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Use LiveData with ViewModel

You have learned in the previous codelabs, how to use a ViewModel to store the app data. ViewModel allows the app's data to survive configuration changes. In this codelab, you'll learn how to integrate LiveData with the data in the ViewModel.

The LiveData class is also part of the Android Architecture Components and is a data holder class that can be observed.

Prerequisites

  • How to download source code from GitHub and open it in Android studio.
  • How to create and run a basic Android app in Kotlin, using activities and fragments.
  • How the activity and fragment life cycles work.
  • How to retain UI data through device-configuration changes using a ViewModel.
  • How to write lambda expressions.

What you'll learn

  • How to use LiveData and MutableLiveData in your app.
  • How to encapsulate the data stored in a ViewModel with LiveData.
  • How to add observer methods to observe changes in the LiveData.
  • How to write binding expressions in a layout file.

What you'll build

  • Use LiveData for the app's data (word, word count and the score) in the Unscramble app.
  • Add observer methods that get notified when the data changes, update the scrambled word text view automatically.
  • Write binding expressions in the layout file, which are triggered when the underlying LiveData is changed. The score, word count and the scrambled word text views are updated automatically.

What you need

  • A computer with Android Studio installed.
  • Solution code from the previous codelab (Unscramble app with ViewModel).

Download the starter code for this codelab

This codelab uses the Unscramble app that you built in the previous codelab ( Store data in ViewModel) as the starter code.

This codelab uses the Unscramble solution code that you are familiar with from the previous codelab. The app displays a scrambled word for the player to unscramble it. The player can try any number of times to guess the correct word. The app data such as the current word, player's score and word count are saved in the ViewModel. However, the app's UI does not reflect the new score and word count values. In this codelab, you will implement the missing features using LiveData.

a20e6e45e0d5dc6f.png

LiveData is an observable data holder class that is lifecycle-aware.

Some characteristics of LiveData:

  • LiveData holds data; LiveData is a wrapper that can be used with any type of data.
  • LiveData is observable, which means that an observer is notified when the data held by the LiveData object changes.
  • LiveData is lifecycle-aware. When you attach an observer to the LiveData, the observer is associated with a LifecycleOwner (usually an activity or fragment). The LiveData only updates observers that are in an active lifecycle state such as STARTED or RESUMED. You can read more about LiveData and observation here.

UI updation in the starter code

In the starter code the updateNextWordOnScreen() method is called explicitly, every time you want to display a new scrambled word in the UI. You call this method during game initialization, and when players press the Submit or Skip button. This method is called from the methods onViewCreated(), restartGame(), onSkipWord(), and onSubmitWord(). With Livedata, you will not have to call this method from multiple places to update the UI. You will do it only once in the observer.

In this task, you will learn how to wrap any data with LiveData, by converting the current word in the GameViewModel to LiveData. In a later task, you will add an observer to these LiveData objects and learn how to observe the LiveData.

MutableLiveData

MutableLiveData is the mutable version of the LiveData, that is, the value of the data stored within it can be changed.

  1. In GameViewModel, change the type of the variable _currentScrambledWord to MutableLiveData<String>. LiveData and MutableLiveData are generic classes, so you need to specify the type of data that they hold.
  2. Change the variable type of _currentScrambledWord to val because the value of the LiveData/MutableLiveData object will remain the same, and only the data stored within the object will change.
private val _currentScrambledWord = MutableLiveData<String>()
  1. Change the backing field currentWordCount type to LiveData<String>, because it is immutable. Android Studio will show some errors which you will fix in the next steps.
val currentScrambledWord: LiveData<String>
   get() = _currentScrambledWord
  1. To access the data within a LiveData object, use the value property. In GameViewModel inside the getNextWord() method, within the else block, change the reference of _currentScrambledWord to _currentScrambledWord.value.
private fun getNextWord() {
 ...
   } else {
       _currentScrambledWord.value = String(tempWord)
       ...
   }
}

In this task you set up an observer in the app component, GameFragment. The observer you will add observes the changes to the app's data currentScrambledWord. LiveData is lifecycle-aware, meaning it only updates observers that are in an active lifecycle state. So the observer in the GameFragment will only be notified when the GameFragment is in STARTED or RESUMED states.

