循环引用非常常见，我们来分析一下为什么会循环引用
#import <Foundation/Foundation.h>
#import "WKPerson.h"
typedef void (^WKBlock)(void);
int main(int argc, const char * argv[]) {
@autoreleasepool {
WKPerson *person = [[WKPerson alloc] init];
person.age = 20;
person.block = ^{
};
}
NSLog(@"---------");
return 0;
}
//WKPerson.h
#import <Foundation/Foundation.h>
typedef void (^WKBlock) (void);
@interface WKPerson : NSObject
@property (copy, nonatomic) WKBlock block;
@property (assign, nonatomic) int age;
@end
//WKPerson.m
#import "WKPerson.h"
@implementation WKPerson
- (void)dealloc
{
// [super dealloc];
NSLog(@"%s", __func__);
}
@end
2018-08-21 10:48:02.483714+0800 mhhhh[16581:274772] -[WKPerson dealloc]
2018-08-21 10:48:02.484591+0800 mhhhh[16581:274772] ---------
Program ended with exit code: 0
- 我们看到了WKPerson已经被释放掉了dealloc方法执行了
- 接下来我要加一句代码，就是这句代码导致循环引用
#import <Foundation/Foundation.h>
#import "WKPerson.h"
typedef void (^WKBlock)(void);
int main(int argc, const char * argv[]) {
@autoreleasepool {
WKPerson *person = [[WKPerson alloc] init];
person.age = 20;
person.block = ^{
NSLog(@"%d",person.age);
};
}
NSLog(@"---------");
return 0;
}
2018-08-21 10:52:00.748869+0800 mhhhh[16622:281567] ---------
Program ended with exit code: 0
我们来分析一下 为什么加了NSLog后会循环引用
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m
//main.m
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
WKPerson *person;
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, WKPerson *_person, int flags=0) : person(_person) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
我们来看看WKPerson类
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc WKPerson.m
struct WKPerson_IMPL {
struct NSObject_IMPL NSObject_IVARS;
int _age;
WKBlock _block;
};
struct NSObject_IMPL {
Class isa;
};
粗略的说 NSObject内部
struct objc_class{
Class isa;
Class superclass;
cache_t cache;//方法缓存
class_data_bits_t bits;//用于具体方法信息
}
struct class_rw_t{
uint32_t flags;
const class_ro_t *ro;
property_list_t *properties //属性列表
}
class_ro_t 属性列表就放着instance 成员变量列表也就是age属性
后面我会专门写一篇NSObject里面的内存 以及isa的指向
接下来我们分析一下 为啥会循环引用
1.person->block 调用了block
2.block内部调用了person.age
3.person isa存着block的内存地址,这个内存地址就是person->block
4.所以当person作用域结束了,还有一根强指针指向他，person不能释放
图花的比较丑
image.png
如果这时候加上__weak
- person->block 调用了block(强指针)
- block内部调用了person.age
- person isa存着block的内存地址,这个内存地址就是person->block (weak)弱指针，也就是下面的那根线变成了虚线，所以当persion作用域结束后，就会调用dispose函数，进行释放，释放后引用计数器-1等于0 所以就能够释放。
