Our widgets are exposed to the real world; they get clicked on, the
mouse moves over them, the user tabs into them; all these things, and
more, generate
events that we can capture. You can create a
binding from an event on a particular widget to a block of
code, using the widget's
bind method.
For instance, suppose we've created a button widget that displays an image.
We'd like the image to change when the user's mouse is over the
button.
image1 = TkPhotoImage.new { file "img1.gif" }
image2 = TkPhotoImage.new { file "img2.gif" }
b = TkButton.new(@root) {
image image1
command proc { doit }
}
b.bind("Enter") { b.configure('image'=>image2) }
b.bind("Leave") { b.configure('image'=>image1) }
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First, we create two GIF image objects
from files on disk, using
TkPhotoImage. Next we create a button (very cleverly named
``b''), which displays the image
image1. We then bind the
``Enter'' event so that it dynamically changes the image displayed by
the button to
image2, and the ``Leave'' event to revert back to
image1.
This example shows the simple events ``Enter'' and ``Leave.'' But the
named event given as an argument to
bind can be composed of
several substrings, separated with dashes, in the order
modifier-modifier-type-detail. Modifiers are listed in the Tk
reference and include
Button1,
Control,
Alt,
Shift, and so on.
Type is the name of the event (taken
from the X11 naming conventions) and includes events such as
ButtonPress,
KeyPress, and
Expose.
Detail is
either a number from 1 to 5 for buttons or a keysym for keyboard
input. For instance, a binding that will trigger on mouse release of
button 1 while the control key is pressed could be specified as:
Control-Button1-ButtonRelease
or
Control-ButtonRelease-1
The event itself can contain certain fields such as the time of the event
and the x and y positions.
bind can pass these items to the callback,
using
event field codes. These are used like
printf
specifications. For instance, to get the x and y coordinates on
a mouse move, you'd specify the call to
bind with three
parameters. The second parameter is the
Proc for the callback, and
the third parameter is the event field string.
canvas.bind("Motion", proc{|x, y| do_motion (x, y)}, "%x %y")
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