Detailed explanation of the code that implements the WordPad function based on Android

// app/
apply plugin: ''
apply plugin: 'kotlin-android'
 
android {
  compileSdkVersion 34
  defaultConfig {
    applicationId ""
    minSdkVersion 21
    targetSdkVersion 34
  }
  buildFeatures { viewBinding true }
  kotlinOptions { jvmTarget = "1.8" }
}
 
dependencies {
  implementation ':appcompat:1.6.1'
  implementation ':core-ktx:1.10.1'
  implementation ':material:1.9.0'
  implementation ':lifecycle-runtime-ktx:2.6.1'
}

// =======================================================
// File: res/layout/activity_main.xml// Description: Main interface layout, including toolbar, DrawingBoardView, color/pen width tools// =======================================================
<?xml version="1.0" encoding="utf-8"?>
<
    xmlns:andro
    xmlns:app="/apk/res-auto"
    android:layout_width="match_parent" android:layout_height="match_parent">
 
  <!-- Top Action Bar -->
  <
      android:
      android:layout_width="match_parent"
      android:layout_height="?attr/actionBarSize"
      android:theme="@style/"
      app:title="Writing Board"/>
 
  <!-- Draw the panel -->
  <
      android:
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:layout_marginTop="?attr/actionBarSize"
      android:background="#FFFFFF"/>
 
  <!-- Bottom toolbar -->
  <LinearLayout
      android:
      android:orientation="horizontal"
      android:gravity="center_vertical"
      android:padding="8dp"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_gravity="bottom"
      android:background="#CCFFFFFF">
 
    <!-- Color panel -->
    <HorizontalScrollView
        android:layout_width="0dp"
        android:layout_weight="1"
        android:layout_height="wrap_content">
      <LinearLayout
          android:
          android:orientation="horizontal"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"/>
    </HorizontalScrollView>
 
    <!-- Pen width slider -->
    <SeekBar
        android:
        android:layout_width="120dp"
        android:layout_height="wrap_content"
        android:max="50"
        android:progress="10"
        android:layout_marginStart="16dp"/>
  </LinearLayout>
 
  <!-- Floating operation button -->
  <
      android:
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_margin="16dp"
      android:src="@drawable/ic_baseline_clear_24"
      app:layout_anchorGravity="bottom|end"
      app:layout_anchor="@id/drawingBoard"/>
 
  <
      android:
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_margin="16dp"
      android:src="@drawable/ic_baseline_undo_24"
      app:layout_anchorGravity="bottom|start"
      app:layout_anchor="@id/drawingBoard"/>
 
  <
      android:
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_margin="16dp"
      android:src="@drawable/ic_baseline_redo_24"
      app:layout_anchorGravity="bottom|start"
      app:layout_anchor="@id/btnUndo"/>
 
  <
      android:
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_margin="16dp"
      android:src="@drawable/ic_baseline_save_24"
      app:layout_anchorGravity="bottom|end"
      app:layout_anchor="@id/btnClear"/>
 
</>
 
// =======================================================
// document:// Description: Custom drawing board, supports drawing, undo, redo, clear, and export// =======================================================
package 
 
import 
import .*
import 
import 
import 
import .*
 
class DrawingBoardView @JvmOverloads constructor(
  context: Context, attrs: AttributeSet? = null
) : View(context, attrs) {
 
  // Collection of brushes and paths  private var paint = Paint(Paint.ANTI_ALIAS_FLAG).apply {
    color = ; strokeWidth = 10f
    style = ; strokeCap = 
    strokeJoin = 
  }
  private var currentPath = Path()
  private val paths = mutableListOf<Pair<Path, Paint>>()
  private val redoStack = Stack<Pair<Path, Paint>>()
 
  // Touch the previous point  private var prevX = 0f; private var prevY = 0f
 
  /** Set the brush color */
  fun setStrokeColor(color: Int) {  = color }
 
  /** Set the brush thickness */
  fun setStrokeWidth(width: Float) {  = width }
 
  /** Undo */
  fun undo() {
    if (()) (())
    invalidate()
  }
 
  /** Redo */
  fun redo() {
    if (()) paths += ()
    invalidate()
  }
 
  /** Clear */
  fun clear() {
    (); ()
    invalidate()
  }
 
  /** Export Bitmap */
  fun exportBitmap(): Bitmap {
    val bmp = (width, height, .ARGB_8888)
    val canvas = Canvas(bmp)
    ()
    for ((p, paint) in paths) (p, paint)
    return bmp
  }
 
