02 - Intents

An Intent is a messaging object you can use to request an action from another app component. Although intents facilitate communication between components in several ways, there are three fundamental use cases:

Starting an activity
Starting a service
Delivering a broadcast

The primary pieces of information in an intent are:

action -- The general action to be performed, such as ACTION_VIEW, ACTION_EDIT, ACTION_MAIN, etc.
data -- The data to operate on, such as a person record in the contacts database, expressed as a Uri.

Some examples of action/data pairs are:

ACTION_VIEW content://contacts/people/1 -- Display information about the person whose identifier is "1".

ACTION_DIAL content://contacts/people/1 -- Display the phone dialer with the person filled in.

Here is a complete list of the attributes containted in the Intent class:

Attribute	Description	Example
Action	Specifies the general action to be performed, such as `ACTION_VIEW` or `ACTION_SEND`.	`Intent.ACTION_VIEW`, `Intent.ACTION_SEND`
Data	The URI of the data that the intent is operating on, typically used with `ACTION_VIEW` or `ACTION_EDIT`.	`intent.data = Uri.parse("https://example.com")`
Category	Provides additional information about the action to be performed, like `CATEGORY_DEFAULT` or `CATEGORY_BROWSABLE`.	`intent.addCategory(Intent.CATEGORY_DEFAULT)`
Component	Specifies the explicit target component (Activity, Service, or BroadcastReceiver) to handle the intent.	`intent.component = ComponentName(context, TargetActivity::class.java)`
Extras	Key-value pairs for additional data, used to pass information between components.	`intent.putExtra("KEY_NAME", "value")`
Flags	Instructions on how the intent should be handled, controlling activity launch modes, task management, etc.	`intent.flags = Intent.FLAG_ACTIVITY_NEW_TASK`
Type	Specifies the MIME type of the data in the intent.	`intent.type = "image/*"`
Package	Specifies the package name to restrict the intent to a specific app.	`intent.setPackage("com.example.app")`
ClipData	Data related to a primary or secondary intent, typically for copy-paste or drag-and-drop functionality.	`intent.clipData = ClipData.newUri(contentResolver, "Label", uri)`

Starting an activity

You can start a new instance of an Activity by passing an Intent to startActivity(). The Intent describes the activity to start and carries any necessary data.

Start Activity from current Activity

val intent = Intent(this, TargetActivity::class.java)
intent.putExtra("KEY_NAME", "Your Data Here") 
//this data will be transmitted to the Target Activity
startActivity(intent)

Get data on Target Activity

val data = intent.getStringExtra("KEY_NAME")

Starting a service

You can start a service to perform a one-time operation (such as downloading a file) by passing an Intent to startService(). The Intent describes the service to start and carries any necessary data.

Starting a service from current activty

val myIntent = Intent(this, MyService::class.java)
myIntent.putExtra("EXTRA1", "https://example.com/path/to/file")
myIntent.putExtra("EXTRA2", "downloaded_file_name.ext")
startService(myIntent)

import android.app.IntentService
import android.content.Intent

class MyService : IntentService("MyService") {

    override fun onHandleIntent(intent: Intent?) {
        val fileUrl = intent?.getStringExtra("EXTRA1") ?: return
        val fileName = intent.getStringExtra("EXTRA2") ?: "downloaded_file"

        try { 
            //Do something with received data
            Log.d("MyService", "Done!")

        } catch (e: Exception) {
            Log.e("MyService", "Error", e)
        }
    }
}

info

With Android 5.0 (API level 21) and later, you can start a service with JobScheduler.

Delivering a broadcast

The system delivers various broadcasts for system events, such as when the system boots up or the device starts charging. You can deliver a broadcast to other apps by passing an Intent to sendBroadcast() or sendOrderedBroadcast().

// Define a custom action for your broadcast
const val CUSTOM_ACTION = "com.example.MY_CUSTOM_BROADCAST"

// Function to send the broadcast
fun sendCustomBroadcast(context: Context) {
    val intent = Intent(CUSTOM_ACTION)
    intent.putExtra("EXTRA_DATA", "This is some extra data!")
    context.sendBroadcast(intent)
}

import android.content.BroadcastReceiver
import android.content.Context
import android.content.Intent
import android.util.Log

class MyBroadcastReceiver : BroadcastReceiver() {
    override fun onReceive(context: Context, intent: Intent) {
        if (intent.action == CUSTOM_ACTION) {
            val extraData = intent.getStringExtra("EXTRA_DATA")
            Log.d("MyBroadcastReceiver", "Received broadcast with data: $extraData")
        }
    }
}

Intent Resolution

There are two primary forms of intents you will use Implicit intents and Explicit intents.

Feature	Implicit Intent	Explicit Intent
Definition	Specifies a general action, no specific target	Targets a specific component or app
Purpose	Allows user/system to choose handler	Directly calls a specific component
Use Cases	Share text, open URL	Start internal activity, launch service
System Behavior	Shows chooser if multiple apps can handle	Launches directly without user choice

Intent filters

When you use 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.

alt text

Intent filters based on three aspects:

Action - at least one needs to be specified
Data (both URI and data type) - 0 or more can be specified
Category - 0 or more can be specified

<activity android:name="ShareActivity" android:exported="false">
    <intent-filter>
        <action android:name="android.intent.action.SEND"/>
        <category android:name="android.intent.category.DEFAULT"/>
        <data android:mimeType="text/plain"/>
    </intent-filter>
</activity>

Exercices

Using the initial project structure from lab01, add buttons for each of the follwing actions:

In MainActivity

Start a new activity which displays the calculated bmi along with some useful tips. (Calculate button)

In SecondActivity

Share the BMI result as text to other apps, using an implicit intent. (share button)
Redirect the user to webpage about bmi.(info button)

note

What type of Intent is used for each of the 3 buttons? (Implicit/Explicit)

Starting an activity​

Starting a service​

Delivering a broadcast​

Intent Resolution​

Intent filters​

Exercices​