标签:
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:
- (void)setCurrentImage:(UIImage *)currentImage
{
if (_currentImage != currentImage) {
[_currentImage release];
_currentImage = [currentImage retain];
// do something
}
}
- (UIImage *)currentImage
{
return _currentImage;
}
关于atomic的实现最开始的方式如下:
- (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:
- (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:
- (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:
- (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。
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:
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:
#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:
- (NSString *)someString
{
return AtomicAutoreleasedGet(someString);
}
- (void)setSomeString:(NSString *)aString
{
AtomicCopiedSetToFrom(someString, aString);
}
someRect存取方法将变成这样:
- (NSRect)someRect
{
NSRect result;
AtomicStructToFrom(result, someRect);
return result;
}
- (void)setSomeRect:(NSRect)aRect
{
AtomicStructToFrom(someRect, aRect);
}
方案:
方案一:copy不可变的字符串
NSString*str = @"aaa";
NSString*copyStr = [str copy];
NSLog(@"str = %p copyStr= %p",str,copyStr);
NSLog(@"指针地址:str = %p copyStr= %p",&str,©Str);
输出结果:str = 0x104d94068
copyStr= 0x104d94068
指针地址:str = 0x7fff529e9aa8
copyStr= 0x7fff529e9aa0
小结:对不可变的字符串的copy
,我们对象的内存地址没有改变,只是指针的地址改变了,所以在这里我们默认进行了一次浅拷贝
,只拷贝了指针。
方案二:copy可变的字符串
NSMutableString*str1 = [NSMutableString stringWithFormat:@"bbb"];
NSString*copyStr1 = [str1 copy];
NSLog(@"str1 = %p copyStr1 = %p",str1,copyStr1);
NSLog(@"str1 = %p copyStr1= %p",&str1,©Str1);
输出结果:str1 = 0x7fa522712cd0
copyStr1 = 0x7fa522717ba0
指针地址:str1 = 0x7fff529e9a98
copyStr1= 0x7fff529e9a90
小结:对可变字符串的copy,我们默认进行了一次深拷贝,直接拷贝了对象。
mutableCopy不可变字符串的
NSString*str2 = @"ccc";
NSMutableString *copyStr2 = [str2 mutableCopy];
NSLog(@"str2 = %p copyStr2 = %p",str2,copyStr2);
输出结果:str2 = 0x10d216108
copyStr2 = 0x7fa522726290
mutableCopy可变字符串
NSMutableString*str3 = [NSMutableString stringWithFormat:@"ddd"];
NSMutableString*copyStr3 = [str3 mutableCopy];
NSLog(@"str3 = %p copyStr3 = %p",str3,copyStr3);
输出结果:str3 = 0x7fa5227153c0
copyStr3 = 0x7fa5227263f0
因为copy默认返回的是不可变的,所以当我们对一个不可变的字符串进行copy的时候,我们只是拷贝了它的指针(浅拷贝)。当我们对一个可变的字符串进行拷贝的时候,因为类型转变了,我们需对其进行深拷贝
。无论对于可变的字符串还是不可变的字符串进行mutableCopy,系统都默认进行深拷贝
,那么为什么对于相同类型的进行mutableCopy返回的仍然是新的对象呢,因为在这里系统要保证,旧的对象和新的对象都是可变的,切他们之前不会相互影响。标签:
原文地址:http://www.cnblogs.com/xsyl/p/5788666.html