在Swift程序中是否需要使用autoreleasepool?

问题描述

此WWDC14演示文稿的第17页，它说

那是什么意思?这是否意味着如果我的代码库中没有任何Objective-C文件，则autoreleasepool {}是不必要的吗?

What does that mean? Does it mean that if my code base doesn't have any Objective-C files, autoreleasepool {} is unnecessary?

在相关问题的答案中，有一个示例autoreleasepool可能有用:

In an answer of a related question, there is an example where autoreleasepool can be useful:

- (void)useALoadOfNumbers {
    for (int j = 0; j < 10000; ++j) {
        @autoreleasepool {
            for (int i = 0; i < 10000; ++i) {
                NSNumber *number = [NSNumber numberWithInt:(i+j)];
                NSLog(@"number = %p", number);
            }
        }
    }
}

如果上面的代码在删除了autoreleasepool的情况下转换为Swift，Swift是否足够聪明，知道number变量应该在第一个}之后释放(就像某些其他语言一样)?

If the code above gets translated into Swift with autoreleasepool dropped, will Swift be smart enough to know that the number variable should be released after the first } (like some other languages does)?

推荐答案

在返回autorelease对象(通过Objective-C代码或使用Cocoa类创建)时，在Swift中使用autoreleasepool模式. Swift函数中的autorelease模式与Objective-C中的模式非常相似.例如，考虑此方法的Swift演绎​​(实例化NSImage/UIImage对象):

The autoreleasepool pattern is used in Swift when returning autorelease objects (created by either your Objective-C code or using Cocoa classes). The autorelease pattern in Swift functions much like it does in Objective-C. For example, consider this Swift rendition of your method (instantiating NSImage/UIImage objects):

func useManyImages() {
    let filename = pathForResourceInBundle

    for _ in 0 ..< 5 {
        autoreleasepool {
            for _ in 0 ..< 1000 {
                let image = NSImage(contentsOfFile: filename)
            }
        }
    }
}

如果您在Instruments中运行此代码，则会看到如下所示的分配图:

If you run this in Instruments, you'll see an allocations graph like the following:

但是，如果在没有自动释放池的情况下执行此操作，则会看到峰值内存使用率更高:

But if you do it without the autorelease pool, you'll see that peak memory usage is higher:

autoreleasepool允许您明确管理在Swift中释放自动释放对象的时间，就像在Objective-C中一样.

The autoreleasepool allows you to explicitly manage when autorelease objects are deallocated in Swift, just like you were able to in Objective-C.

注意:处理Swift本机对象时，通常不会收到自动释放对象.这就是为什么演示文稿中提到了仅在使用Objective-C工作时"才需要的警告，尽管我希望Apple在这一点上更加清楚.但是，如果要处理Objective-C对象(包括Cocoa类)，则它们可能是自动释放对象，在这种情况下，Objective-C @autoreleasepool模式的这种Swift表示法仍然有用.

Note: When dealing with Swift native objects, you generally will not receive autorelease objects. This is why the presentation mentioned the caveat about only needing this when "working with Objective-C", though I wish Apple was more clear on this point. But if you're dealing with Objective-C objects (including Cocoa classes), they may be autorelease objects, in which case this Swift rendition of the Objective-C @autoreleasepool pattern is still useful.

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