为声明的属性合成了哪些等效代码? [英] What equivalent code is synthesized for a declared property?

问题描述

getter 和 setter 方法主体在自动合成后究竟是什么样子的?

How exactly getter and setter methods body looks like after they have been automatically synthesized ?

从官方文档中我发现到目前为止只有推荐的实现技术，但是没有关于编译器在合成过程中使用哪些技术的消息:http://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmAccessorMethods.html#//apple_ref/doc/uid/TP40003539-SW5

From official documentation I found so far only recommended implementation techniques, however no word about which of them used by compiler during synthesizing process: http://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmAccessorMethods.html#//apple_ref/doc/uid/TP40003539-SW5

某些技术推荐包含 autorelease 消息的实现，这对于多线程编程来说不是很安全.我只是想知道自动生成的代码是否遵循一些建议的实现.

Some techniques recommends implementations containing autorelease message, which is not quite secure for multithreaded programming. I'm just wonder if auto-generated code follows to some of the proposed implementations.

例如:

.h

@interface AClass: NSObject{}
    @property (nonatomic, retain) AnotherClass *aProp;
@end

.m

@implementation AClass
    @synthesize aProp

-(id) init {
    if ((self = [super init])) {
        self.aProp = [[AnotherClass alloc] init];    // setter invocation
    }
    return self;
}

-(AnotherClass *) aMethod {
    return self.aProp;     // getter invocation
}
@end

编译器生成的 aProp 的等效访问器代码片段是什么?

What are equivalent accessors code snippets for aProp generated by compiler ?

-(AnotherClass *) aProp {
    // getter body
}

-(void) setAProp: (AnotherClass *) {
    // setter body
}

推荐答案

将属性声明为 nonatomic 时，您将得到以下内容:

When declaring a property as nonatomic, you'll get the following:

// .h
@property (nonatomic, retain) id ivar;

// .m
- (id)ivar {
    return ivar;
}

- (void)setIvar:(id)newValue {
    if (ivar != newValue) {  // this check is mandatory
        [ivar release];
        ivar = [newValue retain];
    }
}

注意检查 ivar != newValue.如果不存在，ivar 可以在 release 之后被释放，并且下面的 retain 会导致内存访问错误.

Note the check ivar != newValue. If it was absent, ivar could be dealloc'ed after release, and the following retain would cause a memory access error.

当您使用 copy 声明您的属性时，代码看起来几乎相同，只是将 retain 替换为 copy.

When you declare your property with copy, the code will look almost the same, with retain replaced by copy.

对于assign，就更简单了:

- (void)setIvar:(id)newValue {
    ivar = newValue;
}

<小时>

现在，当您将属性声明为 atomic(这是默认值)时，事情会变得稍微复杂一些.Apple 的一位工程师在开发论坛上发布了一段与下面类似的片段:

---

Now, when you declare your property as atomic (this one is the default), things get slightly more complicated. A snippet similar to the one below was posted by one of Apple's engineers on the development forums:

- (id)ivar {
    @synchronized (self) {
        return [[self->ivar retain] autorelease];
    }
}

- (void)setIvar:(id)newValue {
    @synchronized (self) {
        if (newValue != self->ivar) {
            [self->ivar release];
            self->ivar = newValue;
            [self->ivar retain];
        }
    }
}

注意两个方法中的 @synchronized 块和 getter 中的附加 retain-autorelease.这两件事都确保您将获得先前的值(保留和自动释放)或新的值，以防在您尝试读取它时某个线程更改了该值.

Note the @synchronized block in both methods and additional retain-autorelease in the getter. Both those things ensure that you will either get the previous value (retained and autoreleased) or a new one in the case the value is changed by some thread while you are trying to read it.

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