Single Activity Kotlin Example

MainActivity.kt

package com.caharkness.demo.kotlin

import android.graphics.Canvas
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.util.TypedValue
import android.view.Gravity
import android.view.ViewGroup
import android.widget.Button
import android.widget.LinearLayout
import android.widget.TextView

class MainActivity : AppCompatActivity()
{
    //
    //  Single Activity Kotlin Demo
    //  Building a UI in Kotlin (with ZERO prior experience, thanks Stack Overflow University)
    //
    //  by Conner Harkness on 12/5/2019
    //
    override fun onCreate(savedInstanceState: Bundle?)
    {
        super.onCreate(savedInstanceState)

        // Commenting out the layout funcionality, let's build our own in Kotlin!
        // This was put here by Android Studio and its blank activity template, we won't be using XML
        //setContentView(R.layout.activity_main)

        // Similar to LinearLayout OurMainLayout = new LinearLayout(this);
        val OurMainLayout = LinearLayout(this)

        OurMainLayout.layoutParams =
            object : LinearLayout.LayoutParams(
                LinearLayout.LayoutParams.MATCH_PARENT,
                LinearLayout.LayoutParams.MATCH_PARENT)
        {
            //
            //  This is the equivalent of new LayoutParams(...) {{ gravity = Gravity.CENTER }}
            //
            init
            {
                gravity = Gravity.CENTER
            }
        }

        OurMainLayout.gravity = Gravity.CENTER
        OurMainLayout.orientation = LinearLayout.VERTICAL

        // "Kotlin doesn't automatically convert between number types." -Kiskae
        // https://stackoverflow.com/questions/44604555/declaring-byte-in-kotlin-does-compile-time-error-the-integer-literal-does-not-c
        //
        // Let's make the color black and call ".toInt()" on it to let .setBackgroundColor() accept an integer literal
        OurMainLayout.setBackgroundColor(0xFF000000.toInt())

        // Similar to TextView OurCaption = new TextView(this)
        val OurCaption = TextView(this)

        //
        //  Here, let's do something Java cannot; define functions on the fly within the Kotlin language, but not as anonymous runnables...
        //  This function allows us to programmatically create pixel measurements using inches
        //
        fun inches(i: Float): Int
        {
            return TypedValue.applyDimension(
                TypedValue.COMPLEX_UNIT_IN,
                i,
                getResources()
                    .getDisplayMetrics()
            ).toInt()
        }

        //
        // Regardless of the DPI of the device, our caption will have a consistent, quarter inch padding!
        //
        OurCaption.setPadding(
            inches(0.25f),
            inches(0.25f),
            inches(0.25f),
            inches(0.25f))

        OurCaption.text = "Hello, world!"
        OurCaption.setTextColor(0xFFFFFFFF.toInt())
        OurCaption.setTextSize(TypedValue.COMPLEX_UNIT_IN, 0.25f) // This allows our TextView's text size to be a quarter inch as well (using TypedValue)

        val OurButton = object : Button(this)
        {
            //
            //  Another example of anonymous classes, this time with a Button view
            //
            init
            {
                text = "Tap me"
                setPadding(
                    inches(1/8f),
                    inches(1/8f),
                    inches(1/8f),
                    inches(1/8f)) // Give the button some beefy padding so it doesn't look like it skips "press" day

                //
                //  Lambda version of an anonymous OnClickListener class?
                //
                setOnClickListener {
                    text = "Tapped"
                    OurCaption.text = "Goodbye, world!" // Interact with a previously defined view outside of our anonymous Button view class...
                }

                layoutParams = ViewGroup.LayoutParams(
                    ViewGroup.LayoutParams.WRAP_CONTENT,
                    ViewGroup.LayoutParams.WRAP_CONTENT)
            }

            // Let's have some fun overriding the Button view's draw method
            override fun draw(canvas: Canvas?)
            {
                //
                //  Unimportant, but make the button violently vibrate from -2 to +2 degrees around its center
                //  "Violently" because the view is only redrawn when interacted with and does not redraw when nothing is happening...
                //
                canvas?.rotate(
                    -2f + (Math.sin(System.currentTimeMillis().toDouble()) * 4).toFloat(),
                    canvas?.width / 2f,
                    canvas?.height / 2f)
                super.draw(canvas)
            }
        }

        OurMainLayout.addView(OurCaption)
        OurMainLayout.addView(OurButton)


        // Literally one semicolon short of being identical to setting the content view in Java, nice!
        setContentView(OurMainLayout)
    }
}