问题

当我们从控制器View Controller跳转到一个启用重复调用定时器的控制器Anohter View Controller后，进行pop操作后，无法销毁该控制器Another View Controller。

1	self.timer = [NSTimer timerWithTimeInterval:1.0 target:self selector:@selector(timerAction) userInfo:nil repeats:YES];

timer作为VC的属性，被VC强引用，创建timer对象时VC作为target被timer强引用，即循环引用。

循环引用

weak指针

既然有循环引用，那我们可以用解决Block循环引用的方案–weak指针来解决吗？答案是不可以的，因为无论是用weak还是strong修饰，在NSTimer的创建过程中都会重新生成一个强引用指针指向VC，导致循环引用。

解决方案

invalidate：

This method is the only way to remove a timer from an NSRunLoop object. The NSRunLoop object removes its strong reference to the timer, either just before the invalidate method returns or at some later point.
If it was configured with target and user info objects, the receiver removes its strong references to those objects as well.

我们可以调用NSTimer的invalidate方法来手动释放timer，当然我们不能在dealloc中调用这个方法，因为本身因为循环引用已经导致当前控制器无法释放，自然也不会走dealloc方法。
所以我们可以考虑在viewWillDisappear这个方法中进行释放，缺点就是可以会忘记，而且并不是很严谨。

类方法：

创建一个继承自NSObject的子类TimerWeakTarget，并创建开启定时器的方法。

TimerWeakTarget文件：

// TimerWeakTarget.h
@interface TimerWeakTarget : NSObject

+ (NSTimer *) scheduledTimerWithTimeInterval:(NSTimeInterval)interval
                                      target:(id)aTarget
                                    selector:(SEL)aSelector
                                    userInfo:(id)userInfo
                                     repeats:(BOOL)repeats;

@end

// TimerWeakTarget.m
@interface TimerWeakTarget ()

@property (weak, nonatomic) id target;
@property (assign, nonatomic) SEL selector;
@property (weak, nonatomic) NSTimer *tempTimer;

@end

@implementation TimerWeakTarget

+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)interval target:(id)aTarget selector:(SEL)aSelector userInfo:(id)userInfo repeats:(BOOL)repeats {
    TimerWeakTarget *timerWeakTarget = [[TimerWeakTarget alloc] init];
    timerWeakTarget.target = aTarget;
    timerWeakTarget.selector = aSelector;
    timerWeakTarget.tempTimer = [NSTimer scheduledTimerWithTimeInterval:interval target:timerWeakTarget selector:@selector(fire:) userInfo:userInfo repeats:repeats];
    return timerWeakTarget.tempTimer;
}


- (void)fire:(NSTimer *)timer {
    if (self.target && [self.target respondsToSelector:self.selector]) {
        [self.target performSelector:self.selector withObject:timer.userInfo];
    } else {
        [self.tempTimer invalidate];
    }
}

@end

控制器内调用：

1	self.timer = [TimerWeakTarget scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerAction) userInfo:nil repeats:YES];

VC强引用timer，timer强引用的target是timerWeakTarget，而timerWeakTarget弱引用VC，解除循环引用。

解除循环引用

WeakProxy：

创建一个继承NSObject的子类WeakProxy，并实现消息转发的相关方法。

WeakProxy文件：

// WeakProxy.h
@interface WeakProxy : NSObject

+ (instancetype)proxyWithTarget:(id)target;

@end

// WeakProxy.m
@interface WeakProxy ()

@property (weak, nonatomic) id weakTarget;

@end

@implementation WeakProxy

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


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


- (id)forwardingTargetForSelector:(SEL)aSelector {
    return self.weakTarget;
}


/* OR
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    return [self.weakTarget methodSignatureForSelector:aSelector];
}


- (void)forwardInvocation:(NSInvocation *)anInvocation {
    SEL sel = [anInvocation selector];
    if ([self.weakTarget respondsToSelector:sel]) {
        [anInvocation invokeWithTarget:self.weakTarget];
    }
}
*/

@end

控制器内的调用：

1	self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:[WeakProxy proxyWithTarget:self] selector:@selector(timerAction) userInfo:nil repeats:YES];

这个方法的原理和类方法相似，将timer的target设置为WeakProxy的实例，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。

GCD：

控制器内：

@interface AnotherViewController ()

@property (strong, nonatomic) dispatch_source_t timer;

@end

@implementation AnotherViewController

- (void)viewDidLoad {
    [super viewDidLoad];
   
    self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0));
    dispatch_source_set_timer(self.timer, DISPATCH_TIME_NOW, 1*NSEC_PER_SEC, 0);
    dispatch_source_set_event_handler(self.timer, ^{
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"fire");
        });
    });
    dispatch_resume(self.timer);
}

@end

结束计时：

1	dispatch_source_cancel(self.timer);

Block:

iOS 10之后新出的API：

1
2
3

self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
    NSLog(@"fire");
}];

NSTimer循环引用解决方案

The Circular Reference Issue with NSTimer

问题