This lesson:

- considering ListView events - onItemClick, onItemSelect, onScroll

Interacting with the list, we might have a necessity to process such events as list item click or scroll. Let’s look how to do that. 

Create a project:

Project name: P0441_SimpleListEvents
Build Target: Android 2.3.3
Application name: SimpleListEvents
Package name: ru.startandroid.develop.p0441simplelistevents
Create Activity: MainActivity

Let’s code the main.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical">
    <ListView
        android:id="@+id/lvMain"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">
    </ListView>
</LinearLayout>

There is only the ListView on the screen.

The same way with the previous lesson we will add the list of names into the resources ( res/values/strings.xml)

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string name="hello">Hello World, MainActivity!</string>
    <string name="app_name">SimpleListEvents</string>
    <string-array name="names">
        <item>Ivan</item>
        <item>Maria</item>
        <item>Petr</item>
        <item>Anton</item>
        <item>Dasha</item>
        <item>Boris</item>
        <item>Kostja</item>
        <item>Igor</item>
        <item>Anna</item>
        <item>Denis</item>
        <item>Vadim</item>
        <item>Olga</item>
        <item>Sergey</item>
    </string-array>
</resources>

Let’s code MainActivity.java:

package ru.startandroid.develop.p0441simplelistevents;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.AdapterView.OnItemSelectedListener;
import android.widget.ArrayAdapter;
import android.widget.ListView;

public class MainActivity extends Activity {

  final String LOG_TAG = "myLogs";

  ListView lvMain;

  /** Called when the activity is first created. */
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    lvMain = (ListView) findViewById(R.id.lvMain);

    ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(
        this, R.array.names, android.R.layout.simple_list_item_1);
    lvMain.setAdapter(adapter);

    lvMain.setOnItemClickListener(new OnItemClickListener() {
      public void onItemClick(AdapterView<?> parent, View view,
          int position, long id) {
        Log.d(LOG_TAG, "itemClick: position = " + position + ", id = "
            + id);
      }
    });

    lvMain.setOnItemSelectedListener(new OnItemSelectedListener() {
      public void onItemSelected(AdapterView<?> parent, View view,
          int position, long id) {
        Log.d(LOG_TAG, "itemSelect: position = " + position + ", id = "
            + id);
      }

      public void onNothingSelected(AdapterView<?> parent) {
        Log.d(LOG_TAG, "itemSelect: nothing");
      }
    });

  }
}

Now look at the code. We define views, create the adapter and assign it to the list. Next, we assign two listeners to the list:

1) OnItemClickListener processes the list item click

Provides us with the onItemClick method (onItemClick(AdapterView<?> parent, View view, int position, long id)), where:

parent is a parent-view of the clicked item;
view is a clicked item;
position  is a number of the item in the list;
id is an identifier;

We will display in logs the id and the position of the item that was clicked.

2) OnItemSelectedListener processes the list item selection

Provides us with the onItemSelected method which parameters are almost similar with the onItemClick method. No need to repeat them.                            

We also have the onNothingSelected method, it processes the situation when neither of the list items is selected.

Now let’s save and launch the app.

Let’s click any item, Peter for example. Look in logs:

itemClick: position = 2, id = 2

It’s alright. Because the position is counted from zero, so Peter has the position number 2. (In our case the position equals to id, I didn’t meet cases when it’s not so, but for sure, they may exist).

Now roll the mouse wheel and push up and down arrows on your keyboard. You can see that the list items are selecting.

So, in logs we can see:

itemSelect: position = 2, id = 2
itemSelect: position = 3, id = 3
itemSelect: position = 4, id = 4
itemSelect: position = 5, id = 5
itemSelect: position = 4, id = 4
itemSelect: position = 3, id = 3
itemSelect: position = 2, id = 2

I.e.  the listener carries information about what list item was selected. I see no usage for this function, but as it exists, I decided to tell about it.

Let’s now click any list item. We can can see that the selection has disappeared. Logs:

itemSelect: nothing
itemClick: position = 3, id = 3

Nothing is selected and the third list item was clicked.

Let’s add one more listener to the list:

    lvMain.setOnScrollListener(new OnScrollListener() {
      public void onScrollStateChanged(AbsListView view, int scrollState) {
        // Log.d(LOG_TAG, "scrollState = " + scrollState);
      }

      public void onScroll(AbsListView view, int firstVisibleItem,
          int visibleItemCount, int totalItemCount) {
        Log.d(LOG_TAG, "scroll: firstVisibleItem = " + firstVisibleItem
            + ", visibleItemCount" + visibleItemCount
            + ", totalItemCount" + totalItemCount);
      }
    });

    lvMain.setOnScrollListener(new OnScrollListener() {
      public void onScrollStateChanged(AbsListView view, int scrollState) {
        Log.d(LOG_TAG, "scrollState = " + scrollState);
      }

      public void onScroll(AbsListView view, int firstVisibleItem,
          int visibleItemCount, int totalItemCount) {
        //Log.d(LOG_TAG, "scroll: firstVisibleItem = " + firstVisibleItem
        //    + ", visibleItemCount" + visibleItemCount
        //    + ", totalItemCount" + totalItemCount);
      }
    });

package ru.startandroid.develop.p0441simplelistevents;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.AbsListView;
import android.widget.AbsListView.OnScrollListener;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.AdapterView.OnItemSelectedListener;
import android.widget.ArrayAdapter;
import android.widget.ListView;

public class MainActivity extends Activity {

  final String LOG_TAG = "myLogs";

  ListView lvMain;

  /** Called when the activity is first created. */
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    lvMain = (ListView) findViewById(R.id.lvMain);

    ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(
        this, R.array.names, android.R.layout.simple_list_item_1);
    lvMain.setAdapter(adapter);

    lvMain.setOnItemClickListener(new OnItemClickListener() {
      public void onItemClick(AdapterView<?> parent, View view,
          int position, long id) {
        Log.d(LOG_TAG, "itemClick: position = " + position + ", id = "
            + id);
      }
    });

    lvMain.setOnItemSelectedListener(new OnItemSelectedListener() {
      public void onItemSelected(AdapterView<?> parent, View view,
          int position, long id) {
        Log.d(LOG_TAG, "itemSelect: position = " + position + ", id = "
            + id);
      }

      public void onNothingSelected(AdapterView<?> parent) {
        Log.d(LOG_TAG, "itemSelect: nothing");
      }
    });

    lvMain.setOnScrollListener(new OnScrollListener() {
      public void onScrollStateChanged(AbsListView view, int scrollState) {
        Log.d(LOG_TAG, "scrollState = " + scrollState);
      }

      public void onScroll(AbsListView view, int firstVisibleItem,
          int visibleItemCount, int totalItemCount) {
        Log.d(LOG_TAG, "scroll: firstVisibleItem = " + firstVisibleItem
            + ", visibleItemCount" + visibleItemCount
            + ", totalItemCount" + totalItemCount);
      }
    });

  }
}

The next lesson:

- creating ExpandableListView