消息转发通常用于解耦，在此有个实际例子就是打破NSTimer的引用循环，
YYKit框架中有一个YYWeakProxy类，就是用来处理引用循环的，YYWeakProxy类继承于NSProxy,
自己可以模仿YYWeakProxy写一个WeakProxy类继承自NSProxy代码如下
@property (nonatomic, weak, readonly) id weakTarget;
+ (instancetype)proxyWithTarget:(id)target;
- (instancetype)initWithTarget:(id)target;
@implementation WeakProxy
+ (instancetype)proxyWithTarget:(id)target {
return [[self alloc] initWithTarget:target];
}
- (instancetype)initWithTarget:(id)target {
_weakTarget = target;
return self;
}
- (void)forwardInvocation:(NSInvocation *)invocation {
SEL sel = [invocation selector];
if ([self.weakTarget respondsToSelector:sel]) {
[invocation invokeWithTarget:self.weakTarget];
}
}
//返回方法的签名。
- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
return [self.weakTarget methodSignatureForSelector:sel];
}
- (BOOL)respondsToSelector:(SEL)aSelector {
return [self.weakTarget respondsToSelector:aSelector];
}
@end
原理跟类方法相似，打破循环引用的环路。将timer的target设置为WeakProxy实例，利用消息转发机制实现执行VC中的计时方法，解决循环引用。
