#140 Performance improvement suggestion

[Anonymous]

Originally created by: yepang...@gmail.com

Dear developers,

I am a fan of apg, and recently I am writing a static code analysis tool to conduct performance analysis for Android apps. I found several violations of "view holder" patterns in apg's code. These violations could affect the ListView scrolling performance.

Currently in apg, in some list adapters the getView() method works likes this

public View getView(int position, View convertView, ViewGroup parent) {
  View item = mInflater.inflate(R.layout.listItem, null);
  ((TextView) item.findViewById(R.id.text)).setText(DATA[position]);
  ((ImageView) item.findViewById(R.id.icon)).setImageBitmap(position & 1) == 1 ? mIcon1 : mIcon2);

  return item;
}

When the users scroll a list view, this method is going to build new views continuously instead of using the recycled "convertView". This wastes a lot of CPU cycles in "view inflation" and RAM in storing view objects. On low end devices, GC will kick in frequently because of RAM pressure. This will reduce the UI smoothness when a list has many items (even a list only has a few items, continuously building new views during scrolling is a waste of computational resource).

Google suggested a faster way for getView(), it works like this:

We define a ViewHolder class with two fields: TextView text and ImageView icon. Then the getView() can be implemented like this:

public View getView(int position, View convertView, ViewGroup parent) {
  ViewHolder holder;
  if(convertView == null){
    //we have no recycled views to use, then build new one
    convertView = mInflater.inflate(R.layout.listItem, null);
    holder = new ViewHolder();
    holder.text = (TextView) convertView.getViewById(R.id.text);
    holder.icon = (ImageView) convertView.findViewById(R.id.icon);
    convertView.setTag(holder)
  } else {
    //use the recycled view to improve performance
    holder = (ViewHolder) convertView.getTag();
  }
  holder.text.setText(DATA[position]);
  holder.icon.setImageBitmap(position & 1) == 1 ? mIcon1 : mIcon2;

  return convertView;
}

In the following adapters, the view holder pattern is not correctly implemented:
org.thialfihar.android.apg.MailListActivity.java
org.thialfihar.android.apg.SelectSecretKeyListAdapter.java
org.thialfihar.android.apg.SelectPublicKeyListAdapter.java
org.thialfihar.android.apg.KeyServerQueryActivity.java

References:
view holder pattern: http://lucasr.org/2012/04/05/performance-tips-for-androids-listview/
http://developer.android.com/training/improving-layouts/smooth-scrolling.html
http://www.youtube.com/watch?v=wDBM6wVEO70

Looking forward to your reply and hope that I can help improve apg :)