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ios-Block

ios-Block

作者: 忽然之间_1988丶 | 来源:发表于2017-08-08 15:55 被阅读0次

    概述：

    • 能够截取自动变量的匿名函数
    • 指向函数的指针
    • 结构体
    • oc对象

    使用：

    - 声明
    格式：返回值类型(^变量名称)(参数列表)
int(^myBlock)(int a,int b)
void(^myBlock)(void)
    - 定义(变量赋值)
    格式：变量 = ^(参数列表){函数体};
myBlock = ^(int a, int b){
     return 1;
};
myBlock = ^(void){

};
    - 调用
    格式：变量(参数列表);
myBlock(1,2);
myBlock();
    - 使用typedef定义Block类型
    格式：typedef 返回值类型(^变量名称)(参数列表)
typedef void(^MyBlock)();
MyBlock myBlock = ^(){

};
    - Block作为函数参数
    int(^MyBlock)(int a, int b) = ^(int a, int b){ // 声明定义一个参数为a,b，返回值为int类型的block
        return 3;
};
[self blockTest:MyBlock]; 
    
void(^MyBlcok)(void) = ^(){
    
};
[self blockTest1:MyBlcok];

- (void)blockTest:(int(^)(int a, int b))myBlock{
}
- (void)blockTest1:(void(^)())myBlock{
}

    注意：为了简化block的声明，一般使用typedef声明block类型

    - Block作为函数返回值
    int(^MyBlock)(int a, int b) = [self blockTest];

- (int(^)(int a, int b))blockTest{
    return ^(int a,int b){
        return a+b;
    };
}

    block类型：

    • 全局区：NSConcreteGlobalBlock
    • 栈区：NSConcreteStackBlock
    • 堆区：NSConcreteMallocBlock
    - NSConcreteGlobalBlock
    block定义在函数之外(和全局变量一个地方)
void (^globalBlock)() = ^{
    NSLog(@"大家好我是NSConcreteGlobalBlock-->");
};
int main(int argc, char * argv[]) {
    @autoreleasepool {
        globalBlock();
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

转换成c++代码：
struct __globalBlock_block_impl_0 {
  struct __block_impl impl;
  struct __globalBlock_block_desc_0* Desc;
  __globalBlock_block_impl_0(void *fp, struct __globalBlock_block_desc_0 *desc, int flags=0) {
    impl.isa = &_NSConcreteGlobalBlock;  <--- 看这，这里。。
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

    注意：在block没有截取任何自动变量的时候，也是NSConcreteGlobalBlock的，但我在ARC环境下，查看c++源码，是NSConcreteStackBlock，具体原因请看唐巧大神的说明，自行验证。

    - NSConcreteStackBlock
    定义在函数内部的block
int main(int argc, char * argv[]) {
    @autoreleasepool {
        void(^StackBlock)() = ^(){
            NSLog(@"大家好，我是StackBlock");
        };
        StackBlock();
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

转换成c++代码：
struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int flags=0) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
    注意:NSConcreteMallocBlock无法直接创建，当执行了以下操作时，系统会自动从NSConcreteStackBlock copy到NSConcreteMallocBlock中：

    1.调用Block的copy实例方法时
    2.Block作为函数返回值返回时
    3.将Block赋值给附有__strong修饰符id类型的类或Block类型成员变量时
    4.将方法名中含有usingBlock的Cocoa框架方法或GCD的API中传递Block时

    截取自动变量：

    - 局部变量
    • 局部变量在block中不可修改。
    • 局部变量是值传递，所以在block外面修改不影响block里截取的变量值。
    oc代码：
int main(int argc, char * argv[]) {
    @autoreleasepool {
        int i = 0;
        void(^MyBlock)() = ^(){
            i++; // 报错
            NSLog(@"大家好，我是MyBlock-->%d",i); // 注释掉上面的报错代码，输出的还是0；
        };
        i++;
        MyBlock();
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

c++代码：
struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int i;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _i, int flags=0) : i(_i) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  int i = __cself->i; // bound by copy
            NSLog((NSString *)&__NSConstantStringImpl__var_folders_k__jbgrnnkx4ds0dh6rz0rglp540000gn_T_main_658820_mi_1,i);
}

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};

int main(int argc, char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        int i = 0;
        void(*MyBlock)() = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, i));

