1. 三种类型

Block有如下三种类型

• NSConcrateGlobalBlock 全局
• NSConcrateStackBlock 栈
• NSConcrateMallocBlock 堆

1. NSConcrateGlobalBlock 位于全局区

当我们声明一个Block,如果这个block没有捕获外部变量，那么这个Block就位于全局区，此时对NSGlobal的retain、copy、release都是无效的，ARC、MRC都是如此

- (void)blockType {
    void(^block1)(void) = ^{
    };
    NSLog(@"-- block1 = %@", [block1 class]);
}

2021-05-17 17:36:02.791048+0800 MemoryManageDemo[37503:5056528] -- block1 = __NSGlobalBlock__

2. NSConcrateStatckBlock 栈区

block捕获了外部变量，但是没有强引用这个block，直接打印出来

    //NSConcrateStackBlock
    int a = 10;
    NSLog(@"-- block2 = %@",[^{
        NSLog(@"-- a = %@",a);
    } class]);

2021-05-17 18:35:27.662072+0800 MemoryManageDemo[57159:5148084] -- block2 = __NSStackBlock__

3. NSConcrateMallocBlock 堆区

ARC环境下只有捕获了外部变量，并且有强引用指向该Block(或做为函数返回值)，系统就会自动将NSStackBlock copy到堆上，就是NSConcrateMallocBlock

    //NSConcrateMallocBlock
    int b = 10;
    void(^block3)(void) = ^{
        NSLog(@"-- b = %d", b);
    };
    NSLog(@"-- block3 = %@", [block3 class]);

2021-05-17 18:37:36.525794+0800 MemoryManageDemo[57425:5151753] -- block3 = __NSMallocBlock__

关于Block源码解析的博客:
Block技巧与底层解析