I am used to building lists in android using adapters. If I need some long-to-get data, I use an asynctask, or a simple runnable, to update the data structure on which the adapter rely, and call notifyDataChanged on the adapter.
Although it is not straightforward, I finally find this is a simple model and it allows a good separation of logic presentation (in the asynctask, update a data structure) and the view (an adapter acting as a view factory, mostly).
Nevertheless, I read recently about loaders introduced in HoneyComb and included in the backward compatibility support-library, I tried them and find the introduce a lot of complexity. They are difficult to handle and add some kind of magic to this whole process through loader managers, add a lot of code and don't decrease the number of classes or collaborating items but I may be wrong and would like to hear some good points on loaders.
- What are they advantages of loaders in terms of lines of code, clarity and effort ?
- What are they advantages of loaders in terms of role separation during data loading, or more broadly, in terms of design ?
- Are they the way to go, should I replace all my list data loading to implement them through loaders ?
Ok, this is a developers' forum, so here is an example. Please, make it better with loaders :
package com.sof.test.loader;
import java.util.ArrayList;
import java.util.List;
import android.app.ListActivity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.view.ViewGroup;
import android.view.ViewGroup.LayoutParams;
import android.widget.ArrayAdapter;
import android.widget.TextView;
/** The activity. */
public class LoaderTestActivity extends ListActivity {
private DataSourceOrDomainModel dataSourceOrDomainModel = new DataSourceOrDomainModel();
private List<Person> listPerson;
private PersonListAdapter personListAdapter;
private TextView emptyView;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
listPerson = new ArrayList<Person>();
personListAdapter = new PersonListAdapter( listPerson );
setListAdapter( personListAdapter );
setUpEmptyView();
new PersonLoaderThread().execute();
}
public void setUpEmptyView() {
emptyView = new TextView( this );
emptyView.setLayoutParams( new LayoutParams( LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT ) );
emptyView.setVisibility(View.GONE);
((ViewGroup)getListView().getParent()).addView(emptyView);
getListView().setEmptyView(emptyView);
}
/** Simulate a long task to get data. */
private class PersonLoaderThread extends AsyncTask<Void, Integer, List<Person>> {
@Override
protected List<Person> doInBackground(Void... params) {
return dataSourceOrDomainModel.getListPerson( new ProgressHandler());
}
@Override
protected void onProgressUpdate(Integer... values) {
emptyView.setText( "Loading data :" + String.valueOf( values[ 0 ] ) +" %" );
}
@Override
protected void onPostExecute(List<Person> result) {
listPerson.clear();
listPerson.addAll( result );
personListAdapter.notifyDataSetChanged();
}
private class ProgressHandler implements ProgressListener {
@Override
public void personLoaded(int count, int total) {
publishProgress( 100*count / total );
}
}
}
/** List item view factory : the adapter. */
private class PersonListAdapter extends ArrayAdapter<Person> {
public PersonListAdapter( List<Person> listPerson ) {
super(LoaderTestActivity.this, 0, listPerson );
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
if( convertView == null ) {
convertView = new PersonView( getContext() );
}
PersonView personView = (PersonView) convertView;
personView.setPerson( (Person) getItem(position) );
return personView;
}
}
}
A small callback interface for progress
package com.sof.test.loader;
/** Callback handler during data load progress. */
public interface ProgressListener {
public void personLoaded(int count, int total );
}
A list item widget
package com.sof.test.loader;
import com.sof.test.loader.R;
import android.content.Context;
import android.view.LayoutInflater;
import android.widget.LinearLayout;
import android.widget.TextView;
/** List Item View, display a person */
public class PersonView extends LinearLayout {
private TextView personNameView;
private TextView personFirstNameView;
public PersonView(Context context) {
super(context);
LayoutInflater inflater= (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
inflater.inflate( R.layout.person_view,this );
personNameView = (TextView) findViewById( R.id.person_name );
personFirstNameView = (TextView) findViewById( R.id.person_firstname );
}
public void setPerson( Person person ) {
personNameView.setText( person.getName() );
personFirstNameView.setText( person.getFirstName() );
}
}
It's xml : res/person_view.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/person_view"
android:layout_width="fill_parent"
android:layout_height="wrap_content" >
<TextView
android:id="@+id/person_name"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true" />
<TextView
android:id="@+id/person_firstname"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_toRightOf="@id/person_name" />
</RelativeLayout>
The data source or model, providing data (slowly)
package com.sof.test.loader;
import java.util.ArrayList;
import java.util.List;
/** A source of data, can be a database, a WEB service or a model. */
public class DataSourceOrDomainModel {
private static final int PERSON_COUNT = 100;
public List<Person> getListPerson( ProgressListener listener ) {
List<Person> listPerson = new ArrayList<Person>();
for( int i=0; i < PERSON_COUNT ; i ++ ) {
listPerson.add( new Person( "person", "" + i ) );
//kids, never do that at home !
pause();
if( listener != null ) {
listener.personLoaded(i,PERSON_COUNT);
}//if
}
return listPerson;
}//met
private void pause() {
try {
Thread.sleep( 100 );
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
The POJO representing a person :
package com.sof.test.loader;
/** A simple POJO to be displayed in a list, can be manipualted as a domain object. */
public class Person {
private String name;
private String firstName;
public Person(String name, String firstName) {
this.name = name;
this.firstName = firstName;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
}//class
