循環(huán)引用非常常見，我們來分析一下為什么會(huì)循環(huán)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
- 我們看到了WKPerson已經(jīng)被釋放掉了dealloc方法執(zhí)行了
- 接下來我要加一句代碼，就是這句代碼導(dǎo)致循環(huán)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
我們來分析一下 為什么加了NSLog后會(huì)循環(huán)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;
}
};
我們來看看WKPerson類
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;
};
粗略的說 NSObject內(nèi)部
struct objc_class{
Class isa;
Class superclass;
cache_t cache;//方法緩存
class_data_bits_t bits;//用于具體方法信息
}
struct class_rw_t{
uint32_t flags;
const class_ro_t *ro;
property_list_t *properties //屬性列表
}
class_ro_t 屬性列表就放著instance 成員變量列表也就是age屬性
后面我會(huì)專門寫一篇NSObject里面的內(nèi)存 以及isa的指向
接下來我們分析一下 為啥會(huì)循環(huán)引用
1.person->block 調(diào)用了block
2.block內(nèi)部調(diào)用了person.age
3.person isa存著block的內(nèi)存地址,這個(gè)內(nèi)存地址就是person->block
4.所以當(dāng)person作用域結(jié)束了,還有一根強(qiáng)指針指向他，person不能釋放
圖花的比較丑
image.png
如果這時(shí)候加上__weak
- person->block 調(diào)用了block(強(qiáng)指針)
- block內(nèi)部調(diào)用了person.age
- person isa存著block的內(nèi)存地址,這個(gè)內(nèi)存地址就是person->block (weak)弱指針，也就是下面的那根線變成了虛線，所以當(dāng)persion作用域結(jié)束后，就會(huì)調(diào)用dispose函數(shù)，進(jìn)行釋放，釋放后引用計(jì)數(shù)器-1等于0 所以就能夠釋放。
