iOS开发之Block

`block`的本质

block本质上也是一个OC对象，它内部也有个isa指针。
block是封装了函数调用以及函数调用环境的OC对象。
block的底层结构如右图所示:

block01.png

`block`的变量`捕获（capture）`

为了保证block内部能够正常访问外部的变量，block有个变量捕获机制。

block02.png
1.局部变量-auto

{
auto int age = 10;//auto可以省略
void(^block)(void)=^{

     //age的值捕获进来（capture）
     NSLog(@"age is %d,height is %d",age,height);

};

age = 20;

block();
}

编译之后的:
{
struct __block_impl {
  void *isa;
  int Flags;
  int Reserved;
  void *FuncPtr;
};
struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int age;//捕获到的值
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _age, int flags=0) : age(_age) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
//  age(_age) C++语法  ==>     age = _age;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  
    //变量捕获
    int age = __cself->age; // bound by copy
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_hl_g93_s7nx05zdzh42fv2fp4rw0000gn_T_main_04a8a7_mi_0,age);
}

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 age = 10;
void(*block) = &__main_block_impl_0(
                                            __main_block_func_0,
                                            &__main_block_desc_0_DATA,
                                            age);
 age = 20;
block->FuncPtr(block);
}

2.局部变量-static

block03.png

{
static int height = 10;
void(^block)(void)=^{

     //height的值捕获进来（capture）
     NSLog(@"height is %d",height);

};

height = 20;

block();
}

编译之后的:
{
struct __block_impl {
  void *isa;
  int Flags;
  int Reserved;
  void *FuncPtr;
};
struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int *height;//捕获到的指针
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int *_height, int flags=0) : height(_height)  {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
//  age(_age) C++语法  ==>     age = _age;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  
    //变量捕获
    int *height = __cself->height; // bound by copy
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_hl_g93_s7nx05zdzh42fv2fp4rw0000gn_T_main_04a8a7_mi_0,(*height));
}

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)};
}

{
static int height = 10;
void(*block) = &__main_block_impl_0(
                                            __main_block_func_0,
                                            &__main_block_desc_0_DATA,
                                             &height);
 height = 20;
block->FuncPtr(block);
}

3.全局变量

block的类型

block有3种类型，可以通过调用class方法或者isa指针查看具体类型，最终都是继承自NSBlock类型.
1、__ NSGlobalBlock __ （ _ NSConcreteGlobalBlock ）
2、__ NSStackBlock __ （ _ NSConcreteStackBlock ）
3、__ NSMallocBlock __ （ _ NSConcreteMallocBlock ）

block04.png

block05.png

没有访问auto变量

即使访问了static 或 全局变量，block的类型仍然是__ NSGlobalBlock__。


int age = 10;

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        void (^block)(void) = ^{
            NSLog(@"age----%d",age);
        };
        
        static int a = 100;
        void (^block1)(void) = ^{
            NSLog(@"a----%d",a);
        };
        
        NSLog(@"block'type is %@;block1'type is %@",[block class],[block1 class]);
        
    }
    return 0;
}

控制台打印的结果：2020-07-05 23:31:53.484623+0800 interview4-block的类型[5014:424923] block'type is __NSGlobalBlock__;block1'type is __NSGlobalBlock__

访问auto变量

说明：
1、在ARC环境下的block类型是__ NSMallocBlock__（因为运行时Runtime做了太多的事导致访问了auto变量之后block是NSMallocBlock类型的block）；
2、在MRC环境下的类型是__ NSMallocBlock__。

`ARC`环境下:`__ NSMallocBlock__ `
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        int a = 100;
        
        void (^block1)(void) = ^{
            
            NSLog(@"a----%d",a);
        };
        
        NSLog(@"block1'type is %@",[block1 class]);
        
    }
    return 0;
}

MRC环境下的类型是__ NSStackBlock __。

block10.png

__ NSStackBlock __调用了copy返回的block类型是__ NSMallocBlock__。

说明:不管是ARC或MRC，__ NSStackBlock __类型的block只要调用了copy返回的类型都是__ NSMallocBlock__。


int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        int a = 100;
        
        void (^block1)(void) = ^{
            
            NSLog(@"a----%d",a);
        };
        
        NSLog(@"block1'type is %@",[[block1 copy] class]);
        
    }
    return 0;
}

每一种类型的block调用copy后的结果如下所示:

block06.png

`Block`的`copy`

在ARC环境下，编译器会根据情况自动将栈上的block复制到堆上，比如以下情况:
1.block作为函数返回值时。

#import <Foundation/Foundation.h>
typedef void(^KXBlock)(void);
KXBlock myBlock(){
    /**
     1.ARC环境下return时，会自动为block 调用一次copy操作，[block copy];
     2.MRC环境下return时，开发人员需要自己调用copy，[block copy]；和[block release]操作。
     */
    
    KXBlock block = ^{
        NSLog(@"-------------");
    };
    return block;
}

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        KXBlock block = myBlock();
        
        block();
        
        NSLog(@"%@",[block class]);
    }
    return 0;
}

2.将block赋值给__strong指针时。

#import <Foundation/Foundation.h>

typedef void(^KXBlock)(void);

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        int age = 10;
        
        KXBlock blcok = ^{
            
            NSLog(@"----%d",age);
            
        };
        
        NSLog(@"%@",[blcok class]);
    }
    return 0;
}

3.block作为Cocoa API中方法名含有UsingBlock的方法参数时。


#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    
    @autoreleasepool {
    
        NSArray *array = @[];
        
        [array enumerateObjectsUsingBlock:^(id  _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
            
        }];
    }
    return 0;
}

4.block作为GCD API的方法参数时。


#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    
    @autoreleasepool {
    
        //01
        static dispatch_once_t onceToken;
        dispatch_once(&onceToken, ^{
            
        });
        
        //02
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            
        });
        
    }
    return 0;
}

MRC下block属性的建议写法
1.@property (copy, nonatomic) void (^block)(void);
ARC下block属性的建议写法
1.@property (strong, nonatomic) void (^block)(void);
2.@property (copy, nonatomic) void (^block)(void);

iOS开发之Block

block的本质

block的变量捕获（capture）

block的类型

Block的copy

`block`的本质

`block`的变量`捕获（capture）`

`Block`的`copy`