the correct URL, and the UI and location bar are all updated correctly. This
was accomplished with three declarative attributes placed directly on the
button: `hx-delete`,
`hx-target` and `hx-push-url`.

This required more work than the `hx-boost` change, but the explicit code makes
it easy to see what the button is doing as a custom hypermedia control. The
resulting solution feels clean; it takes advantage of the built-in features of
the web as a hypermedia system without any URL hacks.

==== One More Thing…​ <_one_more_thing>

#index[hx-confirm]
#index[htmx patterns][confirmation dialog]
There is one additional "bonus" feature we can add to our "Delete Contact"
button: a confirmation dialog. Deleting a contact is a destructive operation and
as it stands right now, if the user inadvertently clicked the "Delete Contact"
button, the application would just delete that contact. Too bad, so sad for the
user.

Fortunately htmx has an easy mechanism for adding a confirmation message on
destructive operations like this: the `hx-confirm` attribute. You can place this
attribute on an element, with a message as its value, and the JavaScript method `confirm()` will
be called before a request is issued, which will show a simple confirmation
dialog to the user asking them to confirm the action. Very easy and a great way
to prevent accidents.

Here is how we would add confirmation of the contact delete operation:

#figure(caption: [Confirming deletion],
```html
<button hx-delete="/contacts/{{ contact.id }}"
  hx-target="body"
  hx-push-url="true"
  hx-confirm="Are you sure you want to delete this contact?"> <1>
  Delete Contact
</button>
```)
1. This message will be shown to the user, asking them to confirm the delete.

Now, when someone clicks on the "Delete Contact" button, they will be presented
with a prompt that asks "Are you sure you want to delete this contact?" and they
will have an opportunity to cancel if they clicked the button in error. Very
nice.

With this final change we now have a pretty solid "delete contact" mechanism: we
are using the correct RESTful routes and HTTP Methods, we are confirming the
deletion, and we have removed a lot of the cruft that normal HTML imposes on us,
all while using declarative attributes in our HTML and staying firmly within the
normal hypermedia model of the web.

==== Progressive Enhancement? <_progressive_enhancement_2>

#index[progressive enhancement]
As we noted earlier about this solution: it is _not_ a progressive enhancement
to our web application. If someone has disabled JavaScript then this "Delete
Contact" button will no longer work. We would need to do additional work to keep
the older form-based mechanism working in a JavaScript-disabled environment.

Progressive Enhancement can be a hot-button topic in web development, with lots
of passionate opinions and perspectives. Like nearly all JavaScript libraries,
htmx makes it possible to create applications that do not function in the
absence of JavaScript. Retaining support for non-JavaScript clients requires
additional work and complexity in your application. It is important to determine
exactly how important supporting non-JavaScript clients is before you begin
using htmx, or any other JavaScript framework, for improving your web
applications.

=== Next Steps: Validating Contact Emails <_next_steps_validating_contact_emails>

#index[validation]
Let’s move on to another improvement in our application. A big part of any web
app is validating the data that is submitted to the server: ensuring emails are
correctly formatted and unique, numeric values are valid, dates are acceptable,
and so forth.

Currently, our application has a small amount of validation that is done
entirely server-side and that displays an error message when an error is
detected.

We are not going to go into the details of how validation works in the model
objects, but recall what the code for updating a contact looks like from Chapter
3:

#figure(caption: [Server-side validation on contact update],
```python
def contacts_edit_post(contact_id=0):
    c = Contact.find(contact_id)
    c.update(
      request.form['first_name'],
      request.form['last_name'],
      request.form['phone'],
      request.form['email']) <1>
    if c.save():
        flash("Updated Contact!")
        return redirect("/contacts/" + str(contact_id))
    else:
        return render_template("edit.html", contact=c) <2>
```)

1. We attempt to save the contact.
2. If the save does not succeed we re-render the form to display error messages.

So we attempt to save the contact, and, if the `save()` method returns true, we
redirect to the contact’s detail page. If the `save()` method does not return
true, that indicates that there was a validation error; instead of redirecting,
we re-render the HTML for editing the contact. This gives the user a chance to
correct the errors, which are displayed alongside the inputs.

Let’s take a look at the HTML for the email input:

#figure(caption: [Validation error messages],
```html
<p>
  <label for="email">Email</label>
  <input name="email" id="email" type="text"
    placeholder="Email" value="{{ contact.email }}">
  <span class="error">{{ contact.errors['email'] }}</span> <1>
</p>
```)
1. Display any errors associated with the email field

We have a label for the input, an input of type `text` and then a bit of HTML to
display any error messages associated with the email. When the template is
rendered on the server, if there are errors associated with the contact’s email,
they will be displayed in this span, which will be highlighted red.

