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Responders

A responder is an object that can receive events, either directly or through the responder chain, by virtue of its inheritance from the NSResponder class. NSApplication, NSWindow, NSDrawer, NSWindowController, NSView and the many descendants of these classes in the Application Kit inherit from NSResponder. This class defines the programmatic interface for the reception of event messages and many action messages. It also defines the general structure of responder behavior. Within the responder chain there is a first responder and a sequence of next responders

For more on the responder chain, see “The Responder Chain.” Cocoa Fundamentals Guide also contains an overview of responders and the responder chain in “The Core Application Architecture.“

In this section:

First Responders
Next Responders

First Responders

A first responder is typically a user-interface object that the user selects or activates with the mouse or keyboard. It is usually the first object in a responder chain to receive an event or action message. An NSWindow object’s first responder is initially itself; however, you can set, programmatically and in Interface Builder, the object that is made first responder when the window is first placed on-screen.

When an NSWindow object receives a mouse-down event, it automatically tries to make the NSView object under the event the first responder. It does so by asking the view whether it wants to become first responder, using the acceptsFirstResponder method defined by this class. This method returns NO by default; responder subclasses that need to be first responder must override it to return YES. The acceptsFirstResponder method is also invoked when the user changes the first responder through the keyboard interface control feature.

You can programmatically change the first responder by sending makeFirstResponder: to an NSWindow object. This message initiates a kind of protocol in which one object loses its first responder status and another gains it. See “Setting the First Responder” for further information.

An NSPanel object presents a variation of first-responder behavior that permits panels to present a user interface that doesn’t take away key focus from the main window. If the panel object representing an inactive window and returning YES from becomesKeyOnlyIfNeeded receives a mouse-down event, it attempts to make the view object under the mouse pointer the first responder, but only if that object returns YES in acceptsFirstResponder and needsPanelToBecomeKey.

Mouse-moved events (type NSMouseMoved) are always sent to the first responder, not to the view under the mouse.

Next Responders

Every responder object has a built-in capability for getting the next responder up the responder chain. The nextResponder method, which returns this object, is the essential mechanism of the responder chain. Figure 1-7 shows the sequence of next responders.

A view’s next responder is always its superview—most of the responder chain, in fact, comprises the views from a window’s first responder up to its content view. When you create a window or add subviews to existing views, either programmatically or in Interface Builder, the Application Kit automatically hooks up the next responders in the responder chain. The addSubview: method of NSView automatically sets the receiver as the new subview’s superview. You should never send setNextResponder: to an NSView object. You can safely add responders to the top end of a window’s responder chain—the NSWindow object itself if it has no delegate or, if it has a delegate, after the delegate.

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