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 变量的一般性问题。我想定义一个变量，使用完成块运行后台网络操作，可能修改块内的变量，然后在块外部访问它。但它总是零。
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.
如何在块外保留变量？这是一个类方法，因此使用实例变量不是一个选项。
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
你可以像在任何其他变量一样在块外使用它们。
You can use them outside block just like any other variable.
在当前代码中，此日志将打印为nil，因为块中的代码将异步执行，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。
If you want meaningful value from myObject, you should really put your log inside the block, after myObject assignment.
查看评论中的执行顺序：
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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