如何在Android中的onStop / onPause活动中保存对象列表

Question

Lately writing some app I've been stopped by one problem.

In this app I have two activities: A and B. In activity AI am downloading list of objects from web each time when GPS coordinates change. To this activity there is connected bound service with GPS and Internet connection call. Those objects are then put into list created from their names and after pressing on any of this I can open the B activity passing this object and in activity B get some more info from this object.

Activity B hasn't got any bound service, it's only taking what is inside object and display it.

Because bound service in activity A is unbound in onStop then when I come back to it using a back arrow it takes a while to download everything again. What is more interesting, the list is still filled, but those are only names, the actual list of objects doesn't exist and pressing just after comeback on any of names will cause OutOfBoundException.

So, my question is, what would you advice to do to save this objects when I am living activity A and retrieve them when come back, before download of the new (or this same) list.

I've tried:

• onRestoreInstanceState() with onSaveInstanceState() , because there you can save parcelable list, but the activity A isn't destroyed so fast, so there is no call for onRestoreInstanceState() ;
• Shared preferences , but it was bad choice, because you can't save list there;
• ContentValues and content resolvers but also, bad choice, because you can't save list.

Do you have any more ideas, how it can be done?

Oh, I've forget to mention, I've taken assumption that there can be mximum 100 objects in list, and each object have 5 fields.

Answer 1

create a Singleton class, and store the objects in a HashMap or ArrayList (whatever is suitable for you) and access that list in other activity.

import java.util.ArrayList;

public class DataHandler {
    private static DataHandler handler;
    private ArrayList<Object> list;

    private DataHandler() {
    }

    public static DataHandler getInstance() {
        if (handler == null)
            handler = new DataHandler();
        return handler;
    }

    public ArrayList<Object> getList() {
        return list;
    }

    public void setList(ArrayList<Object> list) {
        this.list = list;
    }
}

Answer 2

If the reason for not using SharedPreferences and ContentValues is because it can't handle lists then you might consider serializing your list to something like JSON, save it as a String and when loading map it back to objects. It's usually 2 lines of code to transform object <-> JSON for example.

Answer 3

You need to cache the downloaded data in a singleton class like here

class DataController {

public static ArrayList aa;

}

fill this arraylist when you download data and in your activity check if this list is not null then use this arraylist only instead of downloading it. do some more research on singleton class concept and I am sure you will learn something very helpful to you.