Slaven Rezić > Tk > Tk::bind


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Tk::bind - Arrange for X events to invoke callbacks


Retrieve bindings:





Associate and destroy bindings:




The bind method associates callbacks with X events. If callback is specified, bind will arrange for callback to be evaluated whenever the event(s) given by sequence occur in the window(s) identified by $widget or tag. If callback is an empty string then the current binding for sequence is destroyed, leaving sequence unbound. In all of the cases where a callback argument is provided, bind returns an empty string.

If sequence is specified without a callback, then the callback currently bound to sequence is returned, or undef is returned if there is no binding for sequence. If neither sequence nor callback is specified, then the return value is a list whose elements are all the sequences for which there exist bindings for tag.

If no tag is specified then the bind refers to $widget. If tag is specified then it is typically a class name and the bind refers to all instances of the class on the MainWindow associated with $widget. (It is possible for tag to be another "widget object" but this practice is deprecated.) Perl's ref($object) can be used to get the class name of any object. Each window has an associated list of tags, and a binding applies to a particular window if its tag is among those specified for the window. Although the bindtags method may be used to assign an arbitrary set of binding tags to a window, the default binding tags provide the following behavior:

If a tag is the name of an internal window the binding applies to that window.

If the tag is the name of a toplevel window the binding applies to the toplevel window and all its internal windows.

If the tag is the name of a class of widgets, such as Tk::Button, the binding applies to all widgets in that class;

If tag has the value all, the binding applies to all windows descended from the MainWindow of the application.


The sequence argument specifies a sequence of one or more event patterns, with optional white space between the patterns. Each event pat may take one of three forms. In the simplest case it is a single printing ASCII character, such as a or [. The character may not be a space character or the character <. This form of pattern matches a KeyPress event for the particular character. The second form of pattern is longer but more general. It has the following syntax:


The entire event pattern is surrounded by angle brackets, and normally needs to be quoted, as angle brackets are special to perl. Inside the angle brackets are zero or more modifiers, an event type, and an extra piece of information (detail) identifying a particular button or keysym. Any of the fields may be omitted, as long as at least one of type and detail is present. The fields must be separated by white space or dashes.

The third form of pattern is used to specify a user-defined, named virtual event; see Tk::event for details. It has the following syntax:


The entire virtual event pattern is surrounded by double angle brackets. Inside the angle brackets is the user-defined name of the virtual event. Modifiers, such as Shift or Control, may not be combined with a virtual event to modify it. Bindings on a virtual event may be created before the virtual event is defined, and if the definition of a virtual event changes dynamically, all windows bound to that virtual event will respond immediately to the new definition.


Modifiers consist of any of the following values:

 Control        Mod2, M2
 Shift          Mod3, M3
 Lock           Mod4, M4
 Button1, B1    Mod5, M5
 Button2, B2    Meta, M
 Button3, B3    Alt
 Button4, B4    Double
 Button5, B5    Triple
 Mod1,    M1    Quadruple

Where more than one value is listed, separated by commas, the values are equivalent. Most of the modifiers have the obvious X meanings. For example, Button1 requires that button 1 be depressed when the event occurs. For a binding to match a given event, the modifiers in the event must include all of those specified in the event pattern. An event may also contain additional modifiers not specified in the binding. For example, if button 1 is pressed while the shift and control keys are down, the pattern <Control-Button-1> will match the event, but <Mod1-Button-1> will not. If no modifiers are specified, then any combination of modifiers may be present in the event.

Meta and M refer to whichever of the M1 through M5 modifiers is associated with the meta key(s) on the keyboard (keysyms Meta_R and Meta_L). If there are no meta keys, or if they are not associated with any modifiers, then Meta and M will not match any events. Similarly, the Alt modifier refers to whichever modifier is associated with the alt key(s) on the keyboard (keysyms Alt_L and Alt_R).

