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,Methodswizzling,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 MethodSwizzling 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(structobjc_method_List)是存储在 structobjc_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,BOolforInstance)
{
// 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_getTypeEnCoding(origMethod));
class_addMethod(self,
altSel,altSel),
method_getTypeEnCoding(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 [cswizzleMethod: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.cnblogs.com/kesalin/. All rightsreserved.
//
#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.cnblogs.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:
int main (int argc,constchar * 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");
[barswizzleMethod:@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");
[barswizzleMethod:@selector(baseMethod) withMethod:@selector(altBaseMethod)];
[foo baseMethod];
[bar baseMethod];
[bar altBaseMethod];
NSLog(@"========= Method Swizzling test 3 =========");
[barswizzleMethod:@selector(recursionMethod) withMethod:@selector(altRecursionMethod)];
[bar recursionMethod];
}
return0;
}
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
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MethodSwizzling 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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