Does pressing Back always cause Activity to finish()?

Question

I've heard that pressing the back button will essentially cause the current Activity to finish(). Is this always the case? Seems like it would be with the way it pops the Activity off the stack.

The one situation I'm not so sure about is when the root Activity in a Task has back pressed. I'm currently experiencing a very weird effect, described as follows:

On loading my application, the first Activity is for initialization, and once it finishes, it calls my main Activity (a TabActivity). This first init activity has android:noHistory="true" set in the Manifest so pressing Back from my main Activity won't go back to that. It goes to the Launcher. When I click on my App in the Launcher a second time, the initialization activity loads again, and loads the main Activity when done. Almost immediately after, it loads a second instance of my main Activity. But ONLY after the Application has already been run once, and was exited by pressing BACK from the main Activity. It does it every subsequent time until I force quit the app or load a new version from the IDE.

Based on this, I am suspecting some kind of Activity instance is lying around and being reused, since it only happens on the second+ time I run the application (and exit with BACK -- using HOME just returns to the last state of the app, no big deal). Anyone have any thoughts??

Answer 1

I've heard that pressing the back button will essentially cause the current Activity to finish(). Is this always the case?

No it is not. The most activities have this behaviour but not all. For example you could create a Dialog and set it setCancelable(false) and it won't close if you click BACK button.

Furthermore you could customize activity behaviour on BACK button pressed by overriding onBackPressed

Called when the activity has detected the user's press of the back key. The default implementation simply finishes the current activity, but you can override this to do whatever you want.

About your application behaviour..Did you verify if the activity launcher is finished after it loads your main activity? I mean if the onDestroy() method is called. Maybe after it runs the main activity it remains there and when you click back you just go back to the old Launcher...

hope this helps..

Answer 2

Read through the Activity and Task design guidelines on the Android developer site; they explain how the Home and Back buttons work. Obviously, if you override the default behavior (as mentioned by hara above), the back button will not finish the activity.

On your specific issue, check your logcat. You should be able to see there whether it is bringing an old process back to life or starting up a new one. If that is unclear, insert a couple of log statements into onCreate, onPause, onDestroyed, etc., so that you can see exactly what is happening with your process.

Answer 3

Are you running your activities with any special flags, such as singleInstance or singleTop? Those could be causing the oddities you're seeing. The easiest way to track down what's causing your problem is to absolutely fill it with debugging messages. For example:

1. In your initialisation activity, add a log in the beginning of onCreate to get the name of the activity such as this.toString(). More on why you want this line later.
2. When it launches the main tabbed activity, get the name of the launching activity and a message saying it's launched the tabbed one.
3. Override the onPause(), onStop() and onDestroy() callbacks and add debugging lines with this.toString() and also a message telling you which callback it is.

What this will do is tell you whether you've got multiple instances of the initialisation activity lying around. To this by comparing the name of the activities calling your main activity with the ones that were just created and the ones that went through to onDestroy.

If you don't know how to debug, use Log.d(LOG_TAG, "Your message here");. And then define a constant LOG_TAG String somewhere. After that, show the LogCat perspective in Eclispe by going to Window, show perspective (or view, don't remember exactly), other, Android, LogCat. The purpose of having a LOG_TAG constant is that you can set up LogCat to filter to that String and only show you those messages. It will make it easier to see them among the mass of system log messages.

Answer 4

The short answer to the original question is 'no'. This is largely because, unfortunately, not every developer follows the guidelines referenced by previous answers.

Yet the guidleines themselves mention exceptions, when the Back key should not call finish(). the most prominent exception is the Web browser, which has its own "back stack" for each window, so it must have its own custom handling of the Back key.