  1. In GameFragment, delete the method updateNextWordOnScreen() and all the calls to it. You do not require this method, as you will be attaching an observer to the LiveData.
  2. In onSubmitWord(), modify the empty if-else block as follows. The complete method should look like this.
private fun onSubmitWord() {
    val playerWord = binding.textInputEditText.text.toString()

    if (viewModel.isUserWordCorrect(playerWord)) {
        setErrorTextField(false)
        if (!viewModel.nextWord()) {
            showFinalScoreDialog()
        }
    } else {
        setErrorTextField(true)
    }
}
  1. Attach an observer for currentScrambledWord LiveData. In GameFragment at the end of the callback onViewCreated(), call the observe() method on currentScrambledWord.
// Observe the currentScrambledWord LiveData.
viewModel.currentScrambledWord.observe()

Android Studio will display an error about missing parameters. You will fix the error in the next step.

  1. Pass viewLifecycleOwner as the first parameter to the observe() method. The viewLifecycleOwner represents the Fragment's View lifecycle. This parameter helps the LiveData to be aware of the GameFragment lifecycle and notify the observer only when the GameFragment is in active states (STARTED or RESUMED).
  2. Add a lambda as a second parameter with newWord as a function parameter. The newWord will contain the new scrambled word value.
// Observe the scrambledCharArray LiveData, passing in the LifecycleOwner and the observer.
viewModel.currentScrambledWord.observe(viewLifecycleOwner,
   { newWord ->
   })

A lambda expression is an anonymous function that isn't declared, but is passed immediately as an expression. A lambda expression is always surrounded by curly braces { }.

  1. In the function body of the lambda expression, assign newWord to the scrambled word text view.
// Observe the scrambledCharArray LiveData, passing in the LifecycleOwner and the observer.
viewModel.currentScrambledWord.observe(viewLifecycleOwner,
   { newWord ->
       binding.textViewUnscrambledWord.text = newWord
   })
  1. Compile and run app. Your game app should work exactly as before, but now the scrambled word text view is automatically updated in the LiveData observer, not in the updateNextWordOnScreen() method.

As in the previous task, in this task you will add LiveData to the other data in the app, score and word count, so that the UI is updated with correct values of the score and word count during the game.

Step 1: Wrap score and wordcount with LiveData

  1. In GameViewModel, change the type of the _score and _currentWordCount class variables to val.
  2. Change the data type of the variables _score and _currentWordCount to MutableLiveData and initialize them to 0.
  3. Change backing fields type to LiveData<Int>.
private val _score = MutableLiveData(0)
val score: LiveData<Int>
   get() = _score

private val _currentWordCount = MutableLiveData(0)
val currentWordCount: LiveData<Int>
   get() = _currentWordCount
  1. In GameViewModel at the beginning of the reinitializeData() method, change the reference of _score and _currentWordCount to _score.value and _currentWordCount.value respectively.
fun reinitializeData() {
   _score.value = 0
   _currentWordCount.value = 0
   wordsList.clear()
   getNextWord()
}
  1. In the GameViewModel, inside the nextWord() method, change the reference of _currentWordCount to _currentWordCount.value!!.
fun nextWord(): Boolean {
    return if (_currentWordCount.value!! < MAX_NO_OF_WORDS) {
           getNextWord()
           true
       } else false
   }
  1. In GameViewModel, inside the increaseScore() and getNextWord() methods, change the reference of _score and _currentWordCount to _score.value and _currentWordCount.value respectively. Android Studio will show you an error because _score is no longer an integer, it's LiveData, you will fix it in the next steps.
  2. Use the plus() Kotlin function to increase the _score value, which performs the addition with null-safety.
private fun increaseScore() {
    _score.value = (_score.value)?.plus(SCORE_INCREASE)
}
  1. Similarly use inc() Kotlin function to increment the value by one with null-safety.
private fun getNextWord() {
   ...
    } else {
        _currentScrambledWord.value = String(tempWord)
        _currentWordCount.value = (_currentWordCount.value)?.inc()
        wordsList.add(currentWord)
       }
   }
  1. In GameFragment, access the value of score using the value property. Inside the showFinalScoreDialog() method, change viewModel.score to viewModel.score.value.
private fun showFinalScoreDialog() {
   MaterialAlertDialogBuilder(requireContext())
       .setTitle(getString(R.string.congratulations))
       .setMessage(getString(R.string.you_scored, viewModel.score.value))
       ...
       .show()
}

