[Cocoa]深入浅出Cocoa之 Method Swizzling_app

概述[Cocoa]深入浅出Cocoa之 Method Swizzling 罗朝辉(http://blog.csdn.net/kesalin) CC许可，转载请注明出处在前文深入浅出Cocoa之消息中，我简要介绍了ObjC 中消息的基本情况，包括SEL查找，缓存以及消息转发等。在本文中，我要介绍一个很有趣的技术，Method swizzling，通过这个手法，我们可以动态修改方法的实现，从而达到修改类

[Cocoa]深入浅出Cocoa之 Method Swizzling

罗朝辉(http://blog.csdn.net/kesalin)

CC许可，转载请注明出处

在前文深入浅出Cocoa之消息中，我简要介绍了ObjC 中消息的基本情况，包括SEL查找，缓存以及消息转发等。在本文中，我要介绍一个很有趣的技术，Method swizzling，通过这个手法，我们可以动态修改方法的实现，从而达到修改类行为的目的。当然，还有其他办法（如 ClassPosing，category）也可以达到这个目的。Classposing 是针对类级别的，是重量级的手法，category 也差不多，比较重量级，此外 category 还无法避免下面的递归死循环（如果你的代码出现了如下形式的递归调用，应该考虑一下你的设计，而不是使用在这里介绍的 Method Swizzling 手法，：））。

// bar//@implementation bar- (voID) testMethod{    NSLog(@" >> bar testMethod");}@end// bar(barcategory)//@implementation bar(barcategory)- (voID) altRecursionMethod{    NSLog(@" >> bar(barcategory) recursionMethod");    [self altRecursionMethod];}@end

在前文深入浅出Cocoa之消息中提到，ObjC 中的类(class)和实例(instance)都是对象，类对象有自己的类方法列表，实例对象有自己的实例方法列表，这些方法列表（struct objc_method_List）是存储在 struct objc_class 中的。每个方法列表存储近似 SEL:Method 的对，Method 是一个对象，包含方法的具体实现 impl。由此可知，我们只需要修改 SEL 对应的 Method 的 impl 既可以达到修改消息行为的目的。下面来看代码：

voID PerformSwizzle(Class aClass,SEL orig_sel,SEL alt_sel,BOol forInstance){    // First,make sure the class isn't nil    if (aClass != nil) {        Method orig_method = nil,alt_method = nil;        // Next,look for the methods        if (forInstance) {            orig_method = class_getInstanceMethod(aClass,orig_sel);            alt_method = class_getInstanceMethod(aClass,alt_sel);        } else {            orig_method = class_getClassMethod(aClass,orig_sel);            alt_method = class_getClassMethod(aClass,alt_sel);        }        // If both are found,swizzle them        if ((orig_method != nil) && (alt_method != nil)) {            IMP temp;            temp = orig_method->method_imp;            orig_method->method_imp = alt_method->method_imp;            alt_method->method_imp = temp;        } else {#if DEBUG            NSLog(@"PerformSwizzle Error: Original %@,Alternate %@",(orig_method == nil)?@" not found":@" found",(alt_method == nil)?@" not found":@" found");#endif        }    } else {#if DEBUG        NSLog(@"PerformSwizzle Error: Class not found");#endif    }}voID MethodSwizzle(Class aClass,SEL alt_sel){    PerformSwizzle(aClass,orig_sel,alt_sel,YES);}voID ClassMethodSwizzle(Class aClass,NO);}

让我们来分析上面代码：
1，首先，区分类方法和实例方法；
2，取得 SEL 对应的 Method；
3，修改 Method 的 impl，在这里是通过交换实现的。

上面的代码是可以工作的，但还不够完善。Apple 10.5 提供了交换 Method 实现的 API: method_exchangeImplementations 。下面我们使用这个新 API，并以 NSObject category的形式给出新的实现方式：

