NSTimer循环引用解决方案

文章以在TimerViewController中使用计时器为例，在VC中声明一个NSTimer属性。

创建NSTimer对象：

self.timer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(startTimer) 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

解决循环引用的几种方式：

weak指针：

既然是强引用导致循环引用，那么用__weak修饰self就好了，想法是对的，但是做法是无效的。因为无论是weak还是strong修饰，在NSTimer中都会重新生成一个新的强引用指针指向self，导致循环引用的。

类方法：

创建一个继承NSObject的子类TempTarget，并创建开启计时器的方法。

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调用

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

VC强引用timer，因为timer的target是TempTarget实例，所以timer强引用TempTarget实例，而TempTarget实例弱引用VC，解除循环引用。

强引用转移

WeakProxy：

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

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];
    }
}

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

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

VC调用

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

原理跟类方法相似，打破循环引用的环路。将timer的target设置为WeakProxy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:

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(@"执行了");
        });
    });
    //开启计时器
    dispatch_resume(_timer);
}

//结束计时
dispatch_source_cancel(_timer);

Block:

创建NSTimer的分类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调用

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