The Double, Triple and Quadruple modifiers are a convenience for specifying double mouse clicks and other repeated events. They cause a particular event pattern to be repeated 2, 3 or 4 times, and also place a time and space requirement on the sequence: for a sequence of events to match a Double, Triple or Quadruple pattern, all of the events must occur close together in time and without substantial mouse motion in between. For example, <Double-Button-1> is equivalent to <Button-1><Button-1> with the extra time and space requirement.


The type field may be any of the standard X event types, with a few extra abbreviations. Below is a list of all the valid types; where two names appear together, they are synonyms.

    Activate            Destroy            Map
    ButtonPress, Button Enter              MapRequest
    ButtonRelease       Expose             Motion
    Circulate           FocusIn            MouseWheel
    CirculateRequest    FocusOut           Property
    Colormap            Gravity            Reparent
    Configure           KeyPress, Key      ResizeRequest
    ConfigureRequest    KeyRelease         Unmap
    Create              Leave              Visibility

Most of the above events have the same fields and behaviors as events in the X Windowing system. You can find more detailed descriptions of these events in any X window programming book. A couple of the events are extensions to the X event system to support features unique to the Macintosh and Windows platforms. We provide a little more detail on these events here. These include:


These two events are sent to every sub-window of a toplevel when they change state. In addition to the focus Window, the Macintosh platform and Windows platforms have a notion of an active window (which often has but is not required to have the focus). On the Macintosh, widgets in the active window have a different appearance than widgets in deactive windows. The Activate event is sent to all the sub-windows in a toplevel when it changes from being deactive to active. Likewise, the Deactive event is sent when the window's state changes from active to deactive. There are no use- ful percent substitutions you would make when binding to these events.


Some mice on the Windows platform support a mouse wheel which is used for scrolling documents without using the scrollbars. By rolling the wheel, the system will generate MouseWheel events that the application can use to scroll. Like Key events the event is always routed to the window that currently has focus. When the event is received you can use the %D substitution to get the delta field for the event which is a integer value of motion that the mouse wheel has moved. The smallest value for which the system will report is defined by the OS. On Windows 95 & 98 machines this value is at least 120 before it is reported. However, higher resolution devices may be available in the future. The sign of the value determines which direction your widget should scroll. Positive values should scroll up and negative values should scroll down.

The last part of a long event specification is detail. In the case of a ButtonPress or ButtonRelease event, it is the number of a button (1-5). If a button number is given, then only an event on that particular button will match; if no button number is given, then an event on any button will match. Note: giving a specific button number is different than specifying a button modifier; in the first case, it refers to a button being pressed or released, while in the second it refers to some other button that is already depressed when the matching event occurs. If a button number is given then type may be omitted: if will default to ButtonPress. For example, the specifier <1> is equivalent to <ButtonPress-1>.

If the event type is KeyPress or KeyRelease, then detail may be specified in the form of an X keysym. Keysyms are textual specifications for particular keys on the keyboard; they include all the alphanumeric ASCII characters (e.g. ``a'' is the keysym for the ASCII character ``a''), plus descriptions for non-alphanumeric characters (``comma'' is the keysym for the comma character), plus descriptions for all the non-ASCII keys on the keyboard (``Shift_L'' is the keysm for the left shift key, and ``F1'' is the keysym for the F1 function key, if it exists). The complete list of keysyms is not presented here; it is available in other X documentation and may vary from system to system. If necessary, you can use the 'K' notation described below to print out the keysym name for a particular key. If a keysym detail is given, then the type field may be omitted; it will default to KeyPress. For example, <Control-comma> is equivalent to <Control-KeyPress-comma>.


The callback argument to bind is a perl/Tk callback. which will be executed whenever the given event sequence occurs. (See Tk::callbacks for description of the possible forms.) Callback will be associated with the same MainWindow that is associated with the $widget that was used to invoke the bind method, and it will run as though called from MainLoop. If callback contains any Ev(%) calls, then each "nested" Ev(%) "callback" will be evaluated when the event occurs to form arguments to be passed to the main callback. The replacement depends on the character %, as defined in the list below. Unless otherwise indicated, the replacement string is the numeric (decimal) value of the given field from the current event. Perl/Tk has enhanced this mechanism slightly compared to the comparable Tcl/Tk mechanism. The enhancements are not yet all reflected in the list below. Some of the substitutions are only valid for certain types of events; if they are used for other types of events the value substituted is undefined (not the same as undef!).


