NSTimer循环引用的几种解决方案

NSTimer在使用中需要注意，避免循环引用的问题。但是需要纠正一种说法
注意：timer内存泄漏，部分童鞋认为是vc与timer循环引用造成的，这种说法是错误的！

@interface ViewController ()
@property (nonatomic, weak) NSTimer *timer;
@end

- (void)viewDidLoad {
    [super viewDidLoad];

    {
        NSTimer *one = [NSTimer scheduledTimerWithTimeInterval:1.f target:self selector:@selector(tick:) userInfo:nil repeats:YES];
        _timer = one;
    }
}

timer强引用target，即timer强引用vc，然而vc并没有强引用timer，哪来的vc与timer循环引用？但是，如果vc没有强引用timer，timer是如何存活的？
其实，默认将timer加入到currentRunLoop中，currentRunLoop会强引用timer，而currentRunLoop就是mainRunLoop，mainRunLoop一直存活，所以timer可以存活

关系可参考如下图示：

image.png

从以上关系图可见，只要runLoop存活，vc必然存活，所以vc的dealloc方法自然就不会执行。因此，将timer的销毁方法放在dealloc中必然造成内存泄漏！

虽然NSRunLoop提供了addTimer接口，但是并没有提供removeTimer接口，显然，runLoop与timer这条线无法直接销毁，所以只能从vc与timer持有关系入手。

内部其实对target对象做了retain操作， _target = RETAIN(object);

- (id) initWithFireDate: (NSDate*)fd
           interval: (NSTimeInterval)ti
         target: (id)object
           selector: (SEL)selector
           userInfo: (id)info
        repeats: (BOOL)f
{
  if (ti <= 0.0)
    {
      ti = 0.0001;
    }
  if (fd == nil)
    {
      _date = [[NSDate_class allocWithZone: NSDefaultMallocZone()]
        initWithTimeIntervalSinceNow: ti];
    }
  else
    {
      _date = [fd copyWithZone: NSDefaultMallocZone()];
    }
  _target = RETAIN(object);
  _selector = selector;
  _info = RETAIN(info);
  if (f == YES)
    {
      _repeats = YES;
      _interval = ti;
    }
  else
    {
      _repeats = NO;
      _interval = 0.0;
    }
  return self;
}

调用invalidate的时候，会执行destroy操作

- (void) invalidate
{
  /* OPENSTEP allows this method to be called multiple times. */
  _invalidated = YES;
  if (_target != nil)
    {
      DESTROY(_target);
    }
  if (_info != nil)
    {
      DESTROY(_info);
    }
}

下面介绍一下几种解决timer循环引用的方法。

1. 选择合适的时机手动释放timer

调用[self.timer invalidate];

2. timer使用block方式添加Target-Action

这里我们需要自己在NSTimer的分类中添加类方法:

@interface NSTimer (YYAdd)

/**
 Creates and returns a new NSTimer object and schedules it on the current run 
 loop in the default mode.
 
 @discussion     After seconds seconds have elapsed, the timer fires, 
                 sending the message aSelector to target.
 
 @param seconds  The number of seconds between firings of the timer. If seconds
                 is less than or equal to 0.0, this method chooses the
                 nonnegative value of 0.1 milliseconds instead.
 
 @param block    The block to invoke when the timer fires. The timer  maintains
                 a strong reference to the block until it (the timer) is invalidated.
 @param repeats  If YES, the timer will repeatedly reschedule itself until
                 invalidated. If NO, the timer will be invalidated after it fires.
 
 @return A new NSTimer object, configured according to the specified parameters.
 */
+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)seconds block:(void (^)(NSTimer *timer))block repeats:(BOOL)repeats;

/**
 Creates and returns a new NSTimer object initialized with the specified block.
 
 @discussion      You must add the new timer to a run loop, using addTimer:forMode:. 
                  Then, after seconds have elapsed, the timer fires, invoking
                  block. (If the timer is configured to repeat, there is no need
                  to subsequently re-add the timer to the run loop.)
 
 @param seconds  The number of seconds between firings of the timer. If seconds
                 is less than or equal to 0.0, this method chooses the
                 nonnegative value of 0.1 milliseconds instead.
 
 @param block    The block to invoke when the timer fires. The timer instructs
                 the block to maintain a strong reference to its arguments.
 
 @param repeats  If YES, the timer will repeatedly reschedule itself until
                 invalidated. If NO, the timer will be invalidated after it fires.
 
 @return A new NSTimer object, configured according to the specified parameters.
 */
+ (NSTimer *)timerWithTimeInterval:(NSTimeInterval)seconds block:(void (^)(NSTimer *timer))block repeats:(BOOL)repeats;

@end

通过block的方式，获取action，实际的target设置为self，即NSTimer类。这样我们在使用timer时，由于target的改变，就不再有循环引用了。 使用中还需要注意block可能引起的循环引用，所以使用weakSelf：

__weak typeof(self) weakSelf = self;
self.timer = [NSTimer scheduledTimerWithTimeInterval:1 block:^{
     [weakSelf dosomething];
} repeats:YES];

#import "NSTimer+YYAdd.h"
#import "YYCategoriesMacro.h"

YYSYNTH_DUMMY_CLASS(NSTimer_YYAdd)


@implementation NSTimer (YYAdd)

+ (void)_yy_ExecBlock:(NSTimer *)timer {
    if ([timer userInfo]) {
        void (^block)(NSTimer *timer) = (void (^)(NSTimer *timer))[timer userInfo];
        block(timer);
    }
}

+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)seconds block:(void (^)(NSTimer *timer))block repeats:(BOOL)repeats {
    return [NSTimer scheduledTimerWithTimeInterval:seconds target:self selector:@selector(_yy_ExecBlock:) userInfo:[block copy] repeats:repeats];
}

+ (NSTimer *)timerWithTimeInterval:(NSTimeInterval)seconds block:(void (^)(NSTimer *timer))block repeats:(BOOL)repeats {
    return [NSTimer timerWithTimeInterval:seconds target:self selector:@selector(_yy_ExecBlock:) userInfo:[block copy] repeats:repeats];
}

@end

虽然没有了循环引用，但是还是应该记得在dealloc时释放timer。

3. 通过proxy转发的形式解决

如图：

image.png

考虑到循环引用的原因，改方案就是需要打破这些相互引用关系，因此添加一个中间件，弱引用self，同时timer引用了中间件，这样通过弱引用来解决了相互引用

proxy弱引用vc，所以vc可以释放，当vc执行dealloc，在dealloc内部销毁timer即可

参考：YYKit解决方案：

@interface YYWeakProxy : NSProxy

/**
 The proxy target.
 */
@property (nullable, 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

#import "YYWeakProxy.h"


@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