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Android Tutorial for GeckoView: Getting Started

In thus tutorial you’ll learn about GeckoView, an open source library that allows you to render web content on Android using the Gecko web engine.

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  • Kotlin 1.3, Android 8.1, Android Studio 3

What if I told you you could load a web page or any web content without using a WebView. Is it that even possible? Yes, it is with GeckoView.

GeckoView is an open source library that allows you to render web content on Android using the Gecko web engine. In this tutorial, you’ll learn more about GeckoView and other Android tools used for web content.

Note: This tutorial assumes you’re already familiar with the basics of Android development. If you are completely new to Android development, read through our Beginning Android Development tutorials to familiarize yourself with the basics.

What is GeckoView?

GeckoView is an Android library which brings the Gecko web engine to the Android community. It was created by the community and the developers at Mozilla. The folks at Mozilla have a lot of experience with the web and browsers in general.

Have you heard about FireFox? They’re the people behind it. :]

FireFox and GeckoView both use the web engine Gecko. While GeckoView is a new library, it powers many cool open source apps like FireFox Preview, FireFox Focus and FireFox Reality. You can even contriubte to other awesome projects powered by GeckoView.

GeckoView vs WebView

WebView and GeckoView are not the same. GeckoView has its own API and is not a drop in replacement. Here are some major differences between them:

  • While WebView was not designed to create web browsers GeckoView was. It exposes the entire power of the Web to Android applications.
  • WebView can be updated, however consistency in API and behavior can vary from device to device. This makes it harder to develop for. In contrast, GeckoView is self-contained. You’ll ship the whole API with your app so you’ll get what you tested with no surprises.
  • GeckoView complies with web standards. Moreover, it’s backed by Mozilla, an organization with a history of fighting for the open web.
  • GeckoView provides access and control over low level web APIs. It also has built-in support for private mode, tracking protection, WebVR and, coming soon, Web Extensions support.

Finally, GeckoView adds additional space to your final APK. For example, you can see a growth up to 30MB to 40MB from the library alone. This is because GeckoView includes the whole web engine with the app.

If you’re considering GeckoView for your app, ask yourself these questions:

  • Are you developing a web browser-ish or similar app?
  • Do you need guaranteed access to web APIs?
  • Are you developing a game that relies on web technologies?

If you answered yes, GeckoView could be a great option for you. In contrast, if you only want to render some web content without API warranties WebView could be your best bet.

Getting Started

While working on the tutorial, you’ll create an app called AwesomeBrowser. It’s a small web browser that showcases the power of GeckoView. It allows normal browsing and includes tracking protection enabled by default.

Awesome Browser Demo

First, download the AwesomeBrowser project using the Download Materials button at the top or bottom of this tutorial. Then open the project in Android Studio 3.3 or later by selecting Open an existing Android Studio project on the Android Studio welcome screen.

Open an existing Android Studio project

Before continuing with the tutorial, take a look at the project structure.

Exploring the AwesomeBrowser Structure

This is the source code’s structure:

AwesomeBrowser project structure

  • MainActivity.kt Here you’ll handle the geckoView instance and toolbar.
  • activity_main.xml Here you’ll define the main layout.
  • Helpers.kt This contains a set of helper functions and constants.

As you can see in the above image, there are more files I didn’t mention for the sake of brevity. You won’t need to worry about them for this tutorial, but feel free to poke around if you’re curious.

Build and run the app. All you’ll see is a blank screen until you add the UI!

blank screen

Setting Up GeckoView

Begin with the starter project. For the moment, your main goal is to configure GeckoView‘s dependency and then load a page with it.

GeckoView is distributed as a Gradle dependency and its artifacts live in To include it, in the project level build.gradle add the maven repository to the allprojects block right under jcenter().


maven {
    url ""

This includes all the artifacts in the maven Mozilla repository.

