面试题：Objective-C对象释放的复杂场景处理

Question

假设有一个包含多个子对象的复杂Objective-C对象结构，主对象释放时，需要确保所有子对象都能安全释放，且要处理循环引用情况，描述你的实现思路及关键代码片段。

Answer 1

实现思路

1. 强引用与弱引用：对于非循环引用的子对象，主对象通常使用强引用持有。对于可能产生循环引用的关系，使用弱引用（在ARC环境下）或__unsafe_unretained（在MRC环境下）来打破循环。
2. 手动内存管理（MRC）：在MRC环境下，需要在主对象的dealloc方法中手动释放所有强引用的子对象。
3. 自动引用计数（ARC）：在ARC环境下，系统会自动管理内存，但仍需正确处理循环引用以确保对象能被正确释放。

关键代码片段

ARC环境

假设主对象是MainObject，有两个可能产生循环引用的子对象SubObjectA和SubObjectB。

// MainObject.h
#import <Foundation/Foundation.h>
@class SubObjectA;
@class SubObjectB;

@interface MainObject : NSObject
@property (nonatomic, strong) SubObjectA *subObjectA;
@property (nonatomic, weak) SubObjectB *subObjectB;
@end

// SubObjectA.h
#import <Foundation/Foundation.h>
@class SubObjectB;

@interface SubObjectA : NSObject
@property (nonatomic, weak) SubObjectB *subObjectB;
@end

// SubObjectB.h
#import <Foundation/Foundation.h>
@class SubObjectA;

@interface SubObjectB : NSObject
@property (nonatomic, weak) SubObjectA *subObjectA;
@end

MRC环境

// MainObject.h
#import <Foundation/Foundation.h>
@class SubObjectA;
@class SubObjectB;

@interface MainObject : NSObject
@property (nonatomic, retain) SubObjectA *subObjectA;
@property (nonatomic, unsafe_unretained) SubObjectB *subObjectB;
- (void)dealloc;
@end

// MainObject.m
#import "MainObject.h"
#import "SubObjectA.h"
#import "SubObjectB.h"

@implementation MainObject
- (void)dealloc {
    [_subObjectA release];
    [super dealloc];
}
@end

// SubObjectA.h
#import <Foundation/Foundation.h>
@class SubObjectB;

@interface SubObjectA : NSObject
@property (nonatomic, unsafe_unretained) SubObjectB *subObjectB;
- (void)dealloc;
@end

// SubObjectA.m
#import "SubObjectA.h"
#import "SubObjectB.h"

@implementation SubObjectA
- (void)dealloc {
    [super dealloc];
}
@end

// SubObjectB.h
#import <Foundation/Foundation.h>
@class SubObjectA;

@interface SubObjectB : NSObject
@property (nonatomic, unsafe_unretained) SubObjectA *subObjectA;
- (void)dealloc;
@end

// SubObjectB.m
#import "SubObjectB.h"
#import "SubObjectA.h"

@implementation SubObjectB
- (void)dealloc {
    [super dealloc];
}
@end