ListView tips and tricks

Transcript

1. List View: tips & tricks Tibi Giurgiu tiberiu.giurgiu@softvision.ro
2. Overview • a view group that displays a list of scrollable items • designed for scalability and performance • an Adapter pulls content from a source and converts each item result into a view that's placed into the list. java.lang.Object ↳ android.view.View ↳ android.view.ViewGroup ↳ android.widget.AdapterView<T extends android.widget.Adapter> ↳ android.widget.AbsListView ↳ android.widget.ListView
3. Data Source Cursor Collection ... ListView Adapter How it works
4. Virtualization • Problem: large data sets – Memory – Performance • Solution – Populate on demand – Recycle views to reduce object churn
5. Recycling
6. • DOs – use the recycled view – ok to use drawable resources (cached internally) • DON'Ts – perform long operations (network, db, sharedPrefs, files, bitmaps etc.) – call findViewById() if not necessary => ViewHolder pattern – change convertView structure (use view type system for this instead) getView()
7. Performance
8. Multiple Item Types • int getItemViewType(int position) – type of View for a given position – used to provide the right convertView • getViewTypeCount() – how many types to expect – must remain constant
9. Handling dataset changes • notifyDataSetChanged() – new or updated data – list view rescans the adapter for the views on screen – MUST be called everytime you change your adapter – commit adapter changes on the UI thread, call notifyDataSetChanged() in the same UI event • notifyDataSetInvalidated() – no more data available
10. Handling emptiness • AdapterView.setEmptyView(View emptyView) – AdapterView hides/displays it according to the adapter state – the AdapterView and the empty view are mutually exclusive in term of visibility • ListActivity/ListFragment makes it easier – id for the ListView: android.R.id.list – id for the empty view: android.R.id.empty
11. Transcript and stack from bottom • android:transcriptMode – behavior of the list when the content changes – “disabled”, doesn’t scroll – “normal”, scrolls to the bottom if last item is visible – “alwaysScroll”, always scrolls to the bottom • android:stackFromBottom – stack items in reverse order – starts with the last item from the adapter
12. Headers and Footers • ListView.addHeaderView() • ListView.addFooterView() • Must be called before setAdapter() • list.getAdapter() returns a android.widget.HeaderViewListAdapter
13. Don't: android:layout_height="wrap_content" • getView() called too many times • forces ListView to measure a few children at layout time, to know how big it should be • Solution: – android:layout_height="fill_parent" :) – inside LinearLayout, android:layout_height="0dp" and use layout_weight
14. Add clickable areas • ListView blocks clicks of itemview containing at least one focusable descendant – Solution: • android:focusable="false" on all controls • set android:descendantFocusability="blocksDescendants" for the root • Android, by default, recursively dispatches the pressed state of a ViewGroup to all of its children – Solution: don’t ‘accept’ the pressed state when the parent is already in the pressed state.
15. Lazy loading images • the view you’re loading the image for might be recycled before the async operation finishes • attach some piece of information to the view that identifies which row is associated with it
16. private static class ThumbnailTask extends AsyncTask<Void, Void, Bitmap> { private int mPosition; private ViewHolder mHolder; public ThumbnailTask(int position, ViewHolder holder) { mPosition = position; mHolder = holder; } @Override protected Bitmap doInBackground(Void... arg0) { // Download bitmap here } @Override protected void onPostExecute(Bitmap bitmap) { if (mHolder.position == mPosition) { mHolder.thumbnail.setImageBitmap(bitmap); } } }
17. public void onScrollStateChanged(AbsListView view, int scrollState) { if (scrollState == OnScrollListener.SCROLL_STATE_FLING) { mPhotoLoader.pause(); } else if (mDisplayPhotos) { mPhotoLoader.resume(); } } • start an asynchronous operation on every getView() call while scrolling => most of the results would be discarded • setOnScrollListener(), don't load if SCROLL_STATE_FLING
18. Clasic Lazy loading pattern public View getView(int position, View convertView, ViewGroup parent) { // ... setupView Bitmap image = loadImageFromCache(url); if (image != null) { imageView.setImageBitmap(image); } else { AsyncTask loadImageTask = new AsyncTask<Void, Void, Bitmap>() { protected Bitmap doInBackground(Void... params) { return loadImageFromNetwork(url); } protected void onPostExecute(Bitmap image) { if (image != null && imageView.getTag().equals(url)) { imageView.setImageBitmap(image); } } }; // ... setup no image loaded view imageView.setTag(url); loadImageTask.execute(); } }
19. Problems • Imageview redraw is triggered asynchronously • Scrolling behavior suffers
20. Better solution public View getView(int position, View convertView, ViewGroup parent) { // ... setupView Bitmap image = loadImageFromCache(url); if (image != null) { imageView.setImageBitmap(image); } else { AsyncTask loadImageTask = new AsyncTask<Void, Void, Bitmap>() { protected Bitmap doInBackground(Void... params) { return loadImageFromNetwork(url); } protected void onPostExecute(Bitmap image) { if (image != null && imageView.getTag().equals(url)) { adapter.notifyDataSetChanged(); } } }; // ... setup no image loaded view imageView.setTag(url); loadImageTask.execute(); } }
21. Advantages • List adapter chooses best moment to redraw • Scrolling is smooth
22. Source code https://github.com/tibigiurgiu/ListView
23. • Android Developers https://developer.android.com/guide/topics/ui/layout/listview.html • Google IO 10, The world of ListView http://www.google.com/events/io/2010/sessions/world-of-listview-android.html • Cyril Motier, Android Dev Blog http://android.cyrilmottier.com • Lucas Rocha, Performance Tips for Android’s ListView http://lucasr.org/2012/04/05/performance-tips-for-androids-listview/ • Janusz Leidgens, Lazy Loading in Lists http://journeyman.janusz.de/ References