#sidebar[Server-Side Validation Logic][Right now there is a bit of logic in the contact class that checks if there are
  any other contacts with the same email address, and adds an error to the contact
  model if so, since we do not want to have duplicate emails in the database. This
  is a very common validation example: emails are usually unique and adding two
  contacts with the same email is almost certainly a user error.

  Again, we are not going into the details of how validation works in our models,
  but almost all server-side frameworks provide ways to validate data and collect
  errors to display to the user. This sort of infrastructure is very common in Web
  1.0 server-side frameworks.]

The error message shown when a user attempts to save a contact with a duplicate
email is "Email Must Be Unique", as seen in @fig-emailerror.

#figure([#image("images/screenshot_validation_error.png")], caption: [
  Email validation error
])<fig-emailerror>

All of this is done using plain HTML and using Web 1.0 techniques, and it works
well.

However, as the application currently stands, there are two annoyances.
- First, there is no email format validation: you can enter whatever characters
  you’d like as an email and, as long as they are unique, the system will allow
  it.
- Second, we only check the email’s uniqueness when all the data is submitted: if
  a user has entered a duplicate email, they will not find out until they have
  filled in all the fields. This could be quite annoying if the user was
  accidentally reentering a contact and had to put all the contact information in
  before being made aware of this fact.

==== Updating Our Input Type <_updating_our_input_type>

#index[HTML][inputs]
For the first issue, we have a pure HTML mechanism for improving our
application: HTML 5 supports inputs of type `email`. All we need to do is switch
our input from type `text` to type `email`, and the browser will enforce that
the value entered properly matches the email format:

#figure(caption: [Changing the input to type `email`],
```html
<p>
  <label for="email">Email</label>
  <input name="email" id="email" type="email" <1>
    placeholder="Email" value="{{ contact.email }}">
  <span class="error">{{ contact.errors['email'] }}</span>
</p>
```)
1. A change of the `type` attribute to `email` ensures that values entered are
  valid emails.

With this change, when the user enters a value that isn’t a valid email, the
browser will display an error message asking for a properly formed email in that
field.

So a simple single-attribute change done in pure HTML improves our validation
and addresses the first problem we noted.

#sidebar[Server-Side vs. Client-Side Validations][
  Experienced web developers might be grinding their teeth at the code above: this
  validation is done on _the client-side_. That is, we are relying on the browser
  to detect the malformed email and correct the user. Unfortunately, the
  client-side is not trustworthy: a browser may have a bug in it that allows the
  user to circumvent this validation code. Or, worse, the user may be malicious
  and figure out a mechanism around our validation entirely, such as using the
  developer console to edit the HTML.

  This is a perpetual danger in web development: all validations done on the
  client-side cannot be trusted and, if the validation is important,
  _must be redone_ on the server-side. This is less of a problem in
  Hypermedia-Driven Applications than in Single Page Applications, because the
  focus of HDAs is the server-side, but it is worth bearing in mind as you build
  your application.
]

==== Inline Validation <_inline_validation>

#index[htmx patterns][inline validation]
While we have improved our validation experience a bit, the user must still
submit the form to get any feedback on duplicate emails. We can next use htmx to
improve this user experience.

It would be better if the user were able to see a duplicate email error
immediately after entering the email value. It turns out that inputs fire a `change` event
and, in fact, the `change` event is the
_default trigger_ for inputs in htmx. So, putting this feature to work, we can
implement the following behavior: when the user enters an email, immediately
issue a request to the server and validate that email, and render an error
message if necessary.

Recall the current HTML for our email input:

#figure(caption: [The initial email configuration],
```html
<p>
  <label for="email">Email</label>
  <input name="email" id="email" type="email"
    placeholder="Email" value="{{ contact.email }}"> <1>
  <span class="error">{{ contact.errors['email'] }}</span> <2>
</p>
```)
1. This is the input that we want to have drive an HTTP request to validate the
  email.
2. This is the span we want to put the error message, if any, into.

So we want to add an `hx-get` attribute to this input. This will cause the input
to issue an HTTP `GET` request to a given URL to validate the email. We then
want to target the error span following the input with any error message
returned from the server.

