声明变量不能被块捕获

Question

有没有办法防止变量被捕获到块中？

我想要这样的东西：

[context performBlockAndWait:^{
    CoreDataObject *coreDataObject = [self getObject];
    NSString *name = coreDataObject.name;
    [self doThingWithCompletion:^{
        // I never want to use coreDataObject here, but I might want to use name.
    }];
}];

是的，确实不会捕获 coreDataObject。但是如果我不小心在块中使用它，它就会。有没有办法声明 coreDataObject 永远不应该被捕获？

到目前为止，我想到的最好的是：

__block NSString *name;
[context performBlockAndWait:^{
    CoreDataObject *coreDataObject = [self getObject];
    name = coreDataObject.name;
}];
[self doThingWithCompletion:^{
    // coreDataObject is no longer in scope here
}];

但是，如果 doThingWithCompletion 需要 Core Data 锁定，我只是将一个锁变成了两个。

Answer 1

你可以只使用作用域块（不是闭包类型的块，只是 C compound-statement 类型的块）来确保 coreDataObject 不能被使用：

[context performBlockAndWait:^{
    NSString *name;

    {
        CoreDataObject *coreDataObject = [self getObject];
        name = coreDataObject.name;
    }

    [self doThingWithCompletion:^{
        // I never want to use coreDataObject here, but I might want to use name.
    }];
}];

在像这样的简单情况下，您也可以完全消除 coreDataObject 变量：

    NSString *name = [self getObject].name;

声明变量不能被块捕获

Declare that a variable must not be captured by a Block

objective-c

objective-c-blocks