iOS 底层 day09 block 捕获OC对象

一、block 捕获 OC对象

1. 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，比如有以下四种情况

• block 作为函数返回值时
• 将 block 赋值给 __strong 指针时
• block 作为 Cocoa API 中方法名含有 usingBlock 的方法参数时
• block 作为 GCD API 的方法参数时

2. 观察下面代码，person 在断点 1 和断点 2 位置被释放了吗？

#import <Foundation/Foundation.h>
#import "Person.h"

typedef void (^SPBlock)(void);

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        SPBlock block1;
        {
            Person *person1 = [[Person alloc] init];
            NSLog(@"person1,%p",person1);
            block1 = ^{
                NSLog(@"hello,%p",person1);
            };
        }
        {
            Person *person2 = [[Person alloc] init];
            NSLog(@"person2,%p",person2);
            ^{
                NSLog(@"hello,%p",person2);
            };
        }
        NSLog(@"断点1");
    }
    NSLog(@"断点2");
    return 0;
}

• 日志情况如下

Demo[3570:263110] person1,0x1006acd90
Demo[3570:263110] person2,0x1006acc50
Demo[3570:263110] Person - dealloc<Person: 0x1006acc50>
Demo[3570:263110] 断点1
Demo[3570:263110] Person - dealloc<Person: 0x1006acd90>
Demo[3570:263110] 断点2

• 我们可以发现，断点1 处 person2 已经被释放了，而person1 活到了断点 2 位置
• 这是为什么呢?
• 这两处代码，不同之处在于block1有强引用，所以 block1会在 ARC 下会进行一次 copy 操作，然后从栈 block copy成 堆 block，在 copy 的时候会强引用 person1
• 而另一个 block 没有强引用，一直是栈 block，作为栈 block，自己都随时不保，更不能保住 person2

3. 观察下面代码，Person对象在断点 1 和断点 2 位置被释放了吗？

#import <Foundation/Foundation.h>
#import "Person.h"
typedef void (^SPBlock)(void);
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        SPBlock block1;
        SPBlock block2;
        {
            Person *person1 = [[Person alloc] init];
            NSLog(@"person1:%p",person1);
            block1 = ^{
                NSLog(@"hello,%p",person1);
            };
        }
        
        {
            Person *person2 = [[Person alloc] init];
            NSLog(@"person2:%p",person2);
            __weak Person *weakPerson = person2;
            block2 = ^{
                NSLog(@"hello,%p",weakPerson);
            };
        }
         NSLog(@"断点1");
    }
    NSLog(@"断点2");
    return 0;
}

• 打印日志如下

Demo[4124:285733] person1:0x1005acf10
Demo[4124:285733] person2:0x1005b9c50
Demo[4124:285733] Person - dealloc<Person: 0x1005b9c50>
Demo[4124:285733] 断点1
Demo[4124:285733] Person - dealloc<Person: 0x1005acf10>
Demo[4124:285733] 断点2

4. 为了观察例子3 的本质，我们使用如下指令，将上述代码转换成 C++代码

• 指令 xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc -fobjc-arc -fobjc-runtime=ios-8.0.0 main.m
• 然后我们主要观察如下代码

// block1 对应结构体
struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  Person *__strong person1;
};
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};

static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->person1, (void*)src->person1, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->person1, 3/*BLOCK_FIELD_IS_OBJECT*/);}

// block2 对应结构体
struct __main_block_impl_1 {
  struct __block_impl impl;
  struct __main_block_desc_1* Desc;
  Person *__weak weakPerson;
};
static struct __main_block_desc_1 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __main_block_impl_1*, struct __main_block_impl_1*);
  void (*dispose)(struct __main_block_impl_1*);
}
static void __main_block_copy_1(struct __main_block_impl_1*dst, struct __main_block_impl_1*src) {_Block_object_assign((void*)&dst->weakPerson, (void*)src->weakPerson, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static void __main_block_dispose_1(struct __main_block_impl_1*src) {_Block_object_dispose((void*)src->weakPerson, 3/*BLOCK_FIELD_IS_OBJECT*/);}

• 我们可以发现如下内容
• ①person1 在外部是 __strong ，生成结构体内部也是 __strong
• ②person2 在外部是 __weak ，生成的结构体内部也是 __weak
• ③相比上一小节，__main_block_desc 多了 __main_block_copy 和 __main_block_dispose两个函数
• ④当 block 被拷贝到堆上时，内部会自动调用 __main_block_copy
• ⑤当 block 离开作用域被释放时，内部会自动调用 __main_block_dispose
• ⑥_Block_object_assign函数会根据 auto变量的修饰符(__strong、__weak、 _unsafe_unretained)做出相应的操作，形成强引用、弱引用
• ⑦ block 内部就是通过上述的内容，来维持 ARC 引用计数器的平衡

5. 通过练习巩固上面的知识，请问下面的 Person对象多少秒后释放？

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    Person *person1 = [[Person alloc] init];
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"person1:%p", person1);
    });
    
    Person *person2 = [[Person alloc] init];
    __weak Person *weakPerson2 = person2;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"weakPerson2:%p", weakPerson2);
    });
    
    
    Person *person3 = [[Person alloc] init];
    __weak Person *weakPerson3 = person3;
    
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"weakPerson3:%p", weakPerson3);
        
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"person3:%p", person3);
        });
    });
    
    NSLog(@"touchesBegan");
}

• 输出日志如下

14:58:57.708776 Demo[4756:326199] touchesBegan
14:58:57.709004 Demo[4756:326199] Person - dealloc:<Person: 0x6000020ec2d0>
14:58:58.708922 Demo[4756:326199] person1:0x6000020ec2c0
14:58:58.709254 Demo[4756:326199] Person - dealloc:<Person: 0x6000020ec2c0>
14:58:59.859905 Demo[4756:326199] weakPerson2:0x0
14:58:59.860169 Demo[4756:326199] weakPerson3:0x6000020ec2e0
14:59:00.959406 Demo[4756:326199] person3:0x6000020ec2e0
14:59:00.959724 Demo[4756:326199] Person - dealloc:<Person: 0x6000020ec2e0>

• person1 经过 1s 被释放
• person2 点击完，立刻被释放
• person3 经过 3s 被释放