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The responder chain is an ad hoc collection of objects, gathered on the fly when necessary during the life of an application, that can be queried to see if they implement a particular action. This lets certain actions be configured in a generic way, so that at runtime they will be invoked on the object that makes most sense at the time. The chain is arranged in order of specificity, starting with the object that is nearest to the action, and continuing along toward the most generic. Configuring an object to use the responder chain
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CHAPTER 10: Windows and Menus and Sheets
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is done in Interface Builder by connecting to an action on the nib s First Responder icon, which is nothing more than a proxy for the first object in the responder chain that says, at runtime, Yes I can when asked if it implements a particular method. This is all made somewhat more confusing by the fact that each window has its own notion of a first responder, which is typically the control or view that the user last interacted with (thereby making it a likely candidate for receiving key-presses, and the like). Let s try to clarify this with an example. Consider the case of a button whose target/action is configured to call a method called showThing: on the First Responder. When a user clicks the button, each of a list of objects will be asked, in order, if they implement a showThing: method, right up until one of them answers YES, at which point that object s showThing: method is called, and the responder chain s work is done. Here s an example of what the responder chain can look like: 1. The window s first responder (the view that s currently in focus and accepting keyboard input), its superview, the superview s superview, and so on, all the way up the view hierarchy within the window The window itself The window s delegate The application object, NSApp The application object s delegate
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2. 3. 4. 5.
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The responder chain may contain additional objects as well, especially if you re working on a Documentbased application, in which case open documents and their controller s will have a spot in the chain as well. More on that in 11. As soon as any one of those objects says it implements showThing:, then the method is called on that object, and the search is over. Now go to Interface Builder, where MainMenu.xib is still shown. If you re using Snow Leopard, you should see an instance of WindowLabAppDelegate already set up in your nib file. Otherwise, drag a plain object from the Library into your main nib window, and change its class to WindowLabAppDelegate using the Identity Inspector. Then Ctrl-drag from the Application icon to the WindowLabAppDelegate instance and connect the application s delegate outlet. Now open up the empty window contained in your nib, then drag first a wrapping label and then a button from the Library into the empty window. Then lay the window out roughly as shown in Figure 10 3.
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Figure 10 3. A very simple window layout
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CHAPTER 10: Windows and Menus and Sheets
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Next, Ctrl-drag from the button to the app delegate in the main nib window, and connect to its showColorPanel: action, and then Ctrl-drag from the app delegate to the label, connecting the title outlet there. Now save your work, switch back to Xcode, and hit Build & Run. Your new app will appear, and clicking the button will bring up the color panel. Click around on some different colors, and the color of the selected text will immediately change to reflect the new selection. So, considering that the color panel has no direct connection to our app delegate, it s fair to wonder: how does that work How does the changeColor: method in our app delegate get called The key is the use of the responder chain, as described earlier. NSColorPanel uses the responder chain to find an object that implements the changeColor: method. As the application s delegate, our little controller object is one of the last objects queried to see if it implements the method, and since it does, it gets called. Note that if the window implemented the method itself, or if it had a delegate that implemented the method, one of those methods would have been called instead. Now we need to end this section with a reality-check. In reality, what we just did can be more easily (and more handsomely) accomplished by using an NSColorWell, a special control that launches the NSColorPanel when clicked. We d only need to write code to declare a property in a controller class to contain an NSColor, and then use Cocoa Bindings to bind the NSColorWell s Value attribute and the NSTextField s Text Color attribute to the property in our controller. This example is included here, as-is, mainly to show you how to use the color panel from your own code, as well as give you a first look at the responder chain concept.
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