Mobile App Dev 2022W: Tutorial 7

Background

To preserve battery life, the activities of Android apps are often paused or even terminated automatically. To preserve the user experience, apps are notified before various state change events, allowing them to take steps to preserve state so that when they resume the user doesn't even know that they were terminated. Here we explore Android activity lifecycle events and how to respond to them.

Relevant documentation:

• Android activities
• Android activity lifecycle

Questions

1. What activity lifestyle events happen when you do the following with the app:
   1. launch the app
   2. terminate the app (from the app switcher)
   3. rotate the device (change device orientation)
   4. run another app?
   5. switch back to the app?
2. What code is responsible for saving the app's state? When is this code called, relative to the main activity's lifecycle?
3. What code is responsible for restoring the app's state? When is this code called, relative to the main activity's lifecycle?
4. How is data organized in a bundle?
5. How is data saved to a bundle?
6. How is bundle data loaded?
7. How persistent is bundle-stored data? How can this be verified?

Tasks

1. Add lifecycle logging to textanalyzer-5.
2. Change textanalyzer-5 so that when rotated it preserves the current analysis mode.

TapDemo Code

MainActivity.kt

MainActivity.kt source

package carleton.comp1601.tapdemo

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.os.PersistableBundle
import android.util.Log
import android.view.View
import android.widget.TextView

class MainActivity : AppCompatActivity() {
    private lateinit var myMessage: TextView
    private var count = 0

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

        Log.d("TapDemo", "Created")

        myMessage = findViewById(R.id.myMessage)

        if (savedInstanceState != null) {
            with(savedInstanceState) {
                count = getInt("count")
            }
        } else {
            count = 0
        }
    }

    fun myButtonPressed(v: View) {
        count++
        myMessage.text = "You clicked $count times."

        Log.d("TapDemo", "Button Pressed")
    }

    override fun onSaveInstanceState(outState: Bundle) {
        outState.run {
            putInt("count", count)
        }
        super.onSaveInstanceState(outState)

        Log.d("TapDemo", "State saved")
    }

    override fun onDestroy() {
        super.onDestroy()
        Log.d("TapDemo", "Destroyed")
    }

    override fun onResume() {
        super.onResume()
        Log.d("TapDemo", "Resumed")
    }

    override fun onPause() {
        super.onPause()
        Log.d("TapDemo", "Paused")
    }

    override fun onStart() {
        super.onStart()
        Log.d("TapDemo", "Started")
    }

    override fun onStop() {
        super.onStop()
        Log.d("TapDemo", "Stopped")
    }

    override fun onRestart() {
        super.onRestart()
        Log.d("TapDemo", "Restarted")
    }
}

activity_main.xml

activity_main.xml source

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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"
    android:id="@+id/TapDemo"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/myButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="myButtonPressed"
        android:text="Tap me!"
        android:textAppearance="@style/TextAppearance.AppCompat.Display1"
        app:layout_constraintBottom_toTopOf="@+id/myMessage"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/myMessage"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="You haven't tapped yet."
        android:textAppearance="@style/TextAppearance.AppCompat.Display1"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>