Purpose: explain the [Tk] concept of [binding]s.  List the different kinds of [bind]ings available.  Show a practical example of use.

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Bindings are the connections between [widget]s that create [event]s, and the code ([proc]edures--except that, properly
speaking, [Tcl] bindings have always been to [script]s) that handle the events.

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 proc makeButton { buttonName buttonText } {
    button .$buttonName -text $buttonText
    bind .$buttonName <Button-1> "$buttonText"
    bind .$buttonName <Button-2> "destroy .$buttonName"
    bind .$buttonName <Button-3> "puts \"$buttonText\""
    pack .$buttonName 
 }
======

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Bindings are a specialisation of the general concept of event handlers to Tk's external event mechanism, and they are needed because [GUI]s are asynchronous things; you don't know what a user is going to do next, and you have to handle a lot of different possibilities.  The only alternative to the system of having events is to use a finite state machine, but writing one of those for a modern GUI is a terrifying concept!

Most system programming languages use callbacks to handle events, but in Tcl it is far more natural to directly nominate a piece of code to execute whenever a particular event arrives, especially as this can always call a procedure if needed.  It is this code that is called a binding (because of the command used to manage this association; '''bind'''.)

But now for an example:

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  pack [label .l -textvariable msg]
  set msg "Dum de dum de dum..."
  bind .l <Enter> {set msg "Hello friend"}
  bind .l <ButtonPress-1> {set msg "You're touching me!"}
  bind .l <ButtonRelease-1> {set msg "You've let go."}
  bind .l <Leave> {set msg "Dum de dum de dum..."}
  bind .l <3> {bell}
======

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[Ken Jones] explains how to think about default bindings in a
[Usenet] posting
[http://groups.google.com/groups?q=tkbuttonenter+args+group%3acomp.lang.tcl&hl=en&lr=&scoring=d&selm=d8f00cc1.0205140534.2c0dfd53%40posting.google.com&rnum=1].

    :   A quick examination of the default Button bindings indicates that a button needs to receive an `<Enter>`, `<ButtonPress-1>`, `<ButtonRelease-1>` sequence to invoke its associated command. It looks as though it's also a good idea to generate a `<Leave>` to clean things up afterwards (as that's the sequence of events a Button would normally receive if a user were directly interacting with the application). <<br>><<br>>
    :   In case you weren't aware of how to do so, you can examine the default widget bindings in a couple of ways. One is by using the Tcl [bind] command to introspect the bindings. Just give [bind] the widget class name as an argument, and it returns a list of all events that have bindings for that widget class. For example: <<br>><<br>>
    :   `bind Button `<<br>><<br>>
    :   To examine a binding action, execute [bind] with both the target (e.g., the widget class) and the event specification. For example: <<br>><<br>>
    :   `bind Button <Enter> `<<br>><<br>>
    :   You'll notice that the default widget bindings call a variety of pre-defined procedures. For example, in Tcl 8.3 the action for receiving an `<Enter>` event on a Button is to execute `tkButtonEnter`, passing the name of the button widget that received the event as an argument. You can use the [info args] and [info body] commands to then introspect on these pre-defined procedures. <<br>><<br>>
    :   Of course, an even easier way to examine all of the built-in bindings is to browse through the library code that implements them all. These are simply Tcl scripts that are automatically [source]d when a Tk application starts. To find them, go to the directory where Tcl is installed on your system, then go to the `lib/tkX.X` subdirectory (where `X.X` is the version of Tcl/Tk you've got installed). The file `tk.tcl` gets [source]d automatically, and it in turn [source]s most of the other files in that directory. Of note in this situation is the `button.tcl` file, which implements the Button, Checkbutton, and Radiobutton bindings and their actions. 

<<categories>> Essay | GUI