Add multitouch to ImageViewer (#2935)

The code of `ScalableImage` is exactly the same as [here](https://github.com/JetBrains/compose-multiplatform-core/pull/459/files#diff-2df227d37a7fcdb885f4fd1a715c0efd94b8e206d446d553d69a456f83e284f6R19):

1. The initial size of an image is chosen by the area size (phone/windows size)

2. We can zoom using pinch-to-zoom (with taking centroid of touches into account)

3. We can zoom using mouse scroll (also, taking the mouse position into account). On touchpad/macOS it also works great.

4. The code of the old `ScalableState` and `ScalableImage` was complete rewritten.

5. The zoom is not limited by phone/window dimensions, we can zoom out

experimental/examples/imageviewer/shared/src/androidMain/kotlin/example/imageviewer/view/ScalableImage.android.kt

@@ -1,7 +0,0 @@
-package example.imageviewer.view
-
-import androidx.compose.ui.Modifier
-import example.imageviewer.model.ScalableState
-
-actual fun Modifier.addUserInput(state: ScalableState) =
-    addTouchUserInput(state)

experimental/examples/imageviewer/shared/src/commonMain/kotlin/example/imageviewer/model/Config.kt

@@ -1,5 +1,3 @@
 package example.imageviewer.model
 
-const val MAX_SCALE = 5f
-const val MIN_SCALE = 1f
 const val TOAST_DURATION = 3000L

experimental/examples/imageviewer/shared/src/commonMain/kotlin/example/imageviewer/model/ScalableState.kt

@@ -4,81 +4,88 @@ import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.geometry.Offset
-import androidx.compose.ui.unit.IntOffset
+import androidx.compose.ui.geometry.Size
-import androidx.compose.ui.unit.IntRect
+import androidx.compose.ui.geometry.isSpecified
-import androidx.compose.ui.unit.IntSize
 
-class ScalableState() {
+/**
-    var imageSize by mutableStateOf(IntSize(0, 0))
+ * Encapsulate all transformations about showing some target (an image, relative to its center)
-    var boxSize by mutableStateOf(IntSize(1, 1))
+ * scaled and shifted in some area (a window, relative to its center)
-    var offset by mutableStateOf(IntOffset.Zero)
+ */
+class ScalableState {
+    val scaleLimits = 0.2f..10f
+
+    /**
+     * Offset of the camera before scaling (an offset in pixels in the area coordinate system)
+     */
+    var offset by mutableStateOf(Offset.Zero)
+        private set
     var scale by mutableStateOf(1f)
-}
+        private set
 
-val ScalableState.visiblePart
+    private var areaSize: Size = Size.Unspecified
-    get() : IntRect {
+    private var targetSize: Size = Size.Zero
-        val boxRatio = boxSize.width.toFloat() / boxSize.height
-        val imageRatio = imageSize.width.toFloat() / imageSize.height.toFloat()
 
-        val size: IntSize =
+    private var offsetXLimits = Float.NEGATIVE_INFINITY..Float.POSITIVE_INFINITY
-            if (boxRatio > imageRatio) {
+    private var offsetYLimits = Float.NEGATIVE_INFINITY..Float.POSITIVE_INFINITY
-                val height = imageSize.height / scale
-                val targetWidth = height * boxRatio
-                IntSize(minOf(imageSize.width, targetWidth.toInt()), height.toInt())
-            } else {
-                val width = imageSize.width / scale
-                val targetHeight = width / boxRatio
-                IntSize(width.toInt(), minOf(imageSize.height, targetHeight.toInt()))
-            }
 
-        return IntRect(offset = offset, size = size)
+    /**
+     * Limit the target center position, so:
+     * - if the size of the target is less than area,
+     *   the center of the target is bound to the center of the area
+     * - if the size of the target is greater, then limit the center of it,
+     *   so the target will be always in the area
+     */
+    fun limitTargetInsideArea(
+        areaSize: Size,
+        targetSize: Size,
+    ) {
+        this.areaSize = areaSize
+        this.targetSize = targetSize
+        applyLimits()
     }
 
-fun ScalableState.changeBoxSize(size: IntSize) {
+    private fun applyLimits() {
-    boxSize = size
+        if (targetSize.isSpecified && areaSize.isSpecified) {
-    updateOffsetLimits()
+            offsetXLimits = centerLimits(targetSize.width * scale, areaSize.width)
-}
+            offsetYLimits = centerLimits(targetSize.height * scale, areaSize.height)
-
+            offset = Offset(
-fun ScalableState.setScale(scale: Float) {
+                offset.x.coerceIn(offsetXLimits),
-    this.scale = scale
+                offset.y.coerceIn(offsetYLimits),
-}
+            )
-
+        } else {
-fun ScalableState.addScale(diff: Float) {
+            offsetXLimits = Float.NEGATIVE_INFINITY..Float.POSITIVE_INFINITY
-    scale = if (scale + diff > MAX_SCALE) {
+            offsetYLimits = Float.NEGATIVE_INFINITY..Float.POSITIVE_INFINITY
-        MAX_SCALE
+        }
-    } else if (scale + diff < MIN_SCALE) {
-        MIN_SCALE
-    } else {
-        scale + diff
     }
-    updateOffsetLimits()
-}
-
-fun ScalableState.addDragAmount(diff: Offset) {
-    offset -= IntOffset((diff.x + 1).toInt(), (diff.y + 1).toInt())
-    updateOffsetLimits()
-}
-
-fun ScalableState.updateImageSize(width: Int, height: Int) {
-    imageSize = IntSize(width, height)
-    updateOffsetLimits()
-}
 