Step 2: Attach observers to score and word count

In the app, the score and the word count are not updated. You will update them in this task using LiveData observers.

  1. In GameFragment inside the onViewCreated() method, delete the code that updates the score and word count text views.

Remove:

binding.score.text = getString(R.string.score, 0)
binding.wordCount.text = getString(R.string.word_count, 0, MAX_NO_OF_WORDS)
  1. In the GameFragment at the end of onViewCreated() method, attach observer for score. Pass in the viewLifecycleOwner as the first parameter to the observer and a lambda expression for the second parameter. Inside the lambda expression, pass the new score as a parameter and inside the function body, set the new score to the text view.
viewModel.score.observe(viewLifecycleOwner,
   { newScore ->
       binding.score.text = getString(R.string.score, newScore)
   })
  1. At the end of the onViewCreated() method, attach an observer for the currentWordCount LiveData. Pass in the viewLifecycleOwner as the first parameter to the observer and a lambda expression for the second parameter. Inside the lambda expression, pass the new word count as a parameter and in the function body, set the new word count along with the MAX_NO_OF_WORDS to the text view.
viewModel.currentWordCount.observe(viewLifecycleOwner,
   { newWordCount ->
       binding.wordCount.text =
           getString(R.string.word_count, newWordCount, MAX_NO_OF_WORDS)
   })

The new observers will be triggered when the value of score and word count change inside the ViewModel, during the lifetime of the lifecycle owner, that is, the GameFragment.

  1. Run your app to see the magic. Play the game through some words. Score and word count are also updated correctly on the screen. Observe that you are not updating these text views based on some conditions in the code. The score and currentWordCount are LiveData and the corresponding observers are automatically called when the underlying value changes.

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In the previous tasks, your app listens to the data changes in the code. Similarly, apps can listen to the data changes from the layout. With Data Binding, when an observable LiveData value changes, the UI elements in the layout it's bound to are also notified, and the UI can be updated from within the layout.

Concept: Data binding

In the previous codelabs you have seen View Binding, which is a one-way binding. You can bind views to code but not vice versa.

Refresher for View binding:

View binding is a feature that allows you to more easily access views in code. It generates a binding class for each XML layout file. An instance of a binding class contains direct references to all views that have an ID in the corresponding layout. For example, the Unscramble app currently uses view binding, so the views can be referenced in the code using the generated binding class.

Example:

binding.textViewUnscrambledWord.text = newWord
binding.score.text = getString(R.string.score, newScore)
binding.wordCount.text =
                  getString(R.string.word_count, newWordCount, MAX_NO_OF_WORDS)

Using view binding you can't reference the app data in the views (layout files). This can be accomplished using Data binding.

Data Binding

Data Binding Library is also a part of the Android Jetpack library. Data binding binds the UI components in your layouts to data sources in your app using a declarative format, which you will learn later in the codelab.

In simpler terms Data binding is binding data (from code) to views + view binding (binding views to code)

Example using view binding in UI controller

binding.textViewUnscrambledWord.text = viewModel.currentScrambledWord

Example using data binding in layout file

android:text="@{gameViewModel.currentScrambledWord}"

The above example shows how to use the Data Binding Library to assign app data to the views/widget directly in the layout file. Note the use of @{} syntax in the assignment expression.

The main advantage of using data binding is, it lets you remove many UI framework calls in your activities, making them simpler and easier to maintain. This can also improve your app's performance and help prevent memory leaks and null pointer exceptions.

