多线程和自动释放池 [英] Multithreading and autorelease pool

问题描述

当我掌握我的技能与多线程与GCD，我遇到一些问题。假设你有以下方法：

   - （void）方法{
 NSString * string = [NSString string]; //将被自动释放
 
 dispatch_async（dispatch_get_global_queue（DISPATCH_QUEUE_PRIORITY_DEFAULT，0），^ {
 
 //非常非常冗长的操作... 
 
 NSLog （@％@，string）; //是安全吗？
}）; 
} 
  

我想知道这是否正确，因为我认为我应该保留在块执行之前的字符串：事实上我担心事件循环完成并发送 string 一个autorelease消息，然后使用 string 在块中。这将导致程序崩溃。

我是对吗？是否应该向 string 发送一个retain和一个释放消息，或者这是正确的实现？
提前感谢！

解决方案

我想知道这是否正确，因为我想我应该在块执行之前保留字符串：事实上，我担心事件循环完成并发送字符串一个autorelease消息在使用字符串在块中。

不要害怕：

块捕获了周围方法/函数的范围，因为它会自动保留变量，在块内使用。注意，当你在块中使用 self 时，这可能会极大地影响对象的生命周期！

此规则有一个例外，它们是声明为

的变量

  __ block SomeObjectPointerType variableName 
  

更新

因为这个答案有新的评论在ARC下所有对象变量默认为 __ strong ，并且在ARC下引入了一些变化：

< 这也适用于标有 __ block 的变量。如果你想避免强制捕获块中的变量，你应该定义一个局部变量 __ weak 。

< h2> End Update

如果你想了解更多关于块的信息，bbum给了一个很好的会话，叫做 介绍块和大中央调度在iPhone上 （iTunes U链接）在WWDC 2010。

块详细信息部分始于11:30。

As I'm mastering my skills with multithreading with GCD, I've come across some question. Suppose you have the following method:

    - (void)method {
    NSString *string= [NSString string]; //will be autoreleased

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

    //very very lengthy operation...

    NSLog(@"%@", string); //is it safe?
    });
}

I'm wondering if this is correct, because I think I should have retained string before the block execution: in fact I fear that the event loop finishes and sends string an autorelease message before using string in the block. That would crash the program.

Am I right? Should I send a retain and a release message to string or this is the correct implementation? Thanks in advance!

解决方案

I'm wondering if this is correct, because I think I should have retained string before the block execution: in fact I fear that the event loop finishes and sends string an autorelease message before using string in the block.

Fear not:
A block captures the scope of the surrounding method/function in that it automatically retains any object-variable that is used inside of the block. Be aware of that when you use self inside of a block, as this may greatly affect the lifetime of the object!

There is one exception to this rule, and that are variables declared as

__block SomeObjectPointerType variableName

Update

Because there’s a new comment on this answer, I should probably add that things changed a little with the introduction of ARC:

Under ARC all object variables default to __strong, and this holds for variables marked with __block as well. If you want to avoid strong capturing of a variable in a block, you should define a local variable that is __weak.

End Update

If you like to learn more about blocks, bbum gave an excellent session called Introducing Blocks and Grand Central Dispatch on iPhone (iTunes U link) at WWDC 2010.

The "Block Details" section starts at 11:30.

这篇关于多线程和自动释放池的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！