Add a base class for visualization views

Author: juliabeliaeva

visualization/src/androidMain/kotlin/org/jetbrains/kotlinx/dl/visualization/DetectionViewBase.kt

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2022 JetBrains s.r.o. and Kotlin Deep Learning project contributors. All Rights Reserved.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlinx.dl.visualization
+
+import android.content.Context
+import android.graphics.Canvas
+import android.util.AttributeSet
+import android.view.View
+
+/**
+ * Base class for [View] implementations which visualize detected results on top of the image preview.
+ * Derived classes should implement [drawDetection] method to perform actual drawing.
+ */
+abstract class DetectorViewBase<T>(context: Context, attrs: AttributeSet) : View(context, attrs) {
+    /**
+     * Detection result to visualize
+     */
+    var detection: T? = null
+        set(value) {
+            synchronized(this) {
+                field = value
+                postInvalidate()
+            }
+        }
+
+    /**
+     * Draw given detection result on the [Canvas].
+     */
+    abstract fun Canvas.drawDetection(detection: T)
+
+    override fun onDraw(canvas: Canvas) {
+        super.onDraw(canvas)
+
+        synchronized(this) {
+            val detection = detection
+            if (detection != null) {
+                canvas.drawDetection(detection)
+            }
+        }
+    }
+}