NSObject的hash方法

NSObject的hash方法

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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

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

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

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

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

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

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

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

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

总结

一个对象在用作key值时，其 hash 方**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。

原标题：NSObject的hash方法

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