CADisplayLink使用中的循环引用问题的解决

解决循环引用的问题，参考学习了ibireme大神的YYFPSLabel的实现，整理以备用

    // 如果直接用 self 或者 weakSelf，都不能解决循环引用问题
    _link = [CADisplayLink displayLinkWithTarget:[YYWeakProxy proxyWithTarget:self] selector:@selector(tick:)];
//    _link = [CADisplayLink displayLinkWithTarget:self selector:@selector(tick:)];
//    __weak typeof(self) weakSelf = self;
//    _link = [CADisplayLink displayLinkWithTarget:weakSelf selector:@selector(tick:)];
    [_link addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSRunLoopCommonModes];
                                        //NSDefaultRunLoopMode - 标准优先级
                                        //NSRunLoopCommonModes - 高优先级
                                        //UITrackingRunLoopMode - 用于UIScrollView和别的控件的动画
    return self;

@interface YYWeakProxy : NSProxy

/**
 The proxy target.
 */
@property (nonatomic, weak, readonly) id target;

/**
 Creates a new weak proxy for target.
 
 @param target Target object.
 
 @return A new proxy object.
 */
- (instancetype)initWithTarget:(id)target;

/**
 Creates a new weak proxy for target.
 
 @param target Target object.
 
 @return A new proxy object.
 */
+ (instancetype)proxyWithTarget:(id)target;

@end

@implementation YYWeakProxy

- (instancetype)initWithTarget:(id)target {
    _target = target;
    return self;
}

+ (instancetype)proxyWithTarget:(id)target {
    return [[YYWeakProxy alloc] initWithTarget:target];
}

- (id)forwardingTargetForSelector:(SEL)selector {
    return _target;
}

- (void)forwardInvocation:(NSInvocation *)invocation {
    void *null = NULL;
    [invocation setReturnValue:&null];
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector {
    return [NSObject instanceMethodSignatureForSelector:@selector(init)];
}

- (BOOL)respondsToSelector:(SEL)aSelector {
    return [_target respondsToSelector:aSelector];
}

- (BOOL)isEqual:(id)object {
    return [_target isEqual:object];
}

- (NSUInteger)hash {
    return [_target hash];
}

- (Class)superclass {
    return [_target superclass];
}

- (Class)class {
    return [_target class];
}

- (BOOL)isKindOfClass:(Class)aClass {
    return [_target isKindOfClass:aClass];
}

- (BOOL)isMemberOfClass:(Class)aClass {
    return [_target isMemberOfClass:aClass];
}

- (BOOL)conformsToProtocol:(Protocol *)aProtocol {
    return [_target conformsToProtocol:aProtocol];
}

- (BOOL)isProxy {
    return YES;
}

- (NSString *)description {
    return [_target description];
}

- (NSString *)debugDescription {
    return [_target debugDescription];
}

@end

NSProxy（ /BrE ˈprɒksi/）的父类不是NSObject