Android Intents Tutorial

1- Introduction

This document is based on:
  • Android Studio 1.4

2- What is Intent?

Intents are asynchronous messages which allow application components to request functionality from other Android components. Intents allow you to interact with components from the same applications as well as with components contributed by other applications. For example, an activity can start an external activity for taking a picture.

Intents are objects of the android.content.Intent type. Your code can send them to the Android system defining the components you are targeting.

For example, via the startActivity() method you can define that the intent should be used to start an activity. In the target Activity, via startActivity() method, you can determine the sender's intention to start this Activity.

An intent can contain data via a Bundle. This data can be used by the receiving component.
 

Intent can be used to:

  1. Start an Activity
  2. Start sub-activity.
  3. Start a Service.
After learning about  Intent, you can find more details about Android Service at:

3- The types of Intent

Android supports explicit and implicit intents.

An application can define the target component directly in the intent (explicit intent) or ask the Android system to evaluate registered components based on the intent data (implicit intents).

3.1- Explicit Intent

Explicit intents: These explicitly specify the name of the target component to handle the intent; in these, the optional component name’ field (above) is set to a particular value, through the setComponent() or setClass() methods.
Create Intent example:
// Create the Intent with the target of Greeting Activity.
// Intent(FirstActivity, SecondActivity.class)
Intent intent = new Intent(this,GreetingActivity.class);

// The data attached
intent.putExtra("firstName",firstName);
intent.putExtra("lastName", lastName);

// Start Activity (GreetingActivity)
// (No need feedback from the activity is called)
this.startActivity(intent);


// Start Activity and will have a feedback from the activity is called.
this.startActivityForResult(intent, MY_REQUEST_CODE);
Or:
// Way 1.
Intent mIntent = new Intent(this, GreetingActivity.class);
Bundle extras = mIntent.getExtras();
extras.putString("firstName", "<firstName>");
extras.putString("látName", "<lastName>");

// Way 2.
Intent mIntent2 = new Intent(this, GreetingActivity.class);
Bundle mBundle = new Bundle();
mBundle.putString("firstName", "<firstName>");
mBundle.putString("látName", "<lastName>");
mIntent2.putExtras(mBundle);

// Way 3:
// Using putExtra()
Intent mIntent3 = new Intent(this, GreetingActivity.class);
mIntent3.putExtra("firstName", "<firstName>");
mIntent3.putExtra("látName", "<lastName>");
In the target Activity:
Intent intent = this.getIntent();

String firstName= intent.getStringExtra("firstName");
String lastName = intent.getStringExtra("lastName");

// Or
Bundle extras = this.getIntent().getExtras();

String firstName1 = extras.getString("firstName");
String lastName2  = extras.getString("lastName");
OK, you can see the following simple example:

3.2- Explicit Intent example

This is the model of the example:
Create "Empty Activity" project named ExplicitIntentExample:
  • main_activity.xml
By double-clicking on the components on the interface, you can set the ID and text  for them:
EditText 1:
  • ID: text_firstName
  • Properties
    • layout_width: fill_parent
EditText 2:
  • ID: text_lastName
  • Properties
    • layout_width: fill_parent
TextView:
  • ID: text_feedback
  • Text: <Feedback>
  • Properties:
    • layout_width: fill_parent
    • gravity: center_horizontal
Button:
  • ID: button_greeting
  • Text: Show Greeting
  • Properties:
    • onClick: showGreeting
activity_main.xml
<?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:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">

    <EditText
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/text_firstName"
        android:layout_alignParentTop="true"
        android:layout_marginTop="66dp"
        android:layout_centerHorizontal="true" />

    <EditText
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/text_lastName"
        android:layout_below="@+id/text_firstName"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="40dp" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Show Greeting"
        android:id="@+id/button_greeting"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="132dp"
        android:onClick="showGreeting" />

    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="&lt;Feedback>"
        android:id="@+id/text_feedback"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="59dp"
        android:gravity="center_horizontal" />

</RelativeLayout>

 
When users click the button, the program will call a other Activity displays a greeting. You need to create an Activity with name GreetingActivity.
  • File/New/Activity/Empty Activity
  • activity_greeting.xml
Change the attributes for components on the interface:
TextView
  • ID: text_greeting
  • Text: <Greeting>
  • Properties:
    • layout_width: fill_parent
    • gravity: center_horizontal

Button:
  • ID: button_back
  • Properties
    • onClick: backClicked
activity_greeting.xml
<?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:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context="org.o7planning.explicitintentexample.GreetingActivity">

    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="&lt;Greeting>"
        android:id="@+id/text_greeting"
        android:layout_marginBottom="65dp"
        android:layout_above="@+id/button_back"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:gravity="center_horizontal" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Back"
        android:id="@+id/button_back"
        android:layout_centerVertical="true"
        android:layout_centerHorizontal="true"
        android:onClick="backClicked" />
</RelativeLayout>
 
MainActivity.java
package org.o7planning.explicitintentexample;

import android.app.Activity;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

   private EditText textFirstName;
   private EditText textLastName;
   private TextView textFeedback;

   public static final int MY_REQUEST_CODE = 100;

   @Override
   protected void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_main);

       this.textFirstName = (EditText)this.findViewById(R.id.text_firstName);
       this.textLastName = (EditText)this.findViewById(R.id.text_lastName);
       this.textFeedback = (TextView)this.findViewById(R.id.text_feedback);
   }

   // When 'Greeting Activity' completed, it sends back a feedback.
   // (If you have started it by startActivityForResult())
   @Override
   protected void onActivityResult(int requestCode, int resultCode, Intent data) {
       if (resultCode == Activity.RESULT_OK && requestCode == MY_REQUEST_CODE ) {
           String feedback = data.getStringExtra("feedback");
           this.textFeedback.setText(feedback);
       } else {
           this.textFeedback.setText("!?");
       }
   }

