内存管理MRR/ARC，property和内存泄露

MRR：Manual retain-release

推荐：《iPhone/Mac Objective-C内存管理教程和原理剖析》

http://www.cnblogs.com/VinceYuan/archive/2010/03/08/1680488.html

• 生成的对象，有一个引用计数:retainCount，当retainCount = 0，内存就会被释放
• 用alloc/new/copy生成对象时，其 retainCount = 1
• retain 让retainCount + 1，release让retainCount - 1，成对出现：[obj retain]  [obj release]
• 用autorelease能让retainCount的管理简单些，涉及到autorelease pool
• 在生成新的RunLoop时，系统自动创建新的autorelease pool

property的修饰符：

• assign
• copy
• retain：参考默认实现，很重要
• 
  
• 如：@property (retain) objA* obj;
• 
  

retain、copy的默认实现：

-(void) setObjB:(ClassB*) value

{

         if (objB != value)

         {

                   [objB release]; //原来的对象会先release

                   objB = [value retain]; //copy类似

         }

}  

 

assign针对简单数据类型(scalar types) ，如int，float, CGRect, NSInteger等，它只是简单的赋值操作

 

 

 

copy/mutableCopy 浅拷贝和深拷贝

参考：《浅析ObjectiveC 深浅拷贝学习 》http://www.cocoachina.com/bbs/read.php?tid=80463

• 不可改变对象的copy，才是浅拷贝；其他都是深拷贝
• copy 返回的对象都是不可改变的
• 容器类(如NSArray等) 的内容都是浅拷贝的

内存泄露

参考：《Presentation: Finding & Fixing Mem Leaks》

http://www.streamingcolour.com/blog/2010/09/21/presentation-finding-fixing-mem-leaks/

 

常见情况1：

// @property (nonatomic, retain) MyClass* objA;

 

- (void)foo{

  //retainCount = 2

  self.objA = [[MyClass alloc] init];

}

 

- (void)dealloc{

  //retainCount = 1

  [objA release];

  [super dealloc];

}

 

修复方法：

// @property (nonatomic, retain) MyClass* objA;

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- (void)foo{

  //retainCount = 1

  MyClass *obj = [[MyClass alloc] init];

  //retainCount = 2

  self.objA = obj;

  //retainCount = 1

  [obj release]

}

 

- (void)dealloc{

  //retainCount = 0

  [objA release];

  [super dealloc];

}     

 

常见情况2：

 

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       // @property (nonatomic, retain) MyClass* objA;

 

- (void)foo{

  MyClass *obj = [[MyClass alloc] init];

  self.objA = obj;

  [obj release];

  //retainCount = 1

}

 

- (void)dealloc{

  //retainCount = 1

  [super dealloc];

}

 

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修复方法：

// @property (nonatomic, retain) MyClass* objA;

 

- (void)foo{

  MyClass *obj = [[MyClass alloc] init];

  self.objA = obj;

  [obj release];

  //retainCount = 1

}

 

- (void)dealloc{

  //retainCount = 0

  [objA release];

  [super dealloc];

}

 

self.abc = nil 与 _abc = nil的区别

@property (retain) classA *abc;

@synthesize abc = _abc;

 

需要明白 self.abc 的默认实现，它相当于:

[_abc release];

_abc = nil;

前者会发送release消息，而后者可能有内存泄露。

 

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Tools

• Build & Analyze
• Leaks Instument  

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Automatic Reference Counting

• “Automatic Reference Counting (ARC) is a compiler-level feature that simplifies the process of managing object lifetimes (memory management) in Cocoa applications.”
• ARC is a pre-compilation step that adds retain/release/autorelease statements to your code for you
• 需要Xcode 4.2+, Apple LLVM 3.0+ compiler 

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简化了编程（本周上和MRR是一样的），不需要/不能使用下面这些方法

