Navigation Management ¶

Navigation management refers to the implementation of how users navigate through different screens, which include:

• 🐔 navigation between activities
• 🥚 navigation between fragments
• 🍃 navigation to another app
• ...

This usually involves a few classes:

• Intents ✈️: to start an activity
• Navigation Component 🚀: to navigate between fragments

It's worth noting that navigation involves what we call the back stack which may lead to unexpected behaviors if not handled.

Android activities are pilled up in something called the "back stack". On older devices, users can use the "back arrow" to "go back": the current activity is popped out, and we load the previous one. If there are none, then the app is terminated.

It's always the activity at the top that is shown to the user.

At the end of the example, we got two instances of "MainActivity". It's important to consider if this behavior is acceptable or not. If not, you should pass flags to your Intent using Intent#addFlags.

👉 For instance, if the user logs out, he should not be able to press "back", and go back to the "connected area".

• Manual "back" (pop out current)

intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP)

An intent is an object representing an action to be performed, such as navigating to another activity. There are two kinds of intents:

• Explicit: ask specifically for something (ex: start the Activity XXX)
• Implicit: request another application/the system (ex: open link)

Create an intent ¶

// create an intent
val intent = Intent(SOME_PARAMETERS)
// start
startActivity(intent)

Pass arguments to the next activity ¶

To pass arguments to the next activity, use:

// before using startActivity
intent.putExtra("param", "a value")

From the started activity, to get back arguments, use:

// ➡️ Within An Activity
val param = intent?.extras?.getString("param")
// ➡️ Within A Fragment
val param = requireActivity().intent?.extras?.getString("param")
val param = activity?.intent?.extras?.getString("param")

Explicit intent ¶

Ex: to navigate to "MainActivity"

// ➡️ Within An Activity
val intent = Intent(this, MainActivity::class.java)
// ➡️ Within A Fragment
val intent = Intent(requireContext(), MainActivity::class.java)
val intent = Intent(context!!, MainActivity::class.java)

Implicit intent ¶

There are a lot of them here.

val intent = Intent(Intent.ACTION_VIEW, Uri.parse("???"))

val intent = ShareCompat.IntentBuilder.from(this)
        .setText("...")
        .setType("text/plain")
        .intent

val intent = Intent(Intent.ACTION_SEND)
    .setType("text/plain")
    .putExtra(Intent.EXTRA_SUBJECT, "xxx")
    .putExtra(Intent.EXTRA_TEXT, "yyy")
    .putExtra(Intent.EXTRA_EMAIL, "a@b.c")

try {
    startActivity(intent)
} catch (ex: ActivityNotFoundException) {
    // use a toast / ...
}

if (packageManager.resolveActivity(intent, 0) != null) {
    startActivity(intent)
}

The navigation component is a Jetpack component to handle the navigation between fragments.

dependencies {
  implementation "androidx.navigation:navigation-fragment-ktx:2.5.3"
  implementation "androidx.navigation:navigation-ui-ktx:2.5.3"
}

Create a navigation graph ¶

From the resource manager, go to "navigation", then add a new navigation. For instance, "example_navigation". The generated example_navigation.xml is the following:

<?xml version="1.0" encoding="utf-8"?>
<navigation xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/example_navigation">
</navigation>

Add destinations ¶

➡️ To change the "home" fragment, select a fragment, and click on the home icon. The home fragment is the one loaded by default.

Create a NavHost ¶

Now, you need to create a container. This container will host your navigation graph. It will load the default fragment, and show another fragment when prompted.

<androidx.fragment.app.FragmentContainerView
  android:id="@+id/nav_host_fragment"
  android:name="androidx.navigation.fragment.NavHostFragment"

  app:defaultNavHost="true"
  app:navGraph="@navigation/example_navigation" />

➡️On old devices, there is an arrow to go "back". If defaultNavHost is set to true, then "back" will go back to the previous fragment.

Navigation ¶

Inside example_navigation.xml, each fragment should have an Id.

findNavController().navigate(R.id.DESTINATION_ID)

You can also create an action (=link), by linking two destinations. Then, use the action's Id

findNavController().navigate(R.id.action_xxx_to_yyy)

Additional notes ¶

class MainActivity : AppCompatActivity() {
    private lateinit var navController: NavController

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val navHostFragment = supportFragmentManager.findFragmentById(R.id.nav_host_fragment) as NavHostFragment
        navController = navHostFragment.navController
        // show the label in the menu bar
        // and the icon "back" when needed
        setupActionBarWithNavController(navController)
    }

    // pressing "back" in the menu, will go back
    override fun onSupportNavigateUp(): Boolean {
        return navController.navigateUp() || super.onSupportNavigateUp()
    }
}

findNavController().navigate(R.id.action_first_to_second, Bundle().apply {
  // Ex: passing a string
  putString("key", "value")
})

val value = arguments?.getString("key")

// At the top of your build.gradle
buildscript {
    dependencies {
        classpath "androidx.navigation:navigation-safe-args-gradle-plugin:2.5.3"
    }
}

// after plugin { ... }
apply plugin: 'androidx.navigation.safeargs.kotlin'

-findNavController().navigate(R.id.action_first_to_second, Bundle().apply {
-  putString("key", "value")
-})
+val destination = FirstDirections.actionFirstToSecond(key = "value")
+findNavController().navigate(destination)

-val value = arguments?.getString("key")
+val args by navArgs<XXXArgs>()
+val value = args.key

<action
+  app:popUpTo="@id/DESTINATION_ID"
  />

<action
  app:popUpTo="@id/first"
+  app:popUpToInclusive="true"
  />

findNavController().popBackStack()

• Android bottom navigation
• Nested navigation graphs
• Deep Link

// Args must be serializable
@Serializable
@SerialName("id") // rename field

val xxx =  registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result ->
    val yyy = result.data?.getSerializableExtra("yyy") as YYY
}
xxx.launch(intent)