Next, add the following code to the app ‣ build.gradle, after the apply: plugin lines, and before the android block:

ext {
    geckoview_channel = "nightly"
    geckoview_version = "70.0.20190712095934"

Similar to FireFox, GeckoView has different channels for different purposes. These include the latest stable release, the beta tested release and the nightly developer build. In this case, you’ve chosen the nightly build because you’ll get the most updated features and bug fixes.

To keep things neat and tidy, you’ve added a variable for the version of Gecko to use and the channel from which to retrieve it.

Note: Take a look at the API Changelog to see what changed from version to version. You can check the API docs for extra references.

Next, add the following to app ‣ build.gradle below the buildTypes block, inside the android block:

    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8

As GeckoView uses some Java 8 APIs, it requires these compatibility flags.

Finally, for the last Gradle configuration, add the following code inside the dependencies block in the same file.

  implementation "org.mozilla.geckoview:geckoview-${geckoview_channel}:${geckoview_version}"

Above you’re adding the dependency for GeckoView dependency to the project.

Sync your project with the gradle files now that you’ve made these changes.

Adding Some UI

To interact with GeckoView you need two main objects: a GeckoView widget and a GeckoSesssion.

GeckoView Widget
This is the UI component you’re going to use for rendering web content. It’s a normal Android view you can create using either XML or code.

A session represents a tab in a web browser. It contains the URL you want to load and all of its additional configurations.

Imagine that you have a web browser with multiple tabs on it.

Each tab, or GeckoSession, has:

  • State linked to it.
  • Properties like isPrivateMode enabled or disabled.
  • isDesktopMode enable or disabled.
  • Its own browsing history.

You want to keep all this information separated by tab and decoupled from the UI, GeckoView widget.

This is a job for GeckoSession. It’s an object that holds all these configurations in one place making it easy and straightforward to work with it.

This powerful concept decouples the view, GeckoView, from the state, GeckoSession. This way you could have one view, GeckoView, in charge of rendering and multiple sessions, GeckoSession, representing the state of each tab.

This would be tricky to achieve using WebView. Because of the lack of sessions, it applies configurations directly to the view. :[

A great way to customize how a session should behave is by providing a GeckoSessionSettings object. You can do that by passing the settings object to the GeckoSession constructor.

You create a settings object using its builder GeckoSessionSettings.Builder(). Here’s an example of how you might do this:

  val settings = GeckoSessionSettings.Builder()

These are some other things you can configure on a session:

Notice that every GeckoSession holds its own settings object. This allows you to do much customization without creating multiple instances of GeckoView. Remember the multiple tabs example.

Now, that you understand how GeckoView widget and GeckoSession work, you can add them to AwesomeBrowser.

First, open activity_main.xml to add the GeckoView widget. Replace the TextView with the GeckoView widget below.


Next, wire it in the MainActivity.kt file.

Add two private properties to MainActivity: one called geckoView, the UI widget, and another called geckoSession, your tab:

private lateinit var geckoView: GeckoView
private val geckoSession = GeckoSession()

Here, you declare two properties so you can access them from different functions.

Next, create a function called setupGeckoView(). Remember to import all the new references.

  private fun setupGeckoView() {
    // 1
    geckoView = findViewById(

    // 2
    val runtime = GeckoRuntime.create(this)

    // 3

    // 4

Here’s what your code above does:

  1. First you bind the GeckoView object from the XML.
  2. You then create a GeckoRuntime which is responsible for holding all the state for the underlying Gecko engine.
  3. To be able to load an URL you need a GeckoSession. Here you assign it to the GeckoView instance. This way the GeckoView will know which info needs to load.
  4. Finally, you use the GeckoSession instance to load a URL. In this case, about:buildconfig, an internal page that shows all the configurations of this GeckoView build. This can be whatever url you like.

As you know, the GeckoView widget has its own API. This code would look very different if you were using a WebView.

To make use of your new method, add a call to setupGeckoView() to the bottom of onCreate():


You’re almost ready to run the app. But first you need to setup a proper build variant according to the device or emulator architecture that you’re going to test on. Open the Build Variants window in Android Studio and select your architecture:

Awesome Browser Build Variants

You can see your emulator CPU architecture by going to Tools ▸ AVD Manager. You’ll see it on the grid cell under CPU/ABI.

