定时器 · a half fish

Objc - 系列文章之一

Part 17: 当前阅读

Part 18: Togged Pointer

Part 20: AutoreleasePool

Part 22: App Lanuch

Part 23: 安装包瘦身

Part 24: 数据持久化、缓存

NSTimer、CADisplayLink定时器>

NSTimer、CADisplayLink定时器 #

CADisplayLink使用

@interface TMViewController ()
@property (strong, nonatomic) CADisplayLink * link;
@end

- (void)viewDidLoad {
    [super viewDidLoad];
    _link = [CADisplayLink displayLinkWithTarget:(id)target selector:@selector(linkTest)];
    [_link addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];    
}

- (void)dealloc {
    [_link invalidate];
}

NSTimer使用

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

- (void)viewDidLoad {
    [super viewDidLoad];
    _timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:(id)target selector:@selector(linkTest) userInfo:nil repeats:YES];
}

- (void)dealloc {
    [_timer invalidate];
}

以上方式如果将target传为self，则会造成self对timer的强引用，timer又对self强引用，双方释放不了，造成循环引用。

解决方法>

解决方法 #

创建一个不依赖于self的timer

- (void)viewDidLoad {
    [super viewDidLoad];
    
    _timer = [NSTimer timerWithTimeInterval:1 repeats:YES block:^(NSTimer * _Nonnull timer) {
        NSLog(@"执行任务");
    }];
    [[NSRunLoop currentRunLoop] addTimer:_timer forMode:NSDefaultRunLoopMode];
    
}

- (void)dealloc {
    [_timer invalidate];
}

添加中间对象（NSProxy），弱引用self然后将消息转发给self。

- (void)viewDidLoad {
    [super viewDidLoad];
     
    _timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:[TMProxy initWithTarget:self] selector:@selector(linkTest) userInfo:nil repeats:YES];

}

- (void)dealloc {
    [_timer invalidate];
}

// TMProxy实现

@interface TMProxy ()

@property (weak, nonatomic) id target;

@end

@implementation TMProxy

+ (TMProxy *)initWithTarget:(id)target {
    TMProxy *proxy =  [TMProxy alloc];
    proxy.target = target;
    return proxy;
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    return [_target methodSignatureForSelector:sel];
}

- (void)forwardInvocation:(NSInvocation *)invocation {
    [invocation invokeWithTarget:_target];
}

@end

GCD定时器>

GCD定时器 #

由于NSTimer是依赖于RunLoop进行工作的，而RunLoop内部除了需要执行NSTimer事务，同时也可能处理其他事务，所以RunLoop进行计时会又不准确的情况。而GCD不依赖于RunLoop，GCD定时器属于系统层面。所以GCD定时器能够相对准确。

创建GCD定时器>

创建GCD定时器 #

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

@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    
    dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_global_queue(0, 0));
    dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, 2 * NSEC_PER_SEC, 0);
    dispatch_source_set_event_handler(timer, ^{
        NSLog(@"gcd timer counting.");
    });
    dispatch_resume(timer);
		// 防止timer被释放。
    _timer = timer;
}
@end