#if TARGET_OS_IPHONE#import <objc/runtime.h>#import <objc/message.h>#else#import <objc/objc-class.h>#endif// NSObject (MethodSwizzlingcategory)//@interface NSObject (MethodSwizzlingcategory)+ (BOol)swizzleMethod:(SEL)origSel withMethod:(SEL)altSel;+ (BOol)swizzleClassMethod:(SEL)origSel withClassMethod:(SEL)altSel;@end@implementation NSObject (MethodSwizzlingcategory)+ (BOol)swizzleMethod:(SEL)origSel withMethod:(SEL)altSel{    Method origMethod = class_getInstanceMethod(self,origSel);    if (!origSel) {        NSLog(@"original method %@ not found for class %@",NsstringFromSelector(origSel),[self class]);        return NO;    }        Method altMethod = class_getInstanceMethod(self,altSel);	if (!altMethod) {        NSLog(@"original method %@ not found for class %@",NsstringFromSelector(altSel),[self class]);        return NO;    }        class_addMethod(self,origSel,class_getmethodImplementation(self,origSel),method_getTypeEn@R_301_5563@(origMethod));	class_addMethod(self,altSel,altSel),method_getTypeEn@R_301_5563@(altMethod));        method_exchangeImplementations(class_getInstanceMethod(self,class_getInstanceMethod(self,altSel));	return YES;}+ (BOol)swizzleClassMethod:(SEL)origSel withClassMethod:(SEL)altSel{    Class c = object_getClass((ID)self);    return [c swizzleMethod:origSel withMethod:altSel];}@end

代码就不用多解释了，下面我们来看如何使用。先看辅助类Foo：
Foo.h

////  Foo.h//  MethodSwizzling////  Created by LuoZhaohui on 1/5/12.//  copyright (c) 2012 http://blog.csdn.net/kesalin/. All rights reserved.//#import <Foundation/Foundation.h>// Foo//@interface Foo : NSObject- (voID) testMethod;- (voID) baseMethod;- (voID) recursionMethod;@end// bar//@interface bar : Foo- (voID) testMethod;@end// bar(barcategory)//@interface bar(barcategory)- (voID) altTestMethod;- (voID) altBaseMethod;- (voID) altRecursionMethod;@end

Foo.m

////  Foo.m//  MethodSwizzling////  Created by LuoZhaohui on 1/5/12.//  copyright (c) 2012 http://blog.csdn.net/kesalin/. All rights reserved.//#import "Foo.h"// Foo//@implementation Foo- (voID) testMethod{    NSLog(@" >> Foo testMethod");}- (voID) baseMethod{    NSLog(@" >> Foo baseMethod");}- (voID) recursionMethod{    NSLog(@" >> Foo recursionMethod");}@end// bar//@implementation bar- (voID) testMethod{    NSLog(@" >> bar testMethod");}@end// bar(barcategory)//@implementation bar(barcategory)- (voID) altTestMethod{    NSLog(@" >> bar(barcategory) altTestMethod");}- (voID) altBaseMethod{    NSLog(@" >> bar(barcategory) altBaseMethod");}- (voID) altRecursionMethod{    NSLog(@" >> bar(barcategory) recursionMethod");    [self altRecursionMethod];}@end

下面是具体的使用示例：

int main (int argc,const char * argv[]){    @autoreleasepool    {        Foo * foo = [[[Foo alloc] init] autorelease];        bar * bar = [[[bar alloc] init] autorelease];                NSLog(@"========= Method Swizzling test 1 =========");                NSLog(@" Step 1");        [foo testMethod];        [bar testMethod];        [bar altTestMethod];                NSLog(@" Step 2");        [bar swizzleMethod:@selector(testMethod) withMethod:@selector(altTestMethod)];        [foo testMethod];        [bar testMethod];        [bar altTestMethod];                NSLog(@"========= Method Swizzling test 2 =========");        NSLog(@" Step 1");        [foo baseMethod];        [bar baseMethod];        [bar altBaseMethod];                NSLog(@" Step 2");        [bar swizzleMethod:@selector(baseMethod) withMethod:@selector(altBaseMethod)];        [foo baseMethod];        [bar baseMethod];        [bar altBaseMethod];                NSLog(@"========= Method Swizzling test 3 =========");        [bar swizzleMethod:@selector(recursionMethod) withMethod:@selector(altRecursionMethod)];        [bar recursionMethod];    }    return 0;}

输出结果为：注意，test 3 中调用了递归调用“自己”的方法，你能理解为什么没有出现死循环么？

========= Method Swizzling test 1 =========
Step 1
>> Foo testMethod
>> bar testMethod
>> bar(barcategory) altTestMethod
Step 2
>> Foo testMethod
>> bar(barcategory) altTestMethod
>> bar testMethod
========= Method Swizzling test 2 =========
Step 1
>> Foo baseMethod
>> Foo baseMethod
>> bar(barcategory) altBaseMethod
Step 2
>> Foo baseMethod
>> bar(barcategory) altBaseMethod
>> Foo baseMethod
========= Method Swizzling test 3 =========
>> bar(barcategory) recursionMethod
>> Foo recursionMethod