iOS设置拨动开关第一次起作用，但不再起作用

Question

I am using the iPad settings app to change some button sounds and a background image. It all works well and the settings are maintained from one app launch to another in the simulator. Now I have implemented a toggle switch to either set sets of sounds off or on. When the app launches, whatever state the switch is in, it works; eg if the "Alert Sounds" switch is OFF the alert sounds are silent and if I change it to ON the sounds will start working. However, if I turn the switch back OFF the sounds still keep working. However, if the state is ON when the app launches, the sounds work, but will not be silenced when the switch is set to OFF.

Note that this is different than the settings not taking effect until a second round of settings. That was a previous problem I solved (thanks to stack overflow) by using:

- (void)applicationDidBecomeActive:(UIApplication *)application
{
[[NSUserDefaults standardUserDefaults] synchronize];
}

I have methods named:

- (void)defaultsChanged:(NSNotification *)NSUserDefaultsDidChangeNotification

(which is called when the notification is sent)

and

-(void)setValuesFromPreferences

(which is called in ViewDidLoad)

The logic looks like this in both:

// Set alert sounds from preferences
NSString *alertSoundPreference = [userDefaults stringForKey:kAlertSound];

BOOL alertSoundEnabled = [userDefaults boolForKey:kAlertSoundEnabled];

if (alertSoundEnabled) 
{
// Create the URLs for the alert audio files
// Store the alert sound URLs as a CFURLRef instances
// Create system sound objects representing the alert sound files
    }

I do not have an else, because I assume that no sound resources will be specified if alertSoundEnabled is NO.

I have searched for explanations and tutorials that mention this problem but have not found any yet, so I'm asking here. Thanks for any suggestions.

Answer 1

viewDidLoad is not necessarily called when the app becomes active again (nor does viewWill/DidAppear , IIRC), as the whole point of iOS 4+ multitasking is to prevent such loading/unloading and recreation of objects on app-switching.

If I had to guess, the sounds are already allocated when the user had the switch ON at original launch/ viewDidLoad ; however, if your code does nothing to explicitly disassociate them when it loads back up, they would continue playing, as they are all already set up.

As such, I'd try adding an else clause that (upon alertSoundEnabled == NO ) destroys your system sound objects.