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委托（Delegates）示例

假设对象A调用B来执行一项操作，操作一旦完成，对象A就必须知道对象B已完成任务且对象A将执行其他必要操作。

在上面的示例中的关键概念有

• A是B的委托对象
• B引用一个A
• A将实现B的委托方法
• B通过委托方法通知

创建一个委托（Delegates）对象

1. 创建一个单一视图的应用程序

2. 然后选择文件 File -> New -> File...

3. 然后选择Objective C单击下一步

4. 将SampleProtocol的子类命名为NSObject，如下所示

5. 然后选择创建

6.向SampleProtocol.h文件夹中添加一种协议，然后更新代码，如下所示：

#import <Foundation/Foundation.h>
// Protocol definition starts here 
@protocol SampleProtocolDelegate <NSObject>
@required
- (void) processCompleted;
@end
// Protocol Definition ends here
@interface SampleProtocol : NSObject

{
   // Delegate to respond back
   id <SampleProtocolDelegate> _delegate; 

}
@property (nonatomic,strong) id delegate;

-(void)startSampleProcess; // Instance method

@end

7. Implement the instance method by updating the SampleProtocol.m file as shown below.

#import "SampleProtocol.h"

@implementation SampleProtocol

-(void)startSampleProcess{

    [NSTimer scheduledTimerWithTimeInterval:3.0 target:self.delegate 
	selector:@selector(processCompleted) userInfo:nil repeats:NO];
}
@end

8. 将标签从对象库拖到UIView，从而在ViewController.xib中添加UILabel，如下所示:

9. 创建一个IBOutlet标签并命名为myLabel，然后按如下所示更新代码并在ViewController.h里显示SampleProtocolDelegate

#import <UIKit/UIKit.h>
#import "SampleProtocol.h"

@interface ViewController : UIViewController<SampleProtocolDelegate>
{
    IBOutlet UILabel *myLabel;
}
@end

10. 完成授权方法，为SampleProtocol创建对象和调用startSampleProcess方法。如下所示，更新ViewController.m文件

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    SampleProtocol *sampleProtocol = [[SampleProtocol alloc]init];
    sampleProtocol.delegate = self;
    [myLabel setText:@"Processing..."];
    [sampleProtocol startSampleProcess];
	// Do any additional setup after loading the view, typically from a nib.
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

#pragma mark - Sample protocol delegate
-(void)processCompleted{    
    [myLabel setText:@"Process Completed"];
}

@end

11. 将看到如下所示的输出结果，最初的标签也会继续运行，一旦授权方法被SampleProtocol对象所调用，标签运行程序的代码也会更新。