        ((void (*)(__block_impl *))((__block_impl *)MyBlock)->FuncPtr)((__block_impl *)MyBlock);

        return UIApplicationMain(argc, argv, __null, NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("AppDelegate"), sel_registerName("class"))));
    }
}
    - 静态局部变量
    • 局部静态变量在block中可修改。
    • 局部静态变量是地址传递，所以在block外面修改会影响block里截取的变量值。
    oc代码：
int main(int argc, char * argv[]) {
    @autoreleasepool {
        static int i = 0;
        void(^MyBlock)() = ^(){
            i++; // 可以修改，不会报错
            NSLog(@"大家好，我是MyBlock-->%d",i);
        };
        i++;
        MyBlock();

        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

c++代码：
struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int *i;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int *_i, int flags=0) : i(_i) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  int *i = __cself->i; // bound by copy

            (*i)++;
            NSLog((NSString *)&__NSConstantStringImpl__var_folders_k__jbgrnnkx4ds0dh6rz0rglp540000gn_T_main_e314e4_mi_0,(*i));
        }

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};
int main(int argc, char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        static int i = 0;
        void(*MyBlock)() = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, &i));

        i++;
        ((void (*)(__block_impl *))((__block_impl *)MyBlock)->FuncPtr)((__block_impl *)MyBlock);

        return UIApplicationMain(argc, argv, __null, NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("AppDelegate"), sel_registerName("class"))));
    }
}
    - __block修饰局部变量(只能用于修饰普通局部变量，static，全局都不行)
    • __block修饰的局部变量在block中可修改。
    • __block修饰的局部变量，注意看下面c++源码，系统会把用__block修饰的变量包装成一个__Block_byref_i_0结构体对象。main_block_impl_0 中引用的是 Block_byref_i_0 的结构体指针，这样就可以达到修改外部变量的作用，因为传递的是该结构体的地址，所以我们在外面修改变量，会影响block中的值。
    oc代码：
int main(int argc, char * argv[]) {
    @autoreleasepool {
        __block int i = 0;
        void(^MyBlock)() = ^(){
            i++; // 可以修改，不会报错
            NSLog(@"大家好，我是MyBlock-->%d",i);
        };
        
        i++;
        MyBlock();
        
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

c++代码：
struct __Block_byref_i_0 {  // 当用__block来修饰变量时，系统会吧变量包装成一个结构体对象。
  void *__isa;
__Block_byref_i_0 *__forwarding;
 int __flags;
 int __size;
 int i;
};

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __Block_byref_i_0 *i; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_i_0 *_i, int flags=0) : i(_i->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_i_0 *i = __cself->i; // bound by ref

            (i->__forwarding->i)++;
            NSLog((NSString *)&__NSConstantStringImpl__var_folders_k__jbgrnnkx4ds0dh6rz0rglp540000gn_T_main_be9fd2_mi_0,(i->__forwarding->i));
 }

static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->i, (void*)src->i, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->i, 8/*BLOCK_FIELD_IS_BYREF*/);}

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __main_block_impl_0*, struct __main_block_impl_0*);
  void (*dispose)(struct __main_block_impl_0*);
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0), __main_block_copy_0, __main_block_dispose_0};

int main(int argc, char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        __attribute__((__blocks__(byref))) __Block_byref_i_0 i = {(void*)0,(__Block_byref_i_0 *)&i, 0, sizeof(__Block_byref_i_0), 0};
        void(*MyBlock)() = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_i_0 *)&i, 570425344));

        (i.__forwarding->i)++;
        ((void (*)(__block_impl *))((__block_impl *)MyBlock)->FuncPtr)((__block_impl *)MyBlock);

        return UIApplicationMain(argc, argv, __null, NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("AppDelegate"), sel_registerName("class"))));
    }
}
    - 全局变量
    • 全局变量在block中可修改。
    • 全局变量存储在静态数据区，在程序销毁前不会销毁，所以在block中可以直接访问，因为访问的是一份地址，所以在外面修改会影响block里面变量的值。
    oc代码：
int i = 0;
int main(int argc, char * argv[]) {
    @autoreleasepool {
        void(^MyBlock)() = ^(){
            i++; // 可以修改，不会报错
            NSLog(@"大家好，我是MyBlock-->%d",i);
        };
        
        i++;
        MyBlock();
        
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

c++代码：
int i = 0;
struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int flags=0) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
            i++;
            NSLog((NSString *)&__NSConstantStringImpl__var_folders_k__jbgrnnkx4ds0dh6rz0rglp540000gn_T_main_ba21a2_mi_0,i);
}