The number of the last client request processed by the server (the serial field from the event). Valid for all event types.


The above field from the event, formatted as a hexadecimal number. Valid only for Configure events.


The number of the button that was pressed or released. Valid only for ButtonPress and ButtonRelease events.


The count field from the event. Valid only for Expose events.


The detail field from the event. The 'd' is replaced by a string identifying the detail. For Enter, Leave, FocusIn, and FocusOut events, the string will be one of the following:

 NotifyAncestor          NotifyNonlinearVirtual
 NotifyDetailNone        NotifyPointer
 NotifyInferior          NotifyPointerRoot
 NotifyNonlinear         NotifyVirtual

For ConfigureRequest events, the string will be one of:

 Above                   Opposite
 Below                   None
 BottomIf                TopIf

For events other than these, the substituted string is undefined. (Note that this is not the same as Detail part of sequence use to specify the event.)


The focus field from the event (0 or 1). Valid only for Enter and Leave events.


The height field from the event. Valid only for Configure, ConfigureRequest, Create, Expose, and ResizeRequest events.


The window field from the event, represented as a hexadecimal integer.


The keycode field from the event. Valid only for KeyPress and KeyRelease events.


The mode field from the event. The substituted string is one of NotifyNormal, NotifyGrab, NotifyUngrab, or NotifyWhileGrabbed. Valid only for Enter, FocusIn, FocusOut, and Leave events.


The override_redirect field from the event. Valid only for Map, Reparent, and Configure events.


The place field from the event, substituted as one of the strings PlaceOnTop or PlaceOnBottom. Valid only for Circulate and CirculateRequest events.


The state field from the event. For ButtonPress, ButtonRelease, Enter, KeyPress, KeyRelease, Leave, and Motion events, a decimal string is substituted. For Visibility, one of the strings VisibilityUnobscured, VisibilityPartiallyObscured, and VisibilityFullyObscured is substituted.


The time field from the event. Valid only for events that contain a time field.


The width field from the event. Valid only for Configure, ConfigueRequest, Create, Expose, and ResizeREquest events.


The x field from the event. Valid only for events containing an x field.


The y field from the event. Valid only for events containing a y field.


The string "@x,y" where x and y are as above. Valid only for events containing x and y fields. This format is used my methods of Tk::Text and similar widgets.


Substitutes the UNICODE character corresponding to the event, or the empty string if the event doesn't correspond to a UNICODE character (e.g. the shift key was pressed). XmbLookupString does all the work of translating from the event to a UNICODE character. Valid only for KeyPress and KeyRelease events.


The border_width field from the event. Valid only for Configure, ConfigureRequest and Create events.


This reports the delta value of a MouseWheel event. The delta value represents the rotation units the mouse wheel has been moved. On Windows 95 & 98 systems the smallest value for the delta is 120. Future systems may support higher resolution values for the delta. The sign of the value represents the direction the mouse wheel was scrolled.


The send_event field from the event. Valid for all event types.


The keysym corresponding to the event, substituted as a textual string. Valid only for KeyPress and KeyRelease events.


The keysym corresponding to the event, substituted as a decimal number. Valid only for KeyPress and KeyRelease events.


The root window identifier from the event. Valid only for events containing a root field.


The subwindow window identifier from the event, as an object if it is one otherwise as a hexadecimal number. Valid only for events containing a subwindow field.


The type field from the event. Valid for all event types.


The window to which the event was reported (the $widget field from the event) - as an perl/Tk object. Valid for all event types.


The x_root field from the event. If a virtual-root window manager is being used then the substituted value is the corresponding x-coordinate in the virtual root. Valid only for ButtonPress, ButtonRelease, KeyPress, KeyRelease, and Motion events.


