那些位掩码如何工作？ [英] How do those bitmasks actually work?

问题描述

例如，NSCalendar的这个方法需要一个位掩码：

For example, this method from NSCalendar takes a bitmask:

- (NSDate *)dateByAddingComponents:(NSDateComponents *)comps toDate:(NSDate *)date options:(NSUInteger)opts

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So options can be like:

NSUInteger options = kCFCalendarUnitYear;

或类似：

NSUInteger options = kCFCalendarUnitYear | kCFCalendarUnitMonth | kCFCalendarUnitDay;

我没有得到的是，这是怎么做的？我的意思是：他们如何拉出合并到选项的那些值？

What I don't get is, how is this actually done? I mean: How can they pull out those values which are merged into options? If I wanted to program something like this, that can take a bitmask, how would that look?

推荐答案

要做到这一点，你需要想要将AND测试的值与掩码进行比较，然后查看ANDing的结果是否等于掩码本身：

To do this, you want to bitwise AND the value you're testing against the mask, then see if the result of the ANDing equals the mask itself:

if ((options & kCFCalendarUnitYear) == kCFCalendarUnitYear) {
   // do whatever
}

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