Build Variant

Most devices out there are based on ARM CPU architecture, but you have to verify yours.

Now, build and run the app. :]

If you have everything set up correctly, then you’ll see this:

Starter Project

Above you loaded an internal page called about:buildconfig. It’s a page that shows all the configurations of this GeckoView build, but you can replace it with whatever url you like.

Note: When you don’t select the correct build variant for your hardware’s CPU architecture a common problem happens. You’ll get an error similar to this one:
Build Variante Error
You can fix this by first selecting the correct build variant and then running the app again.

Adding a Toolbar

Having to recompile each time you want to load a different site isn’t an ideal situation. In this section, you’ll focus on creating a toolbar that allows you to enter the URL you want to visit.

Instead of having the URL hard-coded, update the layout to add a Toolbar. First, wrap the GeckoView in activity_main.xml with a RelativeLayout. The file should now look like this:


    tools:context=".MainActivity" />


Here, you added a RelativeLayout that will allow you to control how the widgets layout. Now, you can add the Toolbar above the GeckoView widget.

Add this code inside the RelativeLayout and above the GeckoView:


      android:singleLine="true" />


This Toolbar contains an EditText you’ll use to enter new URLs.

Last but not least, to make sure everything looks nice, add this property to the GeckoView to make sure it is rendered below the Toolbar:


With the layout set up, it’s time to connect the input of the EditText with the GeckoView.

Next, go to the MainActivity.kt file and then, add a private property just below of the geckoSession.

 private lateinit var urlEditText: EditText

You added a EditText to allow you to fetch the entered URL.

Then update the setupGeckoView() function replacing the hard-coded URL geckoSession.loadUri("about:buildconfig") with the two lines below:


Here, you updated the initial URL to a constant value. When the app starts it will load this URL. INITIAL_URL is a helper constant and you can see its value in Helpers.kt.

Next, continue updating the URL with the content of the EditText. Add the following method after onCreate, importing for the ActionBar:

  private fun setupToolbar() {
    supportActionBar?.displayOptions = ActionBar.DISPLAY_SHOW_CUSTOM

This indicates to the action bar that you are going to use a custom view, in this case the EditText.

Next, add the onCommit() function. You’ll call this when a new URL is entered:

  fun onCommit(text: String) {
    if ((text.contains(".") || 
            text.contains(":")) && 
        !text.contains(" ")) {
    } else {
      geckoSession.loadUri(SEARCH_URI_BASE + text)


Here you update the new URL in the GeckoView widget. If the text entered is an URL you load it. Otherwise, you trigger a DuckDuckGo search.

SEARCH_URI_BASE is a constant helper. It contains a parameterized url. You just add the search criteria by appending the text.

Then, add the setupUrlEditText() function to link it all together:

  private fun setupUrlEditText() {
    urlEditText = findViewById(

    urlEditText.setOnEditorActionListener(object : 
        View.OnFocusChangeListener, TextView.OnEditorActionListener {

      override fun onFocusChange(view: View?, hasFocus: Boolean) = Unit

      override fun onEditorAction(
          textView: TextView,
          actionId: Int,
          event: KeyEvent?
      ): Boolean {
        return true

Here you add a listener for every time the user hits the enter key. This way it will know that the user wants to load the entered URL, and call onCommit() when this happens.

Finally, add calls to this set up. Inside the onCreate function, add these two new functions, setupUrlEditText() and setupToolbar(), right before the setupGeckoView().

 // 1


Here you:

  1. Set up the toolbar when the activity starts.
  2. Bind the EditText and set a listener for when a new URL is entered.

Phew, that’s all set up now. Build and run the App!

Now you’re able to enter the URL you want or perform a search with a word or a phrase.

AwesomeBrowser Demo

Adding a Progress Bar

Some pages take a long time to load and there’s no clear way to know if it’s loading or not. Has it finished or is still loading? Adding a toolbar will boost the user experience.