The y_root field from the event. If a virtual-root window manager is being used then the substituted value is the corresponding y-coordinate in the virtual root. Valid only for ButtonPress, ButtonRelease, KeyPress, KeyRelease, and Motion events.


It is possible for several bindings to match a given X event. If the bindings are associated with different tag's, then each of the bindings will be executed, in order. By default, a class binding will be executed first, followed by a binding for the widget, a binding for its toplevel, and an all binding. The bindtags method may be used to change this order for a particular window or to associate additional binding tags with the window.

return and Tk->break may be used inside a callback to control the processing of matching callbacks. If return is invoked, then the current callback is terminated but Tk will continue processing callbacks associated with other tag's. If Tk->break is invoked within a callback, then that callback terminates and no other callbacks will be invoked for the event. (Tk->break is implemented via perl's die with a special value which is "caught" by the perl/Tk "glue" code.)

If more than one binding matches a particular event and they have the same tag, then the most specific binding is chosen and its callback is evaluated. The following tests are applied, in order, to determine which of several matching sequences is more specific: (a) an event pattern that specifies a specific button or key is more specific than one that doesn't; (b) a longer sequence (in terms of number of events matched) is more specific than a shorter sequence; (c) if the modifiers specified in one pattern are a subset of the modifiers in another pattern, then the pattern with more modifiers is more specific. (d) a virtual event whose physical pattern matches the sequence is less specific than the same physical pattern that is not associated with a virtual event. (e) given a sequence that matches two or more virtual events, one of the virtual events will be chosen, but the order is undefined.

If the matching sequences contain more than one event, then tests (c)-(e) are applied in order from the most recent event to the least recent event in the sequences. If these tests fail to determine a winner, then the most recently registered sequence is the winner.

If there are two (or more) virtual events that are both triggered by the same sequence, and both of those virtual events are bound to the same window tag, then only one of the virtual events will be triggered, and it will be picked at random:

 $widget->eventAdd('<<Paste>>' => '<Control-y>');
 $widget->eventAdd('<<Paste>>' => '<Button-2>');
 $widget->eventAdd <<Scroll>>' => '<Button-2>');
 $widget->bind('Tk::Entry','<<Paste>>',sub { print 'Paste'});
 $widget->bind('Tk::Entry','<<Scroll>>', sub {print 'Scroll'});

If the user types Control-y, the <<Paste>> binding will be invoked, but if the user presses button 2 then one of either the <<Paste>> or the <<Scroll>> bindings will be invoked, but exactly which one gets invoked is undefined.

If an X event does not match any of the existing bindings, then the event is ignored. An unbound event is not considered to be an error.


When a sequence specified in a bind method contains more than one event pattern, then its callback is executed whenever the recent events (leading up to and including the current event) match the given sequence. This means, for example, that if button 1 is clicked repeatedly the sequence <Double-ButtonPress-1> will match each button press but the first. If extraneous events that would prevent a match occur in the middle of an event sequence then the extraneous events are ignored unless they are KeyPress or ButtonPress events. For example, <Double-ButtonPress-1> will match a sequence of presses of button 1, even though there will be ButtonRelease events (and possibly Motion events) between the ButtonPress events. Furthermore, a KeyPress event may be preceded by any number of other KeyPress events for modifier keys without the modifier keys preventing a match. For example, the event sequence aB will match a press of the a key, a release of the a key, a press of the Shift key, and a press of the b key: the press of Shift is ignored because it is a modifier key. Finally, if several Motion events occur in a row, only the last one is used for purposes of matching binding sequences.


If an error occurs in executing the callback for a binding then the Tk::Error mechanism is used to report the error. The Tk::Error mechanism will be executed at same call level, and associated with the same MainWindow as as the callback was invoked.


Note that for the Canvas widget, the call to bind has to be fully qualified. This is because there is already a bind method for the Canvas widget, which binds individual canvas tags.



Tk::Error Tk::callbacks Tk::bindtags


Event, binding

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