NSUserDefaults用於儲存一些app setting。當你關閉app之後再開啟後，之前儲存的setting依然可以從NSUserDefaults裡讀取。注意：密碼最好不要儲存在NSUserDefaults裡，要儲存也必須用AES/DES加密儲存，更安全的方法是 https://github.com/ldandersen/scifihifi-iphone

example 1: 讀取NSUserDefaults裡的"color" setting

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

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

另外NSUserDefaults還提供了一個registerDefaults方法，通常在appDelegate的didFinishLaunchingWithOptions裡呼叫

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

1 當第一次run上面的app時，"color"="blue"會寫入NSUserDefaults裡

2 然後你的app呼叫了

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

3. exit app and restart it，registerDefaults方法還是會被呼叫，但它check到"color" key已經存在，就不會把它寫入NSUserDefaults中

  這時NSUserDefaults裡"color"="red"

4. 修改registerDefaults方法變成

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

5. 修改registerDefaults方法變成

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

  然後重新run app，之後NSUserDefaults裡依然"color"="red"，不過多了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

* 我沒試過NSUserDefaults在exit app之後被儲存，有人說碰到，並給出程式碼呼叫""的解決方案

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