- Terminology
- index: Child views
- position: Data in Adapter
- id: Unique identifier for data
- getView(int position, View convertView, ViewGroup parent)
- Full data presentation control
- Optimization
- Shoot yourself in the foot (and the face) (搬起石頭砸自己的腳?)
- convertView
- Supplied by ListView
- Matches item types
- Reuse it
- The Right Way to use getView().
- 當 convertView 是 null 時，才 inflate layout，這樣才真正有 reuse。
public View getView(int position, View convertView, ViewGroup parent) {
if (convertView == null) {
convertView = mInflater.inflate(R.layout.item, parent, false);
}
((TextView) convertView.findViewById(R.id.text)).setText(DATA[position]);
((ImageView) convertView.findViewById(R.id.icon)).setImageBitmap(
(position & 1) == 1 ? mIcon1 : mIcon2);
return convertView;
}
- The Fast Way to use getView().
- 將 layout 的 element 儲存在 viewHolder，共用同一份物件。
static class ViewHolder {
TextView text;
ImageView icon;
}
- How to shoot yourself in the foot (?)
- Local view cache
- Accessing views from the adapter
- Change convertView’s structure
- Assumptions about getView calls
- Handling data changes
- notifyDataSetChanged()
- New or updated data
- notifyDataSetInvalidated()
- No more data available
- Focusable
- Do list items focus as a whole? (false)
- Can views within list items have focus? (true)
- Headers and footers - Scrolling
- ListView.addHeaderView()/ListView.addFooterView()
- Must be called before setAdapter()
- isSelectable == Adapter.isEnabled()
[Don’t!]
- android:layout_height=”wrap_content”
- ListView is virtualized, remember?
- wrap_content = “as big as my children”
- ListView supports unevenly sized children
- Measure thousands of children? Android framework cheats, measures only 3 children
- ListView inside a ScrollView
- ScrollView scrolls and ListView scrolls, who will scroll?
- Cache views in the adapter (?)
- Don’t outsmart ListView
- ListView assumes ownership of views
- Complex recycling mechanism
- Numerous optimizations
- Sometimes we even leave bugs there
- Undead views is the worst case
- View both in the recycler and on screen
- Use a ListView when you don’t need one!
- ListView...
- Is for repeating, unbounded data
- Adds (lots of) complexity
* Reference
- 2010 Google IO - The World of List View (android-world-of-listview-android.pdf)
