Action and Target

A menu item behaves as most controls in the GNUstep gui library: it has an action and a target. The typical example of such a control is a button. Say, for example, that you have created a button called myButton. You want something to happen when the user clicks on the button - to do it, you need to specify an action and a target for the button. For example, if you want the method terminate: of NSApp (the application shared object) to be invoked when the user clicks the button, you do as follows:

NSButton *myButton;
id myObject;

// <missing code: create myButton, myObject etc>

[myButton setAction: @selector (terminate:)];
[myButton setTarget: NSApp];

When the user presses myButton, the gui library sends the terminate: message to the object NSApp, passing as argument myButton, executing the equivalent of the following code:

[NSApp terminate: myButton];

This way, when the user clicks the button, the application quits.

The button which was clicked is passed as an argument to the action so that the receiving object can determine (if needed) which button was actually pressed, so that the same method may be used for more than one button. In our case, NSApp simply discards the argument of terminate:.

In brief, the action is the method to be invoked when the user clicks on the button; the target is the object on which to invoke the method. action should be a selector for a method returning void and taking a single argument (of type id, a generic object). The button which was clicked is passed as an argument to the invocation of action on the target.

The case of a menu item is completely similar. When the user selects this menu item, the gui library performs the menu item action (terminate: in this case) on the menu item target. The argument passed to terminate: is the menu item which was selected.

Usually, no target is specified for a menu item, as in our code

  
[menu addItemWithTitle: @"Quit" 
      action: @selector (terminate:)
      keyEquivalent: @"q"];

which only specifies that the action is terminate:. When no target is specified, the gui library tries to determine an appropriate target dynamically at run-time. For now the only important thing to know is that the gui library will try to send the action to certain objects (roughly, the objects inside a window which have the input focus) and, failing these objects, it will try to send the action to NSApp, and as a last resort to NSApp's delegate. In our example, we want the action terminate: to be sent to NSApp (thus terminating the application when the user selects the Quit menu item), so this automatic mechanism works just fine for us (actually, you will soon discover that this automatic mechanism works fine extremely often). So, we don't need to set explicitly a target for our menu item.