COMP 1601 2021W Assignment 3 Solutions

1. [2] Add debugging log messages that report on activity lifecycle
       events.  The log key should be "Converter2A".

First, import the Log class:

   import android.util.Log

Then, define the application name at the top of the MainActivity class
(so we can reuse this code again later more easily):

  private val appName = "Converter2A"

Then add the following code to the end of the MainActivity class
(between lines 84 and 85):

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

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

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

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

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

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

2. [2] Add a Farenheit to Kelvin conversion, making sure that the
       conversion fails if the converted value would be less than 0
       Kelvin (as nothing can be colder than absolute zero).  What
       does the app do when you give it -500F and you ask it to
       convert to Kelvin (with no other changes)?  Why?

Add this entry to the conversions map (between lines 40 and 41 of
Converter.kt):

      "F to K" to Converter("Farenheit", "Kelvin",
            fun (F: Double): Double? {
                val K = ((F - 32.0)*(5.0/9.0) + 273.15)
                if (K >= 0.0) {
                    return K
                } else {
                    return null
                }
            }),

If you try converting -500F to K, you'll get the message "Converting
-500.0 Farenheit to Kelvin failed.".  This happens because this code
returns null when returning a Kelvin value less than zero, and the
return value is tested for this in formatConversion (line 12) and
based on this it returns the failed string (line 15).


3. [2] Add an entry to the menu "Cars to Cows" that has no
       corresponding conversion by modifying the conversions map.

Add the following to the conversions map (say between lines 40 and 41
in Converter.kt):

    "Cars to Cows" to null,

That's it!


4. [4] The "Cars to Cows" conversion still allows input to be entered,
       and that input is ''sometimes'' converted to another value.
       Why does this happen?  How can you change the code so the input
       field is only active when a valid menu selection is made?

If a previous conversion has been selected, conv in MainActivity is
then non-null and will be used.  However, if "Cars to Cows" is the
first one selected, then conv will be null, and no conversion will
take place.

To make the input field only be visible when a valid entry is
selected, replace lines 36-44 with the following:

       if (c != null) {
            conv = c
            result.setText("You picked ${convName}")
            fromStringW.setText("")
            fromStringW.setVisibility(View.VISIBLE);
        } else {
            result.setText("Invalid Menu Selection")
            fromStringW.setVisibility(View.INVISIBLE);
        }

Here we set the visibility appropriately based on whether the selected
entry is valid or not.


5. [2] Converter2A's layout is messed up when in landscape mode (the
       device turned horizontally).  Why?  What is wrong with the
       current layout?

The problem is the fixed width and height of the TextView.  If you
change the layout width and height to be "wrap_content" (lines 37 and
38) the layout works fine in portrait and landscape modes.


6. [2] Fix Converter2A's layout by converting it to a LinearLayout.

Just add a LinearLayout node and make the existing widgets children of
it (after removing the absolute size constraints on the TextView).
Below is what I got.  Note the menu and the entry field are now are on
the left of the screen; that is fine.  (If you did better, please
share!)

<?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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <Button
            android:id="@+id/convMenu"
            style="@style/Widget.MaterialComponents.Button.TextButton"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:onClick="showMenu"
            android:text="Conversions Menu"
            android:textAlignment="center"
            android:textAppearance="@style/TextAppearance.AppCompat.Display1"
            android:textColor="@color/black"
            app:backgroundTint="@color/white" />

        <EditText
            android:id="@+id/fromString"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:ems="10"
            android:inputType="textPersonName" />

        <TextView
            android:id="@+id/result"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:padding="10dp"
            android:text="Please select a conversion type from the menu above."
            android:textAlignment="center"
            android:textAppearance="@style/TextAppearance.AppCompat.Medium" />
    </LinearLayout>

</androidx.constraintlayout.widget.ConstraintLayout>


7. [2] Fix Converter2A's layout while keeping it a ConstrainedLayout.
       Make sure your layout works in both portrait and landscape
       mode!

See question 5 - we just have to change the layout width and height of
the Textview to wrap_content.


8. [4] When rotated, Converter2A doesn't remember the current state of
       the app.  Modify it so it saves and restores the selected
       conversion and the value being input when the main activity is
       destroyed and re-created.

First, we need to create a variable to remember the selected name of the conversion at the top of MainActivity:

   private var convName: String = ""

We make sure convName is updated when a conversion is selected by changing onMenuItemClick as follows:

   fun onMenuItemClick(choice: MenuItem): Boolean {
        val newConvName = choice.getTitle().toString()
        val c = conversions[newConvName]

        if (c != null) {
            conv = c
            convName = newConvName
            result.setText("You picked ${newConvName}")
            fromStringW.setText("")
            fromStringW.setVisibility(View.VISIBLE);
        } else {
            result.setText("Invalid Menu Selection")
            convName = ""
            conv = null
            fromStringW.setVisibility(View.INVISIBLE);
        }

        return true
    }

Note how this version uses a local copy of the conversion name, as a string and sets convName only when it is valid (and to an empty string otherwise).  We also have to toggle the visibility of the entry field appropriately (as done for question 4).

Then, we add a method to save state, saving both the text the user
entered and the selected conversion:

   override fun onSaveInstanceState(outState: Bundle) {
        outState.run {
            putString("fromString", fromStringW.getText().toString())
            putString("convName", convName)
        }
        super.onSaveInstanceState(outState)

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

And finally, we add the following to the end of the onCreate() method:

       if (savedInstanceState != null) {
            with(savedInstanceState) {
                fromStringW.setText(getString("fromString"))
                val c = getString("convName")
                if (c != null) {
                    convName = c
                    conv = conversions[convName]
                    fromStringW.setVisibility(View.VISIBLE);
                } else {
                    convName = ""
                    conv = null
                }
            }
        }