[ListView] The World of List View

[Adapters]

• 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)