深入浅出Cocoa之 Method Swizzling

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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 orig_sel, SEL alt_sel)
{
    PerformSwizzle(aClass, orig_sel, alt_sel, YES);
}

void ClassMethodSwizzle(Class aClass, SEL orig_sel, SEL alt_sel)
{
    PerformSwizzle(aClass, orig_sel, alt_sel, 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_getTypeEncoding(origMethod));
    class_addMethod(self,
                    altSel,
                    class_getMethodImplementation(self, altSel),
                    method_getTypeEncoding(altMethod));

    method_exchangeImplementations(class_getInstanceMethod(self, origSel), 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://www.cppblog.com/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://www.cppblog.com/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：

// Main
//
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;
}

输出结果为下表。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

rentzsch 写了一个完善的开源类jrswizzle 来处理 Method Swizzling，如果你在工程中使用到 Method Swizzling手法，应该优先使用这个类库，：）。

Refference：

MethodSwizzling：http://www.cocoadev.com/index.pl?ExtendingClasses

jrswizzle：https://github.com/rentzsch/jrswizzle