Swizzle in Objcet-C

翻译自：the right way to swizzle in Object-C

在OC中，我们谈到swizzle一般指用我们自己的方法代替原始的方法，调用原始的方法，执行的是我们自己的方法；我们使用runtime.h提供的方法来实现；在运行时，OC的方法被转换为C结构体Method；

//a typedef of struct objc_method defined as:
struct objc_method
{
    SEL method_name         OBJC2_UNAVAILABLE;
    char *method_types      OBJC2_UNAVAILABLE;
    IMP method_imp          OBJC2_UNAVAILABLE;
}

method_name代表的是方法的selector；method_types代表的是编码参数和返回值的类型；method_imp代表的是函数指针指向了真正的函数地址；
　　你可以通过runtime的以下方法拿到Method

Method class_getClassMethod(Class aClass, SEL aSelector);
Method class_getInstanceMethod(Class aClass, SEL aSelector);

我们可以通过IMP的method_setImplementation(Method method, IMP imp)方法改变Method中的imp；这里我们稍后再说，我们先来看看另外一个方法

void method_exchangeImplementations(Method m1, Method m2)

为了能更好的理解这个方法，我们先来看看Method m1和Method m2

Method m1 { //this is the original method. we want to switch this one with
//our replacement method
    SEL method_name = @selector(originalMethodName)
    char *method_types = “v@:“ //returns void, params id(self),selector(_cmd)
    IMP method_imp = 0x000FFFF (MyBundle`[MyClass originalMethodName])
}

Method m2 { //this is the swizzle method. We want this method executed when [MyClass
//originalMethodName] is called
    SEL method_name = @selector(swizzle_originalMethodName)
    char *method_types = “v@:”
    IMP method_imp = 0x1234AABA (MyBundle`[MyClass swizzle_originalMethodName])
}

然后我们调用如下方法

method_exchangeImplementations(m1, m2)

现在我们再看看m1和m2

Method m1 { //this is the original Method struct. we want to switch this one with
//our replacement method
      SEL method_name = @selector(originalMethodName)
      char *method_types = “v@:“ //returns void, params id(self),selector(_cmd)
      IMP method_imp = 0x1234AABA (MyBundle`[MyClass swizzle_originalMethodName])
}

Method m2 { //this is the swizzle Method struct. We want this method executed when [MyClass
//originalMethodName] is called
      SEL method_name = @selector(swizzle_originalMethodName)
      char *method_types = “v@:”
      IMP method_imp = 0x000FFFF (MyBundle`[MyClass originalMethodName])
}

如果我们想执行原始代码我们需要调用[self　swizzle_originalMethodName]，对于_cmd的值将是@selector(swizzle_originalMethodName)；再如果原始代码中有使用过_cmd，而在没有替换方法前，_cmd的值是@selector(originalMethodName)；这样的结果可能会导致未可知的错误；

- (void) originalMethodName //m1
{
    assert([NSStringFromSelector(_cmd) isEqualToString:@“originalMethodNamed”]);
 //this fails after swizzling //using
//method_exchangedImplementations()
//…
}

这里介绍一种方法，方法的核心是通过创建c函数来取代OC的方法；
　　eg：

void __Swizzle_OriginalMethodName(id self, SEL _cmd)
{
//code
}

IMP swizzleImp = (IMP)__Swizzle_OriginalMethodName;

method_setImplementation(method, swizzleImp);

IMP originalImp = method_setImplementation(method,swizzleImp);

接下来列举一个具体的例子；

@interface SwizzleExampleClass : NSObject
- (void) swizzleExample;
- (int) originalMethod;
@end

static IMP __original_Method_Imp;
int _replacement_Method(id self, SEL _cmd)
{
    assert([NSStringFromSelector(_cmd) isEqualToString:@"originalMethod"]);
//code
    int returnValue = ((int(*)(id,SEL))__original_Method_Imp)(self, _cmd);
    return returnValue + 1;
}

@implementation SwizzleExampleClass
- (void) swizzleExample //call me to swizzle
{
    Method m = class_getInstanceMethod([self class],　@selector(originalMethod));
    __original_Method_Imp = method_setImplementation(m,　(IMP)_replacement_Method);
}
- (int) originalMethod
{
//code
    assert([NSStringFromSelector(_cmd) isEqualToString:@"originalMethod"]);
    return 1;
}
@end

然后我们可以验证：

SwizzleExampleClass* example = [[SwizzleExampleClass alloc] init];
int originalReturn = [example originalMethod];
[example swizzleExample];
int swizzledReturn = [example originalMethod];
assert(originalReturn == 1); //true
assert(swizzledReturn == 2); //true

小结：我们可以通过一个C函数和method_setImplementation()来实现我们的swizzle（当然我们也可以通过method_exchangeImplementations()来实现，只是需要注意以上提到的_cmd引发的问题）；需要注意的是我们的C函数需要带有两个参数（id self 和 SEL _cmd；这是由于OC的方法都会传递2个这两个隐藏的参数），另外需要将C函数转换为IMP；最后一点不知道是什么意思；

//you may have to case the IMP call if it returns a void. 
//This is because ARC assumes all IMPs return an id and 
//will try to retain void and primitive types.

IMP anImp; //represents objective-c function
// -UIViewController viewDidLoad;
((void(*)(id,SEL))anImp)(self,_cmd); //call with a cast to prevent
// ARC from retaining void.