在块完成后如何访问 __block 变量? [英] How can I access a __block variable, after the block has completed?

问题描述

我正在使用 Parse.com 进行一些后台操作，但这是关于 __block 变量的一般问题.我想定义一个变量，使用完成块运行后台网络操作，可能在块内修改该变量，然后在块外访问它.但它始终为零.

I'm doing some background operations with Parse.com, but this is a general question about __block variables. I want to define a variable, run a background network operation with a completion block, possibly modify that variable within the block, then access it outside of the block. But it's always nil.

如何将变量保留在块之外?这是在类方法中，因此不能选择使用实例变量.

How can I retain the variable outside of the block? This is inside a class method, so using an instance variable isn't an option.

__block PFObject *myObject = nil;

PFQuery *query = [PFQuery queryWithClassName:@"ClassName"];
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
   if (objects.count) {
       myObject = [objects lastObject];
   }
}];

NSLog(@"%@",myObject);

推荐答案

您可以在块外使用它们，就像任何其他变量一样.

You can use them outside block just like any other variable.

在您当前的代码中，此日志将打印 nil，因为块内的代码是异步执行的，在这种情况下 - 当搜索结果返回时.

In your current code this log will print nil, because code inside the block gets executed asynchronously, in this case - when the search results return.

如果你想从 myObject 获得有意义的值，你应该在 myObject 赋值之后把你的日志放在块中.

If you want meaningful value from myObject, you should really put your log inside the block, after myObject assignment.

在评论中查看执行顺序:

See the order of execution in comments:

 __block PFObject *myObject = nil;             //1

PFQuery *query = [PFQuery queryWithClassName:@"ClassName"];   //2
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {  //3
   if (objects.count)     //5
       myObject = [objects lastObject];   //6
}];                                       //7

NSLog(@"%@",myObject);   //4

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