Android Intents Tutorial

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1- 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:

2- 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- 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:

4- 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("//o7planning.org"));
startActivity(intent);

5- 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).
Some examples with implicit Intent:
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.

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