The value of this property is the same for apps that come from the same vendor running on the same device. A different value is returned for apps on the same device that come from different vendors, and for apps on different devices regardless of vendor.

The value of this property may be nil if the app is running in the background, before the user has unlocked the device the first time after the device has been restarted. If the value is nil, wait and get the value again later.

The value in this property remains the same while the app (or another app from the same vendor) is installed on the iOS device. The value changes when the user deletes all of that vendor’s apps from the device and subsequently reinstalls one or more of them. Therefore, if your app stores the value of this property anywhere, you should gracefully handle situations where the identifier changes.

// 查询
OSStatus SecItemCopyMatching(CFDictionaryRef query, CFTypeRef *result);

// 添加
OSStatus SecItemAdd(CFDictionaryRef attributes, CFTypeRef *result);

// 更新KeyChain中的Item
OSStatus SecItemUpdate(CFDictionaryRef query, CFDictionaryRef attributesToUpdate);

// 删除KeyChain中的Item
OSStatus SecItemDelete(CFDictionaryRef query)

　　 NSString *accessGroup = [NSString stringWithUTF8String:"APPIdentifier.com.cnblogs.smileEvday"];
    if (accessGroup != nil)
    {
#if TARGET_IPHONE_SIMULATOR
        // Ignore the access group if running on the iPhone simulator.
        //
        // Apps that are built for the simulator aren‘t signed, so there‘s no keychain access group
        // for the simulator to check. This means that all apps can see all keychain items when run
        // on the simulator.
        //
        // If a SecItem contains an access group attribute, SecItemAdd and SecItemUpdate on the
        // simulator will return -25243 (errSecNoAccessForItem).
#else
        [dictForQuery setObject:accessGroup forKey:(id)kSecAttrAccessGroup];
#endif
    }

+ (BOOL)settUDIDToKeyChain:(NSString*)udid
{
    NSMutableDictionary *dictForAdd = [[NSMutableDictionary alloc] init];
    
    [dictForAdd setValue:(id)kSecClassGenericPassword forKey:(id)kSecClass];
    [dictForAdd setValue:[NSString stringWithUTF8String:kKeychainUDIDItemIdentifier] forKey:kSecAttrDescription];
    
    [dictForAdd setValue:@"UUID" forKey:(id)kSecAttrGeneric];
    
    // Default attributes for keychain item.
    [dictForAdd setObject:@"" forKey:(id)kSecAttrAccount];
    [dictForAdd setObject:@"" forKey:(id)kSecAttrLabel];
    
    // The keychain access group attribute determines if this item can be shared
    // amongst multiple apps whose code signing entitlements contain the same keychain access group.
    NSString *accessGroup = [NSString stringWithUTF8String:kKeyChainUDIDAccessGroup];
    if (accessGroup != nil)
    {
#if TARGET_IPHONE_SIMULATOR
        // Ignore the access group if running on the iPhone simulator.
        //
        // Apps that are built for the simulator aren‘t signed, so there‘s no keychain access group
        // for the simulator to check. This means that all apps can see all keychain items when run
        // on the simulator.
        //
        // If a SecItem contains an access group attribute, SecItemAdd and SecItemUpdate on the
        // simulator will return -25243 (errSecNoAccessForItem).
#else
        [dictForAdd setObject:accessGroup forKey:(id)kSecAttrAccessGroup];
#endif
    }

    const char *udidStr = [udid UTF8String];
    NSData *keyChainItemValue = http://www.mamicode.com/[NSData dataWithBytes:udidStr length:strlen(udidStr)];"Add KeyChain Item Error!!! Error Code:%ld", writeErr);
            
            [dictForAdd release];
            return NO;
        }
        else {
            NSLog(@"Add KeyChain Item Success!!!");
            [dictForAdd release];
            return YES;
        }
    }
    
    [dictForAdd release];
    return NO;
}

+ (NSString*)getUDIDFromKeyChain
{
    NSMutableDictionary *dictForQuery = [[NSMutableDictionary alloc] init];
    [dictForQuery setValue:(id)kSecClassGenericPassword forKey:(id)kSecClass];
    
    // set Attr Description for query
    [dictForQuery setValue:[NSString stringWithUTF8String:kKeychainUDIDItemIdentifier]
                    forKey:kSecAttrDescription];
    
    // set Attr Identity for query
    NSData *keychainItemID = [NSData dataWithBytes:kKeychainUDIDItemIdentifier
                                            length:strlen(kKeychainUDIDItemIdentifier)];
    [dictForQuery setObject:keychainItemID forKey:(id)kSecAttrGeneric];
    
    // The keychain access group attribute determines if this item can be shared
    // amongst multiple apps whose code signing entitlements contain the same keychain access group.
    NSString *accessGroup = [NSString stringWithUTF8String:kKeyChainUDIDAccessGroup];
    if (accessGroup != nil)
    {
#if TARGET_IPHONE_SIMULATOR
        // Ignore the access group if running on the iPhone simulator.
        //
        // Apps that are built for the simulator aren‘t signed, so there‘s no keychain access group
        // for the simulator to check. This means that all apps can see all keychain items when run
        // on the simulator.
        //
        // If a SecItem contains an access group attribute, SecItemAdd and SecItemUpdate on the
        // simulator will return -25243 (errSecNoAccessForItem).
#else
        [dictForQuery setObject:accessGroup forKey:(id)kSecAttrAccessGroup];
#endif
    }
    
    [dictForQuery setValue:(id)kCFBooleanTrue forKey:(id)kSecMatchCaseInsensitive];
    [dictForQuery setValue:(id)kSecMatchLimitOne forKey:(id)kSecMatchLimit];
    [dictForQuery setValue:(id)kCFBooleanTrue forKey:(id)kSecReturnData];
    
    OSStatus queryErr   = noErr;
    NSData   *udidValue = http://www.mamicode.com/nil;"KeyChain Item: %@ not found!!!", [NSString stringWithUTF8String:kKeychainUDIDItemIdentifier]);
    }
    else if (queryErr != errSecSuccess) {
        NSLog(@"KeyChain Item query Error!!! Error code:%ld", queryErr);
    }
    if (queryErr == errSecSuccess) {
        NSLog(@"KeyChain Item: %@", udidValue);
        
        if (udidValue) {
            udid = [NSString stringWithUTF8String:udidValue.bytes];
        }
    }
    
    [dictForQuery release];
    return udid;
}