Use Android quick scale for image zoom

app/src/main/java/se/ajpanton/notificationlog/ZoomImageView.kt

@@ -8,10 +8,7 @@ import android.util.AttributeSet
 import android.view.GestureDetector
 import android.view.MotionEvent
 import android.view.ScaleGestureDetector
-import android.view.ViewConfiguration
 import androidx.appcompat.widget.AppCompatImageView
-import kotlin.math.abs
-import kotlin.math.exp
 
 /** A fit-to-screen image which supports pinch zoom and one-finger panning. */
 class ZoomImageView @JvmOverloads constructor(
@@ -29,13 +26,7 @@ class ZoomImageView @JvmOverloads constructor(
     private var dragging = false
     private var lastFocusX = 0f
     private var lastFocusY = 0f
-    private var doubleTapZooming = false
+    private var pointerCount = 0
-    private var doubleTapMoved = false
-    private var doubleTapX = 0f
-    private var doubleTapY = 0f
-    private var doubleTapStartY = 0f
-    private var doubleTapBaseScale = 1f
-    private val touchSlop = ViewConfiguration.get(context).scaledTouchSlop
     private val scaleDetector = ScaleGestureDetector(context, object : ScaleGestureDetector.SimpleOnScaleGestureListener() {
         override fun onScaleBegin(detector: ScaleGestureDetector): Boolean {
             lastFocusX = detector.focusX
@@ -44,10 +35,13 @@ class ZoomImageView @JvmOverloads constructor(
         }
 
         override fun onScale(detector: ScaleGestureDetector): Boolean {
-            // A ScaleGestureDetector reports focus movement even when the two
+            // Focus movement while two fingers are down is two-finger panning.
-            // fingers stay the same distance apart, which is two-finger panning.
+            // Quick scale has one pointer, so it remains pure zoom around its
+            // built-in double-tap anchor.
             setScaleAround((currentScale * detector.scaleFactor).coerceIn(minimumScale, minimumScale * MAX_ZOOM), detector.focusX, detector.focusY)
-            transform.postTranslate(detector.focusX - lastFocusX, detector.focusY - lastFocusY)
+            if (pointerCount >= 2) {
+                transform.postTranslate(detector.focusX - lastFocusX, detector.focusY - lastFocusY)
+            }
             lastFocusX = detector.focusX
             lastFocusY = detector.focusY
             constrain()
@@ -60,45 +54,11 @@ class ZoomImageView @JvmOverloads constructor(
             return true
         }
 
-        override fun onDoubleTap(event: MotionEvent): Boolean = true
-
-        override fun onDoubleTapEvent(event: MotionEvent): Boolean {
-            when (event.actionMasked) {
-                MotionEvent.ACTION_DOWN -> {
-                    doubleTapZooming = true
-                    doubleTapMoved = false
-                    doubleTapX = event.x
-                    doubleTapY = event.y
-                    doubleTapStartY = event.y
-                    doubleTapBaseScale = currentScale
-                }
-                MotionEvent.ACTION_MOVE -> if (doubleTapZooming) {
-                    val distance = doubleTapStartY - event.y
-                    if (abs(distance) > touchSlop) doubleTapMoved = true
-                    val target = (doubleTapBaseScale * exp((distance / DOUBLE_TAP_DRAG_DISTANCE).toDouble()).toFloat())
-                        .coerceIn(minimumScale, minimumScale * MAX_ZOOM)
-                    setScaleAround(target, doubleTapX, doubleTapY)
-                    constrain()
-                }
-                MotionEvent.ACTION_UP -> if (doubleTapZooming) {
-                    if (!doubleTapMoved) {
-                        val target = if (currentScale > minimumScale * 1.1f) minimumScale else minimumScale * DOUBLE_TAP_ZOOM
-                        setScaleAround(target, doubleTapX, doubleTapY)
-                        constrain()
-                    }
-                    doubleTapZooming = false
-                }
-                MotionEvent.ACTION_CANCEL -> doubleTapZooming = false
-            }
-            return true
-        }
     })
 
     init {
         scaleType = ScaleType.MATRIX
-        // We implement double-tap-and-drag ourselves below. Leaving Android's
+        scaleDetector.isQuickScaleEnabled = true
-        // quick-scale enabled makes both handlers modify the same matrix.
-        scaleDetector.isQuickScaleEnabled = false
     }
 
     override fun setImageBitmap(bitmap: Bitmap?) {
@@ -112,6 +72,7 @@ class ZoomImageView @JvmOverloads constructor(
     }
 
     override fun onTouchEvent(event: MotionEvent): Boolean {
+        pointerCount = event.pointerCount
         scaleDetector.onTouchEvent(event)
         gestureDetector.onTouchEvent(event)
         when (event.actionMasked) {
@@ -120,7 +81,7 @@ class ZoomImageView @JvmOverloads constructor(
                 lastY = event.y
                 dragging = false
             }
-            MotionEvent.ACTION_MOVE -> if (event.pointerCount == 1 && !scaleDetector.isInProgress && !doubleTapZooming) {
+            MotionEvent.ACTION_MOVE -> if (event.pointerCount == 1 && !scaleDetector.isInProgress) {
                 val dx = event.x - lastX
                 val dy = event.y - lastY
                 if (dx != 0f || dy != 0f) {
@@ -186,7 +147,5 @@ class ZoomImageView @JvmOverloads constructor(
 
     private companion object {
         const val MAX_ZOOM = 6f
-        const val DOUBLE_TAP_ZOOM = 2.5f
-        const val DOUBLE_TAP_DRAG_DISTANCE = 280f
     }
 }