NSObject的hash方法，NSObjecthash方法

NSObject的hash方法，NSObjecthash方法

NSObject的hash方法

 

說明

本樣本僅僅示範一個對象什麼時候執行hash方法。

 

細節

1. 必要的Model類，重載了hash方法用以反映Hash方法是否被調用了

2. 測試

////  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. 測試 isEqual: 方法執行的時候是否會執行 hash 方法，列印情況裡面是沒有的

////  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 添加對象，這時候會執行hash方法，至於為何會執行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 添加對象測試一下，發現不會執行 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 時，hash 方法不會執行

////  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 時，hash 方法執行了，不過崩潰了，因為 Model 類沒有實現 NSCopying 協議

////  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 在初始化的時候添加了 model 並不會讓 model 執行 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. 在建立不可變數組時，model 作為 key 會執行 hash 方法，但同樣會崩潰，因為 Model 類沒有實現 NSCopying 協議

////  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

 

總結

一個對象在用作key值時，其 hash 方法會被調用，用以產生一個唯一識別碼，NSDictionary 需要根據唯一 key 值（根據 hash 演算法產生的值）尋找對象， NSSet 需要根據 hash 值來確保過濾掉重複的對象。