2 NSDates应该相等不是？ [英] 2 NSDates that should be equal aren't?

问题描述

我使用的是Stig Brautaset的JSON库（http://code.google.com/p/json-framework），我需要序列化NSDate。我在考虑把它转换成一个字符串之前JSON化它，但是，我遇到了这种奇怪的行为：

I'm using the JSON library from Stig Brautaset(http://code.google.com/p/json-framework) and I need to serialize an NSDate. I was considering converting it into a string before JSONifying it, however, I ran into this weird behavior:

为什么这些NSDates被认为是相等的？

Why aren't these NSDates considered equal?

NSDate *d = [[NSDate alloc] init];
NSDate *dd = [NSDate dateWithString:[d description]];

NSLog(@"%@", d);
NSLog(@"%@", dd);
if( [d isEqualToDate:dd] ){
    NSLog(@"Yay!");
}

推荐答案

对象，您从原始对象中丢失了一些亚秒级的精度 - 换句话说， -description 会消失小数秒，并返回

When you describe the original date object you lose some sub-second precision from the original object — in other words, -description shaves off fractional seconds, and returns

国际格式的接收器的字符串表示 YYYY-MM-DD HH：MM：SS±HHMM ，其中±HHMM 表示以GMT和分钟为单位的时区偏移

A string representation of the receiver in the international format YYYY-MM-DD HH:MM:SS ±HHMM, where ±HHMM represents the time zone offset in hours and minutes from GMT

当你根据描述创建一个新的日期对象时，你会得到它在整秒钟，因为字符串只精确到一整秒。因此 -isEqualToDate： 返回 NO ，因为

When you create a new date object based on the description, you get it in whole seconds because the string is only precise to a whole second. So -isEqualToDate: returns NO because there is a difference of a fraction of a second between your two date objects, which it's sensitive to.

此方法检测日期之间的次秒差异。如果您想比较不太细粒度的日期，请使用 timeIntervalSinceDate：来比较这两个日期。

所以你可以这样做（ NSTimeInterval 措施在几秒钟内）：

So you'd do something like this instead (NSTimeInterval measures in seconds):

if ([d timeIntervalSinceDate:dd] == 0) {
    NSLog(@"Yay!");
}

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