NSObject的hash方法

雨点打透心脏的1/2处 2022-03-27 12:53 266阅读 0赞

NSObject的hash方法

![607542-20160415220445441-1616011676.gif][]

说明

本示例仅仅演示一个对象什么时候执行hash方法。

细节

1. 必要的Model类，重载了hash方法用以反映Hash方法是否被调用了

![607542-20160415220713066-1078426601.png][]

![607542-20160415220722863-102049627.png][]

2. 测试

//
    //  ViewController.m
    //  Hash
    //
    //  Created by YouXianMing on 16/4/15.
    //  Copyright © 2016年 YouXianMing. All rights reserved.
    //
    
    #import "ViewController.h"
    #import "Model.h"
    
    @interface ViewController ()
    
    @end
    
    @implementation ViewController
    
    - (void)viewDidLoad {
        
        [super viewDidLoad];
        
        Model *model = [Model new];
        
        [model hash];
        model = nil;
    }
    
    @end

![607542-20160415221018207-766126229.gif][]

3. 测试 isEqual: 方法执行的时候是否会执行 hash 方法，打印情况里面是没有的

//
    //  ViewController.m
    //  Hash
    //
    //  Created by YouXianMing on 16/4/15.
    //  Copyright © 2016年 YouXianMing. All rights reserved.
    //
    
    #import "ViewController.h"
    #import "Model.h"
    
    @interface ViewController ()
    
    @end
    
    @implementation ViewController
    
    - (void)viewDidLoad {
        
        [super viewDidLoad];
        
        Model *modelA = [Model new];
        Model *modelB = [Model new];
        
        if ([modelA isEqual:modelB]) {
            
            NSLog(@"YES");
            
        } else {
        
            NSLog(@"NO");
        }
    }
    
    @end

![607542-20160415221610457-1641468858.gif][]

4. 用 NSMutableSet 添加对象，这时候会执行hash方法，至于为何会执行2回 \_(:з」∠)\_ ？

//
    //  ViewController.m
    //  Hash
    //
    //  Created by YouXianMing on 16/4/15.
    //  Copyright © 2016年 YouXianMing. All rights reserved.
    //
    
    #import "ViewController.h"
    #import "Model.h"
    
    @interface ViewController ()
    
    @end
    
    @implementation ViewController
    
    - (void)viewDidLoad {
        
        [super viewDidLoad];
        
        Model        *model = [Model new];
        NSMutableSet *set   = [NSMutableSet set];
        
        [set addObject:model];
    }
    
    @end

![607542-20160415221928035-407468273.gif][]

5. 用 NSMutableArray 添加对象测试一下，发现不会执行 hash 方法

//
    //  ViewController.m
    //  Hash
    //
    //  Created by YouXianMing on 16/4/15.
    //  Copyright © 2016年 YouXianMing. All rights reserved.
    //
    
    #import "ViewController.h"
    #import "Model.h"
    
    @interface ViewController ()
    
    @end
    
    @implementation ViewController
    
    - (void)viewDidLoad {
        
        [super viewDidLoad];
        
        Model          *model = [Model new];
        NSMutableArray *array = [NSMutableArray array];
        
        [array addObject:model];
    }
    
    @end

![607542-20160415222117395-2019442176.gif][]

6. 用作 NSMutableDictionary 中的 object 时，hash 方法不会执行

//
    //  ViewController.m
    //  Hash
    //
    //  Created by YouXianMing on 16/4/15.
    //  Copyright © 2016年 YouXianMing. All rights reserved.
    //
    
    #import "ViewController.h"
    #import "Model.h"
    
    @interface ViewController ()
    
    @end
    
    @implementation ViewController
    
    - (void)viewDidLoad {
        
        [super viewDidLoad];
        
        Model               *model      = [Model new];
        NSMutableDictionary *dictionary = [NSMutableDictionary dictionary];
        
        [dictionary setObject:model forKey:@"A"];
        [dictionary objectForKey:@"A"];
    }
    
    @end

![607542-20160415223053410-1706657666.gif][]

7. 用作 NSMutableDictionary 中的 key 时，hash 方法执行了，不过崩溃了，因为 Model 类没有实现 NSCopying 协议

//
    //  ViewController.m
    //  Hash
    //
    //  Created by YouXianMing on 16/4/15.
    //  Copyright © 2016年 YouXianMing. All rights reserved.
    //
    
    #import "ViewController.h"
    #import "Model.h"
    
    @interface ViewController ()
    
    @end
    
    @implementation ViewController
    
    - (void)viewDidLoad {
        
        [super viewDidLoad];
        
        Model               *model      = [Model new];
        NSMutableDictionary *dictionary = [NSMutableDictionary dictionary];
        
        [dictionary setObject:@"A" forKey:model];
    }
    
    @end

![607542-20160415223313801-2124391133.gif][]

8. NSSet 在初始化的时候添加了 model 并不会让 model 执行 hash 方法

//
    //  ViewController.m
    //  Hash
    //
    //  Created by YouXianMing on 16/4/15.
    //  Copyright © 2016年 YouXianMing. All rights reserved.
    //
    
    #import "ViewController.h"
    #import "Model.h"
    
    @interface ViewController ()
    
    @end
    
    @implementation ViewController
    
    - (void)viewDidLoad {
        
        [super viewDidLoad];
        
        Model *model = [Model new];
        
        NSSet *set = [NSSet setWithObjects:model, nil];
        
        if ([[set anyObject] isEqual:model]) {
            
            NSLog(@"A");
        }
        
        set = nil;
    }
    
    @end

![607542-20160415223922129-1369708721.gif][]

9. 在创建不可变数组时，model 作为 key 会执行 hash 方法，但同样会崩溃，因为 Model 类没有实现 NSCopying 协议

//
    //  ViewController.m
    //  Hash
    //
    //  Created by YouXianMing on 16/4/15.
    //  Copyright © 2016年 YouXianMing. All rights reserved.
    //
    
    #import "ViewController.h"
    #import "Model.h"
    
    @interface ViewController ()
    
    @end
    
    @implementation ViewController
    
    - (void)viewDidLoad {
        
        [super viewDidLoad];
        
        Model        *model      = [Model new];
        NSDictionary *dictionary = @{model : @"A"};
        dictionary = nil;
    }
    
    @end

![607542-20160415224342707-1424457526.gif][]

总结

一个对象在用作key值时，其 hash 方法会被调用，用以生成一个唯一标识符，NSDictionary 需要根据唯一 key 值（根据 hash 算法生成的值）查找对象， NSSet 需要根据 hash 值来确保过滤掉重复的对象。

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[607542-20160415221928035-407468273.gif]: /images/20220327/6c1939c5067842d487fdc90d20a23d44.png
[607542-20160415222117395-2019442176.gif]: /images/20220327/7b60474b88e543bb9141a55c9a4ca751.png
[607542-20160415223053410-1706657666.gif]: /images/20220327/423297d7a661470397b252d6ffe22d54.png
[607542-20160415223313801-2124391133.gif]: /images/20220327/7e53c8bda71e4ea8af456e70380693d0.png
[607542-20160415223922129-1369708721.gif]: /images/20220327/81038f26dd4e4b8081e7efc914e6bbb2.png
[607542-20160415224342707-1424457526.gif]: /images/20220327/45b32dab029147ae819a9dda3b051fa0.png