enumerateObjectsUsingBlock: 的 BOOL *stop 参数是什么? [英] What is the BOOL *stop argument for enumerateObjectsUsingBlock: used for?

问题描述

我最近一直在使用 enumerateObjectsUsingBlock: 来满足我的快速枚举需求，但我很难理解 BOOL *stop 在枚举块.

I've been using enumerateObjectsUsingBlock: a lot lately for my fast-enumeration needs, and I'm having a hard time understanding the usage of BOOL *stop in the enumeration block.

NSArray 类引用状态

stop:对布尔值的引用.该块可以将值设置为 YES 以停止对数组的进一步处理.stop 参数是一个 out-only争论.您应该只在阻止.

stop: A reference to a Boolean value. The block can set the value to YES to stop further processing of the array. The stop argument is an out-only argument. You should only ever set this Boolean to YES within the Block.

所以当然我可以在我的块中添加以下内容来停止枚举:

So then of course I can add the following in my block to stop the enumeration:

if (idx == [myArray indexOfObject:[myArray lastObject]]) {
    *stop = YES;
}

据我所知，不将 *stop 明确设置为 YES 不会产生任何负面影响.枚举似乎在数组末尾自动停止.那么在块中使用 *stop 真的有必要吗?

From what I've been able to tell, not explicitly setting *stop to YES doesn't have any negative side effects. The enumeration seems to automatically stop itself at the end of the array. So is using *stop really necessary in a block?

推荐答案

Block 的 stop 参数允许您过早停止枚举.它相当于普通 for 循环中的 break.如果你想遍历数组中的每个对象，你可以忽略它.

The stop argument to the Block allows you to stop the enumeration prematurely. It's the equivalent of break from a normal for loop. You can ignore it if you want to go through every object in the array.

for( id obj in arr ){
    if( [obj isContagious] ){
        break;    // Stop enumerating
    }

    if( ![obj isKindOfClass:[Perefrigia class]] ){
        continue;    // Skip this object
    }

    [obj immanetizeTheEschaton];
}

<小时>

[arr enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
    if( [obj isContagious] ){
        *stop = YES;    // Stop enumerating
        return;
    }

    if( ![obj isKindOfClass:[Perefrigia class]] ){
        return;    // Skip this object
    }

    [obj immanentizeTheEschaton];
}];

这是一个输出参数，因为它是对来自调用范围的变量的引用.它需要在您的 Block 内设置，但在 enumerateObjectsUsingBlock: 内读取，与 NSError 通常从框架调用传递回您的代码的方式相同.

That is an out parameter because it is a reference to a variable from the calling scope. It needs to be set inside your Block, but read inside of enumerateObjectsUsingBlock:, the same way NSErrors are commonly passed back to your code from framework calls.

- (void)enumerateObjectsUsingBlock:(void (^)(id obj, NSUInteger idx, BOOL *stop))block {
    // N.B: This is probably not how this method is actually implemented!
    // It is just to demonstrate how the out parameter operates!

    NSUInteger idx = 0;
    for( id obj in self ){

        BOOL stop = NO;

        block(obj, idx++, &stop);

        if( stop ){
            break;
        }
    }
}

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