NSTimer使用

一、介绍

NSTimer.h文件里的一些方法、属性的讲解

1、NSInvocation创建

+ (NSTimer *)timerWithTimeInterval:(NSTimeInterval)ti invocation:(NSInvocation *)invocation repeats:(BOOL)yesOrNo;
+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)ti invocation:(NSInvocation *)invocation repeats:(BOOL)yesOrNo;

需要一个NSInvocation对象创建。前者创建一个定时器，但是需要在创建定时器后手动将timer加入NSRunLoop 中；后者自动以NSDefaultRunLoopMode添加到当前线程RunLoop中。

例子：

- (void)setupTimer{
    SEL sel = @selector(run);
    NSMethodSignature *sign = [[self class] instanceMethodSignatureForSelector:sel];
    NSInvocation *invo = [NSInvocation invocationWithMethodSignature:sign];
    invo.target = self;
    invo.selector = sel;
    [invo invoke];
    
    NSTimer *timer = [NSTimer timerWithTimeInterval:1 invocation:invo repeats:YES];
    // NSRunLoopCommonModes NSDefaultRunLoopMode
    [[NSRunLoop currentRunLoop] addTimer:timer forMode:NSDefaultRunLoopMode];
    
    
}

- (void)run{
    NSLog(@"run");
}

2、指定target和selector

+ (NSTimer *)timerWithTimeInterval:(NSTimeInterval)ti target:(id)aTarget selector:(SEL)aSelector userInfo:(nullable id)userInfo repeats:(BOOL)yesOrNo;
+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)ti target:(id)aTarget selector:(SEL)aSelector userInfo:(nullable id)userInfo repeats:(BOOL)yesOrNo;

这两个是常用的创建方式，区别同上

3、block

+ (NSTimer *)timerWithTimeInterval:(NSTimeInterval)interval repeats:(BOOL)repeats block:(void (^)(NSTimer *timer))block ;
+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)interval repeats:(BOOL)repeats block:(void (^)(NSTimer *timer))block;

block创建方式，区别同上

4、实例方法

- (instancetype)initWithFireDate:(NSDate *)date interval:(NSTimeInterval)interval repeats:(BOOL)repeats block:(void (^)(NSTimer *timer))block ;
- (instancetype)initWithFireDate:(NSDate *)date interval:(NSTimeInterval)ti target:(id)t selector:(SEL)s userInfo:(nullable id)ui repeats:(BOOL)rep ;

实例方法创建方式，区别同上

// 执行（不加入runloop,用fire执行一次）
- (void)fire;

@property (copy) NSDate *fireDate;
@property (readonly) NSTimeInterval timeInterval;

// 容差  
@property NSTimeInterval tolerance API_AVAILABLE(macos(10.9), ios(7.0), watchos(2.0), tvos(9.0));
// 停止定时器再次触发并请求将其从runloop中移除
- (void)invalidate;
@property (readonly, getter=isValid) BOOL valid;

@property (nullable, readonly, retain) id userInfo;

二、实例

1、子线程开启定时器

@interface BViewController ()

@property (nonatomic,strong) NSTimer *timer;
@property (nonatomic,strong) NSThread *thread;

@end

@implementation BViewController

- (void)dealloc{
    NSLog(@"注销");
}

- (void)viewDidLoad {
    [super viewDidLoad];
    NSLog(@"%@ %@",[NSThread currentThread],[NSRunLoop currentRunLoop]);
    self.thread = [[NSThread alloc] initWithTarget:self selector:@selector(perTask) object:nil];
    [self.thread start];
    
}

- (void)perTask{
    NSLog(@"%@ %@",[NSThread currentThread],[NSRunLoop currentRunLoop]);
    self.timer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(run) userInfo:nil repeats:YES];
    // 子线程runloop是默认不运行，必须手动开启
    [[NSRunLoop currentRunLoop] run];
}

- (void)run{
    NSLog(@"run里的线程%@",[NSThread currentThread]);
    if ([NSThread currentThread].isCancelled) {
        [self.timer invalidate];
    }
    NSLog(@"run");
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    [self.thread cancel];
//    if (self.thread.isCancelled) {
//        [self.timer invalidate];
//    }
    NSLog(@"touch的线程%@",[NSThread currentThread]);
    
}

这个例子有几个注意点：
1、子线程runloop默认不运行，必须手动run
2、invalidate：文档上有一段说明

必须从定时器当前线程发送invalidate。 如果从另一个线程发送此消息，则与定时器关联的输入源可能不会从其runloop中删除，这可能会阻止线程正常退出。

所以销毁timer必须在子线程中即run方法里

2、循环问题
这个NStimer经典问题：当前VC持有timer，timer的target又是self(VC),从而产生循环引用。

怎么解决？

一般是按实例1中的方法，在退出VC前invalidate掉timer,但是往往我们会遗漏或者某些场景不好操作。如何优雅的解决循环引用呢？

把timer的target设为自己，这样就不会循环引用。我们用一个Category封装一下：

+ (NSTimer *)ca_scheduledTimerWithTimeInterval:(NSTimeInterval)interval
                                       repeats:(BOOL)repeats
                                         block:(void(^)(void))block{
    return [self scheduledTimerWithTimeInterval:interval target:self selector:@selector(ca_blockInvoke:) userInfo:[block copy] repeats:repeats];
}

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

这样的话，timer事件在block中执行。

注：
1、NSTimer不是一种实时机制，这点官网文档上作了说明

A timer is not a real-time mechanism. If a timer’s firing time occurs during a long run loop callout or while the run loop is in a mode that isn't monitoring the timer, the timer doesn't fire until the next time the run loop checks the timer. Therefore, the actual time at which a timer fires can be significantly later.

2、当然了，想让NSTimer在scrollView滚动时有效需要将runloop mode换成NSRunLoopCommonModes(默认mode是NSDefaultRunLoopMode，scrollView滚动时mode切换成UITrackingRunLoopMode，所以NSTimer失效)