Step 1: Change view binding to data binding

  1. In the build.gradle(Module) file, enable the dataBinding property under the buildFeatures section.

Replace

buildFeatures {
   viewBinding = true
}

with

buildFeatures {
   dataBinding = true
}

Do a gradle sync when prompted by Android Studio.

  1. To use data binding in any Kotlin project, you should apply the kotlin-kapt plugin. This step is already done for you in the build.gradle(Module) file.
plugins {
   id 'com.android.application'
   id 'kotlin-android'
   id 'kotlin-kapt'
}

Above steps auto generates a binding class for every layout XML file in the app. If the layout file name is activity_main.xml then your autogen class will be called ActivityMainBinding.

Step 2: Convert layout file to data binding layout

Data binding layout files are slightly different and start with a root tag of <layout> followed by an optional <data> element and a view root element. This view element is what your root would be in a non-binding layout file.

  1. Open game_frament.xml, select code tab.
  2. To convert the layout to a Data Binding layout, wrap the root element in a <layout> tag. You'll also have to move the namespace definitions (the attributes that start with xmlns:) to the new root element. Add <data></data> tags inside <layout> tag above the root element. Android Studio offers a handy way to do this automatically: Right-click the root element (ScrollView), select Show Context Actions > Convert to data binding layout

f356fc45e8fe91b1.png

  1. Your layout should look something like this:
<layout xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:app="http://schemas.android.com/apk/res-auto"
   xmlns:tools="http://schemas.android.com/tools">

   <data>

   </data>

   <ScrollView
       android:layout_width="match_parent"
       android:layout_height="match_parent">

       <androidx.constraintlayout.widget.ConstraintLayout
         ...
       </androidx.constraintlayout.widget.ConstraintLayout>
   </ScrollView>
</layout>
  1. In GameFragment, at the beginning of the onCreateView() method, change the instantiation of the binding variable to use data binding.

Replace

binding = GameFragmentBinding.inflate(inflater, container, false)

With

binding = DataBindingUtil.inflate(inflater, R.layout.game_fragment, container, false)
  1. Compile the code; you should be able to compile without any issues. Your app now uses data binding and the views in the layout can access the app data.

In this task you will add properties in the layout file to access the app data from the viewModel. You will initialize the layout variables in the code.

  1. In game_frament.xml, inside the <data> tag and a child tag called <variable>, declare a property called gameViewModel and of the type GameViewModel. You will use this to bind the data in ViewModel to the layout.
<data>
   <variable
       name="gameViewModel"
       type="com.example.android.unscramble.ui.game.GameViewModel" />
</data>

Notice the type of gameViewModel contains the package name. Make sure this package name matches with the package name in your app.

  1. Below the gameViewModel declaration, add another variable inside the <data> tag of type Integer, and name it maxNoOfWords. You will use this to bind to the variable in ViewModel to store the number of words per game.
<data>
   ...
   <variable
       name="maxNoOfWords"
       type="int" />
</data>
  1. In GameFragment at the beginning of the onViewCreated()method, initialize the layout variables gameViewModel and maxNoOfWords.
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
   super.onViewCreated(view, savedInstanceState)

   binding.gameViewModel = viewModel

   binding.maxNoOfWords = MAX_NO_OF_WORDS
...
}
  1. The LiveData is lifecycle-aware observable, so you have to pass the lifecycle owner to the layout. In the GameFragment, inside the onViewCreated()method, below the initialization of the binding variables, add the following code.
   // Specify the fragment view as the lifecycle owner of the binding.
   // This is used so that the binding can observe LiveData updates
   binding.lifecycleOwner = viewLifecycleOwner

Recall that you implemented a similar functionality when implementing LiveData observers. You passed viewLifecycleOwner as one of the parameters to the LiveData observers.

Binding expressions are written within the layout in the attribute properties (such as android:text) referencing the layout properties. Layout properties are declared at the top of the data binding layout file, via the <variable> tag. When any of the dependent variables change, the ‘DB Library' will run your binding expressions (and thus updates the views). This change-detection is a great optimization which you get for free, when you use a Data Binding Library.

