Handle events in android

Android — Event Handling

Events are a useful way to collect data about a user’s interaction with interactive components of Applications. 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 listener 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 listener 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 listener for the event, the event listener calls the Event Handlers, which is the method that actually handles the event.

Event Listeners & Event Handlers

This is called when the user either clicks or touches or focuses upon any widget like button, text, image etc. You will use onClick() event handler to handle such event.

This is called when the user either clicks or touches or focuses upon any widget like button, text, image etc. for one or more seconds. You will use onLongClick() event handler to handle such event.

This is called when the widget looses its focus ie. user goes away from the view item. You will use onFocusChange() event handler to handle such event.

This is called when the user is focused on the item and presses or releases a hardware key on the device. You will use onKey() event handler to handle such event.

This is called when the user presses the key, releases the key, or any movement gesture on the screen. You will use onTouch() event handler to handle such event.

This is called when the user selects a menu item. You will use onMenuItemClick() event handler to handle such event.

This is called when the context menu is being built(as the result of a sustained «long click)

There are many more event listeners available as a part of View class like OnHoverListener, OnDragListener etc which may be needed for your application. So I recommend to refer official documentation for Android application 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 listener for any event, but I’m going to list 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 calling the View class’s isInTouchMode() method.

Focus

A view or widget is usually highlighted 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)

onTouchEvent()

Event Handling Examples

Event Listeners Registration Using an Anonymous Inner Class

Here you will create an anonymous implementation of the listener and will be useful if each class is applied 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 applied 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 click event. Similar way you can implement your listener for any other required event type.

Event Handler	Event Listener & Description
onClick()

Step	Description
1	You will use Android studio IDE to create an Android application and name it as myapplication under a package com.example.myapplication as explained in the Hello World Example chapter.
2	Modify src/MainActivity.java file to add click event listeners and handlers for the two buttons defined.
3	Modify the detault content of res/layout/activity_main.xml file to include Android UI controls.
4	No need to declare default string constants.Android studio takes care default constants.
5	Run the application to launch Android emulator and verify the result of the changes done in the aplication.

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

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

Here abc indicates about tutorialspoint logo

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

Following is the default content of AndroidManifest.xml −

Let’s try to run your myapplication application. 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 click 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 application, it will display following Emulator window −

Now you try to click on two buttons, one by one and you will see that font of the Hello World text will change, which happens because registered click event handler method is being called against each click 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 handling. Events related to menu, spinner, pickers widgets are little different but they are also based on the same concepts as explained above.

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Android Input Events (Event Listeners, Event Handling)

In android, Input Events are used to capture the events, such as button clicks, edittext touch, etc. from the View objects that defined in a user interface of our application, when the user interacts with it.

To handle input events in android, the views must have in place an event listener. The View class, from which all UI components are derived contains a wide range event listener interfaces and each listener interface contains an abstract declaration for a callback method. To respond to an event of a particular type, the view must register an appropriate event listener and implement the corresponding callback method.

For example, if a button is to respond to a click event it must register to View.onClickListener event listener and implement the corresponding onClick() callback method. In application when a button click event is detected, the Android framework will call the onClick() method of that particular view.

Generally, to handle input events we use Event Listeners and Event Handling in android applications to listen for user interactions and to extend a View class, in order to build a custom component.

Android Event Listeners

In android, Event Listener is an interface in the View class that contains a single call-back method. These methods will be called by the Android framework when the View which is registered with the listener is triggered by user interaction with the item in UI.

The following are the call-back methods included in the event listener interface.

Method	Description
onClick()	This method is called when the user touches or focuses on the item using navigation-keys or trackball or presses on the «enter» key or presses down on the trackball.
onLongClick()	This method is called when the user touches and holds the item or focuses on the item using navigation-keys or trackball and presses and holds «enter» key or presses and holds down on the trackball (for one second).
onFocusChange()	This method is called when the user navigates onto or away from the item, using the navigation-keys or trackball.
onKey()	This method is called when the user is focused on the item and presses or releases a hardware key on the device.
onTouch()	This method is called when the user performs a touch event, including a press, a release, or any movement gesture on the screen.
onCreateContextMenu()	This method is called when a Context Menu is being built.

There are many more event listeners available as a part of View class to use it in our android applications.

Android Event Listeners Registration

In android, 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.

Following are the different ways to register event listeners in our android applications.

• By specifying event handlers directly in activity_main.xml file, we can register event listeners
• By using Activity class that implements a listener interface, we can register event listeners
• By using an anonymous class.

Following is the example of registering a Button onClick event listener in android application.

@Override
protected void onCreate(Bundle savedInstanceState) <
….
// Capture button from our layout
Button button = (Button)findViewById(R.id.btnShow);
// Register onClick listener with the below implementation
button.setOnClickListener( btnListener );

….
>
// Anonymous implementation of OnClickListener
private View.OnClickListener btnListener = new View.OnClickListener() <
public void onClick(View v) <
// do something when the button is clicked
>
>;

This is how we can register our event listeners in different ways based on our requirements.

Android Event Handlers

In android, Event Handlers are useful to define several callback methods when we are building custom components from view.

Following are the some of commonly used callback methods for event handling in android applications.

Method	Description
onKeyDown()	This method is called when a new key event occurs.
onKeyUp()	This method is called when a key up event occurs.
onTrackballEvent()	This method is called when a trackball motion event occurs.
onTouchEvent()	This method is called when a touch screen motion event occurs.
onFocusChanged()	This method is called when the view gains or loses focus.

Now we will see how to define input Events using Even Listeners and Event Handling with examples in android applications.

Android Input Events Example

Following is the example of defining an input controls Button, TextView in user interface and showing the text in TextView when users click on Button in the android application.

Create a new android application using android studio and give names as InputEventsExample. In case if you are not aware of creating an app in android studio check this article Android Hello World App.

Now open an activity_main.xml file from \res\layout path and write the code like as shown below

activity_main.xml

If you observe above code we created a one Button and TextView control in XML Layout file to show the text in textview when we click on Button.

Once we are done with the creation of layout with required controls, we need to load the XML layout resource from our activity onCreate() callback method, for that open main activity file MainActivity.java from \java\com.tutlane.inputeventsexample path and write the code like as shown below.

MainActivity.java

package com.tutlane.inputeventsexample;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity <
Button btn ;
TextView tView ;
@Override
protected void onCreate(Bundle savedInstanceState) <
super .onCreate(savedInstanceState);
setContentView(R.layout. activity_main );
btn = (Button)findViewById(R.id. btnClick );
tView = (TextView)findViewById(R.id. txtResult );
btn .setOnClickListener( new View.OnClickListener() <
@Override
public void onClick(View v) <
tView .setText( «You Clicked On Button» );
>
>);
>
>

If you observe above code we registered our Button click event using setOnClickListener event listener and binding a data to TextView control when we click on button.

Generally, during the launch of our activity, onCreate() callback method will be called by android framework to get the required layout for an activity.

Output of Android Input Events Example

When we run above example using android virtual device (AVD) we will get a result like as shown below

If you observe above result, we are getting binding a text to textview control on button click. This is how we can handle input events in android applications based on our requirements.

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