Let’s make those changes to our HTML:

#figure(caption: [Our updated HTML],
```html
<p>
  <label for="email">Email</label>
  <input name="email" id="email" type="email"
    hx-get="/contacts/{{ contact.id }}/email" <1>
    hx-target="next .error" <2>
    placeholder="Email" value="{{ contact.email }}">
  <span class="error">{{ contact.errors['email'] }}</span>
</p>
```)
1. Issue an HTTP `GET` to the `email` endpoint for the contact.
2. Target the next element with the class `error` on it.

Note that in the `hx-target` attribute we are using a _relative positional_ selector, `next`.
This is a feature of htmx and an extension to normal CSS. Htmx supports prefixes
that will find targets
_relative_ to the current element.

#sidebar[Relative Positional Expressions in Htmx][
/ `next`: #[
  Scan forward in the DOM for the next matching element, e.g.,
  `next .error`
  ]

/ `previous`: #[
  Scan backwards in the DOM for the closest previous matching element, e.g., `previous .alert`
  ]

/ `closest`: #[
  Scan the parents of this element for matching element, e.g.,
  `closest table`
  ]

/ `find`: #[
  Scan the children of this element for matching element, e.g.,
  `find span`
  ]

/ `this`: #[
    the current element is the target (default)
  ]
]

By using relative positional expressions we can avoid adding explicit ids to
elements and take advantage of the local structure of HTML.

So, in our example with added `hx-get` and `hx-target` attributes, whenever
someone changes the value of the input (remember, `change` is the _default_ trigger
for inputs in htmx) an HTTP `GET` request will be issued to the given URL. If
there are any errors, they will be loaded into the error span.

==== Validating Emails Server-Side <_validating_emails_server_side>
Next, let’s look at the server-side implementation. We are going to add another
endpoint, similar to our edit endpoint in some ways: it is going to look up the
contact based on the ID encoded in the URL. In this case, however, we only want
to update the email of the contact, and we obviously don’t want to save it!
Instead, we will call the `validate()`
method on it.

That method will validate the email is unique and so forth. At that point we can
return any errors associated with the email directly, or the empty string if
none exist.

#figure(caption: [Code for our email validation endpoint],
```python
@app.route("/contacts/<contact_id>/email", methods=["GET"])
def contacts_email_get(contact_id=0):
    c = Contact.find(contact_id) <1>
    c.email = request.args.get('email') <2>
    c.validate() <3>
    return c.errors.get('email') or "" <4>
```)
1. Look up the contact by id.
2. Update its email (note that since this is a `GET`, we use the `args` property
  rather than the `form` property).
3. Validate the contact.
4. Return a string, either the errors associated with the email field or, if there
  are none, the empty string.

With this small bit of server-side code in place, we now have the following user
experience: when a user enters an email and tabs to the next input field, they
are immediately notified if the email is already taken.

Note that the email validation is _still_ done when the entire contact is
submitted for an update, so there is no danger of allowing duplicate email
contacts to slip through: we have simply made it possible for users to catch
this situation earlier by use of htmx.

It is also worth noting that this particular email validation
_must_ be done on the server side: you cannot determine that an email is unique
across all contacts unless you have access to the data store of record. This is
another simplifying aspect of Hypermedia-Driven Applications: since validations
are done server-side, you have access to all the data you might need to do any
sort of validation you’d like.

Here again we want to stress that this interaction is done entirely within the
hypermedia model: we are using declarative attributes and exchanging hypermedia
with the server in a manner very similar to how links or forms work. But we have
managed to improve our user experience dramatically.

==== Taking The User Experience Further <_taking_the_user_experience_further>
Despite the fact that we haven’t added a lot of code here, we have a fairly
sophisticated user interface, at least when compared with plain HTML-based
applications. However, if you have used more advanced Single Page Applications
you have probably seen the pattern where an email field (or a similar sort of
input) is validated _as you type_.

This seems like the sort of interactivity that is only possible with a
sophisticated, complex JavaScript framework, right?

Well, no.

It turns out that you can implement this functionality in htmx, using pure HTML
attributes.

#index[hx-trigger][change]
#index[hx-trigger][keyup]
#index[event][change]
#index[event][keyup]
In fact, all we need to do is to change our trigger. Currently, we are using the
default trigger for inputs, which is the `change` event. To validate as the user
types, we would want to capture the `keyup` event as well:

#figure(caption: [Triggering With `keyup` events],
```html
<p>
  <label for="email">Email</label>
  <input name="email" id="email" type="email"
    hx-get="/contacts/{{ contact.id }}/email"
    hx-target="next .error"
    hx-trigger="change, keyup" <1>
    placeholder="Email" value="{{ contact.email }}">
  <span class="error">{{ contact.errors['email'] }}</span>
</p>
```)
1. An explicit `keyup` trigger has been added along with `change`.