Syntax for binding expressions

Binding expressions start with an @ symbol and are wrapped inside curly braces {}. In the following example, the TextView text is set to the firstName property of the user variable:

Example:

<TextView android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:text="@{user.firstName}" />

Step 1: Add binding expression to the current word

In this step, you bind the current word text view to the LiveData object in the ViewModel.

  1. In game_fragment.xml, add a text attribute to the textView_unscrambled_word text view. Use the new layout variable, gameViewModel and assign @{gameViewModel.currentScrambledWord} to the text attribute.
<TextView
   android:id="@+id/textView_unscrambled_word"
   ...
   android:text="@{gameViewModel.currentScrambledWord}"
   .../>
  1. In GameFragment, remove the LiveData observer code for currentScrambledWord: You don't need the observer code in fragment any more. The layout receives the updates of the changes to the LiveData directly.

Remove

viewModel.currentScrambledWord.observe(viewLifecycleOwner,
   { newWord ->
       binding.textViewUnscrambledWord.text = newWord
   })
  1. Run your app, your app should work as before. But now the scrambled word text view uses the binding expressions to update the UI, not the LiveData observers.

Step 2: Add binding expression to the score and the word count

Resources in data binding expressions

A data binding expression can reference app resources with the following syntax.

Example:

android:padding="@{@dimen/largePadding}"

In the above example, the padding attribute is assigned a value of largePadding from the dimen.xml resource file.

You can also pass layout properties as resource parameters.

Example:

android:text="@{@string/example_resource(user.lastName)}"

strings.xml

<string name="example_resource">Last Name: %s</string>

In the above example, example_resource is a string resource with %s placeholder. You are passing user.lastName as a resource parameter in the binding expression, where user is a layout variable.

In this step you will add binding expressions to the score and word count text views, passing in the resource parameters. This step is similar to what you did for textView_unscrambled_word above.

  1. In game_fragment.xml, update the text attribute for word_count text view with the following binding expression. Use word_count string resource and pass in gameViewModel.currentWordCount, and maxNoOfWords as resource parameters.
<TextView
   android:id="@+id/word_count"
   ...
   android:text="@{@string/word_count(gameViewModel.currentWordCount, maxNoOfWords)}"
   .../>
  1. Update the text attribute for score text view with the following binding expression. Use score string resource and pass in gameViewModel.score as a resource parameter.
<TextView
   android:id="@+id/score"
   ...
   android:text="@{@string/score(gameViewModel.score)}"
   ... />
  1. Remove LiveData observers from the GameFragment. You don't need them any longer, binding expressions update the UI when the corresponding LiveData changes.

Remove:

viewModel.score.observe(viewLifecycleOwner,
   { newScore ->
       binding.score.text = getString(R.string.score, newScore)
   })

viewModel.currentWordCount.observe(viewLifecycleOwner,
   { newWordCount ->
       binding.wordCount.text =
           getString(R.string.word_count, newWordCount, MAX_NO_OF_WORDS)
   })
  1. Run your app and play through some words. Now your code uses LiveData and binding expressions to update the UI.

8b35e5d705554e5f.png 534cb03ffcf41c16.png

Congratulations! You have learned how to use LiveData with LiveData observers and LiveData with binding expressions.

As you've been learning throughout this course, you want to build apps that are accessible to as many users as possible. Some users may use Talkback to access and navigate your app. TalkBack is the Google screen reader included on Android devices. TalkBack gives you spoken feedback so that you can use your device without looking at the screen.

With Talkback enabled, ensure that a player can play the game.

