假设对象A调用对象B来执行操作.一旦操作完成,对象A应该知道B已完成任务并采取必要的操作.这是在代表的帮助下实现的.
上例中的关键概念是 :
A是B的委托对象.
B将引用A.
A将实现B的委托方法.
B将通过委托方法通知A.
步骤1 : 首先,创建一个单一的视图应用程序.
第2步 : 然后选择File → 新的 → 文件...
第3步 : 然后选择Objective C Class并单击Next.
步骤4 : 给类命名,比如SampleProtocol,子类为NSObject,如下所示.
第5步 : 然后选择创建.
步骤6 : 将协议添加到SampleProtocol.h文件中,更新后的代码如下<
#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
第7步 : 通过更新SampleProtocol.m文件来实现实例方法,如下所示.
#import "SampleProtocol.h" @implementation SampleProtocol -(void)startSampleProcess { [NSTimer scheduledTimerWithTimeInterval:3.0 target:self.delegate selector:@selector(processCompleted) userInfo:nil repeats:NO]; } @end
第8步 : 通过将标签从对象库拖到UIView,在ViewController.xib中添加UILabel,如下所示.
第9步 : 为标签创建一个IBOutlet并将其命名为myLabel并按如下所示更新代码以在ViewController.h中采用SampleProtocolDelegate.
#import <UIKit/UIKit.h> #import "SampleProtocol.h" @interface ViewController : UIViewController<SampleProtocolDelegate> { IBOutlet UILabel *myLabel; } @end
步骤10 实现委托方法,为SampleProtocol创建对象并调用startSampleProcess方法.更新的ViewController.m文件如下 :
#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
步骤11 我们将看到如下输出.最初,标签显示"processing ...",一旦SampleProtocol对象调用委托方法,它就会更新.