由于蘋果最近有了一些新動作，所以keychain這個功能可能沒有辦法做到iOS刪除App也無法刪除的數據，具體的內容可以看我最新的文章iOS 10.3 keychain 功能可能出現重大更改。

看標題是不是聽起來挺唬人的，沒錯就是這么唬人，之前我沒有發現iOS有這個功能，但是裝了幾個百度的應用之后我發現：卸載百度的應用之后，過段時間再次安裝的時候你就會發現百度賬號自動登錄了，沒錯你可以去試試。看到這個現象之后我就尋求實現方法。下面貼出來：
iOS的keychain服務提供了一種安全的保存私密信息（密碼，序列號，證書等）的方式。每個ios程序都有一個獨立的keychain存儲。從ios 3.0開始，跨程序分享keychain變得可行。
下面就使用keychain來實現存取用戶名和密碼。
蘋果已經有現成的類封裝好了keychain,KeychainItemWrapper.h和KeychainItemWrapper.m文件,可以在GenericKeychain實例里找到。
但是這里我只需要存取用戶名和密碼，就不用蘋果提供的類了，自己寫個簡單的類來實現就好。
代碼如下：
HPKeychain.h

#import <Foundation/Foundation.h>

@interface HPKeychain : NSObject

+ (void)save:(NSString *)service data:(id)data;
+ (id)load:(NSString *)service;
+ (void)delete:(NSString *)service;

@end

HPKeychain.m

#import "HPKeychain.h"

@implementation HPKeychain


+ (NSMutableDictionary *)getKeychainQuery:(NSString *)service {
    return [NSMutableDictionary dictionaryWithObjectsAndKeys:
            (id)kSecClassGenericPassword,(id)kSecClass,
            service, (id)kSecAttrService,
            service, (id)kSecAttrAccount,
            (id)kSecAttrAccessibleAfterFirstUnlock,(id)kSecAttrAccessible,
            nil];
}

+ (void)save:(NSString *)service data:(id)data {
    //Get search dictionary
    NSMutableDictionary *keychainQuery = [self getKeychainQuery:service];
    //Delete old item before add new item
    SecItemDelete((CFDictionaryRef)keychainQuery);
    //Add new object to search dictionary(Attention:the data format)
    [keychainQuery setObject:[NSKeyedArchiver archivedDataWithRootObject:data] forKey:(id)kSecValueData];
    //Add item to keychain with the search dictionary
    SecItemAdd((CFDictionaryRef)keychainQuery, NULL);
}

+ (id)load:(NSString *)service {
    id ret = nil;
    NSMutableDictionary *keychainQuery = [self getKeychainQuery:service];
    //Configure the search setting
    //Since in our simple case we are expecting only a single attribute to be returned (the password) we can set the attribute kSecReturnData to kCFBooleanTrue
    [keychainQuery setObject:(id)kCFBooleanTrue forKey:(id)kSecReturnData];
    [keychainQuery setObject:(id)kSecMatchLimitOne forKey:(id)kSecMatchLimit];
    CFDataRef keyData = NULL;
    if (SecItemCopyMatching((CFDictionaryRef)keychainQuery, (CFTypeRef *)&keyData) == noErr) {
        @try {
            ret = [NSKeyedUnarchiver unarchiveObjectWithData:(__bridge NSData *)keyData];
        } @catch (NSException *e) {
            NSLog(@"Unarchive of %@ failed: %@", service, e);
        } @finally {
        }
    }
    if (keyData)
        CFRelease(keyData);
    return ret;
}

+ (void)delete:(NSString *)service {
    NSMutableDictionary *keychainQuery = [self getKeychainQuery:service];
    SecItemDelete((CFDictionaryRef)keychainQuery);
}




@end

使用方法

#import "ViewController.h"
#import "HPKeychain.h"


NSString * const KEY_USERNAME_PASSWORD = @"com.company.app.usernamepassword";
NSString * const KEY_USERNAME = @"com.company.app.username";
NSString * const KEY_PASSWORD = @"com.company.app.password";


@interface ViewController ()
@property (weak, nonatomic) IBOutlet UITextField *nameTextfield;
@property (weak, nonatomic) IBOutlet UITextField *passwordTextField;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    //從keychain中取出用戶名和密碼：
    NSMutableDictionary *usernamepasswordKVPairs = (NSMutableDictionary *)[HPKeychain load:KEY_USERNAME_PASSWORD];
    _nameTextfield.text = [usernamepasswordKVPairs objectForKey:KEY_USERNAME];
    _passwordTextField.text = [usernamepasswordKVPairs objectForKey:KEY_PASSWORD];
}
- (IBAction)saveAction:(UIButton *)sender {
    //把用戶名和密碼存入keychain：
    NSMutableDictionary *usernamepasswordKVPairs = [NSMutableDictionary dictionary];
    [usernamepasswordKVPairs setObject:_nameTextfield.text forKey:KEY_USERNAME];
    [usernamepasswordKVPairs setObject:_passwordTextField.text forKey:KEY_PASSWORD];
    [HPKeychain save:KEY_USERNAME_PASSWORD data:usernamepasswordKVPairs];
    
    
    UIAlertController * alert = [UIAlertController alertControllerWithTitle:@"提示" message:@"保存成功" preferredStyle:UIAlertControllerStyleAlert];
    UIAlertAction * action = [UIAlertAction actionWithTitle:@"我知道了" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
        NSLog(@"點擊了確認");
    }];
    [alert addAction:action];
    [self presentViewController:alert animated:YES completion:nil];
}


- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

刪除一個keychain item:
[HPKeychain delete:KEY_USERNAME_PASSWORD];

這里可以查看代碼