Callbacks

Basic usage

Several classes use callbacks to respond to user events. The simplest of these take no arguments

Example: respond to a button press

def button_callback():
    screen.text("Button pressed")

but = gui.Button((40,40),(80,80),label="Button",callback = button_callback)

Notice that button_callback has no brackets when passed to the Button. Other callbacks will take a value dependent on the state of the widget. For example, the callback for a slider will pass it’s current value as a float

Example: display the value of a slider

def slider_callback(value):
    screen.rectangle((0,0),(320,200),"black","topleft")
    screen.text("%d" % int(value))

slider = gui.Slider((0,200),(320,20),align="topleft",
                    max_val = 200, callback = slider_callback)

Passing extra arguments to callbacks

Sometimes it is useful to pass an extra value to the callback, if you have several widgets, where you want to use the same callback. This can be done using lambda.

Example: display which button was pressed

def button_callback(name):
    screen.rectangle((0,80),(320,240),"black","topleft")
    screen.text("Button %s pressed" % name)

but1 = gui.Button((40,40),(80,80),label="1",callback = lambda : button_callback("1"))
but2 = gui.Button((130,40),(80,80),label="2",callback = lambda : button_callback("2"))
but3 = gui.Button((220,40),(80,80),label="3",callback = lambda : button_callback("3"))

If the callback should be passed a value from the widget, then you need to use the form lambda x: as below.

Example: display which slider has changed

def slider_callback(value,name):
    screen.rectangle((0,0),(320,100),"black","topleft")
    screen.text("Slider %s: %d" % (name,int(value)),(160,50))

sld1 = gui.Slider((120,110),(230,20),max_val=200,
                  callback = lambda x: slider_callback(x,"1"))
sld2 = gui.Slider((120,150),(230,20),max_val=200,
                  callback = lambda x: slider_callback(x,"2"))
sld3 = gui.Slider((120,190),(230,20),max_val=200,
                  callback = lambda x: slider_callback(x,"3"))

For more information about lambda the Mouse vs Python blog is a good summary of the subject.