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iOS定时器

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NSTimer如何使用

方法1: Creates a timer and schedules it on the current run loop in the default mode.

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

方法2: Initializes a timer object with the specified object and selector.

You must add the new timer to a run loop, using addTimer:forMode:. Then, after ti seconds have elapsed, the timer fires, sending the message aSelector to target. (If the timer is configured to repeat, there is no need to subsequently re-add the timer to the run loop.)

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

[[NSRunLoop mainRunLoop] addTimer:self.timer forMode:NSDefaultRunLoopMode];

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。

由于NSTimer会强引用Target，RunLoop会强引用NSTimer，若ViewController再强引用NSTimer，NSTimer找不到合适的时机释放的时候，很容易造成内存泄漏，这个问题如何解决？

如何解决NSTimer找不到释放时机的问题？

方案1，使用官方API带Block的方式来创建NSTimer

具体的创建方法如下，但是仅支持iOS 10以上的系统。

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+ (NSTimer *)timerWithTimeInterval:(NSTimeInterval)interval repeats:(BOOL)repeats block:(void (^)(NSTimer *timer))block API_AVAILABLE(macosx(10.12), ios(10.0), watchos(3.0), tvos(10.0));

在ViewController中的使用方法：

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__weak typeof(self)weakSelf = self;
self.timer = [NSTimer timerWithTimeInterval:1.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
    NSLog(@"第%ld次执行", (long)weakSelf.count);
    weakSelf.count ++;
}];
[[NSRunLoop currentRunLoop] addTimer:self.timer forMode:NSRunLoopCommonModes];

方案2，使用Block的方式来创建NSTimer

具体实现方式是，需要创建一个NSTimer的分类，让外部传入一个Block，NSTimer每次到时间就执行传入进来的Block。新建一个分类：
NSTimer+BlockSupport.h 

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#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface NSTimer (BlockSupport)

+ (NSTimer *)cd_timerWithTimeInterval:(NSTimeInterval)interval
                                       repeats:(BOOL)repeats
                                         block:(void(^)(void))block;

@end

NS_ASSUME_NONNULL_END

NSTimer+BlockSupport.m

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#import "NSTimer+BlockSupport.h"

@implementation NSTimer (BlockSupport)

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

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

@end

在ViewController中使用方法：

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__weak typeof(self)weakSelf = self;
self.timer = [NSTimer cd_timerWithTimeInterval:1.0 repeats:YES block:^{
    NSLog(@"第%ld次执行", (long)weakSelf.count);
    weakSelf.count ++;
}];
[[NSRunLoop currentRunLoop] addTimer:self.timer forMode:NSRunLoopCommonModes];

方案3，自定义NSTimer的Target

封装自定义NSTimer的Target和NSTimer的工具类，让NSTimer在合适的时机释放。
CDTimerTarget.h

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#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface CDTimerTarget : NSObject

// 目标对象
@property (nonatomic,weak) id target;
// 目标方法
@property (nonatomic,assign) SEL selector;

@end

NS_ASSUME_NONNULL_END

CDTimerTarget.m

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#import "CDTimerTarget.h"

@implementation CDTimerTarget

- (void)fire:(NSTimer *)timer {
    // target为弱引用,所以当target对象销毁时,target 为nil
    if (self.target) {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
        [self.target performSelector:self.selector withObject:timer.userInfo];
#pragma clang diagnostic pop
    }else {
        [timer invalidate];
    }
}

@end

CDTimer.h

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#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface CDTimer : NSObject

+ (NSTimer *)scheduleTimerWithTimerInterval:(NSTimeInterval)interval
                                     target:(id)aTarget
                                   selector:(SEL)aSelector
                                   userInfo:(id)userInfo
                                    repeats:(BOOL)repeats;

@end

NS_ASSUME_NONNULL_END

CDTimer.m

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#import "CDTimer.h"
#import "CDTimerTarget.h"

@implementation CDTimer

+ (NSTimer *)scheduleTimerWithTimerInterval:(NSTimeInterval)interval
                                     target:(id)aTarget
                                   selector:(SEL)aSelector
                                   userInfo:(id)userInfo
                                    repeats:(BOOL)repeats {
    
    CDTimerTarget *timerTarget = [[CDTimerTarget alloc]init];
    timerTarget.target = aTarget;
    timerTarget.selector = aSelector;
    NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:interval
                                                      target:timerTarget
                                                    selector:@selector(fire:)
                                                    userInfo:userInfo
                                                     repeats:repeats];
    return timer;
}

@end

在ViewController中的用法：

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self.timer = [CDTimer scheduleTimerWithTimerInterval:1.0 target:self selector:@selector(repeatAction) userInfo:nil repeats:YES];

方案4，自定义NSProxy子类，作为NSTimer的Target

TargetProxy.h

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#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface TargetProxy : NSProxy

+ (instancetype)proxyWithTarget:(id)target;

@end

NS_ASSUME_NONNULL_END

TargetProxy.m

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#import "TargetProxy.h"

@interface TargetProxy()

//target, 这里必须要用weak, 因为某个对象会强引TimeProxy对象, TimeProxy对象不能再强引target, 否则会形成循环引用
@property (nonatomic, weak)id target;

@end

@implementation TargetProxy

+ (instancetype)proxyWithTarget:(id)target {
    TargetProxy *proxy = [[self class] alloc];
    proxy.target = target;
    return proxy;
}

//获取target类中的sel方法的方法签名
- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    return [self.target methodSignatureForSelector:sel];
}

- (void)forwardInvocation:(NSInvocation *)invocation {
    //判断target是否实现了该方法
    if ([self.target respondsToSelector:invocation.selector]) {
        //让target调用该方法
        [invocation invokeWithTarget:self.target];
    }else {
        //找不到该方法
        [invocation doesNotRecognizeSelector:invocation.selector];
    }
}

@end

在ViewController中的使用方法：

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self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:[TargetProxy proxyWithTarget:self] selector:@selector(repeatAction) userInfo:nil repeats:YES];

参考链接

NSTimer

NSProxy解决NSTimer和CADisplayLink的循环引用

《Effective Objective-C 2.0 编写高质量iOS与OS X代码的52个有效方法》