如何知道何时使“NSTimer”无效

Question

这是我的问题：

我有一个模型类，其中有一个NSTimer，我希望计时器在模型对象的整个生命周期内运行。初始化很简单：我在init 方法中只有以下代码行：

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

但是，我的问题是，如何在模型从内存中释放时使此计时器无效？ 现在，这通常很容易，但是据我所知，当您安排NSTimer 操作系统维护一个指向 Timer 对象的强指针。

我应该如何处理这个问题？有没有在模型从内存中释放之前被调用的方法？

Answer 1

[NSTimer scheduledTimerWithTimeInterval:...]保留目标，所以如果目标是self，那么你的模型类实例永远不会被释放。

作为一种解决方法，可以使用单独的对象（在以下示例中称为 TimerTarget）。 TimerTarget 有一个对ModelClass 的弱引用，以避免保留循环。

这个“帮助类”看起来像这样。它的唯一目的是将定时器事件转发给“真正的目标”。

@interface TimerTarget : NSObject
@property(weak, nonatomic) id realTarget;
@end

@implementation TimerTarget

- (void)timerFired:(NSTimer*)theTimer
{
    [self.realTarget performSelector:@selector(timerFired:) withObject:theTimer];
}

@end

现在，在您的模型类中，您可以在dealloc 中创建一个计时器并使其无效：

@interface ModelClass ()
@property(strong, nonatomic) NSTimer *timer;
@end

@implementation ModelClass

- (id)init
{
    self = [super init];
    if (self) {
        TimerTarget *timerTarget = [[TimerTarget alloc] init];
        timerTarget.realTarget = self;
        self.timer = [NSTimer scheduledTimerWithTimeInterval:1
                                                 target:timerTarget
                                               selector:@selector(timerFired:)
                                               userInfo:nil repeats:YES];
    }
    return self;
}

- (void)dealloc
{
    [self.timer invalidate]; // This releases the TimerTarget as well!
    NSLog(@"ModelClass dealloc");
}

- (void)timerFired:(NSTimer*)theTimer
{
    NSLog(@"Timer fired");
}

@end

所以我们有

modelInstance ===> timer ===> timerTarget ---> modelInstance
(===> : strong reference, ---> : weak reference)

请注意，不再有从计时器到模型类实例的（强）引用。

我已使用以下代码对此进行了测试，该代码创建了一个 ModelClass 实例并在 5 秒后释放它：

__block ModelClass *modelInstance = [[ModelClass alloc] init];
int64_t delayInSeconds = 5.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
    modelInstance = nil;
});

输出：

2013-01-23 23:54:11.483 timertest[16576:c07] Timer fired
2013-01-23 23:54:12.483 timertest[16576:c07] Timer fired
2013-01-23 23:54:13.483 timertest[16576:c07] Timer fired
2013-01-23 23:54:14.483 timertest[16576:c07] Timer fired
2013-01-23 23:54:15.483 timertest[16576:c07] Timer fired
2013-01-23 23:54:15.484 timertest[16576:c07] ModelClass dealloc

Answer 2

基于@Martin R 的想法，我创建了更易于使用的自定义类，并添加了一些检查以避免崩溃。

@interface EATimerTarget : NSObject

// Initialize with block to avoid missing call back
- (instancetype)initWithRealTarget:(id)realTarget timerBlock:(void(^)(NSTimer *))block;

// For NSTimer @selector() parameter
- (void)timerFired:(NSTimer *)timer;

@end

@interface EATimerTarget ()
@property (weak, nonatomic) id realTarget;
@property (nonatomic, copy) void (^timerBlock)(NSTimer *); // use 'copy' to avoid retain counting
@end

@implementation EATimerTarget

- (instancetype)initWithRealTarget:(id)realTarget timerBlock:(void (^)(NSTimer *))block {
    self = [super init];
    if (self) {
        self.realTarget = realTarget;
        self.timerBlock = block;
    }
    return self;
}

- (void)timerFired:(NSTimer *)timer {
    // Avoid memory leak, timer still run while our real target is dealloc
    if (self.realTarget) {
        self.timerBlock(timer);
    }
    else {
        [timer invalidate];
    }
}

@end

这是我的示例类

@interface MyClass
@property (nonatomic, strong) NSTimer *timer;
@end

@implementation MyClass

- (id)init
{
    self = [super init];
    if (self) {
         // Using __weak for avoiding retain cycles
         __weak typeof(self) wSelf = self;
    EATimerTarget *timerTarget = [[EATimerTarget alloc] initWithRealTarget:self timerBlock: ^(NSTimer *timer) {
        [wSelf onTimerTick:timer];
    }];
         self.timer = [NSTimer timerWithTimeInterval:1 target:timerTarget selector:@selector(timerFired:) userInfo:nil repeats:YES];
         [[NSRunLoop mainRunLoop] addTimer:self.timer forMode:NSDefaultRunLoopMode];
    }
    return self;
}

- (void)dealloc
{
    [self.timer invalidate]; // This releases the EATimerTarget as well! 
    NSLog(@"### MyClass dealloc");
}

- (void)onTimerTick:(NSTimer *)timer {
     // DO YOUR STUFF!
     NSLog(@"### TIMER TICK");
}

@end