[Cocoa]深入浅出Cocoa之 Method Swizzling
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[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 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
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
test3 解释:在函数体 {} 之间的部分是真正的 IMP,而在这之前的是 SEL。通常情况下,SEL 是与 IMP 匹配的,但在 swizzling 之后,情况就不同了。下图就是调用的时序图。
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
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