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

问题描述

在官方文档中，我发现迄今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了哪些内容： http://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmAccessorMethods.html#//apple_ref/doc/uid/TP40003539-SW5

一些技术推荐包含 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适用于某些提议的实现。

---

例如：

.h

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

.m

  @implementation AClass 
 @synthesize aProp 
 
  - （id）init {
 if（ = [super init]））{
 self.aProp = [[AnotherClass alloc] init]; // setter调用
} 
 return self; 
} 
 
  - （AnotherClass *）aMethod {
返回self.aProp; // getter invocation 
} 
 @end 
  

什么是等价的访问器代码片段对于 aProp 由编译器生成？

   - （AnotherClass *）aProp { 
 // getter body 
 
 $ b  - （void）setAProp :( AnotherClass *）{
 // setter body 
} 
  

解决方案

当声明一个属性为 nonatomic
$ p $ // .h
@property（nonatomic，retain） id ivar;

// .m
- （id）ivar {
return ivar;

$ b $ - （void）setIvar：（id）newValue {
if（ivar！= newValue）{//此检查是强制性的
[ivar release] ;
ivar = [newValue retain];




请注意检查 ivar！ = newValue 。如果它不存在，ivar可以在 release 之后解除分配，并且下面的 retain 会导致内存访问错误。

当你用 copy 声明你的属性时，代码看起来几乎是一样的，用<$ c $ copy 。

，它更简单：

   - （void）setIvar：（id）newValue {
 ivar = newValue; 
 
 code 
 
 
 
 $ b 
现在，当你声明你的属性为 atomic （这是默认的），事情会变得稍微复杂一些。苹果的一位工程师在发展论坛上发布了一段类似于下面的代码片段： 
 $ b 
   - （id）ivar {
 @synchronized（self）{
 return [[self> ivar retain] autorelease]; 
 
 
 $ b  - （void）setIvar：（id）newValue {
 @synchronized（self）{
 if（newValue！= self- > ivar）{
 [自主>伊娃发行]; 
自我> ivar = newValue; 
 [self> ivar retain]; 
 
 
 
 $ / code $ / pre 
 
注意 @synchronized 在这两个方法中都会阻塞，并且在getter中增加 retain-autorelease 。这两件事情确保你可以得到以前的值（保留和自动释放），或者在你尝试读取值时由某个线程改变值的情况下获得新值。
 
How exactly getter and setter methods body looks like after they have been automatically synthesized ?

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

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.



For example:

.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
What are equivalent accessors code snippets for aProp generated by compiler ?
-(AnotherClass *) aProp {
    // getter body
}

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

解决方案
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];
    }
}
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.

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

For assign, it is even simpler:
- (void)setIvar:(id)newValue {
    ivar = newValue;
}




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];
        }
    }
}
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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