With this tiny change, every time a user types a character we will issue a
request and validate the email. Simple.

==== Debouncing Our Validation Requests <_debouncing_our_validation_requests>

#index[debouncing]
Simple, yes, but probably not what we want: issuing a new request on every key
up event would be very wasteful and could potentially overwhelm your server.
What we want instead is only issue the request if the user has paused for a
small amount of time. This is called
"debouncing" the input, where requests are delayed until things have
"settled down".

Htmx supports a `delay` modifier for triggers that allows you to debounce a
request by adding a delay before the request is sent. If another event of the
same kind appears within that interval, htmx will not issue the request and will
reset the timer.

This turns out to be exactly what we want for our email input: if the user is
busy typing in an email we won’t interrupt them, but as soon as they pause or
leave the field, we’ll issue a request.

Let’s add a delay of 200 milliseconds to the `keyup` trigger, which is long
enough to detect that the user has stopped typing.:

#figure(caption: [Debouncing the `keyup` event],
```html
<p>
  <label for="email">Email</label>
  <input name="email" id="email" type="email"
    hx-get="/contacts/{{ contact.id }}/email"
    hx-target="next .error"
    hx-trigger="change, keyup delay:200ms" <1>
    placeholder="Email" value="{{ contact.email }}">
  <span class="error">{{ contact.errors['email'] }}</span>
</p>
```)
1. We debounce the `keyup` event by adding a `delay` modifier.

Now we no longer issue a stream of validation requests as the user types.
Instead, we wait until the user pauses for a bit and then issue the request.
Much better for our server, and still a great user experience.

==== Ignoring Non-Mutating Keys <_ignoring_non_mutating_keys>
There is one last issue we should address with the keyup event: as it stands we
will issue a request no matter _which_ keys are pressed, even if they are keys
that have no effect on the value of the input, such as arrow keys. It would be
better if there were a way to only issue a request if the input value has
changed.

#index[event modifier][changed]
And it turns out that htmx has support for that exact pattern, by using the `changed` modifier
for events. (Not to be confused with the `change`
event triggered by the DOM on input elements.)

By adding `changed` to our `keyup` trigger, the input will not issue validation
requests unless the keyup event actually updates the inputs value:

#figure(caption: [Only sending requests when the input value changes],
```html
<p>
  <label for="email">Email</label>
  <input name="email" id="email" type="email"
    hx-get="/contacts/{{ contact.id }}/email"
    hx-target="next .error"
    hx-trigger="change, keyup delay:200ms changed" <1>
    placeholder="Email" value="{{ contact.email }}">
  <span class="error">{{ contact.errors['email'] }}</span>
</p>
```)
1. We do away with pointless requests by only issuing them when the input’s value
  has actually changed.

That’s some pretty good-looking and powerful HTML, providing an experience that
most developers would think requires a complicated client-side solution.

With a total of three attributes and a simple new server-side endpoint, we have
added a fairly sophisticated user experience to our web application. Even
better, any email validation rules we add on the server side will _automatically_ just
work using this model: because we are using hypermedia as our communication
mechanism there is no need to keep a client-side and server-side model in sync
with one another.

A great demonstration of the power of the hypermedia architecture!

=== Another Application Improvement: Paging <_another_application_improvement_paging>

#index[htmx patterns][paging]
Let’s move on from the contact editing page for a bit and improve the root page
of the application, found at the `/contacts` path and rendering the `index.html` template.

Currently, Contact.app does not support paging: if there are 10,000 contacts in
the database we will show all 10,000 contacts on the root page. Showing so much
data can bog a browser (and a server) down, so most web applications adopt a
concept of "paging" to deal with data sets this large, where only one "page" of
a smaller number of items is shown, with the ability to navigate around the
pages in the data set.

Let’s fix our application so that we only show ten contacts at a time with a "Next"
and "Previous" link if there are more than 10 contacts in the contact database.

The first change we will make is to add a simple paging widget to our
`index.html` template.

We will conditionally include two links:
- If we are beyond the "first" page, we will include a link to the previous page
- If there are ten contacts in the current result set, we will include a link to
  the next page

This isn’t a perfect paging widget: ideally we’d show the number of pages and
offer the ability to do more specific page navigation, and there is the
possibility that the next page might have 0 results in it since we aren’t
checking the total results count, but it will do for now for our simple
application.