  1. Enable Talkback on your device by following these instructions.
  2. Return to the Unscramble app.
  3. Explore your app with Talkback using these instructions. Swipe right to navigate through screen elements in sequence, and swipe left to go in the opposite direction. Double-tap anywhere to select. Verify that you can reach all elements of your app with swipe gestures.
  4. Ensure that a Talkback user is able to navigate to each item on the screen.
  5. Observe that Talkback tries to read the scrambled word as a word. This may be confusing to the player since this is not a real word.
  6. A better user experience would be to have Talkback read aloud the individual characters of the scrambled word. Within the GameViewModel, convert the scrambled word String to a Spannable string. A spannable string is a string with some extra information attached to it. In this case, we want to associate the string with a TtsSpan of TYPE_VERBATIM, so that the text-to-speech engine reads aloud the scrambled word verbatim, character by character.
  7. In GameViewModel, use the following code to modify how the currentScrambledWord variable is declared:
val currentScrambledWord: LiveData<Spannable> = Transformations.map(_currentScrambledWord) {
    if (it == null) {
        SpannableString("")
    } else {
        val scrambledWord = it.toString()
        val spannable: Spannable = SpannableString(scrambledWord)
        spannable.setSpan(
            TtsSpan.VerbatimBuilder(scrambledWord).build(),
            0,
            scrambledWord.length,
            Spannable.SPAN_INCLUSIVE_INCLUSIVE
        )
        spannable
    }
}

This variable is now a LiveData<Spannable> instead of LiveData<String>. You don't have to worry about understanding all the details of how this works, but the implementation uses a LiveData transformation to convert the current scrambled word String into a Spannable string that can be handled appropriately by the accessibility service. In the next codelab, you will learn more about LiveData transformations, which allow you to return a different LiveData instance based on the value of corresponding LiveData.

  1. Run the Unscramble app, explore your app with Talkback. TalkBack should read out the individual characters of the scrambled word now.

For more information on how to make your app more accessible, check out these principles.

It is a good practice to delete the dead, unused, unwanted code for the solution code. This makes the code easy to maintain, which also makes it easier for new teammates to understand the code better.

  1. In GameFragment, delete getNextScrambledWord()and onDetach() methods.
  2. In GameViewModel delete onCleared() method.
  3. Delete any unused imports, at the top of the source files. They will be greyed out.

You don't need the log statements any more, you can delete them from the code if you prefer.

  1. [Optional] Delete the Log statements in the source files(GameFragment.kt and GameViewModel.kt) you added in the previous codelab, to understand the ViewModel lifecycle.

The solution code for this codelab is in the project shown below.

To get the code for this codelab and open it in Android Studio, do the following.

Get the code

  1. Click on the provided URL. This opens the GitHub page for the project in a browser.
  2. On the GitHub page for the project, click the Code button, which brings up a dialog.

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  1. In the dialog, click the Download ZIP button to save the project to your computer. Wait for the download to complete.
  2. Locate the file on your computer (likely in the Downloads folder).
  3. Double-click the ZIP file to unpack it. This creates a new folder that contains the project files.

Open the project in Android Studio

  1. Start Android Studio.
  2. In the Welcome to Android Studio window, click Open an existing Android Studio project.

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Note: If Android Studio is already open, instead, select the File > New > Import Project menu option.

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  1. In the Import Project dialog, navigate to where the unzipped project folder is located (likely in your Downloads folder).
  2. Double-click on that project folder.
  3. Wait for Android Studio to open the project.
  4. Click the Run button j7ptomO2PEQNe8jFt4nKCOw_Oc_Aucgf4l_La8fGLCMLy0t9RN9SkmBFGOFjkEzlX4ce2w2NWq4J30sDaxEe4MaSNuJPpMgHxnsRYoBtIV3-GUpYYcIvRJ2HrqR27XGuTS4F7lKCzg to build and run the app. Make sure it works as expected.
  5. Browse the project files in the Project tool window to see how the app was implemented.
  • LiveData holds data; LiveData is a wrapper that can be used with any data
  • LiveData is observable, which means that an observer is notified when the data held by the LiveData object changes.
  • LiveData is lifecycle-aware. When you attach an observer to the LiveData, the observer is associated with a LifecycleOwner (usually an Activity or Fragment). The LiveData only updates observers that are in an active lifecycle state such as STARTED or RESUMED. You can read more about LiveData and observation here.
  • Apps can listen to the LiveData changes from the layout using Data Binding and binding expressions.
  • Binding expressions are written within the layout in the attribute properties (such as android:text) referencing the layout properties.

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