面试题:NSTimer循环引用的解决方案

前言

在使用NSTimer,如果使用不得当特别会引起循环引用,造成内存泄露。所以怎么避免循环引用问题,下面我提出几种解决NSTimer的几种循环引用。

原因

当你在ViewController(简称VC)中使用timer属性,由于VC强引用timer,timer的target又是VC造成循环引用。当你在VC的dealloc方法中销毁timer,
发现VC被pop,VC的dealloc方法没走,VC在等timer释放才走dealloc,timer释放在dealloc中,所以引起循环引用。

解决方案

方案一

//1、类别中使用copy block的形式使timer只引用自己而不引用TimerViewController(可以释放)
//如果在block里面直接调用self,还是会保留环的。因为block对self强引用,self对timer强引用,timer又通过userInfo参数保留block(强引用block),这样就构成一个环block->self->timer->userinfo->block,所以要打破这个环的话要在block里面弱引用self。

  • (NSTimer )timerWithLFJBlockSupport{
    __weak __typeof(&
    self)weakSelf = self;
    NSTimer *timer = [NSTimer lfj_scheduledTimerWithTimeInterval:1 repeats:YES block:^{
    [weakSelf countAddWith:28];
    }];
    return timer;
    }

import "NSTimer+LFJBlockSupport.h"

@implementation NSTimer (LFJBlockSupport)
/*
该方案主要要点:
将计时器所应执行的任务封装成"Block",在调用计时器函数时,把block作为userInfo参数传进去。
userInfo参数用来存放"不透明值",只要计时器有效,就会一直保留它。
在传入参数时要通过copy方法,将block拷贝到"堆区",否则等到稍后要执行它的时候,该blcok可能已经无效了。
计时器现在的target是NSTimer类对象,这是个单例,因此计时器是否会保留它,其实都无所谓。此处依然有保留环,然而因为类对象(class object)无需回收,所以不用担心。
*/

  • (NSTimer*)lfj_scheduledTimerWithTimeInterval:(NSTimeInterval)timeInterval repeats:(BOOL)repeats block:(void(^)(void))block{

    return [self scheduledTimerWithTimeInterval:timeInterval target:self selector:@selector(lfj_blockSelector:) userInfo:[block copy] repeats:repeats];
    }
    +(void)lfj_blockSelector:(NSTimer *)timer{
    void(^block)(void) = timer.userInfo;
    if (block) {
    block();
    }
    }

@end
方案二

//2、使用系统block(不过只支持iOS 10以后)(可以释放)

  • (NSTimer )timerWithsystemBlock{
    __weak __typeof(&(
    self))weakSelf = self;
    NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:1 repeats:YES block:^(NSTimer * _Nonnull timer) {
    [weakSelf countAddWith:28];
    }];
    return timer;
    }
    方案三

//3、使用中间类弱引用(weak)target

  • (NSTimer *)timerWithWeakTimer{
    NSTimer *timer = [LFJWeakTimer weak_lfjScheduledTimerWithTimeInterval:1 target:self selector:@selector(countAddWith:) userInfo:nil repeats:YES];
    return timer;
    }
    //3、使用中间类弱引用(weak)target
  • (NSTimer *)timerWithProxyTimerTimer{
    NSTimer *timer = [LFJProxyTimer weak_lfjScheduledTimerWithTimeInterval:1 target:self selector:@selector(countAddWith:) userInfo:nil repeats:YES];
    return timer;
    }

import "LFJWeakTimer.h"

@interface LFJWeakTimer()
@property (nonatomic,weak)id target;

@end
@implementation LFJWeakTimer

  • (void)forwardInvocation:(NSInvocation *)anInvocation OBJC_SWIFT_UNAVAILABLE(""){
    [anInvocation invokeWithTarget:self.target];
    }

  • (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector OBJC_SWIFT_UNAVAILABLE(""){
    //消息转发(返回值methodSignature不能为空(调用这个方法的类没有实现这个方法就会为空))
    return [self.target methodSignatureForSelector:aSelector];
    }