Let’s look at the jinja template code for this in `index.html`.

#figure(caption: [Adding paging widgets to our list of contacts],
```html
<div>
  <span style="float: right"> <1>
    {% if page > 1 %}
      <a href="/contacts?page={{ page - 1 }}">Previous</a> <2>
    {% endif %}
    {% if contacts|length == 10 %}
      <a href="/contacts?page={{ page + 1 }}">Next</a> <1>
    {% endif %}
  </span>
</div>
```)
1. Include a new div under the table to hold our navigation links.
2. If we are beyond page 1, include an anchor tag with the page decremented by one.
3. If there are 10 contacts in the current page, include an anchor tag linking to
  the next page by incrementing it by one.

Note that here we are using a special jinja filter syntax
`contacts|length` to compute the length of the contacts list. The details of
this filter syntax is beyond the scope of this book, but in this case you can
think of it as invoking the `contacts.length` property and then comparing that
with `10`.

Now that we have these links in place, let’s address the server-side
implementation of paging.

We are using the `page` request parameter to encode the paging state of the UI.
So, in our handler, we need to look for that `page` parameter and pass that
through to our model, as an integer, so the model knows which page of contacts
to return:

#figure(caption: [Adding paging to our request handler],
```python
@app.route("/contacts")
def contacts():
    search = request.args.get("q")
    page = int(request.args.get("page", 1)) <1>
    if search is not None:
        contacts_set = Contact.search(search)
    else:
        contacts_set = Contact.all(page) <2>
    return render_template("index.html",
      contacts=contacts_set, page=page)
```)
1. Resolve the page parameter, defaulting to page 1 if no page is passed in.
2. Pass the page through to the model when loading all contacts so it knows which
  page of 10 contacts to return.

This is fairly straightforward: we just need to get another parameter, like the `q` parameter
we passed in for searching contacts earlier, convert it to an integer and then
pass it through to the `Contact`
model, so it knows which page to return.

And, with that small change, we are done: we now have a very basic paging
mechanism for our web application.

And, believe it or not, it is already using AJAX, thanks to our use of
`hx-boost` in the application. Easy!

==== Click To Load <_click_to_load>

#index[htmx patterns][click to load]
This paging mechanism is fine for a basic web application, and it is used
extensively on the internet. But it has some drawbacks associated with it: every
time you click the "Next" or "Previous" buttons you get a whole new page of
contacts and lose any context you had on the previous page.

Sometimes a more advanced paging UI pattern might be better. Maybe, rather than
loading in a new page of elements and replacing the current elements, it would
be nicer to append the next page of elements
_inline_, after the current elements.

This is the common "click to load" UX pattern, found in more advanced web
applications.

#figure(
  caption: [A Click To Load UI], image("images/screenshot_click_to_load.png"),
)<fig-clicktoload>

In @fig-clicktoload, you have a button that you can click, and it will load the next set of
contacts directly into the page, rather than "paging" to the next page. This
allows you to keep the current contacts "in context" visually on the page, but
still progress through them as you would in a normal, paged user interface.

Let’s see how we can implement this UX pattern in htmx.

It’s actually surprisingly simple: we can just take the existing "Next" link and
repurpose it a bit using nothing but a few htmx attributes!

#index[hx-select][example]
We want to have a button that, when clicked, appends the rows from the next page
of contacts to the current, existing table, rather than re-rendering the whole
table. This can be achieved by adding a new row to our table that has just such
a button in it:

#figure(caption: [Changing to "click to load"],
```html
<tbody>
{% for contact in contacts %}
  <tr>
    <td>{{ contact.first }}</td>
    <td>{{ contact.last }}</td>
    <td>{{ contact.phone }}</td>
    <td>{{ contact.email }}</td>
    <td>
      <a href="/contacts/{{ contact.id }}/edit">Edit</a>
      <a href="/contacts/{{ contact.id }}">View</a></td>
  </tr>
{% endfor %}
{% if contacts|length == 10 %} <1>
  <tr>
    <td colspan="5" style="text-align: center">
      <button hx-target="closest tr" <2>
        hx-swap="outerHTML" <3>
        hx-select="tbody > tr" <4>
        hx-get="/contacts?page={{ page + 1 }}">
        Load More
      </button>
    </td>
  </tr>
{% endif %}
</tbody>
```)
1. Only show "Load More" if there are 10 contact results in the current page.
2. Target the closest enclosing row.
3. Replace the entire row with the response from the server.
4. Select out the table rows from the response.