  override fun onTouchEvent(e: MotionEvent): Boolean {
    val x = ; val y = 
    when () {
      MotionEvent.ACTION_DOWN -> {
        currentPath = Path().apply { moveTo(x, y) }
        prevX = x; prevY = y
        // Clear the redo stack with new operation        ()
      }
      MotionEvent.ACTION_MOVE -> {
        val mx = (x + prevX) / 2
        val my = (y + prevY) / 2
        (prevX, prevY, mx, my)
        prevX = x; prevY = y
      }
      MotionEvent.ACTION_UP -> {
        // Complete a stroke and store the path and its brush attributes        val p = Path(currentPath)
        val paintCopy = Paint(paint)
        paths += Pair(p, paintCopy)
      }
    }
    invalidate()
    return true
  }
 
  override fun onDraw(canvas: Canvas) {
    (canvas)
    // Draw historical paths in turn    for ((p, paint) in paths) (p, paint)
    // Draw the current path    (currentPath, paint)
  }
}
 
// =======================================================
// document:// Description: Image saving and sharing tool// =======================================================
package 
 
import 
import 
import 
import 
import 
import .*
 
object ImageUtil {
  /** Save to album and return to Uri */
  fun saveBitmapToGallery(ctx: Context, bmp: Bitmap, name: String = "draw_${()}"): Uri? {
    return if (.SDK_INT >= Build.VERSION_CODES.Q) {
      val values = ContentValues().apply {
        put(.DISPLAY_NAME, "$")
        put(.MIME_TYPE, "image/png")
        put(.RELATIVE_PATH, "Pictures/DrawingBoard")
        put(.IS_PENDING, 1)
      }
      val uri = (.EXTERNAL_CONTENT_URI, values)
      uri?.let {
        (it)?.use { os -> (, 100, os) }
        (); (.IS_PENDING, 0)
        (it, values, null, null)
      }
      uri
    } else {
      val dir = File((null), "DrawingBoard")
      if (!()) ()
      val file = File(dir, "$")
      FileOutputStream(file).use { fos -> (, 100, fos) }
      (file)
    }
  }
}
 
// =======================================================
// document:// Description: Main interface logic: Initialize artboard, tool binding, save and share// =======================================================
package 
 
import 
import 
import 
import 
import 
import 
import 
import 
import 
import 
import .*
 
class MainActivity : AppCompatActivity() {
  private lateinit var binding: ActivityMainBinding
  private val scope = CoroutineScope( + Job())
 
  // Temporary Uri after sharing  private var savedImageUri: Uri? = null
 
  // Share authorization  private val shareLauncher = registerForActivityResult(
    ()
  ) { /* nothing */ }
 
  override fun onCreate(savedInstanceState: Bundle?) {
    (savedInstanceState)
    binding = (layoutInflater)
    setContentView()
 
    // Initialize the color panel    initColorPalette()
 
    // Pen width control    (object: SimpleSeekListener(){
      override fun onProgressChanged(sb: , p: Int, u: Boolean) {
        (())
      }
    })
 
    // Top button binding     { () }
      { () }
      { () }
      { saveDrawing() }
  }
 
  private fun initColorPalette() {
    val colors = listOf(, , , , )
    for (c in colors) {
      val btn = ImageButton(this).apply {
        val size = (.color_btn_size)
        layoutParams = (size, size).apply {
          marginEnd = 16
        }
        setBackgroundColor(c)
        setOnClickListener { (c) }
      }
      (btn)
    }
  }
 
  private fun saveDrawing() {
    // Save and share asynchronously     {
      val bmp = withContext() { () }
      savedImageUri = (this@MainActivity, bmp)
      if (savedImageUri != null) {
        shareImage(savedImageUri!!)
      } else {
        (this@MainActivity, "Save failed", Toast.LENGTH_SHORT).show()
      }
    }
  }
 
  private fun shareImage(uri: Uri) {
    val contentUri = if ( == "file") {
      (this, "$", (!!).toFile())
    } else uri
    val intent = Intent(Intent.ACTION_SEND).apply {
      type = "image/png"
      putExtra(Intent.EXTRA_STREAM, contentUri)
      addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION)
    }
    ((intent, "Share drawing"))
  }
 
  override fun onDestroy() {
    ()
    ()
  }
}
 
// =======================================================
// document:// Description: Simple SeekBar listening, omit callback implementation// =======================================================
package 
 
import 
 
abstract class SimpleSeekListener:  {
  override fun onStartTrackingTouch(p0: SeekBar?) {}
  override fun onStopTrackingTouch(p0: SeekBar?) {}
}