typedef uint64_t dispatch_time_t;
dispatch_time_t

1、

[NSDate dateWithTimeIntervalSinceNow:delay]


+ (dispatch_time_t)wallTimeWithDate:(NSDate *)date {
    NSCParameterAssert(date != nil);

    double seconds = 0;
//分解： 第二個參數：整數部分。返回值：小數部分。
12341234123412341234.818
//這兩個數的意義分別對應： 秒數， 微秒數
    double frac = modf(date.timeIntervalSince1970, &seconds);

    struct timespec walltime = {
        .tv_sec = (time_t)fmin(fmax(seconds, LONG_MIN), LONG_MAX),
        .tv_nsec = (long)fmin(fmax(frac * NSEC_PER_SEC, LONG_MIN), LONG_MAX)
    };

    return dispatch_walltime(&walltime, 0);
}

2、

dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.1 * NSEC_PER_SEC))