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