Block的引用循环问题 (ARC & non-ARC)

2010年WWDC发布iOS4时Apple对Objective-C进行了一次重要的升级：支持Block。说到底这东西就是闭包，其他高级语音例如Java和C++已有支持，第一次使用Block感觉满简单好用的，但是慢慢也遇到很多坑。本文聊聊ARC和non-ARC下Block使用中的引用循环问题，最近遇到了好几次这种问题，还是深入记录下。先来套题目热热身，貌似能够全部答对的人蛮少的

Block实现原理

首先探究下Block的实现原理，由于Objective-C是C语言的超集，既然OC中的NSObject对象其实是由C语言的struct+isa指针实现的，那么Block的内部实现估计也一样，以下三篇Blog对Block的实现机制做了详细研究：

• A look inside blocks: Episode 1
• A look inside blocks: Episode 2
• A look inside blocks: Episode 3

虽然实现细节看着头痛，不过发现Block果然是和OC中的NSObject类似，也是用struct实现出来的东西。这个是LLVM项目compiler-rt分析的block头文Block_private.h头文件中关于Block的struct声明：

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struct Block_descriptor {    unsigned long int reserved;    unsigned long int size;    void (*copy)(void *dst, void *src);    void (*dispose)(void *);};struct Block_layout {    void *isa;    int flags;    int reserved;    void (*invoke)(void *, ...);    struct Block_descriptor *descriptor;    /* Imported variables. */};

我们发现Block_layout中也有一个isa指针，像极了NSobject内部实现struct中的isa指针。这里的isa可能指向三种类型之一的Block：

• _NSConcreteGlobalBlock：全局类型Block，在编译器就已经确定，直接放在代码段__TEXT上。直接在NSLog中打印的类型为__NSGlobalBlock__。
• _NSConcreteStackBlock：位于栈上分配的Block，即__NSStackBlock__。
• _NSConcreteMallocBlock：位于堆上分配的Block，即__NSMallocBlock__。

为什么会有这么多种类呢？首先来看全局类型Block，看例子：

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void addBlock(NSMutableArray *array) {  [array addObject:^{    printf("global block\n");  }];} void example() {  NSMutableArray *array = [NSMutableArray array];  addBlock(array);  void (^block)() = [array objectAtIndex:0];  block();}

为什么addBlock中添加到array中的Block属于全局Block呢？因为它不需要运行时(Runtime)任何的状态来改变行为，不需要放在堆上或者栈上，直接编译后在代码段中即可，就像个c函数一样。这种类型的Block在ARC和non-ARC情况下没有差别。

这个Block访问了作用域外的变量d，在实现上就是这个block会多一个成员变量对应这个d，在赋值^block时会将方法exmpale中的d变量值复制到成员变量中，从而实现访问。

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void example() {  int d = 5;  void (^block)() = ^() {      printf("%d\n", d);  };  block();}

如果要修改d呢？：

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void example() {  int d = 5;  void (^block)() = ^() {      d++;      printf("%d\n", d);  };  block();  printf("%d\n", d);}

由于局部变量d和这个block的实现不在同一作用域，仅仅在调用过程中用到了值传递，所以不能直接修改，而需要加一个标识符__block int d = 5;，那么block就可以实现对这个局部变量的修改了。如果是这种block标识的变量，在Block实现中不再是简单的一个成员变量，而是对应一个新的结构体表示这个block变量。block的本质是引入了一个新的Block_byref{$var_name}{$index}结构体，被block关键字修饰的变量就被放到这个结构体中。另外，block结构体通过引入Block_byref{$var_name}{$index}指针类型的成员，得以间接访问到Block的外部变量。这样对Block外的变量访问从值传递转变为引用，从而有了修改内容的能力。

正常我们使用Block是在栈上生成的，离开了栈作用域便释放了，如果copy一个Block，那么会将这个Block copy到堆上分配，这样就不再受栈的限制，可以随意使用啦。例如：

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typedef void (^TestBlock)(); TestBlock getBlock() {  char e = 'E';  void (^returnedBlock)() = ^{    printf("%c\n", e);  };  return returnedBlock;} void example() {  TestBlock block = getBlock();  block();}

函数getBlock中声明并赋值的returnedBlock，一开始是在栈上分配的，属于NSStackBlock，如果是non-ARC情况下return这个NSStackBlock，那么其实已经被销毁了，在函数中example()使用时就会crash。如果是ARC情况下，getBlock返回的block会自动copy到堆上，那么block的类型就是NSMallocBlock，可以在example()中继续使用。要在Non-ARC情况下正常运行，那么就应该修改为：

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TestBlock getBlock() {  char e = 'E';  void (^returnedBlock)() = ^{    printf("%c\n", e);  };  return [[returnedBlock copy] autorelease];}

Block中的循环引用问题

扯了这么多，回到Block的循环引用问题，由于我们很多行为会导致Block的copy，而当Block被copy时，会对block中用到的对象产生强引用(ARC下)或者引用计数加一(non-ARC下)。

如果遇到这种情况：

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@property(nonatomic, readwrite, copy) completionBlock completionBlock;//========================================self.completionBlock = ^ {        if (self.success) {            self.success(self.responseData);        }    }};

对象有一个Block属性，然而这个Block属性中又引用了对象的其他成员变量，那么就会对这个变量本身产生强应用，那么变量本身和他自己的Block属性就形成了循环引用。在ARC下需要修改成这样：

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@property(nonatomic, readwrite, copy) completionBlock completionBlock;//========================================__weak typeof(self) weakSelf = self;self.completionBlock = ^ {    if (weakSelf.success) {        weakSelf.success(weakSelf.responseData);    }};

也就是生成一个对自身对象的弱引用，如果是倒霉催的项目还需要支持iOS4.3，就用__unsafe_unretained替代__weak。如果是non-ARC环境下就将__weak替换为__block即可。non-ARC情况下，__block变量的含义是在Block中引入一个新的结构体成员变量指向这个__block变量，那么__block typeof(self) weakSelf = self;就表示Block别再对self对象retain啦，这就打破了循环引用。