一个method swizzling实验

今天想用method swizzling的时候，突然想到了一个问题，如果swizzle了一个父类（而不是当前类）的方法，会怎么样呢？

于是写了点代码试验一下。

首先定义一个基类XSQBaseObject，其中有一个baseMethod方法：

//XSQBaseObject.h

#import <Foundation/Foundation.h>

@interface XSQBaseObject : NSObject

- (void)baseMethod;

@end

//XSQBaseObject.m

#import "XSQBaseObject.h"

@implementation XSQBaseObject

- (void)baseMethod {
    NSLog(@"this is base method");
}

@end

然后定义一个子类XSQChildObject继承自XSQBaseClass，子类里没有写任何方法。

//XSQChildObject.h

#import "XSQBaseObject.h"

@interface XSQChildObject : XSQBaseObject

@end

//XSQChildObject.m

#import "XSQChildObject.h"

@implementation XSQChildObject

@end

然后给XSQChildObject增加一个category，用来做swizzling：

//XSQChildObject+Swizzling.h

#import "XSQChildObject.h"

@interface XSQChildObject (Swizzling)

@end

//XSQChildObject+Swizzling.m

#import "XSQChildObject+Swizzling.h"
#import <objc/runtime.h>

@implementation XSQChildObject (Swizzling)

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class class = [self class];
        // When swizzling a class method, use the following:
        // Class class = object_getClass((id)self);
        
        SEL originalSelector = @selector(baseMethod);
        SEL swizzledSelector = @selector(xxx_baseMethod);
        
        Method originalMethod = class_getInstanceMethod(class, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
        
        BOOL didAddMethod =
        class_addMethod(class,
                        originalSelector,
                        method_getImplementation(swizzledMethod),
                        method_getTypeEncoding(swizzledMethod));
        
        if (didAddMethod) {
            class_replaceMethod(class,
                                swizzledSelector,
                                method_getImplementation(originalMethod),
                                method_getTypeEncoding(originalMethod));
        } else {
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
}

#pragma mark - Method Swizzling

- (void)xxx_baseMethod {
    NSLog(@"swizzle~~");
    [self xxx_baseMethod];
}

@end

swizzling的代码参考自Objective-C Runtime 运行时之四：Method Swizzling

然后给XSQChildObject的实例发送消息：

XSQBaseObject *baseObject = [[XSQBaseObject alloc] init];
[baseObject baseMethod];

XSQChildObject *childObject = [[XSQChildObject alloc] init];
[childObject baseMethod];

竟然平安无事的得到了结果：

2016-05-09 20:46:06.220 XSQSwizzlingDemo[4235:1321820] this is base method
2016-05-09 20:46:08.055 XSQSwizzlingDemo[4235:1321820] swizzle~~
2016-05-09 20:46:08.056 XSQSwizzlingDemo[4235:1321820] this is base method

明明我还在担心，父类的方法如果被swizzle了，应该会出现这样的情景：

给XSQBaseObject的实例发送baseMethod消息时，实际调用的是xxx_baseMethodIMP的实现，而xxx_baseMethodIMP中还给[self xxx_baseMethod]发送了消息，而XSQBaseObject中没有xxx_baseMethod这个selector，走到这里应该崩溃才对。

仔细看一下swizzling的代码，发现这段代码中已经对我担心的问题做了处理：

/*
 * class_addMethod方法会给这个类添加一个方法
 * 如果这个类（本身，不包括父类）已经有了originalSelector，则无法添加成功，同时返回NO
 * 所以这里的意义是：如果这个类（自身，不包括父类）没有originSelector，则给它添加一个方法，而方法实现对应于swizzledMethod
 */
BOOL didAddMethod =
class_addMethod(class,
                originalSelector,
                method_getImplementation(swizzledMethod),
                method_getTypeEncoding(swizzledMethod));

/*
 * 如果给这个类添加originSelector成功，则让这个类的swizzledSelector的实现变成originalMethod
 */
if (didAddMethod) {
    class_replaceMethod(class,
                        swizzledSelector,
                        method_getImplementation(originalMethod),
                        method_getTypeEncoding(originalMethod));
}

这段代码应对的就是，如果originalSelector是父类中的方法，而子类也没有重写它，这时就不能直接交换两个方法的实现，而是要给子类也添加一个originalSelector的实现。

如果把上面这段代码删除，全部用method_exchangeImplementations来做swizzle：

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class class = [self class];
        // When swizzling a class method, use the following:
        // Class class = object_getClass((id)self);
        
        SEL originalSelector = @selector(baseMethod);
        SEL swizzledSelector = @selector(xxx_baseMethod);
        
        Method originalMethod = class_getInstanceMethod(class, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
        
        method_exchangeImplementations(originalMethod, swizzledMethod);
    });
}

果然运行时崩溃了：

2016-05-09 21:10:55.523 XSQSwizzlingDemo[4247:1325552] swizzle~~
2016-05-09 21:10:55.526 XSQSwizzlingDemo[4247:1325552] -[XSQBaseObject xxx_baseMethod]: unrecognized selector sent to instance 0x155e189d0
2016-05-09 21:10:55.527 XSQSwizzlingDemo[4247:1325552] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[XSQBaseObject xxx_baseMethod]: unrecognized selector sent to instance 0x155e189d0'

和图片中画的一致，给XSQBaseObject的实例发送baseMethod消息时，执行了xxx_baseMethod的实现（所以第一行输出了“swizzle~~”），然后发送了[self xxx_baseMethod]消息给XSQBaseObject的实例，自然抛出了unrecognized selector异常。

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参考

Objective-C Runtime 运行时之四：Method Swizzling