Performance improvement suggestion

[GoogleCodeExporter]

Dear developers,

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

Currently in BreadcrumbsAdapter class, the getView() method works likes this

public View getView(int position, View convertView, ViewGroup parent) {
  if(convertView == null) {
    convertView = mInflater.inflate(R.layout.listItem, null);
  }
  ((TextView) convertView.findViewById(R.id.text)).setText(DATA[position]);

  return convertView;
}

When the users scroll the list view (I am not sure if it is a kind of list 
view), this implementation avoids inflations for each view (by using the 
recycled convertView), which saves CPU cycles and RAM. However, the method 
still invokes findViewById() every time when it is called. Android 
documentation says that findViewById is an expensive call, it recursively 
traverses a view tree to find a view matching the give ID. Google developers 
actually suggested a better way to implement getView(). It works like this:

We define a ViewHolder class with the field: TextView text . 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);
    convertView.setTag(holder)
  } else {
    //use the recycled view to improve performance
    holder = (ViewHolder) convertView.getTag(); 
  }
  holder.text.setText(DATA[position]);

  return convertView;
}

This avoids calling findViewById frequently and will improve performance when 
the list contains many items or on low end devices.

you may find more useful information in thees 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

In the last Google IO video, we find that the current implementation of 
getView() in BreadcrumbsAdapter is a right way, but not a fast way. The video 
actually provides three ways: a slow way, a right way and a fast way.

Looking forward to your reply and hope I can help improve open-gpstracker :)

Original issue reported on code.google.com by yepang...@gmail.com on 26 Jun 2013 at 12:05