Block 之 block的本质

1、Block的格式、使用

• 1、立即执行的block
  ^{ /*代码内容*/ }();

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
}

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    //第一种写法：立马执行
    ^{
        NSLog(@"this is a block");
    }();
}
@end

• 2、无参数无返回值的Block
  void(^Block变量名)(void) = ^(){ /*代码内容*/ };
void(^Block变量名)(void) = ^{ /*代码内容*/ };

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
}

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    //第二种：先定义好这个无返回值无参数的block，在需要时代码调用
    //现在的编辑器要求定义的时，形参列表的位置写void
    void (^emptyBlock)(void) = ^{
        NSLog(@"这是个无参数,无返回值的Block");
    };
    emptyBlock();
}

@end

• 3、有参数无返回值的Block
  void(^Block变量名)(形参列表) = ^(形参列表){ /*代码内容*/ };

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
}

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    //第三种:有参数无返回值的Block
    void (^sumBlock)(int ,int ) = ^(int a,int b){
        NSLog(@"%d + %d = %d",a,b,a+b);
    };
    
    //调用这个sumBlock的Block，得到的结果是20
    sumBlock(10,10);
}

@end

• 4、有参数有返回值的Block
  返回值类型 (^Block变量名)(形参列表) = ^(形参列表){ /*代码内容*/ };

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
}

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    //第四种.有参数有返回值的Block
    NSString* (^logBlock)(NSString *,NSString *) = ^(NSString *str1,NSString *str2){
        return [NSString stringWithFormat:@"%@%@",str1,str2];
    };
    //调用logBlock
    NSLog(@"%@", logBlock(@"我是",@"Block"));
}

@end

• 5、Block结合typedef使用
  包含有返回值block 和无返回值block

#import "ViewController.h"

typedef void(^AgeBlock)(int age1,int age2);
typedef NSString*(^NameBlock) (NSString *name1,NSString *name2);


@interface ViewController ()
@property(nonatomic,copy)AgeBlock ageBlock;
@property(nonatomic,copy)NameBlock nameBlock;
@end


@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.ageBlock = ^(int age1, int age2) {
        int oldAge =  age1>age2?age1:age2;
        NSLog(@"输出较大的年龄：%d",oldAge);
    };

    self.nameBlock = ^NSString *(NSString *name1, NSString *name2) {
        NSString *fullName = [NSString stringWithFormat:@"%@ %@",name1,name2];
        NSLog(@"fullName is %@",fullName);
        return fullName;
    };
}

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    //无返回值的block
    self.ageBlock(12, 13);
    
    //有返回值的block
    NSString *myNameStr = self.nameBlock(@"zhang", @"san");
    NSLog(@"myName is %@",myNameStr);
}

@end

2、Block的本质

block本质上就是一个OC对象，是一个封装了函数调用以及函数调用环境的OC对象。

下面ViewController.m文件中有个引用了外部变量的block，我们可以通过clang 命令来观察其源码：

xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc ViewController.m

可以找到对应block的源码：

屏幕快照 2018-10-26 下午5.20.49.png

• block底层源码结构

//block的底层是这样的一个结构体
struct __ViewController__touchesBegan_withEvent__block_impl_0 {
    struct __block_impl impl;
    struct __ViewController__touchesBegan_withEvent__block_desc_0* Desc;
    int age; //这里可能还有block要用到的外部局部变量,根据具体block而定
    
    //这是一个赋值过程，并不是block的结构
    __ViewController__touchesBegan_withEvent__block_impl_0(void *fp, struct __ViewController__touchesBegan_withEvent__block_desc_0 *desc, int _age, int flags=0) : age(_age) {
        impl.isa = &_NSConcreteStackBlock;
        impl.Flags = flags;
        impl.FuncPtr = fp;
        Desc = desc;
    }
};

//结构体的第一个成员
struct __block_impl {
    void *isa; //是一个isa，即一个OC对象
    int Flags;
    int Reserved;
    void *FuncPtr;//block的代码块内容，要执行的函数地址
};

//结构体的第二个成员
struct __ViewController__touchesBegan_withEvent__block_desc_0 {
    size_t reserved;
    size_t Block_size;//block的内存大小
} __ViewController__touchesBegan_withEvent__block_desc_0_DATA = { 0, sizeof(struct __ViewController__touchesBegan_withEvent__block_impl_0)};

屏幕快照 2018-10-26 下午4.53.55副本.png

3、总结

• <1>block的原理是怎样的？本质是什么？
  block的本质其实是一个封装了函数调用以及函数调用环境的OC对象。