Android上的活动娱乐

Question

I am reading the official Android tutorial on Managing Activity Lifecycle. Please have a look at the following points as I quote.

(Normal app behavior) "When your activity is destroyed because the user presses Back or the activity finishes itself, the system's concept of that Activity instance is gone forever because the behavior indicates the activity is no longer needed."

(System induced destruction) "If the system destroys the activity due to system constraints (rather than normal app behavior), then althought the actual Activity instance is gone, the system remembers that it existed such that if the user navigates back to it, the system creates a new instance of the activity using a set of saved data that describes the state of the activity when it was destroyed."

"The saved data that the system uses to restore the previous state is a collection of key value pairs stored in a Bundle object. By default the saved information is about the activity's view hierarchy, such as the text entered in a widget, the scroll position, etc. To save additional state information for your activity, such as member variables, you must implement onSaveInstanceState() and add key-value pairs to the Bundle object."

Please see if I can sum up correctly from the above:

The system is resource conscious and can destroy an activity to recover memory. If the system does that then it leaves an option of recovering the destroyed activity to its previous state if needed later.

State = view-state (default, done automatic) + member-variable-state (manual, up to the programmer).

My question is, if the system is destroying an activity in order to free some memory, then doesn't it defeat the purpose if it allows us to keep a "copy" of the activity so that the activity can be restored to the point exactly as it was prior to destruction?

Answer 1

No, it does not defeat the purpose:

• A Bundle is optimized for being serializable, so the Android framework could just write it to disk.
• The data you need to represent your state is much smaller than the amount of RAM you need for a running copy of your app. For example, if you show an image to the user, you'd probably just have to save the image's location, not all of its data. Also, every Android app is a whole new Dalvik VM, so if all the activities are from different applications, killing an activity means one Dalvik VM less – and every Dalvik VM probably needs a relatively big amount of RAM.