NSObject的hash方法
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。