  • (void)dealloc{
    NSLog(@"weakTimer------释放");
    }

  • (NSTimer *)weak_lfjScheduledTimerWithTimeInterval:(NSTimeInterval)ti target:(id)aTarget selector:(SEL)aSelector userInfo:(nullable id)userInfo repeats:(BOOL)yesOrNo{
    LFJWeakTimer weakTimer = [LFJWeakTimer new];
    weakTimer.target = aTarget;
    return [NSTimer scheduledTimerWithTimeInterval:ti target:weakTimer selector:aSelector userInfo:userInfo repeats:yesOrNo];
    //NSTimer 持有---->weakTimer 弱引用--->aTarget(当NSTimer 调用invalidate的时候释放weakTimer)
    //NSTimer 的target 都是强引用(即使使用__weak __typeof(&
    self)weakself = self 对weakself也是强引用,weakself也会和NSTimer引起循环引用)
    }

@end

@interface LFJProxyTimer : NSProxy

  • (instancetype)proxyWithObjc:(id)object;
  • (NSTimer *)weak_lfjScheduledTimerWithTimeInterval:(NSTimeInterval)ti target:(id)aTarget selector:(SEL)aSelector userInfo:(nullable id)userInfo repeats:(BOOL)yesOrNo;
    @end

import "LFJProxyTimer.h"

@interface LFJProxyTimer()
@property (nonatomic,weak)id target;

@end
@implementation LFJProxyTimer

  • (instancetype)initWithObjc:(id)object {

    self.target = object;
    return self;
    }
    //通过类方法创建创建

  • (instancetype)proxyWithObjc:(id)object{
    return [[self alloc] initWithObjc:object];

}

  • (void)forwardInvocation:(NSInvocation *)anInvocation{
    [anInvocation invokeWithTarget:self.target];
    }

  • (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{
    //消息转发(返回值methodSignature不能为空(调用这个方法的类没有实现这个方法就会为空))
    return [self.target methodSignatureForSelector:aSelector];
    }

  • (void)dealloc{
    NSLog(@"weakTimer------释放");
    }

  • (NSTimer *)weak_lfjScheduledTimerWithTimeInterval:(NSTimeInterval)ti target:(id)aTarget selector:(SEL)aSelector userInfo:(nullable id)userInfo repeats:(BOOL)yesOrNo{
    LFJProxyTimer proxyTimer = [LFJProxyTimer proxyWithObjc:aTarget];
    proxyTimer.target = aTarget;
    return [NSTimer scheduledTimerWithTimeInterval:ti target:proxyTimer selector:aSelector userInfo:userInfo repeats:yesOrNo];
    //NSTimer 持有---->weakTimer 弱引用--->aTarget(当NSTimer 调用invalidate的时候释放weakTimer)
    //NSTimer 的target 都是强引用(即使使用__weak __typeof(&
    self)weakself = self 对weakself也是强引用,weakself也会和NSTimer引起循环引用)
    }
    @end
    方案四

//4、在当前类释放前释放NSTimer(在ViewController执行dealloc前释放timer(不推荐))

  • (NSTimer *)timerWithNormal{
    objc_setAssociatedObject(self, &nameKey, @"1", OBJC_ASSOCIATION_ASSIGN);
    NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(countAddWith:) userInfo:@34 repeats:YES];
    return timer;
    }

  • (void)viewDidDisappear:(BOOL)animated{
    [super viewDidDisappear:animated];
    NSString *dd = objc_getAssociatedObject(self, &nameKey);
    if (dd) {
    [self releaseTimer];
    }
    }

  • (void)dealloc{
    NSLog(@"释放");
    [self releaseTimer];
    }

  • (void)restartTimer {
    [self startTimer];
    }

  • (void)releaseTimer{
    if (self.timer) {
    [self.timer invalidate];
    self.timer = nil;
    }
    }

作者:学不来的凡人
链接:https://www.jianshu.com/p/fb52c60416ac
来源:
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