ios - iOS如何在不子类化的情况下将自定义代码添加到类方法中？

Question

如果我在应用中使用核心的iOS类，例如NSURLCache；我想在调用[[NSURLCache sharedURLCache] removeAllCachedResponses]时打印一条日志消息。 如何在不显式地继承NSURLCache子类并将项目中的所有引用替换为自定义类的情况下扩展类似的类方法？ 是否可以使用类别进行此操作？

Answer 1

您可以使用Method Swizzling来完成。这是a page with a nice overview of the technique，我从中借来了以下大部分代码:

@interface SwizzleNSURLCache : NSURLCache
+(void)load;
-(void)swzl_removeAllCachedResponses;
@end

+(void)load {
    Class class = [SwizzleNSURLCache class];
    SEL originalSelector = @selector(removeAllCachedResponses);
    SEL swizzledSelector = @selector(swzl_removeAllCachedResponses);

    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);
    }
}

-(void) swzl_removeAllCachedResponses {
    // This is your replacement method. You can do whatever you want here.
    NSLog(@"Running removeAllCachedResponses");
    // If you need to call the actual implementation, do it like this:
    [self swzl_removeAllCachedResponses];
}

您无需使用此子类替换 NSURLCache的引用。您只需要从程序中的某个位置加载一次即可。

注意:我不确定是否有更简便的方法。方法混乱无疑是一门沉重的大炮，但它可以完成工作。