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NSTimer使用详解

1.开发中如何使用NSTimer

1.   self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerFired) userInfo:nil repeats:YES];复制代码

2.  self.timer = [NSTimer timerWithTimeInterval:1.0 target:self selector:@selector(timerFired) userInfo:nil repeats:YES];
    [[NSRunLoop mainRunLoop] addTimer:self.timer forMode:NSDefaultRunLoopMode];
复制代码

这两个方法是等价的,区别是第一个方法默认创建了一个NSTimer并自动添加到了当前线程的Runloop中去,第二个需要我们手动添加。如果当前线程是主线程的话,某些UI事件,比如UIScrollView的拖拽操作,会将Runloop切换成UITrackingRunLoopMode,这时候,默认的NSDefaultRunLoopMode模式中注册的事件是不会被执行的。所以为了设置一个不会被UI干扰的Timer,我们需要手动将timer的当前RunloopMode设置为NSRunLoopCommonModes,这个模式等效于NSDefaultRunLoopMode和UITrackingRunLoopMode的结合。

2.NSTimer无法释放的原因分析

上面的使用方法是没问题的,但是大家在使用过程中一定遇到过因使用了NSTimer,导致所在的UIViewController内存泄漏的问题,这种原因是怎么出现的呢?
其中许多人都认为是UIViewController和NSTimer循环引用的问题,彼此强引用,导致了彼此无法释放,那么问题真的是这样吗?
  1. 验证如下:

     NSTimer *timer = [NSTimer timerWithTimeInterval:1.0 target:self selector:@selector(timerFired) userInfo:nil repeats:YES];
     [[NSRunLoop mainRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];
    //结果:将NSTimer设置成局部变量,你会发现两者仍释放不了。复制代码
  2. 将self设置成弱引用,又会是什么现象呢?

     __weak typeof(self) weakSelf = self;
     NSTimer *timer = [NSTimer timerWithTimeInterval:1.0 target:weakSelf selector:@selector(timerFired) userInfo:nil repeats:YES];
     [[NSRunLoop mainRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];
    //结果:两者仍然无法释放。复制代码
  3. 如果我们将target强制释放,强制破坏循环引用呢?

     TimerAction *Test = [TimerAction new];
     NSTimer *timer = [NSTimer timerWithTimeInterval:1.0 target:Test selector:@selector(test) userInfo:nil repeats:YES];
     [[NSRunLoop mainRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];
     CFRelease((__bridge CFTypeRef)(Test));
    //结果:Test顺利释放,但Timer仍在运行。并且在Timer触发事件时崩溃复制代码
  4. 在timer建立后面断点,查看运行的时候内存图


结果:其实只有timer单向的指向target,target并未指向timer,是因为timer运行的时候释放不了,导致被强引用的target也无法释放。并非循环引用导致不释放。

3.解决NSTimer的内存泄漏问题

一般呢解决NSTimer的内存泄漏问题,通常有两种方法,第一种是找对合适的时机释放NSTimer,通常人们会想到两个调用时机。
  1. -(void)dealloc
     {
        [self.timer invalidate];
     }
    //NSTimer,通常人们会想到两个调用时机。
    复制代码
  2. -(void)viewWillDisappear:(BOOL)animated
     {
        [super viewWillDisappear:animated];
        [self.timer invalidate];
     }
    //这种情况是可以解决循环引用的问题,内存可以释放,但是又会引来新的问题,当导航控制器push到下一个页面时,当前VC并没有被释放,这时候我们可能并不想销毁NSTimer,我们通常希望VC该销毁的时候,同时销毁NSTimer,所以调用invalidate方法的时机很难找复制代码
那么就是第二种了,想办法破除强引用,让NSTimer和VC同生共死,这种方法呢也有两种方式

1.使用block的方式:

 #import <Foundation/Foundation.h>
 typedef void(^JSTimerBlcok)(NSTimer *timer);
 @interface NSTimer (Category)

 + (NSTimer *)js_scheduledTimerWithTimeInterval:(NSTimeInterval)timeInterval executeBlock:(JSTimerBlcok)block repeats:(BOOL)repeats;

 @end

 #import "NSTimer+Category.h"

 @implementation NSTimer (Category)

 +(NSTimer *)js_scheduledTimerWithTimeInterval:(NSTimeInterval)timeInterval executeBlock:(JSTimerBlcok)block repeats:(BOOL)repeats
 {

 NSTimer *timer = [self scheduledTimerWithTimeInterval:timeInterval target:self selector:@selector(js_executeTimer:) userInfo:[block copy] repeats:repeats];

 return timer;

 }

 +(void)js_executeTimer:(NSTimer *)timer

 {
     JSTimerBlcok block = timer.userInfo;
     if (block) {
     block(timer);
     }
 }
 @end
使用案例: - (void)viewDidLoad { [super viewDidLoad]; __weak typeof(self) weakSelf = self; self.timer = [NSTimer js_scheduledTimerWithTimeInterval:1.0 executeBlock:^(NSTimer *timer){         __strong typeof(weakSelf) strongSelf = weakSelf;        [strongSelf timerFired:timer];       } repeats:YES]; }复制代码

