NSUserDefaults, do something on first load only

问题:

I'm trying to do show an alert the first time the app is launched. I'd also like to show this alert if they go into a settings page later on and want to see it again. I figured I could set a boolean in the NSUserDefaults. I'm not quite sure how to do that though (assuming this approach is right). I thought in applicationDidFinishLaunching:options, I could get the value for key @"FirstTimeLaunched", and in applicationWillTerminate, I could set the object for that key to NO. If this is the case, how do I get the BOOL to be YES for the first time?

Also, do I need to set it to NO in didEnterBackground mode as well? Thanks.

回答1:

Just think it backwards, instead of expecting a YES, set a variable when the app is actually launched, and remove it when the user says so in the settings.

At launch:

if (![[NSUserDefaults standardUserDefaults] boolForKey:@"hasBeenLaunched"]) {
    // show your only-one-time view
    [[NSUserDefaults standardUserDefaults] setBool:YES forKey:@"hasBeenLaunched"];
    [[NSUserDefaults standardUserDefaults] synchronize];
}

And when the user wants to see it again, just remove the key:

[[NSUserDefaults standardUserDefaults] removeObjectForKey:@"hasBeenLaunched"];

When accessing a key that does not exists, NSUserDefaults will return nil.

回答2:

I would take the opposite approach and first launch check for a key called appHasLaunchedPreviously.

If it's yes then bypass the alert. If it's not YES then show the alert and immediately set it ready for next time.

回答3:

First time App is launched, so I don't think you will ever need to set that BOOL to NO. UserDefaults gets deleted if App is removed. and we set it when App is first launched application didFinishLaunchingWithOptions:

[[NSUserDefaults standardUserDefaults] setBool:YES forKey:@"FirstTimeLaunched"];
[[NSUserDefaults standardUserDefaults] synchronize];

If you set Bool to NO in applicationWillTerminate, then everytime user kills app from tray you will show alert on app launched. Again it depends on your business requirements.