保存ListView的状态以进行屏幕方向

Question

I went through many posts for screen orientation. But my scenario is little bit different. My ListView have custom row layout. With ImageView bitmap being loaded from internet using AsyncTask. What I want is to, is it possible to save the ArrayAdapter of my ListView or the whole ListView so that its state is preserved upon orientation screen change.

Please suggests some posts or if some codes are posted it will be very useful.

public View getView(int position, View convertView, ViewGroup parent) {
        final Contents entry = contentList.get(position);
        View row = convertView;
        LayoutInflater infltr = null;
            if (row == null) {
                infltr = (LayoutInflater) context
                        .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
                row = inflater.inflate(R.layout.highlight_contents_layout,
                        parent, false);
        }

            ImageView imgViewImage = (ImageView) row
                    .findViewById(R.id.imgView_Image);
            if (imgViewImage.getTag() == null) {
                new DownloadImage(imgViewImage, entry).execute(); //Async Task to download Image
            }

        }
        return row;
    }

and in onCreate i have a function call to populate the list the function call is :

init(){
listContent = (ListView) findViewById(R.id.listview); //vairable declared at top of my class;
contentlist = new ArrayList<Contents>(); //Variable Declared at top of class;

//... Populated the contenlist... in here
//..
//..
adapter = new ListAdapter(getApplicationContext(),
                R.layout._contents_layout, contentList,
                thumb_image_bytes);
        listContent.setAdapter(adapter);
        adapter.notifyDataSetChanged();
}

Answer 1

You can preserve state of your ListView by using onRetainNonConfigurationInstance() method. This method is executed before activity is destroyed during orientation change and it can return any object you want, for example your list adapter. Leter, in onCreate() method that object can be restored by calling getLastNonConfigurationInstance( );

So create field in your activity, for example mAdapter . That field will hold reference to your list adapter. Override onRetainNonConfigurationInstance() like this:

@Override
public Object onRetainNonConfigurationInstance() {
    return mAdapter;
}

And restore list adapter in onCreate() method:

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mList = (ListView) findViewById(android.R.id.list);

    Object obj = getLastNonConfigurationInstance();

    if(null != obj){//if there is saved adapter - restore it
        mAdapter = (ArrayAdapter<String>)obj;
    } else { //if not - create new one
        mAdapter = new ArrayAdapter<String>(getApplicationContext(),
                android.R.layout.simple_list_item_1, listData);
    }
        mList.setAdapter(mAdapter);
}

Edit:

This doesn't look good for me:

ImageView imgViewImage = (ImageView) row
                    .findViewById(R.id.imgView_Image);
      if (imgViewImage.getTag() == null) {
           new DownloadImage(imgViewImage, entry).execute(); //Async Task to download Image
}

Please, check if imgViewImage.getTag() ever returns false. I think You shouldn't execute AsyncTask in getView() because it will be executed while you are scrolling list so you could get list items with no imager(I think).

Answer 2

Why dont you store the downloaded data in variables, and upon change of orientation, reload the elements with the stored data. This link will help you catch the screen orientation change event.

Answer 3

I am not 100% sure what you are asking. But you can set the orientation so that it is fixed to a certain value. I had a problem where I had a list view change on orientation so I fixed the orientation so that it would only produce the output in portrait mode.

I added this tag inside my activity tags as an option

<activity android:name="preferences.Preferences" android:screenOrientation="portrait"></activity>. 

I hope this is the answer that you were looking for. This will preserve the state. If this does not answer your question. I will endeavour to help you further.