2.使用NSProxy来初始化一个子类,这里我们直接用YYWeakProcy

#import <Foundation/Foundation.h>
 
 NS_ASSUME_NONNULL_BEGIN

@interface YYWeakProxy : NSProxy

@property (nullable, nonatomic, weak, readonly) id target;

- (instancetype)initWithTarget:(id)target;

+ (instancetype)proxyWithTarget:(id)target;

@end

NS_ASSUME_NONNULL_END
 
 #import "YYWeakProxy.h"
 
 
 @implementation YYWeakProxy
 
 - (instancetype)initWithTarget:(id)target {
 _target = target;
 return self;
 }
 
 + (instancetype)proxyWithTarget:(id)target {
 return [[YYWeakProxy alloc] initWithTarget:target];
 }
 
 - (id)forwardingTargetForSelector:(SEL)selector {
 return _target;
 }
 
 - (void)forwardInvocation:(NSInvocation *)invocation {
 void *null = NULL;
 [invocation setReturnValue:&null];
 }
 
 - (NSMethodSignature *)methodSignatureForSelector:(SEL)selector {
 return [NSObject instanceMethodSignatureForSelector:@selector(init)];
 }
 
 - (BOOL)respondsToSelector:(SEL)aSelector {
 return [_target respondsToSelector:aSelector];
 }
 
 - (BOOL)isEqual:(id)object {
 return [_target isEqual:object];
 }
 
 - (NSUInteger)hash {
 return [_target hash];
 }
 
 - (Class)superclass {
 return [_target superclass];
 }
 
 - (Class)class {
 return [_target class];
 }
 
 - (BOOL)isKindOfClass:(Class)aClass {
 return [_target isKindOfClass:aClass];
 }
 
 - (BOOL)isMemberOfClass:(Class)aClass {
 return [_target isMemberOfClass:aClass];
 }
 
 - (BOOL)conformsToProtocol:(Protocol *)aProtocol {
 return [_target conformsToProtocol:aProtocol];
 }
 
 - (BOOL)isProxy {
 return YES;
 }
 
 - (NSString *)description {
 return [_target description];
 }
 
 - (NSString *)debugDescription {
 return [_target debugDescription];
 }
 
 @end
使用案例:
 - (void)initTimer {
    YYWeakProxy *proxy = [YYWeakProxy proxyWithTarget:self];
    _timer = [NSTimer timerWithTimeInterval:0.1 target:proxy selector:@selector(tick:) userInfo:nil repeats:YES];
 }
//至于具体的原理,让NSTimer定时中的方法由YYWeakProxy转发给VC执行.但是NStimer持有的却不是VC.这样就不会循环引用.复制代码

4.开发中如何创建更精确的定时器

大家应该知道,NSTimer的精确度一般能达到1ms,也就是小于1毫秒时,误差会很大,那么如何创建一个误差很小,甚至没有误差的定时器呢
  1. 纳秒级精度的Timer

    #include <mach mach.h="">
     #include <mach mach_time.h="">
     static const uint64_t NANOS_PER_USEC = 1000ULL;
    
     static const uint64_t NANOS_PER_MILLISEC = 1000ULL * NANOS_PER_USEC;
     static const uint64_t NANOS_PER_SEC = 1000ULL * NANOS_PER_MILLISEC;
     static mach_timebase_info_data_t timebase_info;
    
     static uint64_t nanos_to_abs(uint64_t nanos) {
    
     return nanos * timebase_info.denom / timebase_info.numer;
    
     }
     void waitSeconds(int seconds) {
    
     mach_timebase_info(&timebase_info);
    
     uint64_t time_to_wait = nanos_to_abs(seconds * NANOS_PER_SEC);
    
     uint64_t now = mach_absolute_time();
    
     mach_wait_until(now + time_to_wait);
    
     }</mach></mach>
    //理论上这是iPhone上最精准的定时器,可以达到纳秒级别的精度复制代码
  2. CADisplayLink

    CADisplayLink * displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(logInfo)];
     [displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
    //CADisplayLink是一个频率能达到屏幕刷新率的定时器类。iPhone屏幕刷新频率为60帧/秒,也就是说最小间隔可以达到1/60s。复制代码
  3. GCD定时器

    NSTimeInterval interval = 1.0;
     _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0));
    
     dispatch_source_set_timer(_timer, dispatch_walltime(NULL, 0), interval * NSEC_PER_SEC, 0);
    
     dispatch_source_set_event_handler(_timer, ^{
    
     NSLog(@"GCD timer test");
    
     });
     dispatch_resume(_timer);
    //RunLoop是dispatch_source_t实现的timer,所以理论上来说,GCD定时器的精度比NSTimer只高不低。复制代码

参考资料:

www.cnblogs.com/pioneerMax/…

blog.csdn.net/allangold/a…

www.aliyun.com/jiaocheng/3…l


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