自我弱的 NSTimer；为什么不调用 dealloc？

Question

考虑具有强（或弱，相同）的视图控制器NSTimer 属性:

__weak __typeof(self) ws = self;
self.timer = [NSTimer scheduledTimerWithTimeInterval:2 target:ws selector:@selector(timerTigger:) userInfo:nil repeats:YES];

但为什么这个视图控制器不调用 dealloc 方法，无论我通过 strong 还是 weak 引用 self？

详细代码如下：

#import "SecondViewController.h"

@interface SecondViewController ()

@property (nonatomic, weak) NSTimer *timer;

@end

@implementation SecondViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor whiteColor];
    __weak __typeof(self) ws = self;
    self.timer = [NSTimer scheduledTimerWithTimeInterval:2 target:ws selector:@selector(timerTigger:) userInfo:nil repeats:YES];

}

- (void)timerTigger:(id)timer {
    NSLog(@"do someting");
}

- (void)dealloc {
    NSLog(@"SecondViewController dealloc");
}

Answer 1

NSTimer 保持对其 target 的强引用，直到计时器失效。您无法选择 NSTimer 是建立弱参考还是强参考。当您传递弱引用时，只要在您启动计时器时 ws 不是 nil（在这种情况下显然不会），NSTimer 将建立对 target 指出的任何内容的强烈引用。 scheduledTimerWithTimeInterval 是否建立强引用或弱引用不是您传递给它的指针的某些固有特征，而是该方法如何处理它所提供的指针的问题。

要修复这种强引用行为，您可以采用以下模式之一：

使用 NSTimer 的基于块的再现并在块内使用 "weak self" 模式；

@interface ViewController ()

@property (nonatomic, weak) NSTimer *timer;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    typeof(self) __weak weakSelf = self;

    self.timer = [NSTimer scheduledTimerWithTimeInterval:2 repeats:true block:^(NSTimer * _Nonnull timer) {
        [weakSelf timerTigger];
    }];
}

- (void)timerTigger {
    NSLog(@"do something");
}

- (void)dealloc {
    [self.timer invalidate];
}


@end

使用 GCD 计时器，它也是基于块的，因此也可以轻松配置为不保持强引用；或

@interface ViewController ()

@property (nonatomic, strong) dispatch_source_t timer;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    typeof(self) __weak weakSelf = self;

    self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_main_queue());
    dispatch_source_set_timer(self.timer, DISPATCH_TIME_NOW, 1 * NSEC_PER_SEC, 0 * NSEC_PER_SEC);
    dispatch_source_set_event_handler(self.timer, ^{
        [weakSelf timerTigger];
    });
    dispatch_resume(self.timer);
}

- (void)timerTigger {
    NSLog(@"do something");
}

@end

将 NSTimer 与 target/selector 一起使用，但 invalidate 计时器在某个地方是合乎逻辑的（例如 viewDidDisappear 等）。

@interface ViewController ()

@property (nonatomic, weak) NSTimer *timer;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    self.timer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(timerTigger:) userInfo:nil repeats:true];
}

- (void)viewDidDisappear:(BOOL)animated {
    [super viewDidDisappear:animated];

    [self.timer invalidate];
}

- (void)timerTigger:(NSTimer *)timer {
    NSLog(@"do something");
}

@end

Answer 2

你可以试试这个：

@interface Person : NSObject
@property(nonatomic, strong) DemoViewController_19 *vc;
@end

@implementation Person
@end

@interface DemoViewController_19 ()
@property(nonatomic, strong) Person *person;
@end

@implementation DemoViewController_19

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    self.person = [Person new];
    __weak typeof(self) weaks = self;
    self.person.vc = weaks;
}

@end

在运行之后你可以看到 vc dealloc 没有被调用。取决于Person属性的强属性

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