iOS: NSUserDefaults issues

NSUserDefaults用于存储一些app setting。当你关闭app之后再打开后，之前存储的setting依然可以从NSUserDefaults里读取。注意：密码最好不要存储在NSUserDefaults里，要存储也必须用AES/DES加密存储，更安全的方法是 https://github.com/ldandersen/scifihifi-iphone

把密码存储在NSUserDefault or plist里的后果： http://www.shushao.com/news/info/item/203510-ios

example 1: 读取NSUserDefaults里的"color" setting

 [[NSUserDefaults standardUserDefaults] stringForKey:@"color"];

example 2: 写入NSUserDefaults里的"color" setting为"red"

 [[NSUserDefaults standardUserDefaults] setObject:@"red" forKey:@"color"];

另外NSUserDefaults还提供了一个registerDefaults方法，通常在appDelegate的didFinishLaunchingWithOptions里调用

 - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
 {
     NSDictionary *defaultValues = [NSDictionary dictionaryWithObjectsAndKeys: @"blue", @"color",nil];
     [[NSUserDefaults standardUserDefaults] registerDefaults:defaultValues];

     return YES;
 }

registerDefaults 方法调用时会check NSUserDefaults里是否已经存在了相同的key，如果有则不会把其覆盖。举个例子：

1 当第一次run上面的app时，"color"="blue"会写入NSUserDefaults里

2 然后你的app调用了

 [[NSUserDefaults standardUserDefaults] setObject:@"red" forKey:@"color"];

  这时NSUserDefaults里"color"="red"

3. exit app and restart it，registerDefaults方法还是会被调用，但它check到"color" key已经存在，就不会把它写入NSUserDefaults中

  这时NSUserDefaults里"color"="red"

4. 修改registerDefaults方法变成

 NSDictionary *defaultValues = [NSDictionary dictionaryWithObjectsAndKeys: @"black", @"color",nil];
 [[NSUserDefaults standardUserDefaults] registerDefaults:defaultValues];

  然后重新run app，之后NSUserDefaults里依然"color"="red"

5. 修改registerDefaults方法变成

 NSDictionary *defaultValues = [NSDictionary dictionaryWithObjectsAndKeys: @"black", @"color",@"tomson", @"username",nil];
     [[NSUserDefaults standardUserDefaults] registerDefaults:defaultValues];

  然后重新run app，之后NSUserDefaults里依然"color"="red"，不过多了a pair of key-value:  username=tomson

注意：

* If you terminate your app by pressing "Stop" in Xcode (in the Simulator or on the device), your User Defaults might get saved

* 我没试过NSUserDefaults在exit app之后被保存，有人说碰到，并给出代码调用""的解决方案

ref links:

http://stackoverflow.com/questions/6033385/nsuserdefaults-not-being-saved-in-simulator

http://stackoverflow.com/questions/2622754/why-is-nsuserdefaults-not-saving-my-values