NSTimer使用詳解

時間 2019-11-08

標籤 nstimer 使用詳解欄目 iOS 简体版

原文原文鏈接

1.開發中如何使用NSTimer

1.   self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerFired) userInfo:nil repeats:YES];複製代碼

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

這兩個方法是等價的，區別是第一個方法默認建立了一個NSTimer並自動添加到了當前線程的Runloop中去，第二個須要咱們手動添加。若是當前線程是主線程的話，某些UI事件，好比UIScrollView的拖拽操做，會將Runloop切換成UITrackingRunLoopMode,這時候，默認的NSDefaultRunLoopMode模式中註冊的事件是不會被執行的。因此爲了設置一個不會被UI干擾的Timer，咱們須要手動將timer的當前RunloopMode設置爲NSRunLoopCommonModes，這個模式等效於NSDefaultRunLoopMode和UITrackingRunLoopMode的結合。html

2.NSTimer沒法釋放的緣由分析

上面的使用方法是沒問題的，可是你們在使用過程當中必定遇到過因使用了NSTimer，致使所在的UIViewController內存泄漏的問題，這種緣由是怎麼出現的呢？
其中許多人都認爲是UIViewController和NSTimer循環引用的問題，彼此強引用，致使了彼此沒法釋放，那麼問題真的是這樣嗎？

驗證以下：bash

NSTimer *timer = [NSTimer timerWithTimeInterval:1.0 target:self selector:@selector(timerFired) userInfo:nil repeats:YES];
 [[NSRunLoop mainRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];
//結果：將NSTimer設置成局部變量，你會發現二者仍釋放不了。複製代碼

將self設置成弱引用，又會是什麼現象呢?

__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];
//結果：二者仍然沒法釋放。複製代碼

若是咱們將target強制釋放，強制破壞循環引用呢?app

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));
//結果：Test順利釋放，但Timer仍在運行。而且在Timer觸發事件時崩潰複製代碼

在timer創建後面斷點,查看運行的時候內存圖oop

結果：其實只有timer單向的指向target,target並未指向timer,是由於timer運行的時候釋放不了,致使被強引用的target也沒法釋放。並不是循環引用致使不釋放。ui

3.解決NSTimer的內存泄漏問題

通常呢解決NSTimer的內存泄漏問題，一般有兩種方法，第一種是找對合適的時機釋放NSTimer，一般人們會想到兩個調用時機。

-(void)dealloc
 {
    [self.timer invalidate];
 }
//NSTimer，一般人們會想到兩個調用時機。
複製代碼

-(void)viewWillDisappear:(BOOL)animated
 {
    [super viewWillDisappear:animated];
    [self.timer invalidate];
 }
//這種狀況是能夠解決循環引用的問題，內存能夠釋放，可是又會引來新的問題，當導航控制器push到下一個頁面時，當前VC並無被釋放，這時候咱們可能並不想銷燬NSTimer，咱們一般但願VC該銷燬的時候，同時銷燬NSTimer，因此調用invalidate方法的時機很難找複製代碼

那麼就是第二種了，想辦法破除強引用，讓NSTimer和VC同生共死，這種方法呢也有兩種方式

1.使用block的方式：atom

#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]; }複製代碼

2.使用NSProxy來初始化一個子類，這裏咱們直接用YYWeakProcyspa

#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];
 }
//至於具體的原理，讓NSTimer定時中的方法由YYWeakProxy轉發給VC執行.可是NStimer持有的卻不是VC.這樣就不會循環引用.複製代碼

4.開發中如何建立更精確的定時器

你們應該知道，NSTimer的精確度通常能達到1ms，也就是小於1毫秒時，偏差會很大，那麼如何建立一個偏差很小，甚至沒有偏差的定時器呢

納秒級精度的Timer.net

#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>
//理論上這是iPhone上最精準的定時器，能夠達到納秒級別的精度複製代碼

CADisplayLink線程

CADisplayLink * displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(logInfo)];
 [displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
//CADisplayLink是一個頻率能達到屏幕刷新率的定時器類。iPhone屏幕刷新頻率爲60幀/秒，也就是說最小間隔能夠達到1/60s。複製代碼

GCD定時器debug

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實現的timer，因此理論上來講，GCD定時器的精度比NSTimer只高不低。複製代碼

參考資料：

www.cnblogs.com/pioneerMax/…

blog.csdn.net/allangold/a…

www.aliyun.com/jiaocheng/3…l