OS中atomic的实现解析
转自：http://my.oschina.net/majiage/blog/267409
摘要 atomic属性线程安全，会增加一定开销，但有些时候必须自定义atomic。这时候，我们就需要知道atomic的实现原理及方法了。这篇文章主要就是讲解自定义atomic的实现。
atomic原子性与non-atomic非原子性
iOS中有两个属性non-atomic和atomic，前者是非原子性的（线程不安全），后者是原子性的（线程安全），一般情况下不会去重写它们，但某些时候确实有重写的需求。
那些int、float之类的类型，你重写想出错都很难。但是强引用类型（retain）就需要注意了。
简单的说一下非原子性的nonatomic实现，方式如下：
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- (void)setCurrentImage:(UIImage *)currentImage{ if (_currentImage != currentImage) { [_currentImage release]; _currentImage = [currentImage retain]; // do something }}- (UIImage *)currentImage{ return _currentImage;} |
atomic实现：
关于atomic的实现最开始的方式如下：
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- (void)setCurrentImage:(UIImage *)currentImage{ @synchronized(self) { if (_currentImage != currentImage) { [_currentImage release]; _currentImage = [currentImage retain]; // do something } }}- (UIImage *)currentImage{ @synchronized(self) { return _currentImage; }} |
具体讲就是retain的同步版本，本来以为没问题，但在用GCD重绘currentImage的过程中，有时候currentImage切换太频繁。在完成之前就把之前的currentImage释放了，程序仍然会崩溃。还需要在resize过程中增加retain和release操作，代码如下：
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- (UIImage *)resizedImage:(CGSize)newSize interpolationQuality:(CGInterpolationQuality)quality { // For multithreading [self retain]; BOOL drawTransposed; CGAffineTransform transform = CGAffineTransformIdentity; // In iOS 5 the image is already correctly rotated. See Eran Sandler's // addition here: http://eran.sandler.co.il/2011/11/07/uiimage-in-ios-5-orientation-and-resize/ if([[[UIDevice currentDevice]systemVersion]floatValue] >= 5.0) { drawTransposed = NO; } else { switch(self.imageOrientation) { case UIImageOrientationLeft: case UIImageOrientationLeftMirrored: case UIImageOrientationRight: case UIImageOrientationRightMirrored: drawTransposed = YES; break; default: drawTransposed = NO; } transform = [self transformForOrientation:newSize]; } transform = [self transformForOrientation:newSize]; UIImage *image = [self resizedImage:newSize transform:transform drawTransposed:drawTransposed interpolationQuality:quality]; [self release]; return image;} |
原始版本的resize函数如下：
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- (UIImage *)resizedImage:(CGSize)newSize interpolationQuality:(CGInterpolationQuality)quality { BOOL drawTransposed; CGAffineTransform transform = CGAffineTransformIdentity; // In iOS 5 the image is already correctly rotated. See Eran Sandler's // addition here: http://eran.sandler.co.il/2011/11/07/uiimage-in-ios-5-orientation-and-resize/ if([[[UIDevice currentDevice]systemVersion]floatValue] >= 5.0) { drawTransposed = NO; } else { switch(self.imageOrientation) { case UIImageOrientationLeft: case UIImageOrientationLeftMirrored: case UIImageOrientationRight: case UIImageOrientationRightMirrored: drawTransposed = YES; break; default: drawTransposed = NO; } transform = [self transformForOrientation:newSize]; } transform = [self transformForOrientation:newSize]; return [self resizedImage:newSize transform:transform drawTransposed:drawTransposed interpolationQuality:quality];} |
但是之前在没有重写getter之前，用atomic的getter程序不会崩溃。于是我就想现在的getter和atomic自己实现的getter肯定有区别。
最后，答案出现：在getter的return之前retain，再autorelease一次就可以了。getter函数就变成了这样：
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- (UIImage *)currentImage{ @synchronized(self) { UIImage *image = [_currentImage retain]; return [image autorelease]; }} |
这样可以确保currentImage在调用过程中不会因为currentImage被释放或者改变，使它的retainCount次数变为0，再在调用时让程序直接崩溃。
Runtime方法
在Memory and thread-safe custom property methods这篇文章中还提到了一种Objective-C的runtime解决方案。
Objective-C的runtime中实现了以下函数：
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id <strong>objc_getProperty</strong>(id self, SEL _cmd, ptrdiff_t offset, BOOL atomic);void <strong>objc_setProperty</strong>(id self, SEL _cmd, ptrdiff_t offset, id newValue, BOOL atomic, BOOL shouldCopy);void <strong>objc_copyStruct</strong>(void *dest, const void *src, ptrdiff_t size, BOOL atomic, BOOL hasStrong); |
这几个函数被实现了，但没有被声名。如果要使用他们，必须自己声名。它们比用@synchronized实现的要快。因为它的实现方式与一般情况不同，静态变量只在接收并发时才会锁住。
声名方式：
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#define <strong>AtomicRetainedSetToFrom</strong>(dest, source) objc_setProperty(self, _cmd, (ptrdiff_t)(&dest) - (ptrdiff_t)(self), source, YES, NO)#define <strong>AtomicCopiedSetToFrom</strong>(dest, source) objc_setProperty(self, _cmd, (ptrdiff_t)(&dest) - (ptrdiff_t)(self), source, YES, YES)#define <strong>AtomicAutoreleasedGet</strong>(source) objc_getProperty(self, _cmd, (ptrdiff_t)(&source) - (ptrdiff_t)(self), YES)#define <strong>AtomicStructToFrom</strong>(dest, source) objc_copyStruct(&dest, &source, sizeof(__typeof__(source)), YES, NO) |
用这些宏定义，上面something的copy getter和setter方法将变成这样：
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- (NSString *)someString{ return AtomicAutoreleasedGet(someString);}- (void)setSomeString:(NSString *)aString{ AtomicCopiedSetToFrom(someString, aString);} |
someRect存取方法将变成这样：
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- (NSRect)someRect{ NSRect result; AtomicStructToFrom(result, someRect); return result;}- (void)setSomeRect:(NSRect)aRect{ AtomicStructToFrom(someRect, aRect);} |