   // The method is called when the user clicks on "Show Greeting" button.
   public void showGreeting(View view)  {
        String firstName= this.textFirstName.getText().toString();
        String lastName= this.textLastName.getText().toString();

       Intent intent = new Intent(this,GreetingActivity.class);
       intent.putExtra("firstName", firstName);
       intent.putExtra("lastName", lastName);

       // Start Activity and no need feedback.
       // this.startActivity(intent);

       // Start Activity and get feedback.
       this.startActivityForResult(intent, MY_REQUEST_CODE);
   }

}
GreetingActivity.java
package org.o7planning.explicitintentexample;

import android.app.Activity;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;

public class GreetingActivity extends AppCompatActivity {

   private String firstName;
   private String lastName;

   @Override
   protected void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_greeting);

       // Intent is passed into
       Intent intent = this.getIntent();

       this.firstName= intent.getStringExtra("firstName");
       this.lastName = intent.getStringExtra("lastName");

       String greeting = "Hello "+ firstName+" "+ lastName;

       TextView textGreeting =(TextView) this.findViewById(R.id.text_greeting);

       textGreeting.setText(greeting);
   }

   // When completed this Activity, send feedback to the caller.
   @Override
   public void finish() {
       // Prepare data intent
       Intent data = new Intent();
       data.putExtra("feedback", "I'm "+ this.firstName+", Hi!");
       // Activity finished ok, return the data
       this.setResult(Activity.RESULT_OK, data);
       super.finish();
   }


   // The method is called when the user clicks the Back button.
   public void backClicked(View view)  {
       // Calling onBackPressed().
       // Gọi phương thức onBackPressed().
       this.onBackPressed();
   }

 
}
Running application:

3.3- Intent Filter

When you create new an Activity or Service, you need to declare it to AndroidManifest.xml, normally, when you create it with helps of wizard in Android Studio, registration code is automatically created  in AndroidManifest.xml. For example:
You may notice that in the declaration of Activity on  AndroidManifest.xml may have  <intent-filter> tag, this tag specifies the type of Intent sent to it (Activity) that it can accept.
When you create an implicit intent, the Android system finds the appropriate component to start by comparing the contents of the intent to the intent filters declared in the manifest file of other apps on the device. If the intent matches an intent filter, the system starts that component and delivers it the Intent object.
Example:
** AndroidManifest.xml **
.....

<activity android:name=".HelloWorld"
   android:label="@string/app_name">
   <intent-filter>
       <action android:name="android.intent.action.VIEW"/>
       <category android:name="android.intent.category.DEFAULT"/>
       <category android:name="android.intent.category.BROWSABLE"/>
       <data android:scheme="http" android:host="o7planning.org"/>
   </intent-filter>
</activity>

...
For example, create an implicit intent can call the above Activity (It matches the intent filters).
// An implicit intent, requested to view a URL

Intent intent = new Intent (Intent.ACTION_VIEW, Uri.parse("http://o7planning.org"));
startActivity(intent);

3.4- Implicit intent

Implicit intents: These do not specify a target component, but include enough information for the system to determine which of the available components is best to run for that intent. Consider an app that lists the available restaurants near you. When you click a particular restaurant option, the application has to ask another application to display the route to that restaurant. To achieve this, it could either send an explicit intent directly to the Google Maps application, or send an implicit intent, which would be delivered to any application that provides the Maps’ functionality (e.g., Yahoo Maps).

3.5- Simple example with the implicit intent

This simple example, when you click a Button which requires to see website by URL, you create an  implicitly Intent, Intent sent to the Android system to decide which  components will be  opened , maybe in your equipment installs many different browsers (Firefox, Chrome, ..), the device will open it in your default browser or your preferred browser
In addition examples also include:
  • Send an email
Create Android project with name "ImplicitIntentExample".
Set ID, Text for Button:
Button 1:
  • ID: button_google
  • Text: Go Google
  • Properties:
    • onClick: goGoogle
Button 2:
  • ID: button_email
  • Text: Send Email
  • Properties:
    • onClick: sendEmail
activity_main.xml
<?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:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Go Google"
        android:id="@+id/button_google"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="86dp"
        android:onClick="goGoogle" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Send Email"
        android:id="@+id/button_email"
        android:layout_below="@+id/button_google"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="45dp"
        android:onClick="sendEmail" />

</RelativeLayout>

 
MainActivity.java
package org.o7planning.implicitintentexample;

import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;

public class MainActivity extends AppCompatActivity {


   @Override
   protected void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_main);
   }

   // The method is called when the user clicks on "Go Google" button.
   public void goGoogle(View view)  {
       String url="http://google.com";

       // An implicit intent, request a URL.
       Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse(url));
       this.startActivity(intent);
   }

   // The method is called when the user clicks on "Send Email" button.
   public void sendEmail(View view)  {

       // List of recipients
       String[] recipients=new String[]{"friendemail@gmail.com"};

   
       String subject="Hi, how are you!";

     
       String content ="This is my test email";

       Intent intentEmail = new Intent(Intent.ACTION_SEND, Uri.parse("mailto:"));
       intentEmail.putExtra(Intent.EXTRA_EMAIL, recipients);
       intentEmail.putExtra(Intent.EXTRA_SUBJECT, subject);
       intentEmail.putExtra(Intent.EXTRA_TEXT, content);

       intentEmail.setType("text/plain");

       startActivity(Intent.createChooser(intentEmail, "Choose an email client from..."));
   }



}
Running app:
  • Send Email  (VIEW SLIDER).

3.6- Implicit intent example, photographed from the camera and save to file

An example with implicit Intent, when you click a button, the application sent to the Android system  a request to take a photograph by Camera and save image to a file.