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};
int main(int argc, char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 

        void(*MyBlock)() = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));

        i++;
        ((void (*)(__block_impl *))((__block_impl *)MyBlock)->FuncPtr)((__block_impl *)MyBlock);

        return UIApplicationMain(argc, argv, __null, NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("AppDelegate"), sel_registerName("class"))));
    }
}
    - 静态全局变量
    • 静态全局变量在block中可修改。
    • 静态全局变量存储在静态数据区，在程序销毁前不会销毁，所以在block中可以直接访问，因为访问的是一份地址，所以在外面修改会影响block里面变量的值。
    oc代码：
static int i = 0;
int main(int argc, char * argv[]) {
    @autoreleasepool {
        
        void(^MyBlock)() = ^(){
            i++; // 可以修改，不会报错
            NSLog(@"大家好，我是MyBlock-->%d",i);
        };
        
        i++;
        MyBlock();
        
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

c++代码：
static int i = 0;

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int flags=0) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
       i++;
       NSLog((NSString *)&__NSConstantStringImpl__var_folders_k__jbgrnnkx4ds0dh6rz0rglp540000gn_T_main_e20576_mi_0,i);
 }

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};

int main(int argc, char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 

        void(*MyBlock)() = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));

        i++;
        ((void (*)(__block_impl *))((__block_impl *)MyBlock)->FuncPtr)((__block_impl *)MyBlock);

        return UIApplicationMain(argc, argv, __null, NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("AppDelegate"), sel_registerName("class"))));
    }
}

    内存管理：

    - (void)viewDidLoad {
    
    [super viewDidLoad];
    
    self.view.backgroundColor = [UIColor whiteColor];
    
    int i = 0;
    MyBlock = ^(){
        NSLog(@"我是MyBlock--->%d",i);
    };
    MyBlock();
    
    NSLog(@"myBlock--->%@",MyBlock);
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    MyBlock();
}
    • MRC
      引用了局部变量i,所以block在NSStackBlock中，出了viewDidLoad方法，MyBlock会自动销毁，touchesBegan再调用MyBlock就会报野指针错误。
      需要手动调用Block_copy方法把block复制到NSConcreteMallocBlock中，记得调用Block_release方法使计数器-1。
    • ARC
      会自动调用copy方法，把block复制到NSConcreteMallocBlock中。

    注意：在Block中无论是MRC/ARC，没有调用局部变量时，都是NSConcreteGlobalBlock类型的，也就不会出现block销毁了，在调用的情况了。

    image.png

    循环引用：

    • 调用系统的block不会循环引用，self并不持有该方法。
    [UIView animateWithDuration:0.5 animations:^{
        NSLog(@"%@", self);
}];
    • 当self持有block时，block中又调用了self,会循环引用。
    @property (nonatomic, copy)MyBlock myBlock;
self.myBlock = ^(){
   self.xxxx;
};

// 不会循环引用
__weak typeof(self)weakSelf = self;
self.myBlock = ^(){
  weakSelf.xxxx;
};
    • 自定义对象中有个block属性，在调用该对象的block属性中，调用该对象的其他属性，会循环引用。
    typedef void(^MyBlock)();

@interface MyBlockModel : NSObject
@property (nonatomic, copy) MyBlock myBlock;
@property (nonatomic, copy) NSString *text;
@end

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor whiteColor];
    
    MyBlockModel *model = [[MyBlockModel alloc] init];
    model.myBlock = ^(){
        NSLog(@"--->%@",model.text);
    };

   // 不会循环引用
    MyBlockModel *model1 = [[MyBlockModel alloc] init];
    __weak MyBlockModel *weakModel = model1;
    model1.myBlock = ^(){
        NSLog(@"--->%@",weakModel.text);
    };
}
    • 注意：在异步(多线程)环境下。防止self被释放，应该在block内部用强引用引用该弱引用。
    __weak typeof(self)weakSelf = self;
self.myBlock = ^(){
    __strong typeof (weakSelf)strongSelf = weakSelf;
    if(strongSelf){
        strongSelf.xxxx; 
    }
};

    参考：
    http://www.jianshu.com/p/51d04b7639f1
    http://www.cocoachina.com/ios/20161025/17198.html
    http://blog.devtang.com/2013/07/28/a-look-inside-blocks/

    小提示:
    1.如和把oc代码转成c++代码：
    通过终端cd到你要编译的文件所在目录，输入：clang -rewrite-objc xxxx.m即可。(xxxx:你要编译的文件名称)。
    2.如果出现main.m:9:9: fatal error: 'UIKit/UIKit.h' file not found怎么办？
    clang -x objective-c -rewrite-objc -isysroot /Applications/Xcode8.3.3.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk xxxx.m(xxxx:你要编译的文件名称)。

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