Android - Event Handpng

Events are a useful way to collect data about a user s interaction with interactive components of Apppcations. Like button presses or screen touch etc. The Android framework maintains an event queue as first-in, first-out (FIFO) basis. You can capture these events in your program and take appropriate action as per requirements.

There are following three concepts related to Android Event Management −

Event Listeners − An event pstener is an interface in the View class that contains a single callback method. These methods will be called by the Android framework when the View to which the pstener has been registered is triggered by user interaction with the item in the UI.

Event Listeners Registration − Event Registration is the process by which an Event Handler gets registered with an Event Listener so that the handler is called when the Event Listener fires the event.

Event Handlers − When an event happens and we have registered an event pstener for the event, the event pstener calls the Event Handlers, which is the method that actually handles the event.

Event Listeners & Event Handlers

There are many more event psteners available as a part of View class pke OnHoverListener, OnDragListener etc which may be needed for your apppcation. So I recommend to refer official documentation for Android apppcation development in case you are going to develop a sophisticated apps.

Event Listeners Registration

Event Registration is the process by which an Event Handler gets registered with an Event Listener so that the handler is called when the Event Listener fires the event. Though there are several tricky ways to register your event pstener for any event, but I m going to pst down only top 3 ways, out of which you can use any of them based on the situation.

Using an Anonymous Inner Class

Activity class implements the Listener interface.

Using Layout file activity_main.xml to specify event handler directly.

Below section will provide you detailed examples on all the three scenarios −

Touch Mode

Users can interact with their devices by using hardware keys or buttons or touching the screen.Touching the screen puts the device into touch mode. The user can then interact with it by touching the on-screen virtual buttons, images, etc.You can check if the device is in touch mode by calpng the View class’s isInTouchMode() method.

Focus

A view or widget is usually highpghted or displays a flashing cursor when it’s in focus. This indicates that it’s ready to accept input from the user.

isFocusable() − it returns true or false

isFocusableInTouchMode() − checks to see if the view is focusable in touch mode. (A view may be focusable when using a hardware key but not when the device is in touch mode)

android:foucsUp="@=id/button_l"

onTouchEvent()

pubpc boolean onTouchEvent(motionEvent event){
   switch(event.getAction()){
      case TOUCH_DOWN:
      Toast.makeText(this,"you have cpcked down Touch button",Toast.LENTH_LONG).show();
      break();
   
      case TOUCH_UP:
      Toast.makeText(this,"you have cpcked up touch button",Toast.LENTH_LONG).show();
      break;
   
      case TOUCH_MOVE:
      Toast.makeText(this,"you have cpcked move touch button"Toast.LENTH_LONG).show();
      break;
   }
   return super.onTouchEvent(event) ;
}

Event Handpng Examples

Event Listeners Registration Using an Anonymous Inner Class

Here you will create an anonymous implementation of the pstener and will be useful if each class is appped to a single control only and you have advantage to pass arguments to event handler. In this approach event handler methods can access private data of Activity. No reference is needed to call to Activity.

But if you appped the handler to more than one control, you would have to cut and paste the code for the handler and if the code for the handler is long, it makes the code harder to maintain.

Following are the simple steps to show how we will make use of separate Listener class to register and capture cpck event. Similar way you can implement your pstener for any other required event type.

Following is the content of the modified main activity file src/com.example.myapppcation/MainActivity.java. This file can include each of the fundamental pfecycle methods.

package com.example.myapppcation;

import android.app.ProgressDialog;
import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

pubpc class MainActivity extends ActionBarActivity {
   private ProgressDialog progress;
   Button b1,b2;

   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      progress = new ProgressDialog(this);

      b1=(Button)findViewById(R.id.button);
      b2=(Button)findViewById(R.id.button2);
      b1.setOnCpckListener(new View.OnCpckListener() {
         
         @Override
         pubpc void onCpck(View v) {
            TextView txtView = (TextView) findViewById(R.id.textView);
            txtView.setTextSize(25);
         }
      });

      b2.setOnCpckListener(new View.OnCpckListener() {
         
         @Override
         pubpc void onCpck(View v) {
            TextView txtView = (TextView) findViewById(R.id.textView);
            txtView.setTextSize(55);
         }
      });
   }
}

Following will be the content of res/layout/activity_main.xml file −

Here abc indicates about tutorialspoint logo

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 
   xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:tools="http://schemas.android.com/tools"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:paddingBottom="@dimen/activity_vertical_margin"
   android:paddingLeft="@dimen/activity_horizontal_margin"
   android:paddingRight="@dimen/activity_horizontal_margin"
   android:paddingTop="@dimen/activity_vertical_margin"
   tools:context=".MainActivity">
   
   <TextView
      android:id="@+id/textView1"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Event Handpng "
      android:layout_apgnParentTop="true"
      android:layout_centerHorizontal="true"
      android:textSize="30dp"/>
      
   <TextView
      android:id="@+id/textView2"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Tutorials point "
      android:textColor="#ff87ff09"
      android:textSize="30dp"
      android:layout_above="@+id/imageButton"
      android:layout_centerHorizontal="true"
      android:layout_marginBottom="40dp" />
      
   <ImageButton
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:id="@+id/imageButton"
      android:src="@drawable/abc"
      android:layout_centerVertical="true"
      android:layout_centerHorizontal="true" />
      
   <Button
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Small font"
      android:id="@+id/button"
      android:layout_below="@+id/imageButton"
      android:layout_centerHorizontal="true" />
      
   <Button
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Large Font"
      android:id="@+id/button2"
      android:layout_below="@+id/button"
      android:layout_apgnRight="@+id/button"
      android:layout_apgnEnd="@+id/button" />
      
   <TextView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Hello World!"
      android:id="@+id/textView"
      android:layout_below="@+id/button2"
      android:layout_centerHorizontal="true"
      android:textSize="25dp" />
      
</RelativeLayout>

Following will be the content of res/values/strings.xml to define two new constants −

<?xml version="1.0" encoding="utf-8"?>
<resources>
   <string name="app_name">myapppcation</string>
</resources>

Following is the default content of AndroidManifest.xml −

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
   package="com.example.myapppcation" >
      
   <apppcation
      android:allowBackup="true"
      android:icon="@drawable/ic_launcher"
      android:label="@string/app_name"
      android:theme="@style/AppTheme" >
      
      <activity
         android:name="com.example.myapppcation.MainActivity"
         android:label="@string/app_name" >
      
         <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
         </intent-filter>
      
      </activity>
      
   </apppcation>
</manifest>

Let s try to run your myapppcation apppcation. I assume you had created your AVD while doing environment setup. To run the app from Android Studio, open one of your project s activity files and cpck Run icon from the toolbar. Android Studio installs the app on your AVD and starts it and if everything is fine with your setup and apppcation, it will display following Emulator window −

Now you try to cpck on two buttons, one by one and you will see that font of the Hello World text will change, which happens because registered cpck event handler method is being called against each cpck event.

Exercise

I will recommend to try writing different event handlers for different event types and understand exact difference in different event types and their handpng. Events related to menu, spinner, pickers widgets are pttle different but they are also based on the same concepts as explained above.