设计模式之Delegate

#设计模式 #Ruby #Objective-C

代理模式在有的地方叫委托模式, 是软件设计模式中的一项基本技巧。

在委托模式中，有两个对象参与处理同一个请求，接受请求的对象将请求委托给另一个对象来处理。

说白了, 就是为其他对象提供一个代理, 以控制对这个对象的访问，降低对象间的耦合.

在Objective-C中, delegate模式是很常见的, 像UITableViewDelegate, UIScrollViewDelegate等都是, 实现起来也是非常简单.

例如, 点击了屏幕上的VideoView后, 可以告诉它的代理对象做一些更新, 比方改变屏幕背景颜色.

VideoView.h

#import <UIKit/UIKit.h>

@protocol VideoViewDelegate;

@interface VideoView : UIView
{
	id<VideoViewDelegate> delegate;
}

@property (assign) id<VideoViewDelegate> delegate;

@end

@protocol VideoViewDelegate <NSObject>
@required
- (void)touchVideoView:(VideoView *)videoView;
@end

VideoView.m

#import "VideoView.h"

@implementation VideoView

#pragma mark
#pragma mark Touch Events

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    if (self.delegate && [delegate respondsToSelector:@selector(touchVideoView:)]) 
    {
        [delegate touchVideoView:self];
    }
}

@end

VideoViewController.h 中声明代理协议:

#import "VideoView.h"

@interface VideoViewController : UIVIewController <VideoViewDelegate>
{
}

@end

VideoViewController.m 中初始化videoView, 并给其设置delegate对象:

#import "VideoViewController.h"

@implementation VideoViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view from its nib.

    CGRect rect = CGRectMake(50, 50, 100, 100);
	VideoView *videoView = [[VideoView alloc] initWithFrame:rect];
	videoView.backgroundColor = [UIColor redColor];
	videoView.delegate = self;
	[scrollView addSubview:videoView];
}

#pragma mark
#pragma mark Video Vide Delegate

- (void)touchVideoView:(VideoView *)videoView
{
    // do something when touch vide view
	self.view.backgroudColor = [UIColor blueColor];
}

@end

在Rails中, delegate被作为Module的类方法集成到了ActiveSupport中:

class User < ActiveRecord::Base
  has_many :actions 
  delegate :inbox, :to => :actions
end
 
class Action < ActiveRecord::Base
  belongs_to :user
 
  class << self
    def inbox
      find(:all, :conditions => ['inbox = 1'])
    end
  end
end
 
user = User.find(1)
user.inbox # instead of user.actions.inbox

参考: http://api.rubyonrails.org/classes/Module.html#method-i-delegate