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This chapter is from the book

Using Protocols and Delegates

Many people consider protocols and delegates to be advanced topics in Objective-C. However, as you will see, they are critical in the table views that are often used to manipulate Core Data objects as well as in Core Data itself. They are also used throughout the iOS and Mac OS frameworks. This section explains that there are several pieces to the puzzle, but they fit together the same way in every case. Once you've worked through a few of them, they will become very natural. In particular, you will see that a lot of the details need no attention from you when you use a protocol or delegate. This section shows you how they work, but soon you will appreciate the fact that they are another part of the operating system that just works without too much of your attention.

Looking Up the Background of Protocols and Delegates

Object-oriented programming offered (and continues to offer) very powerful ways to build and maintain code. One issue arose quite early—multiple inheritance. For simple classroom examples, it is easy to propose a base class of Toy, with subclasses of Ball, Jump rope, and Puzzle. It is also easy to propose a base class of Sports Equipment with subclasses of Ball, Jump rope, Puzzle, Net, and Score board.

Both of these object hierarchies refer to real-life objects, and both make sense to most people. However, as soon as you start programming with those objects, you might find that you want an object such as a ball to have some variables and behaviors that descend from Toy and some that descend from Sports Equipment. In other words, can a ball have two superclasses (or ancestors)?

Many proposals have been made and implemented for solving the multiple inheritance problem. Objective-C started out addressing that issue and has evolved a structure that handles multiple inheritance. However, it also covers a number of other long-time object-oriented programming issues.

The Objective-C approach that has evolved allows you to share functionality between two objects without using inheritance. To be sure, inheritance is used throughout Objective-C, but the very deep inheritance hierarchies that often evolve in languages such as C++ are far less common in Objective-C. Instead, you can take a defined chunk of functionality and share it directly.

A major distinction between extending a class by subclassing it and extending a class by adding a protocol to it is that a subclass can add or modify methods and can also add new instance variables. Protocols, like categories that are described briefly at the end of this section, only add methods.

Using an Example of a Protocol

An example of this is found in the iOS sample code for Multiple Detail Views (http://developer.apple.com/library/ios/#samplecode/MultipleDetailViews/). This code addresses an issue that arises with some iPad apps. iOS supports a split view in which the main view fills the screen when the device is vertically oriented; when the device is horizontally oriented, the right and larger part of the screen shows the detail view, but, at the left, a list of items controls what is shown in the larger view.

In the vertical orientation, a control bar at the top of the window contains a button that will let you open a popover with the list of items that can be shown at the left.

The problem arises because the control bar at the top of the window can be a navigation bar or a toolbar. These are two different types of controls. The button to bring up the popover needs to be shown (in portrait mode) and hidden (in landscape mode). The code to implement this differs whether the button is added to a toolbar or to a navigation bar.

The key to this consists of four steps:

  • Declaring a protocol—A protocol is declared. It is a set of methods presented as they would be in an interface.
  • Adopting the protocol—Any class in this sample app that wants to be able to use this protocol must adopt it in its header. Adopting a protocol means that the class declared in the header must implement methods from the protocol. (Note that ones marked optional do not have to be implemented. This is another Objective-C 2.0 improvement.)
  • Implementing the protocol—Any class that adopts the protocol must implement all required methods and might implement other methods. The implementations might use variables and other methods of the particular class that adopts the protocol.
  • Using the protocol.

The code is described in the following sections.

The first step is to define the protocol in RootViewController.h, as shown in Listing 3.6.

Listing 3.6 Defining the Protocol

@protocol SubstitutableDetailViewController
- (void)showRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem;
- (void)invalidateRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem;
@end

Beginning with Objective-C 2.0, you can indicate which methods are required or optional. The default is required, so the code in Listing 3.6 actually is the same as the code shown in Listing 3.7.

Listing 3.7 Marking Protocol Methods Required or Optional

@protocol SubstitutableDetailViewController
@required
- (void)showRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem;
- (void)invalidateRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem;
@end

After you have declared a protocol, you need to adopt it. Listing 3.8 shows the code from the sample app for a view with a toolbar. Although the protocol is declared in RootViewController.h, it is adopted in FirstDetailViewController.h (and in the second one, too).

Listing 3.8 Protocol Adoption with a Toolbar

@interface FirstDetailViewController : UIViewController <
SubstitutableDetailViewController> {

  UIToolbar *toolbar;
}

Listing 3.9 shows the protocol adopted by another view that uses a navigation bar.

Listing 3.9 Protocol Adoption with a Navigation Bar

@interface SecondDetailViewController : UIViewController <
SubstitutableDetailViewController> {

  UINavigationBar *navigationBar;
}

Each of the classes that has adopted the protocol must implement its methods. Listing 3.10 shows the implementation of the protocol with a toolbar in FirstDetailViewController.m.

Listing 3.10 Implementation of the Protocol with a Toolbar

#pragma mark -
#pragma mark Managing the popover

- (void)showRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem {

// Add the popover button to the toolbar.