For this job, you need a ProgressBar and way to know which state a page is in. Fortunately, GeckoView provides an API for that. The ProgressDelegate is an interface for observing the progress of a web page.

The ProgressDelegate interface has different callbacks for the different statuses:

  • onPageStart: Called when the content has started loading.
  • onPageStop: Called when the content has finished loading.
  • onProgressChange: Called every time the progress of a web page has changed.
  • onSecurityChange: Indicates when the security status updates. It passes a SecurityInformation object that contains useful information about the site security.

Now, to put all that together in code.

First, go to the activity_main.xml file. Add a ProgressBar to the RelativeLayout, right after the Toolbar and before the GeckoView:

    android:layout_alignBottom="@id/toolbar" />

This adds the ProgressBar at the bottom of the toolbar.

Next, add the ProgressBar to the MainActivity.kt file. First add a progressBar property below the urlEditText.

private lateinit var progressView: ProgressBar

This adds a private property to control the progress of the progress bar from different points in the code.

Next, add createProgressDelegate() to the end of MainActivity.

private fun createProgressDelegate(): GeckoSession.ProgressDelegate {
    return object : GeckoSession.ProgressDelegate {

      override fun onPageStop(session: GeckoSession, success: Boolean) = Unit

      override fun onSecurityChange(
          session: GeckoSession, 
          securityInfo: GeckoSession.ProgressDelegate.SecurityInformation
      ) = Unit

      override fun onPageStart(session: GeckoSession, url: String) = Unit

      override fun onProgressChange(session: GeckoSession, progress: Int) {
        progressView.progress = progress

        if (progress in 1..99) {
          progressView.visibility = View.VISIBLE
        } else {
          progressView.visibility = View.GONE

This is your implementation of ProgressDelegate. Since you’re only interested in changes in progress, onProgressChange() is the only function you’ve added an implementation for.

Inside it, you’re updating the progress bar depending on the progress of the page. You’re hiding or showing the progress bar depending on if the page has finished loading or not.

After that, at the end of setupGeckoView(), add these two lines.

 progressView = findViewById(
 geckoSession.progressDelegate = createProgressDelegate()

Here, you add the ProgressDelegate. This is how the geckoSession allows you to listen for new updates in the progress of a web page.

Now build and run the app. You’ll see a the progress as the page loads.

AwesomeBrowser Demo

Tracking Protection

Have you ever felt that some ads follow you to whatever site you visit? For example, you were looking at a pair of shoes on an e-commerce site. Then, on every page you visit after that you’re harassed with ads for that same pair of shoes.

No, the shoes aren’t haunted and it’s not a coincidence. Many companies make a living tracking your preferences and behavior on the web and then selling it, without even you knowing.

Tracking protection, an amazing builit-in feature of GeckoView, stops these sites from following you. Time to see how you can integrate this feature into AwesomeBrowser.

GeckoView provides an API for enabling tracking protection and getting information about the trackers. ContentBlocking.Delegate is an interface that allows you to set a listener on a session and get notified when a content element is blocked from loading. It also provides you with useful data like the categories of the blocked content via a BlockEvent object.

Time to update the UI to show how many trackers have been blocked per site.

First, update activity_main.xml to add a TextView at the end of the RelativeLayout.

    android:text="0" />

trackers_count is a where you’re going to store how many trackers were blocked. When it’s pressed a pop-up window will appear showing all the details about the trackers.

Next, in MainActivity.kt add the logic for showing the number of trackers blocked and the individual details of each one. Don’t forget to import all the new references.

Add two properties, trackersCount and trackersBlockedList, after the progressView.

private lateinit var trackersCount: TextView
private var trackersBlockedList: List<ContentBlocking.BlockEvent> = 

Here, you add two private properties, trackersCount and trackersBlockedList. These two contain how many trackers were blocked and the list of each of them.

