iOS 4 makes it easy to leave apps without quitting, but what happens when you actually do want to terminate an application? Quitting applications can really help with older apps that have not been updated for iOS 4 "multitasking" and that do not properly return to the state they previously were in when resumed.
The most common type of app that fits this profile supplies Bonjour services that might not properly continue after resuming the application. Another app that reportedly has this issue is Gmail. TUAW reader Kefz writes, "Gmail does not work unless you force quit the mail application. If it is sitting in the multitasking task bar, [notifications aren't] automatically pushed to the phone...You'd have to open the mail app and have it check for new mail."
There's are two ways you can quit, according to Apple's documentation, although neither one is particularly obvious. Read on to learn what they are.
Force Quitting To force an application to quit, Apple recommends that you bring the app you want to quit to the foreground. Then press and hold the sleep/wake button for several seconds, until the Slide to Power Off control appears. Release the sleep/wake button and hold down the Home button for another 7-10 seconds. Your screen will flash briefly and you will return to the main iOS 4 Springboard home page with its icons. This method works for all operating systems from iPhone OS 3 forward, and is the preferred method listed in the iOS 4 documentation.
Removing the Application from the Recents List There's actually a much simpler approach for quitting apps, and that's to use your recent app list. Double-click the home button to display the recently accessed applications. Press and hold any of the icons shown, then navigate to the application you want to quit and tap the red circled minus button. This sends a signal to the application in question that allows it to quit. The application will be re-added to the recents list the next time you launch it.
If you continue to experience any reliability issues or if your iPhone simply isn't working right, you can always restart your phone by pressing the Sleep/Wake button for a few seconds and then sliding across to shut the phone down.