• retain /release

• autorelease

• retainCount

• NSAllocateObject / NSDeallocateObject  

property的修饰符：

• strong：相当于retain
• weak：相当于assign

变量修饰符：

• __strong
  • This means any object created usingalloc/init is retained for the lifetime of its current scope.
  • is the default so you don’t need to type it.
  • #相当于retain,在不再使用的时候被释放 (默认)
• __weak
  • means the object can be destroyed at anytime.
  • This is only useful if the object is somehow strongly referenced somewhere else.
  • When destroyed, a variable with __weak is set to nil.
  • #与assign很像,不同在于如果指向的数据被释放了,那么这个指向nil
• __unsafe_unretained
  • is just like __weak but the poiner is not set to nil when the object is deallocated.
  • Instead the pointer is left dangling (i.e. it no longer points to anything useful).
  • #相当于assign,指向的数据如果被释放,这个指向原来的地址
• __autoreleasing
• not to be confused with calling autorelease on an object before returning it from a method, this is used for passing objects by reference, for example when passing NSError objects by reference such as [myObject performOperationWithError:&tmp];
• #标明传给函数的(id*)型参数是自动释放的,(函数中(id*)型参数默认的也是这种类型)  

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Toll-Free Bridging/CF和ObjC转换

• 在没有开启ARC时,可以进行强制转换.
  • CFStringRef aCFString = (CFStringRef)aNSString;
  • NSString *aNSString = (NSString *)aCFString;
• __bridge

  • 简单赋值,不会影响两边对象的retain count.

    id my_id;

    CFStringRef my_cfref;

    NSString *a = (__bridge NSString*)my_cfref;

    CFStringRef b = (__bridge CFStringRef)my_id;

• __bridge_retained or CFBridgingRetain
  • 保留赋值，赋值后也保留不释放右边的对象
• __bridge_transfer or CFBridgingRelease
  • 释放赋值，赋值后释放右边的对象
• 例子

  • -(void)test

    {

        CFStringRef coreFoundationString = CFStringCreateWithCString(CFAllocatorGetDefault(),"C String", kCFStringEncodingUTF8);    // 创建 retainCount = 1

        id unknownObjectType = (__bridge  id)coreFoundationString; // 简单赋值,不变,retainCount = 1

        CFStringRef anotherString = (__bridge_retained  CFStringRef)unknownObjectType; // 保留赋值,加一,retainCount = 2

        NSString *objCString = (__bridge_transfer  NSString *)coreFoundationString; // 释放赋值,减一,retainCount =1;由于NSString*默认strong,加一,retainCount = 2

        NSLog(@"String = %@", objCString);

        objCString = nil;   // 不再指向原内存,原内存减一,retainCount = 1

        CFRelease(anotherString);   // 释放,减一,retainCount = 0

    }

• 转换宏

  • #if __has_feature(objc_arc)

    #   define objc_retainedObject(o) ((__bridge_transfer id)(objc_objectptr_t)(o))

    #   define objc_unretainedObject(o) ((__bridge id)(objc_objectptr_t)(o))

    #   define objc_unretainedPointer(o) ((__bridge objc_objectptr_t)(id)(o))

    #else

    #   define objc_retainedObject(o) ((id)(objc_objectptr_t)(o))

    #   define objc_unretainedObject(o) ((id)(objc_objectptr_t)(o))

    #   define objc_unretainedPointer(o) ((objc_objectptr_t)(id)(o))

    #endif     

@synthesize 和Dynamic ivars

@interface ClassA {

  //NSObject _objB; // instance varible or ivar，可以不写，编译器会自动生成

}

@property (retain) NSObject *objB; 

@end

 

@implement ClassA

@synthesize objB = _objB;

@end

 

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@synthesize myProperty = myIvar; // a dynamic ivar named myIvar  will be generated

@synthesize anotherProperty;   // a dynamic ivar named anotherProperty  will be generated

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1. @synthesize是新特征，简化setter/getter方法；
2. 能用retain/copy/assing，strong/wak管理内存
3. 在init和dealloc函数中，不建议用synthesized accessors，它会trigger KVO notifications；它本质上是setter/getter，在不需要调用setter/getter的时候，就不要用
4. 
   
5. 32bit Mac OS X不支持Dynamic ivar，必须手动什么iVar，否则无法编译
6. 调试原因（我还不清楚这是什么意思，因为在self变量里可以找到ivar）：For some reason the XCode debugger doesn't show properties that don't have corresponding ivars explicitly declared.       

self.abc = nil 与 _abc = nil的本质区别