NSMutableArray *itemsArray = [toolbar.items mutableCopy];
  [itemsArray insertObject:barButtonItem atIndex:0];
  [toolbar setItems:itemsArray animated:NO];
  [itemsArray release];
}

- (void)invalidateRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem {

// Remove the popover button from the toolbar.

NSMutableArray *itemsArray = [toolbar.items mutableCopy];
  [itemsArray removeObject:barButtonItem];
  [toolbar setItems:itemsArray animated:NO];
  [itemsArray release];
}

In Listing 3.11, you see how you can implement the protocol with a navigation bar. (This code is from SecondDetailViewController.m.)

Listing 3.11 Implementation of the Protocol with a Navigation Bar

#pragma mark -
#pragma mark Managing the popover

- (void)showRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem {

// Add the popover button to the left navigation item.
  [navigationBar.topItem setLeftBarButtonItem:barButtonItem animated:NO];
}

- (void)invalidateRootPopoverButtonItem:(UIBarButtonItem *)barButtonItem {

// Remove the popover button.
  [navigationBar.topItem setLeftBarButtonItem:nil animated:NO];
}

The next step is to adopt another protocol. This protocol, UISplitViewControllerDelegate, is part of the Cocoa framework, so you do not have to write it. All you have to do is adopt it as the RootViewController class in the example does. The Interface is shown in Listing 3.12 together with the adoption of the protocol in RootViewController.h. To repeat, what that adoption statement (in the < and >) means is that all required methods of the protocol will be implemented by this class.

Listing 3.12 Adopting the UISplitViewControllerDelegate Protocol

@interface RootViewController : UITableViewController
  <UISplitViewControllerDelegate> {
UISplitViewController *splitViewController;

UIPopoverController *popoverController;
UIBarButtonItem *rootPopoverButtonItem;
}

Having promised to implement the required and (possibly) optional methods of the UISplitViewControllerDelegate protocol, RootViewController.m must do so. The sample app implements two of the methods as shown in Listing 3.13. In doing so, it has fulfilled the promise made when it adopted the UISplitViewControllerDelegate protocol.

There are two critical lines, one in each method of Listing 3.13. Those lines are the same in both methods and are underlined. It is easiest to start reading them from the middle. The heart of each line is the assignment of a local variable, *detailViewController, using the split view controller's array of view controllers and selecting item one.

This local variable is declared as being of type UIViewController and adopting the SubstitutableDetailViewController protocol shown previously in Listing 3.6. Because it adopts the protocol, it is safe to assume that it implements all the required methods. Because nothing is marked optional, both methods are required, so it is certain that they will be there (if they are not, that assignment statement will fail).

Listing 3.13 Implementing the protocol in RootViewController.m

- (void)splitViewController:(UISplitViewController*)svc
  willHideViewController:(UIViewController *)aViewController
  withBarButtonItem:(UIBarButtonItem*)barButtonItem
  forPopoverController:(UIPopoverController*)pc {

// Keep references to the popover controller and the popover button, and tell the
// detail view controller to show the button.
  barButtonItem.title = @"Root View Controller";

self.popoverController = pc;

self.rootPopoverButtonItem = barButtonItem;

UIViewController <SubstitutableDetailViewController> *detailViewController =
  
   [splitViewController.viewControllers objectAtIndex:1];
  
   [detailViewController showRootPopoverButtonItem:rootPopoverButtonItem];
}

- (void)splitViewController:(UISplitViewController*)svc
  willShowViewController:(UIViewController *)aViewController
  invalidatingBarButtonItem:(UIBarButtonItem *)barButtonItem {

// Nil out references to the popover controller and the popover button, and tell
// the detail view controller to hide the button.

UIViewController <SubstitutableDetailViewController> *detailViewController =
      
   [splitViewController.viewControllers objectAtIndex:1];
      
   [detailViewController invalidateRootPopoverButtonItem:rootPopover
        
   ButtonItem];

self.popoverController = nil;

self.rootPopoverButtonItem = nil;
}

You might have to trace through the code again, but it is worth it to get the hang of it. The point is that this locally declared class inherits from a standard class in the framework (UIViewController). However, by creating and adopting its own protocol, two separate classes with two different ways of implementing control bars can both promise to do the same thing, albeit in different ways because they have different types of control bars to work with.

Using Delegates

Protocols are often paired with delegates, another key Objective-C concept. As noted previously in this hour, instead of calling procedures, messages are sent to objects in Objective-C. That makes the use of delegates possible. A class can declare a delegate for itself. That delegate processes messages sent to the object itself. Frequently, functionality is wrapped up in a protocol as you have seen here, and some of those protocols are designed to be used by delegates.

For example, you saw in Listing 3.12 that the RootViewController class adopts the UISplitViewControllerDelegate protocol. This means that a RootViewController can be named as the delegate of an object that requires that protocol to be implemented.

  • This is a high-level view of delegates and protocols. You will find more examples and much more detail in Part IV, "Using Data Sources and Table Views." If it is a little fuzzy now, do not worry.
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