GNU Smalltalk Library Reference: BLOX.BDropDown-callbacks

callback: Answer a DirectedMessage that is sent when the receiver is clicked, or nil if none has been set up.
callback: aReceiver message: aSymbol: Set up so that aReceiver is sent the aSymbol message (the name of a zero- or one-argument selector) when the receiver is clicked. If the method accepts an argument, the receiver is passed.
invokeCallback: Generate a synthetic callback