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委托（Delegates）示例

假設(shè)對(duì)象A調(diào)用B來執(zhí)行一項(xiàng)操作，操作一旦完成，對(duì)象A就必須知道對(duì)象B已完成任務(wù)且對(duì)象A將執(zhí)行其他必要操作。

在上面的示例中的關(guān)鍵概念有

• A是B的委托對(duì)象
• B引用一個(gè)A
• A將實(shí)現(xiàn)B的委托方法
• B通過委托方法通知

創(chuàng)建一個(gè)委托（Delegates）對(duì)象

1. 創(chuàng)建一個(gè)單一視圖的應(yīng)用程序

2. 然后選擇文件 File -> New -> File...

3. 然后選擇Objective C單擊下一步

4. 將SampleProtocol的子類命名為NSObject，如下所示

5. 然后選擇創(chuàng)建

6.向SampleProtocol.h文件夾中添加一種協(xié)議，然后更新代碼，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. 將標(biāo)簽從對(duì)象庫拖到UIView，從而在ViewController.xib中添加UILabel，如下所示:

9. 創(chuàng)建一個(gè)IBOutlet標(biāo)簽并命名為myLabel，然后按如下所示更新代碼并在ViewController.h里顯示SampleProtocolDelegate

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

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

10. 完成授權(quán)方法，為SampleProtocol創(chuàng)建對(duì)象和調(diào)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. 將看到如下所示的輸出結(jié)果，最初的標(biāo)簽也會(huì)繼續(xù)運(yùn)行，一旦授權(quán)方法被SampleProtocol對(duì)象所調(diào)用，標(biāo)簽運(yùn)行程序的代碼也會(huì)更新。