文章以在TimerViewController中使用計時器為例，在VC中聲明一個NSTimer屬性。

創(chuàng)建NSTimer對象：

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

timer作為VC的屬性，被VC強引用，創(chuàng)建timer對象時VC作為target被timer強引用，即循環(huán)引用。

image

解決循環(huán)引用的幾種方式：

weak指針：

既然是強引用導(dǎo)致循環(huán)引用，那么用__weak修飾self就好了，想法是對的，但是做法是無效的。因為無論是weak還是strong修飾，在NSTimer中都會重新生成一個新的強引用指針指向self，導(dǎo)致循環(huán)引用的。

類方法：

創(chuàng)建一個繼承NSObject的子類TempTarget，并創(chuàng)建開啟計時器的方法。

TempTarget .h文件

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

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

TempTarget .m文件

@implementation TempTarget

+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)interval target:(id)tempTarget selector:(SEL)selector userInfo:(id)userInfo repeats:(BOOL)repeats {
    TempTarget *target = [[TempTarget alloc] init];
    target.tempTarget = tempTarget;
    target.selector = selector;
    target.tempTimer = [NSTimer scheduledTimerWithTimeInterval:interval target:target selector:@selector(timerSelector:) userInfo:userInfo repeats:repeats];
    return target.tempTimer;
}

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

@end

VC調(diào)用

self.timer = [TempTarget scheduledTimerWithTimeInterval:1 target:self selector:@selector(timerStart:) userInfo:nil repeats:YES];

VC強引用timer，因為timer的target是TempTarget實例，所以timer強引用TempTarget實例，而TempTarget實例弱引用VC，解除循環(huán)引用。

image

WeakProxy：

創(chuàng)建一個繼承NSProxy的子類WeakProxy，并實現(xiàn)消息轉(zhuǎn)發(fā)的相關(guān)方法。

WeakProxy.h文件

@property (nonatomic, weak, readonly) id weakTarget;

+ (instancetype)proxyWithTarget:(id)target;
- (instancetype)initWithTarget:(id)target;

WeakProxy.m文件

@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];
    }
}

//返回方法的簽名。
- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    return [self.weakTarget methodSignatureForSelector:sel];
}

- (BOOL)respondsToSelector:(SEL)aSelector {
    return [self.weakTarget respondsToSelector:aSelector];
}
@end

VC調(diào)用

self.timer = [NSTimer scheduledTimerWithTimeInterval:1 target:[WeakProxy proxyWithTarget:self] selector:@selector(timerStart:) userInfo:nil repeats:YES];
- (void)dealloc {
    [_timer invalidate];
    NSLog(@"VC銷毀了");
}

原理跟類方法相似，打破循環(huán)引用的環(huán)路。將timer的target設(shè)置為WeakProxy實例，利用消息轉(zhuǎn)發(fā)機制實現(xiàn)執(zhí)行VC中的計時方法，解決循環(huán)引用。

GCD:

VC的.m文件

@interface ViewController ()
@property (nonatomic, strong) dispatch_source_t timer;
@end

- (void)viewDidLoad {
    [super viewDidLoad];
   dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    dispatch_source_set_timer(_timer, dispatch_time(DISPATCH_TIME_NOW, 0), 0.1*NSEC_PER_SEC, 0);
    dispatch_source_set_event_handler(_timer, ^{
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"執(zhí)行了");
        });
    });
    //開啟計時器
    dispatch_resume(_timer);
}

//結(jié)束計時
dispatch_source_cancel(_timer);

Block:

創(chuàng)建NSTimer的分類NSTimer+UnretainCycle。

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

+ (void)blcokInvoke:(NSTimer *)timer {

    void (^block)(NSTimer *timer) = timer.userInfo;
    if (block) {
        block(timer);
    }
}

VC調(diào)用

self.timer = [NSTimer mxScheduledTimerWithTimeInterval:0.5 repeats:YES block:^(NSTimer *timer) {
        NSLog(@"執(zhí)行了");
    }];

來自：http://m.itdecent.cn/p/aaf7b13864d9