View source for Mobile App Development 2021W Lecture 20

==Video==

Video from the lecture given on March 29, 2021 [https://homeostasis.scs.carleton.ca/~soma/mad-2021w/lectures/comp1601-2021w-lec20-20210329.m4v is now available].  Note that this recording ends a bit abruptly and early as there were technical issues.

==Notes==

<pre>
Lecture 20
----------

* Tutorial 8 is finally posted
  - will be finalized this afternoon

* Tutorial 9 will come out by next Monday

* Assignment 4 will be due still on April 12
  - will come out this week, hopefully by Wednesday
  - will give away answers in class

* No Tutorial 10 (schedule will be updated)

* Final exam on April 23, 2 PM
  - same format as the midterm, just 2 hours instead of 80 min

* Solutions for A3 will be posted on Wednesday
  - turn in by Tuesday night if you haven't already


Kotlin is a bit funny about numbers
 - it does NO automatic conversions
 - a number with no decimals is an Int
 - a number with decimals is a Double
 - but then, how do you write a Float as a constant?
    - append with an F

Floats vs Doubles
 - floats use 4 bytes, doubles use 8 bytes
 - so, doubles allow for much greater precision
    - larger exponent and mantissa
 - both are binary representations of numbers in
   scientific notation

Android intents & activities go together
 - intents are messages to an app to do something
 - activities are screens that can be displayed in response to intents
    - an intent doesn't have to cause a screen to be
      displayed, but if you want to display a new screen
      (including the first one), it will start with an
      intent

 - so, to switch screens we generate and receive intents
 - also, to call outside apps, we send intents

Intents have two kinds: explicit and implicit
 - explicit specifies exactly what to do
 - implicit allows the receiver to interpret,
   may even be receivable by multiple apps
</pre>

==Code==

Below is the layout created during class.  The app was just the default Kotlin hello world (empty activity).

<syntaxhighlight lang="xml" line>
<?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">

    <TextView
        android:id="@+id/valueLabel"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="33dp"
        android:layout_marginBottom="215dp"
        android:text="Value"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toStartOf="@+id/value" />

    <EditText
        android:id="@+id/value"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="74dp"
        android:ems="8"
        android:inputType="numberDecimal"
        app:layout_constraintBaseline_toBaselineOf="@+id/valueLabel"
        app:layout_constraintEnd_toEndOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>
</syntaxhighlight>