背景

公司的一个UIWebView要加个进度条，但是UIWebView没有进度啊。所以做个假的，用定时器。但是发现来回的进UIWebView的控制器，会崩溃。发现是在NSTimer那里崩溃的。那是因为NSTimer导致内存泄漏了。先说说为什么会发生内存泄漏吧。

写个小栗子🌰。

//
//  ViewController.m
//  timer
//
//  Created by wyb on 2017/4/12.
//  Copyright © 2017年 中天易观. All rights reserved.
//

#import "ViewController.h"
#import "TimeViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
}


- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    TimeViewController *vc = [[TimeViewController alloc]init];
    [self presentViewController:vc animated:YES completion:nil];
}

@end

#import "TimeViewController.h"

@interface TimeViewController ()

@property(nonatomic,weak)NSTimer *timer;

@end

@implementation TimeViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.view.backgroundColor = [UIColor greenColor];
    
    self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timeAction:) userInfo:nil repeats:YES];
    
    
}

- (void)timeAction:(NSTimer *)timer {
    
    NSLog(@"%s",__func__);
    
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    [self dismissViewControllerAnimated:YES completion:nil];
}

发现个问题，当我跳到TimeViewController控制器的时候，定时器干活了。当我点击屏幕开始dismissViewController它的时候，发现定时器还在干活。那就是timer没停止。

delloc方法里销毁它。

- (void)dealloc
{
    [self.timer invalidate];
    self.timer = nil;
}

发现不起作用。我就是遇到这个问题导致的内存泄漏。本以为把它销毁了。然而并没有卵用。dealloc根本没走。

根源

@property(nonatomic,weak)NSTimer *timer;

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

timer创建成功会被添加到Runloop里，它就被Runloop强引用了。它的target是self。所以它强引用了self。属性那里是weak没事。
当我们想销毁TimeViewController的时候，因为它被timer强引用所以它销毁不了。多会timer死了，它才能死。但是它要是死不了delloc方法不会走。delloc方法走不了timer就死不了。只有执行了[self.timer invalidate]，timer才会死。所以这就是循环引用了。

解决1

  __weak typeof(self) weakSelf = self;
    self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:weakSelf selector:@selector(timeAction:) userInfo:nil repeats:YES];

发现不起作用。还是引用self。这个里面表面看着可以，但是内部肯定还强引用。我这里也不太明白，知道的告诉下。

解决2

既然self被引用。我可以换个别的对象啊。让timer调用别的对象的timeAction，那么跟self就没半毛钱关系了。循环引用就迎刃而解了。写了个YBTimer。上代码吧

//
//  YBTimer.h
//  xxx
//
//  Created by wyb on 2017/4/11.
//  Copyright © 2017年 xxx. All rights reserved.
//

#import <Foundation/Foundation.h>

typedef void (^YBTimerBlock)(id userInfo);

@interface YBTimer : NSObject

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

+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)interval
                                      block:(YBTimerBlock)block
                                   userInfo:(id)userInfo
                                    repeats:(BOOL)repeats;

@end

//
//  YBTimer.m
//  xxx
//
//  Created by wyb on 2017/4/11.
//  Copyright © 2017年 xxx. All rights reserved.
//

#import "YBTimer.h"

//------------------------------YBTimerTargetBegin-------------------------------------


@interface YBTimerTarget : NSObject

@property (nonatomic, weak) id target;
@property (nonatomic, assign) SEL selector;
@property (nonatomic, weak) NSTimer* timer;

@end

@implementation YBTimerTarget

- (void)timeAction:(NSTimer *)timer {
    if(self.target) {

        [self.target performSelector:self.selector withObject:timer.userInfo afterDelay:0.0f];

    } else {
        [self.timer invalidate];
    }
}

@end


//------------------------------YBTimerTargetEnd-------------------------------------

@implementation YBTimer

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

+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)interval
                                      block:(YBTimerBlock)block
                                   userInfo:(id)userInfo
                                    repeats:(BOOL)repeats {
    NSMutableArray *userInfoArray = [NSMutableArray arrayWithObject:[block copy]];
    if (userInfo != nil) {
        [userInfoArray addObject:userInfo];
    }
    return [self scheduledTimerWithTimeInterval:interval
                                         target:self
                                       selector:@selector(timerBlock:)
                                       userInfo:[userInfoArray copy]
                                        repeats:repeats];
}

+ (void)timerBlock:(NSArray*)userInfo {
    YBTimerBlock block = userInfo[0];
    id info = nil;
    if (userInfo.count == 2) {
        info = userInfo[1];
    }
    
    if (block) {
        block(info);
    }
}

@end

NSTimer 进阶 http://www.jianshu.com/p/19aab8570ce3