一、探究
NSDateFormatter * dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"yyyy-MM-dd"];
NSString * current = [dateFormatter stringFromDate:[NSDate date]];
关于 NSDateFormatter 创建耗时的资料很多，下面开始测试一下，究竟有多耗时。
double begin = 0.0;
double end = 0.0;
NSDateFormatter * formatter = nil;
{
begin = CACurrentMediaTime();
for (int i = 0; i < 1000; i++) {
formatter = [[NSDateFormatter alloc] init];
[formatter setDateFormat:@"yyyy-MM-dd"];
[formatter stringFromDate:[NSDate date]];
}
end = CACurrentMediaTime();
NSLog(@"NSDateFormatter: %8.2f ms", (end - begin) * 1000);
}
{
begin = CACurrentMediaTime();
formatter = [[NSDateFormatter alloc] init];
for (int i = 0; i < 1000; i++) {
[formatter setDateFormat:@"yyyy-MM-dd"];
[formatter stringFromDate:[NSDate date]];
}
end = CACurrentMediaTime();
NSLog(@"NSDateFormatter once: %8.2f ms", (end - begin) * 1000);
}
-----------Xcode 10.1 iPhone 6s(10.0)----------
2019-03-01 10:08:42.184 Demo[95118:1359994] NSDateFormatter: 48.73 ms
2019-03-01 10:08:42.188 Demo[95118:1359994] NSDateFormatter once: 3.57 ms
2019-03-01 10:11:18.871 Demo[95164:1361958] NSDateFormatter: 61.18 ms
2019-03-01 10:11:18.875 Demo[95164:1361958] NSDateFormatter once: 3.85 ms
2019-03-01 10:12:03.123 Demo[95178:1362677] NSDateFormatter: 79.80 ms
2019-03-01 10:12:03.129 Demo[95178:1362677] NSDateFormatter once: 6.08 ms
上面可以看出两者之间消耗时间差距很大。创建单例很有必要。
那是 [[NSDateFormatter alloc] init] 初始化消耗太高吗？
NSDateFormatter * formatter = nil;
double begin = 0.0;
double end = 0.0;
double a = 0, b = 0, c = 0;
for (int i = 0; i < 1000; i++) {
begin = CACurrentMediaTime();
formatter = [[NSDateFormatter alloc] init];
end = CACurrentMediaTime();
a += (end - begin);
begin = CACurrentMediaTime();
[formatter setDateFormat:@"yyyy-MM-dd"];
end = CACurrentMediaTime();
b += (end - begin);
begin = CACurrentMediaTime();
[formatter stringFromDate:[NSDate date]];
end = CACurrentMediaTime();
c += (end - begin);
}
NSLog(@"NSDateFormatter:alloc %8.2f ms", a * 1000);
NSLog(@"NSDateFormatter:setFormat %8.2f ms", b * 1000);
NSLog(@"NSDateFormatter:stringFromDate %8.2f ms", c * 1000);
-------------Xcode 10.1 iPhone 6s(10.0)-------------
2019-03-01 10:11:18.939 Demo[95164:1361958] NSDateFormatter:alloc 7.01 ms
2019-03-01 10:11:18.939 Demo[95164:1361958] NSDateFormatter:setFormat 0.28 ms
2019-03-01 10:11:18.939 Demo[95164:1361958] NSDateFormatter:stringFromDate 55.98 ms
2019-03-01 10:12:03.198 Demo[95178:1362677] NSDateFormatter:alloc 7.69 ms
2019-03-01 10:12:03.199 Demo[95178:1362677] NSDateFormatter:setFormat 0.25 ms
2019-03-01 10:12:03.199 Demo[95178:1362677] NSDateFormatter:stringFromDate 60.97 ms
2019-03-01 10:18:43.946 Demo[95261:1366071] NSDateFormatter:alloc 6.01 ms
2019-03-01 10:18:43.946 Demo[95261:1366071] NSDateFormatter:setFormat 0.20 ms
2019-03-01 10:18:43.946 Demo[95261:1366071] NSDateFormatter:stringFromDate 49.06 ms
从上面可以看出，实际最耗时的方法是 stringFromDate:/dateFromString:。再往下细究。
double begin = 0.0;
