使用KeychainItemWrapper保存到iOS密钥链时出错

Question

每次我尝试将数据存储到keychain中时，都会得到一个错误

错误来自以下行的dictionaryToSecItemFormat方法

NSString *passwordString = [dictionaryToConvert objectForKey:(id)kSecValueData];
[returnDictionary setObject:[passwordString dataUsingEncoding:NSUTF8StringEncoding] forKey:(id)kSecValueData]'

错误是

我像这样调用KeychainItemWrapper方法

KeychainItemWrapper *keychain = [[KeychainItemWrapper alloc] initWithIdentifier:@"credentials" accessGroup:nil];
[keychain setObject:username forKey:kSecAttrAccount];
[keychain release];

错误是

 *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 
  '-[NSConcreteData dataUsingEncoding:]: unrecognized selector sent to instance"

Answer 1

dataUsingEncoding是来自NSString类的方法，将username设置为NSString。