How do I lay out my widgets? Where is my XML layout file?

In Android, you write layouts in XML, but in Flutter you write your layouts
with a widget tree.

The following example shows how to display a simple widget with padding:

You can view some of the layouts that Flutter has to offer in the
widget catalog.

How do I add or remove a component from my layout?

In Android, you call addChild() or removeChild()
on a parent to dynamically add or remove child views.
In Flutter, because widgets are immutable there is
no direct equivalent to addChild().  Instead,
you can pass a function to the parent that returns a widget,
and control that child’s creation with a boolean flag.

For example, here is how you can toggle between two
widgets when you click on a FloatingActionButton:

How do I animate a widget?