How to reduce a healthlevel by 1 when a button is clicked in kotlin?

Question

I'm starting out with kotlin. and I'm having trouble understanding OnClickListener. Here After setting the initial health level to 10, I need to reduce the health level by 1 and display it. So far I have initiliased the healthlevel and set the onclick, but How do declare the function to reduce it by 1 and call it when the button is clicked?

val TAG = "MyMessage"

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    Log.i("LIFECYCLE", "OnCreate")
}

private var healthLevel: Int = 10 //Set the initial health level to 10
private lateinit var healthLevelTextView: TextView
private lateinit var sneezeBtn: Button
private lateinit var takeMedicationButton: Button
private lateinit var blowNoseButton: Button

override fun onSaveInstanceState(outState: Bundle) {
    super.onSaveInstanceState(outState)
    outState.putInt("Answer", healthLevel)
    Log.i(TAG, "Called SaveInstanceState()")
}

sneezeBtn.setOnClickListener{ _ ->
    // the function goes here 
}

Answer 1

Vitor has the answer, but just as a couple of alternatives...

You might want to create a function to update and display your value together:

fun setHealth(health: Int) {
    healthLevel = health
    healthLevelTextView.text = healthLevel
}

That couples the update with the display change, so the two things always happen together. And if you always set healthLevel with this function (instead of setting the variable directly), the display and value will always be in sync

override fun onCreate(savedInstanceState: Bundle?) {
    // set your view variables, so the TextView is ready to update
    setHealth(INITIAL_HEALTH_VALUE)

And if you like, Kotlin lets us put a setter function on the variable itself

var healthLevel: Int = 10
    set(value) {
        field = value
        healthLevelTextView.text = health
    }

so every time you change the value of healthLevel, it also updates the display. You can use the observable delegate too

var healthLevel: Int by Delegates.observable(10) { _, oldValue, newValue ->
    healthLevelTextView.text = newValue
}

These are more advanced than just setting the value and updating the view yourself, just pointing out that they exist as alternatives and ways to keep your logic in one place. Also with these examples, they only run the code after you first set a value - they have a default of 10 in both cases, but initialising that default won't run the setter code or the observable function. So you'd still need to go healthLevel = 10 to get the text to display the initial value.

Answer 2

You can change your healthLevel variable by using:

sneezeBtn.setOnClickListener {
    healthLevel--
    healthLevelTextView.setText(healthLevel.toString())
}

As a side note, if you use lateinit you lose one of the best features of Kotlin, which is Null Safety. If you don't know how to use that yet, I recommend you to start learning it as soon as possible.

You can also use a very nice feature of Android with kotlin, where the objects for your views are generated automatically in your Activity, you just have to type your XML views id, instead of having to use findViewById everywhere.

Answer 3

I assume, your code sample is within an Activity class and you have a layout file activity_main.xml which looks something like this:

    ...
        <TextView
            android:id="@+id/healthLevelTextViewId"
            ...
            />

        <Button
            android:id="@+id/sneezeBtnId"
            ...
            />
    ...

This would be the code with your desired functionality:

class YourActivity : Activity {

    private var healthLevel: Int = 10

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        
        // retrieve references to the text view and button
        val healthLevelTextView = findViewById<TextView>(R.id.healthLevelTextViewId)
        val sneezeBtn = findViewById<Button>(R.id.sneezeBtnId)

        // the OnClickListener must be initialized within a method body, as it is not a method itself.
        sneezeBtn.setOnClickListener { _ ->
            // implementation from the answer from Vitor Ramos
            healthLevel--
            healthLevelTextView.setText(healthLevel.toString())
        }
    }
}

You can further improve the code:

• Add apply plugin: 'kotlin-android-extensions' to your app's build.gradle file. Then you can reference your views directly using their IDs. So instead of

      val sneezeBtn = findViewById<Button>(R.id.sneezeBtnId)
      sneezeBtn.setOnClickListener { ... }
  

  you can use the Id directly like this:

      sneezeBtnId.setOnClickListener { ... }
  

  AndroidStudio will give you a hint to add an import for sneezeBtnId.
• Use View Models, LiveData and Data Binding. This is the recommended, but a little bit more advanced technique.