Detect when app is opened for the first time after launch

I want to display a help message on a view controller when the app is installed and opened for the very first time ONLY.

Is开发者_StackOverflow there a method I can use to do this?

You can display the help message once, and then store a boolean value in NSUserDefaults to indicate that it should not be shown again:

NSUserDefaults * userDefaults = [NSUserDefaults standardUserDefaults];
BOOL appHasBeenLaunchedBefore = [userDefaults boolForKey:@"HasBeenLaunched"];
if (!appHasBeenLaunchedBefore)
{
    [self showHelpMessage];
}
[userDefaults setBool:YES forKey:"HasBeenLaunched"];

Use Grand Central Dispatch's dispatch_once() and check some persistent storage.

static dispatch_once_t pred;
dispatch_once(&pred,^{
    NSUserDefaults * userDefaults = [NSUserDefaults standardUserDefaults];
    BOOL hasLaunched = [userDefaults boolForKey:kAppHasLaunched];
    if (!hasLaunched) {
        [self showFirstLaunchMessage];
        [userDefaults setBool:YES forKey:kAppHasLaunched];
    }
});

This is the easiest way to ensure code only gets run once in your app per launch (in this case the block will only be executed once). More information about this is in the Grand Central Dispatch Reference. In this block you simply need to check some persistent storage, such as your preferences, to see if your app has ever launched before and display a message as appropriate.

Use a key in the user defaults. For example:

NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
BOOL launchedBefore = [userDefaults boolForKey:@"hasRunBefore"];

if(!hasRunBefore)
{
    NSLog(@"this is my first run");
    [userDefaults setBool:YES forKey:@"hasRunBefore"];
}

The user defaults are backed up by iTunes, so it'll normally be the user's first launch rather than the first launch per device.

EDIT: this is explicitly the same answer as e.James gave before me. The 'other answers have been posted' bar failed to appear. I'll leave it up for the example code but don't deserve credit fir a first answer.