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动机：我正在开发一个应用程序，它可以让多个（客户端）手机从（服务器）手机读取音频数据。这个想法是他们必须在完全相同的时间一起播放这首歌。

前提：我必须想办法让所有手机都以绝对的某个时间戳开始（即它与任何一部手机的使用设置时钟无关等）..根据一些研究，我想出了最好的方法这样做是为了使用CFAbsoluteTimeGetCurrent();这里的想法是我得到服务器与每部电话通信所需的延迟（b/c GKSession 显然是串行而不是并行完成的），将该延迟添加到当前时间，然后让每部电话从该开始时间开始播放歌曲。这个开始时间必须是绝对的，不能是相对的。

问题：我如何用数字表示未来的时间，以后可以用来构造CFDateRef。（它必须以数字表示的原因是我必须能够以数据包的形式发送它..）

示例：这是一段代码，解释了我想要实现的目标：

-(void)viewDidLoad
{
    _timer = [Timer new];
    [_timer setCurrentTimeAsReferencepoint];

    [self performSelector:@selector(performAction) withObject:NULL afterDelay:5];
}


-(void)performAction
{
    double diff = [_timer getTimeElapsedinAbsTime];
    double timeInFuture = diff + [Timer getCurTime];
    NSLog(@"this is time in future in abs terms %f",timeInFuture);

    CFDateRef futureDate = CFDateCreate(NULL, timeInFuture);
    CFDateRef dateNow = CFDateCreate(NULL, [Timer getCurTime]);

    Boolean keepTesting = true;

    while (keepTesting) {
        if (CFDateCompare(dateNow, futureDate,NULL) == 0)   // ie both times are equal
        {
            NSLog(@"now is the time!");
            keepTesting = false;
        } else {
            NSLog(@"now isn't the time.. skip");
        }
    }
}

在 Timer.m 中：

-(void)setCurrentTimeAsReferencepoint
{
    _referencePoint = [[self class] getCurTime];
    NSLog(@"this is reference point %f",_referencePoint);  
}

+(double)getCurTime
{
    return (double)CFAbsoluteTimeGetCurrent();
}

// not used in the above code.. but used when i compare the time of the server phone with the client phone
+(double)getTimeDifference:(double)time1
                     time2:(double)time2
{    
    CFDateRef newDate = CFDateCreate(NULL, time2);
    CFDateRef oldDate = CFDateCreate(NULL, time1);

    CFTimeInterval difference = CFDateGetTimeIntervalSinceDate(newDate, oldDate);    

    NSLog(@"this is time difference %f",fabs(difference));
    CFRelease(oldDate); CFRelease(newDate); 

    // fabs = absolute value
    return fabs(difference);    
}
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1 回答 1

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事实证明它比我想象的要简单（注意：getCurTime 在问题中定义）：

-(void)performAction
{
    double curTime = [Timer getCurTime];
    double timeInFuture = 2 + curTime;
    NSLog(@"this is time in future in abs terms %f and this is curTime %f",timeInFuture, curTime);

    CFDateRef futureDate = CFDateCreate(NULL, timeInFuture);
    CFDateRef dateNow = CFDateCreate(NULL, curTime);

    Boolean keepTesting = true;

    while (keepTesting) {
        dateNow = CFDateCreate(NULL, [Timer getCurTime]);
        if (CFDateCompare(dateNow, futureDate,NULL) >= 0)   // ie both times are equal or we've 
                                                            // gone past the future date time
        { 
            NSLog(@"now is the time!");
            keepTesting = false;
            return;
        } else {
            NSLog(@"now isn't the time.. skip");
        }
    }
}
于 2012-10-08T06:31:07.630 回答