iOS中定时器NSTimer的一些用法总结

NSTimer在IOS开发中会经常用到，尤其是小型游戏，然而对于初学者时常会注意不到其中的内存释放问题，将其基本用法总结如下：

一、初始化方法：有五种初始化方法，分别是

+ (NSTimer *)timerWithTimeInterval:(NSTimeInterval)ti invocation:(NSInvocation *)invocation repeats:(BOOL)yesOrNo;

- (void)viewDidLoad {
    [super viewDidLoad];
    //初始化一个Invocation对象
    NSInvocation * invo = [NSInvocation invocationWithMethodSignature:[[self class] instanceMethodSignatureForSelector:@selector(init)]];
    [invo setTarget:self];
    [invo setSelector:@selector(myLog)];
    NSTimer * timer = [NSTimer timerWithTimeInterval:1 invocation:invo repeats:YES];
    //加入主循环池中
    [[NSRunLoop mainRunLoop]addTimer:timer forMode:NSDefaultRunLoopMode];
    //开始循环
    [timer fire];
}

+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)ti invocation:(NSInvocation *)invocation repeats:(BOOL)yesOrNo;

  NSTimer * timer = [NSTimer scheduledTimerWithTimeInterval:1 invocation:invo repeats:YES];

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

NSTimer * timer = [NSTimer timerWithTimeInterval:1 target:self selector:@selector(myLog) userInfo:nil repeats:NO]

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

NSTimer * timer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(myLog:) userInfo:@"123" repeats:YES]

- (instancetype)initWithFireDate:(NSDate *)date interval:(NSTimeInterval)ti target:(id)t selector:(SEL)s userInfo:(id)ui repeats:(BOOL)rep 

 NSTimer * timer = [[NSTimer alloc]initWithFireDate:[NSDate distantPast] interval:1 target:self selector:@selector(myLog:) userInfo:nil repeats:YES];
    [[NSRunLoop mainRunLoop]addTimer:timer forMode:NSDefaultRunLoopMode];

注意：这五种初始化方法的异同：

        1、参数repeats是指定是否循环执行，YES将循环，NO将只执行一次。

        2、timerWithTimeInterval这两个类方法创建出来的对象如果不用 addTimer: forMode方法手动加入主循环池中，将不会循环执行。并且如果不手动调用fair，则定时器不会启动。

        3、scheduledTimerWithTimeInterval这两个方法不需要手动调用fair，会自动执行，并且自动加入主循环池。

        4、init方法需要手动加入循环池，它会在设定的启动时间启动。

二、成员变量

@property (copy) NSDate *fireDate;

这是设置定时器的启动时间，常用来管理定时器的启动与停止

    //启动定时器
    timer.fireDate = [NSDate distantPast];
    //停止定时器
    timer.fireDate = [NSDate distantFuture];

@property (readonly) NSTimeInterval timeInterval;

这个是一个只读属性，获取定时器调用间隔时间。

@property NSTimeInterval tolerance;

这是7.0之后新增的一个属性，因为NSTimer并不完全精准，通过这个值设置误差范围。

@property (readonly, getter=isValid) BOOL valid;

获取定时器是否有效

@property (readonly, retain) id userInfo;

获取参数信息

三、关于内存释放

如果我们启动了一个定时器，在某个界面释放前，将这个定时器停止，甚至置为nil，都不能是这个界面释放，原因是系统的循环池中还保有这个对象。所以我们需要这样做：

-(void)dealloc{
    NSLog(@"dealloc:%@",[self class]);
}
- (void)viewDidLoad {
    [super viewDidLoad];
    timer= [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(myLog:) userInfo:nil repeats:YES];
    UIButton *btn = [[UIButton alloc]initWithFrame:CGRectMake(0, 0, 100, 100)];
    btn.backgroundColor=[UIColor redColor];
    [btn addTarget:self action:@selector(btn) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:btn];
}
-(void)btn{
    if (timer.isValid) {
        [timer invalidate];
    }
    timer=nil;
    [self dismissViewControllerAnimated:YES completion:nil];
}

在官方文档中我们可以看到 [timer invalidate]是唯一的方法将定时器从循环池中移除。

OS程序进入后台后十分钟之内就会被系统kill掉，怎么解决呢？我想要程序进入后台后仍然运行计时功能，否则就无法达到考试的目的，之后在网上查阅了相关资料最后终于找到答案，其精髓就是：利用苹果给出的三种类型的程序可以保持在后台运行：音频播放类，位置更新类，另外一个记不太清楚了，我利用了苹果给出的音频播放类的这个“特权”来满足我程序上的要求，详细步骤如下：

1、步骤一：在Info.plist中，添加"Required background modes"键，value为：App plays audio

步骤二：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease];
// Override point for customization after application launch.
    NSError *setCategoryErr = nil;
    NSError *activationErr  = nil;
    [[AVAudioSession sharedInstance]
     setCategory: AVAudioSessionCategoryPlayback
     error: &setCategoryErr];
    [[AVAudioSession sharedInstance]
     setActive: YES
     error: &activationErr];
    self.window.backgroundColor = [UIColor whiteColor];
    [self.window makeKeyAndVisible];
return YES;
}