Let’s go through each attribute in detail here.

First, we are using `hx-target` to target the "closest" `tr` element, that is,
the closest _parent_ table row.

Second, we want to replace this _entire_ row with whatever content comes back
from the server.

Third, we want to yank out only the `tr` elements in the response. We are
replacing this `tr` element with a new set of `tr` elements, which will have
additional contact information in them, as well as, if necessary, a new "Load
More" button that points to the _next_ next page. To do this, we use a CSS
selector `tbody > tr` to ensure we only pull out the rows in the body of the
table in the response. This avoids including rows in the table header, for
example.

Finally, we issue an HTTP `GET` to the url that will serve the next page of
contacts, which looks just like the "Next" link from above.

Somewhat surprisingly, no server-side changes are necessary for this new
functionality. This is because of the flexibility that htmx gives you with
respect to how it processes server responses.

So, four attributes, and we now have a sophisticated "Click To Load" UX, via
htmx.

==== Infinite Scroll <_infinite_scroll>

#index[htmx patterns][infinite scroll]
Another common pattern for dealing with large sets of things is known as the "Infinite
Scroll" pattern. In this pattern, as the last item of a list or table of
elements is scrolled into view, more elements are loaded and appended to the
list or table.

Now, this behavior makes more sense in situations where a user is exploring a
category or series of social media posts, rather than in the context of a
contact application. However, for completeness, and to just show what you can do
with htmx, we will implement this pattern as well.

It turns out that we can repurpose the "Click To Load" code to implement this
new pattern quite easily: if you think about it for a moment, infinite scroll is
really just the "Click To Load" logic, but rather than loading when a click
event occurs, we want to load when an element is "revealed" in the view portal
of the browser.

As luck would have it, htmx offers a synthetic (non-standard) DOM event,
`revealed` that can be used in tandem with the `hx-trigger` attribute, to
trigger a request when, well, when an element is revealed.

#index[hx-select][example]
So let’s convert our button to a span and take advantage of this event:

#figure(caption: [Changing to "infinite scroll"],
```html
{% if contacts|length == 10 %}
  <tr>
    <td colspan="5" style="text-align: center">
      <span hx-target="closest tr"
        hx-trigger="revealed"
        hx-swap="outerHTML"
        hx-select="tbody > tr"
        hx-get="/contacts?page={{ page + 1 }}">Loading More...</span>
    </td>
  </tr>
{% endif %}
```)
1. We have converted our element from a button to a span, since the user will not
  be clicking on it.
2. We trigger the request when the element is revealed, that is when it comes into
  view in the portal.

All we needed to do to convert from "Click to Load" to "Infinite Scroll" was to
update our element to be a span and then add the `revealed` event trigger.

The fact that switching to infinite scroll was so easy shows how well htmx
generalizes HTML: just a few attributes allow us to dramatically expand what we
can achieve in the hypermedia.

And, again, we are doing all this while taking advantage of the RESTful model of
the web. Despite all this new behavior, we are still exchanging hypermedia with
the server, with no JSON API response to be seen.

As the web was designed.

#html-note[Caution With Modals and "display: none"][
#index[modal window]
#index[display: none]
_Think twice about modals._ Modal windows have become popular, almost standard,
in many web applications today.

Unfortunately, modal windows do not play well with much of the infrastructure of
the web and introduce client-side state that can be difficult (though not
impossible) to integrate cleanly with the hypermedia-based approach.

Modal windows can be used safely for views that don’t constitute a resource or
correspond to a domain entity:

- Alerts

- Confirmation dialogs

- Forms for creating/updating entities

Otherwise, consider using alternatives such as inline editing, or a separate
page, rather than a modal.

_Use `display: none;` with care_. The issue is that it is not purely cosmetic
--- it also removes elements from the accessibility tree and keyboard focus.
This is sometimes done to present the same content to visual and aural
interfaces. If you want to hide an element visually without hiding it from
assistive technology (e.g. the element contains information that is communicated
through styling), you can use this utility class:

#figure(
```css
.vh {
    clip: rect(0 0 0 0);
    clip-path: inset(50%);
    block-size: 1px;
    inline-size: 1px;
    overflow: hidden;
    white-space: nowrap;
}
```)

`vh` is short for "visually hidden." This class uses multiple methods and
workarounds to make sure no browser removes the element’s function.
]