Continue by adding a function to create the blocking delegate. Add createBlockingDelegate() to the end of the MainActivity class:

  private fun createBlockingDelegate(): ContentBlocking.Delegate {
    return object : ContentBlocking.Delegate {
      override fun onContentBlocked(session: GeckoSession, event: ContentBlocking.BlockEvent) {
        trackersBlockedList = trackersBlockedList + event
        trackersCount.text = "${trackersBlockedList.size}"

Here you set a listener for notifying when content is blocked. Inside the listener, you’re updating the state of the trackersBlockedList, adding a new blocked content to the list and then updating the counter, trackersCount.

Now use your delegate by adding this to the end of setupGeckoView():

 geckoSession.settings.useTrackingProtection = true
 geckoSession.contentBlockingDelegate = createBlockingDelegate()

This enables tracking protection. So, you’re telling this session that tracking protection will be activated.

Now that you have the delegate set up, you can show the list of blocked URLs when the count is tapped.

Start by adding an extension function categoryToString() to categorize different blocked events:

  private fun ContentBlocking.BlockEvent.categoryToString(): String {
    val stringResource = when (categories) {
      ContentBlocking.NONE -> R.string.none
      ContentBlocking.AT_ANALYTIC -> R.string.analytic
      ContentBlocking.AT_AD ->
      ContentBlocking.AT_TEST -> R.string.test
      ContentBlocking.SB_MALWARE -> R.string.malware
      ContentBlocking.SB_UNWANTED -> R.string.unwanted
      ContentBlocking.AT_SOCIAL ->
      ContentBlocking.AT_CONTENT -> R.string.content
      ContentBlocking.SB_HARMFUL -> R.string.harmful
      ContentBlocking.SB_PHISHING -> R.string.phishing
      else -> R.string.none
    return getString(stringResource)

Here you map every tracker category to a String one.

Then add getFriendlyTrackersUrls() to use this extension function and get a list of formatted URLS:

  private fun getFriendlyTrackersUrls(): List<Spanned> {
    return { blockEvent ->
      val host = Uri.parse(blockEvent.uri).host
      val category = blockEvent.categoryToString()
          "<b><font color='#D55C7C'>[$category]</font></b> <br/> $host", 

Here’s what you did. You styled the tracker’s list, adding a highlighting color to each blocked entry.

Next, add setupTrackersCounter() below the previous function to show the blocked events:

private fun setupTrackersCounter() {
  // 1
  trackersCount = findViewById(
  trackersCount.text = "0"
  // 2
  trackersCount.setOnClickListener {
    if (trackersBlockedList.isNotEmpty()) {
      val friendlyURLs = getFriendlyTrackersUrls()

Here’s what you just did:

  1. Set the initial trackers count.
  2. Added a listener to show a dialog with the blocked events when the counter is tapped.

Then add a call to this function at the end of setupGeckoView():


One last thing before this is finished. The count needs to reset when a new URL is opened. Add the clearTrackersCount() to MainActivity.

private fun clearTrackersCount() {
  trackersBlockedList = emptyList()
  trackersCount.text = "0"

This clears all the tracker’s information.

Finally, add a call to clearTrackersCount() at the beginning of the onCommit() function.


Here, you re-set the trackers count when a new URL is entered so you won’t mix trackers from different sessions.

Finally, build run the app.

After the changes, AwesomeBrowser is complete. Try going to different websites to see the counter number increment and reset. Tap on the number to see the full list.

Finished AwesomeBrowser

If you have any issues you can always take a look at the final project. You can find the final project by using the Download Materials button at the top or bottom of the tutorial.

Where to Go From Here?

GeckoView is a huge and amazing project and while this tutorial couldn’t cover everything it has to offer, hopefully it awakened your curiosity.

To get a better idea of everything GeckoView has to offer, take a look at the official website, also you can find the GeckoView source code here. Additionally, the GeckoView team has a sample app where they show all its features.

Here’s a special shout-out to Emily Toop, a member of the GeckoView team, as well as Dan Callahan, developer advocate at Mozilla, for all their help during this tutorial. If you have any questions about the APIs or the project, they’ll be more than happy to help.

I hope you enjoyed this tutorial as much as I did. If you have any comments or questions, please join in on the forum discussion below!

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