步骤三：将以下代码添加到appDelegate文件中的- (void)applicationDidEnterBackground:(UIApplication *)application函数，也可添加到在具体类中注册的应用进入后台后的通知方法

- (void)applicationDidEnterBackground:(UIApplication *)application{

    UIApplication*   app = [UIApplication sharedApplication];
    __block    UIBackgroundTaskIdentifier bgTask;
    bgTask = [app beginBackgroundTaskWithExpirationHandler:^{
        dispatch_async(dispatch_get_main_queue(), ^{
if (bgTask != UIBackgroundTaskInvalid)
            {
                bgTask = UIBackgroundTaskInvalid;
            }
        });
    }];
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        dispatch_async(dispatch_get_main_queue(), ^{
if (bgTask != UIBackgroundTaskInvalid)
            {
                bgTask = UIBackgroundTaskInvalid;
            }
        });
    });
}
问题解决之后遇到的一个新问题，我的页面上有一个UIScrollView和一个定时器用来记录当前考试模式下的剩余时间，问题出现了：当我滑动滚动试图时，定时器的方法便不在运行（即被UI主线程阻塞）。google一下找到了解决办法：将定时器放在非主线程中执行将更新UI的操作放到主线程，这样UI主线程和定时器就能互不干扰的相互工作了，以下是主要代码：
#import "CountdownTool.h"

@interface CountdownTool()
{
    UILabel *_lblShow;
    NSTimer *_timer;
}
@property (nonatomic, assign) NSInteger hour;
@property (nonatomic, assign) NSInteger minute;
@property (nonatomic, assign) NSInteger second;
@property (nonatomic, copy) NSString  *strHour;
@property (nonatomic, copy) NSString  *strMinute;
@property (nonatomic, copy) NSString  *strSecond;
@property (nonatomic, assign) NSInteger totalSeconds;
@end
@implementation CountdownTool
@synthesize hour = _hour;
@synthesize minute = _minute;
@synthesize second = _second;
@synthesize totalSeconds = _totalSeconds;

- (void)dealloc
{
    [_lblShow release];
    [_strHour release];
    [_strMinute release];
    [_strSecond release];
    [super dealloc];
}

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        _lblShow = [[UILabel alloc] initWithFrame:self.bounds];
        _lblShow.backgroundColor = [UIColor clearColor];
        _lblShow.font = [UIFont systemFontOfSize:15];
        _lblShow.textColor = [UIColor yellowColor];
        _lblShow.textAlignment = NSTextAlignmentCenter;
        _lblShow.numberOfLines = 1;
        [self addSubview:_lblShow];
    }
    return self;
}

- (id)initWithFrame:(CGRect)frame andMinutesNum:(NSInteger)minute
{
    if (self = [self initWithFrame:frame]) {
        self.totalSeconds = minute * 60;
        //多线程启动定时器
       [NSThread detachNewThreadSelector:@selector(startTimer) toTarget:self withObject:nil];
    }
    return self;
}
- (void)startTimer
{
    _timer = [NSTimer scheduledTimerWithTimeInterval:1.0f target:self selector:@selector(timerFire) userInfo:nil repeats:YES];
    [[NSRunLoop currentRunLoop] run];
}
- (void)handleWithTotalSeconds
{
    self.hour = _totalSeconds/3600;
    self.minute = _totalSeconds%3600/60;
    self.second = _totalSeconds%3600%60;
    if (_hour <= 0) {
        _lblShow.text = [NSString stringWithFormat:@"%@:%@",_strMinute,_strSecond];
    }else{
        _lblShow.text = [NSString stringWithFormat:@"%@:%@:%@",_strHour,_strMinute,_strSecond];
    }
}
- (void)setHour:(NSInteger)hour
{
    _hour = hour;
    if (_hour < 10) {
        self.strHour = [NSString stringWithFormat:@"0%d",_hour];
    }else{
        self.strHour = [NSString stringWithFormat:@"%d",_hour];
    }
}
- (void)setMinute:(NSInteger)minute
{
    _minute = minute;
    if (_minute < 10) {
        self.strMinute = [NSString stringWithFormat:@"0%d",_minute];
    }else{
        self.strMinute = [NSString stringWithFormat:@"%d",_minute];
    }
}
- (void)setSecond:(NSInteger)second
{
    _second = second;
    if (_second < 10) {
        self.strSecond = [NSString stringWithFormat:@"0%d",_second];
    }else{
        self.strSecond = [NSString stringWithFormat:@"%d",_second];
    }
}
- (void)setTotalSeconds:(NSInteger)totalSeconds
{
    _totalSeconds = totalSeconds;
    [self performSelectorOnMainThread:@selector(handleWithTotalSeconds) withObject:nil waitUntilDone:YES];
}
- (void)timerFire
{
    if (_totalSeconds == 0) {
        [_timer invalidate];
        return;
    }
    self.totalSeconds -= 1;
}
@end