double end = 0.0;
NSDateFormatter * formatter = [[NSDateFormatter alloc] init];
for (int i = 0; i < 1000; i++) {
[formatter setDateFormat:@"yyyy-MM-dd"];
begin = CACurrentMediaTime();
[formatter stringFromDate:[NSDate date]];
end = CACurrentMediaTime();
NSLog(@"%8.2f ms", (end - begin) * 1000);
}
-------------Xcode 10.0 iPhone 6s(10.0)-------------
2019-03-01 10:27:06.218 Demo[95456:1372764] 1.43 ms
2019-03-01 10:27:06.218 Demo[95456:1372764] 0.03 ms
2019-03-01 10:27:06.219 Demo[95456:1372764] 0.02 ms
2019-03-01 10:27:06.219 Demo[95456:1372764] 0.01 ms
2019-03-01 10:27:06.219 Demo[95456:1372764] 0.01 ms
2019-03-01 10:27:06.219 Demo[95456:1372764] 0.01 ms
2019-03-01 10:27:06.219 Demo[95456:1372764] 0.01 ms
2019-03-01 10:27:06.219 Demo[95456:1372764] 0.02 ms
2019-03-01 10:27:06.220 Demo[95456:1372764] 0.01 ms
2019-03-01 10:27:06.220 Demo[95456:1372764] 0.01 ms
2019-03-01 10:27:06.220 Demo[95456:1372764] 0.01 ms
2019-03-01 10:27:06.220 Demo[95456:1372764] 0.01 ms
2019-03-01 10:27:06.220 Demo[95456:1372764] 0.01 ms
2019-03-01 10:27:06.221 Demo[95456:1372764] 0.01 ms
2019-03-01 10:27:06.221 Demo[95456:1372764] 0.01 ms
从上面可以看出，只有首次调用 stringFromDate:/dateFromString: 方法才会很耗时。再往下细究。
还有人说应该针对 format 格式创建对应的单例对象。
double begin = 0.0;
double end = 0.0;
// 不同的对象不同的 format 格式
{
begin = CACurrentMediaTime();
NSDateFormatter * formatter1 = [[NSDateFormatter alloc] init];
NSDateFormatter * formatter2 = [[NSDateFormatter alloc] init];
NSDateFormatter * formatter3 = [[NSDateFormatter alloc] init];
NSDateFormatter * formatter4 = [[NSDateFormatter alloc] init];
for (int i = 0; i < 1000; i++) {
[formatter1 setDateFormat:@"yyyy-MM-dd"];
[formatter1 stringFromDate:[NSDate date]];
[formatter2 setDateFormat:@"MM-dd-yyyy"];
[formatter2 stringFromDate:[NSDate date]];
[formatter3 setDateFormat:@"MM-dd"];
[formatter3 stringFromDate:[NSDate date]];
[formatter4 setDateFormat:@"MM-yyyy"];
[formatter4 stringFromDate:[NSDate date]];
}
end = CACurrentMediaTime();
printf("NSDateFormatter: different format %8.2f ms\n", (end - begin) * 1000);
}
// 同一个对象不同的 format 格式
{
begin = CACurrentMediaTime();
NSDateFormatter * formatter = [[NSDateFormatter alloc] init];
for (int i = 0; i < 1000; i++) {
[formatter setDateFormat:@"yyyy-MM-dd"];
[formatter stringFromDate:[NSDate date]];
[formatter setDateFormat:@"MM-dd-yyyy"];
[formatter stringFromDate:[NSDate date]];
[formatter setDateFormat:@"MM-dd"];
[formatter stringFromDate:[NSDate date]];
[formatter setDateFormat:@"MM-yyyy"];
[formatter stringFromDate:[NSDate date]];
}
end = CACurrentMediaTime();
printf("NSDateFormatter: %8.2f ms\n", (end - begin) * 1000);
}
---------------Xcode 10.1 iPhone 6s(10.0)---------------
NSDateFormatter: different format 23.26 ms
NSDateFormatter: 16.25 ms
如果不计 NSDateFormatter 对象的初始化时间，那么打印输出：
NSDateFormatter:different format 23.81 ms
NSDateFormatter: 23.02 ms
两者相差不大，创建一个单例即可。dateFormatter 初次使用时消耗较大，设置 format 格式却并没有什么影响。