Protocol简单来说就是一系列方法的列表，其中声明的方法可以被任何类实现。这中模式一般称为代理（delegation)模式。

在IOS和OS X开发中，Apple采用了大量的代理模式来实现MVC中View(UI控件）和Controller(控制器）的解耦。

下面我们先来看一下我们熟悉的Android中的按钮监听过程，然后再对比理解delegation。

首先我建立一个很简单的Android工程，在Layout中放置一个按钮，如下：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >
	<Button 
	    android:id="@+id/mybutton"
	    android:layout_width="wrap_content"
	    android:layout_height="wrap_content"
	    android:text="按钮"/>
</LinearLayout>

package com.example.helloword;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.View.OnLongClickListener;
import android.widget.Button;
import android.widget.Toast;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button button = (Button) findViewById(R.id.mybutton);
        button.setOnClickListener(new MyOnClickListener());
    }
    
    class MyOnClickListener implements OnClickListener{

		@Override
		public void onClick(View arg0) {
			Toast.makeText(MainActivity.this, 
					"点击了按钮", Toast.LENGTH_SHORT).show();
		}
    }
    
    class MyonLongClickListener implements OnLongClickListener{

		@Override
		public boolean onLongClick(View arg0) {
			Toast.makeText(MainActivity.this, 
					"长按了按钮", Toast.LENGTH_SHORT).show();
			return false;
		}
    	
    }
}

    /**
     * Interface definition for a callback to be invoked when a view is clicked.
     */
    public interface OnClickListener {
        /**
         * Called when a view has been clicked.
         *
         * @param v The view that was clicked.
         */
        void onClick(View v);
    }

    public void setOnClickListener(OnClickListener l) {
        if (!isClickable()) {
            setClickable(true);
        }
        getListenerInfo().mOnClickListener = l;
    }

    ListenerInfo getListenerInfo() {
        if (mListenerInfo != null) {
            return mListenerInfo;
        }
        mListenerInfo = new ListenerInfo();
        return mListenerInfo;
    }

接下来我们来模拟一下IOS中按钮监听的实现。

Button.h文件

#import <Foundation/Foundation.h>
@class Button;

//<>代表实现某个协议
//这里相当于OnClickListener
@protocol ButtonDelegate <NSObject>

//将Button对象传递给监听器，来判断具体的调用实例
- (void) onClick:(Button *)btn;

@end

@protocol ButtonLongClickDelegate <NSObject>

- (void) onLongClick:(Button *)btn;

@end

@interface Button : NSObject

//delegate就是按钮的监听器
//id代表任何OC对象
@property (nonatomic, retain) id<ButtonDelegate> delegate;
@property (nonatomic, retain) id<ButtonLongClickDelegate> longClickDeleate;

//模拟系统调用click方法
- (void)click;

//模拟系统调用longclick方法
- (void)longClick;

@end

#import "Button.h"

@implementation Button

- (void)click {
    //按钮被点击了，就应该通知监听器（这里是模拟）
    //如果onClick方法被实现，调用onClick方法
    if([_delegate respondsToSelector:@selector(onClick:)]){
        [_delegate onClick:self];
    }else{
        NSLog(@"onClick监听器未实现");
    }
}

- (void)longClick {
    //按钮被长按（模拟系统）
    if([_delegate respondsToSelector:@selector(onClick:)]){
        [_longClickDeleate onLongClick:self];
    }else{
        NSLog(@"longClick监听器未实现");
    }
}

- (void)dealloc {
    [_delegate release];
    [_longClickDeleate release];
    [super dealloc];
}

@end

#import <Foundation/Foundation.h>
@protocol ButtonDelegate;

//实现按钮点击协议
@interface ButtonListener : NSObject <ButtonDelegate>

@end

#import "ButtonListener.h"
#import "Button.h"

@implementation ButtonListener

- (void)onClick:(Button *)btn {
    NSLog(@"按钮被点击了");
}
@end

#import <Foundation/Foundation.h>

//对协议进行提前声明，跟@class的用途是一样的
@protocol ButtonLongClickDelegate;

@interface ButtonLongClickListener : NSObject <ButtonLongClickDelegate>

@end

#import "ButtonLongClickListener.h"
#import "Button.h"

@implementation ButtonLongClickListener

- (void)onLongClick:(Button *)btn{
    NSLog(@"按钮被长按了");
}

@end

#import <Foundation/Foundation.h>
#import "Button.h"
#import "ButtonListener.h"
#import "ButtonLongClickListener.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        //初始化一个按钮
        Button *button = [[[Button alloc] init] autorelease];
        //初始化一个按钮的监听器
        ButtonListener *listener = [[[ButtonListener alloc] init] autorelease];
        //初始化一个按钮长按监听器
        ButtonLongClickListener *longClickListener =
            [[[ButtonLongClickListener alloc] init] autorelease];
        //设置按钮的监听器
        button.delegate = listener;
        //设置长按按钮监听器
        button.longClickDeleate = longClickListener;
        //点击按钮
        [button click];
        //长按按钮
        [button longClick];
    }
    return 0;
}

2014-11-16 13:52:35.215 ProtocalTest[845:82273] 按钮被点击了

2014-11-16 13:52:35.216 ProtocalTest[845:82273] 按钮被长按了

Objective-C基础笔记（5）Protocol