-private fun ScalableState.updateOffsetLimits() {
+    private fun centerLimits(imageSize: Float, areaSize: Float): ClosedFloatingPointRange<Float> {
-    if (offset.x + visiblePart.width > imageSize.width) {
+        val areaCenter = areaSize / 2
-        changeOffset(x = imageSize.width - visiblePart.width)
+        val imageCenter = imageSize / 2
-    }
+        val extra = (imageCenter - areaCenter).coerceAtLeast(0f)
-    if (offset.y + visiblePart.height > imageSize.height) {
+        return -extra / 2..extra / 2
-        changeOffset(y = imageSize.height - visiblePart.height)
     }
-    if (offset.x < 0) {
+
-        changeOffset(x = 0)
+    fun addPan(pan: Offset) {
+        offset += pan
+        applyLimits()
     }
-    if (offset.y < 0) {
+
-        changeOffset(y = 0)
+    /**
+     * @param focus on which point the camera is focused in the area coordinate system.
+     * After we apply the new scale, the camera should be focused on the same point in
+     * the target coordinate system.
+     */
+    fun addScale(scaleMultiplier: Float, focus: Offset = Offset.Zero) {
+        setScale(scale * scaleMultiplier, focus)
     }
-}
 
-private fun ScalableState.changeOffset(x: Int = offset.x, y: Int = offset.y) {
+    fun setScale(scale: Float, focus: Offset = Offset.Zero) {
-    offset = IntOffset(x, y)
+        val newScale = scale.coerceIn(scaleLimits)
+        val focusInTargetSystem = (focus - offset) / this.scale
+        // calculate newOffset from this equation:
+        // focusInTargetSystem = (focus - newOffset) / newScale
+        offset = focus - focusInTargetSystem * newScale
+        this.scale = newScale
+        applyLimits()
+    }
 }

experimental/examples/imageviewer/shared/src/commonMain/kotlin/example/imageviewer/utils/EventUtils.kt

@@ -0,0 +1,26 @@
+package example.imageviewer.utils
+
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.rememberUpdatedState
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.composed
+import androidx.compose.ui.input.pointer.*
+
+fun Modifier.onPointerEvent(
+    eventType: PointerEventType,
+    pass: PointerEventPass = PointerEventPass.Main,
+    onEvent: AwaitPointerEventScope.(event: PointerEvent) -> Unit
+): Modifier = composed {
+    val currentEventType by rememberUpdatedState(eventType)
+    val currentOnEvent by rememberUpdatedState(onEvent)
+    pointerInput(pass) {
+        awaitPointerEventScope {
+            while (true) {
+                val event = awaitPointerEvent(pass)
+                if (event.type == currentEventType) {
+                    currentOnEvent(event)
+                }
+            }
+        }
+    }
+}

experimental/examples/imageviewer/shared/src/commonMain/kotlin/example/imageviewer/view/FullscreenImageScreen.kt

@@ -8,6 +8,7 @@ import androidx.compose.runtime.*
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.clip
+import androidx.compose.ui.draw.clipToBounds
 import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.graphics.ImageBitmap
 import androidx.compose.ui.graphics.painter.BitmapPainter
@@ -54,46 +55,34 @@ internal fun FullscreenImageScreen(
     Box(Modifier.fillMaxSize().background(color = ImageviewerColors.fullScreenImageBackground)) {
         if (imageWithFilter != null) {
             val scalableState = remember { ScalableState() }
-            scalableState.updateImageSize(imageWithFilter.width, imageWithFilter.height)
+
-            val visiblePartOfImage: IntRect = scalableState.visiblePart
+            ScalableImage(
-            Box(
+                scalableState,
-                Modifier.fillMaxSize()
+                imageWithFilter,
-                    .onGloballyPositioned { coordinates ->
+                modifier = Modifier.fillMaxSize().clipToBounds(),
-                        scalableState.changeBoxSize(coordinates.size)
+            )
-                    }
+
-                    .addUserInput(scalableState)
+            Column(
+                Modifier
+                    .align(Alignment.BottomCenter)
+                    .clip(RoundedCornerShape(topStart = 8.dp, topEnd = 8.dp))
+                    .background(ImageviewerColors.filterButtonsBackground)
+                    .padding(16.dp),
+                horizontalAlignment = Alignment.CenterHorizontally
             ) {
-                Image(
+                FilterButtons(
-                    modifier = Modifier.fillMaxSize(),
+                    picture = picture,
-                    painter = BitmapPainter(
+                    filters = availableFilters,
-                        imageWithFilter,
+                    selectedFilters = selectedFilters,
-                        srcOffset = visiblePartOfImage.topLeft,
+                    onSelectFilter = {
-                        srcSize = visiblePartOfImage.size
+                        if (it !in selectedFilters) {
-                    ),
+                            selectedFilters += it
-                    contentDescription = null,
+                        } else {
+                            selectedFilters -= it
+                        }
+                    },
                 )
-                Column(
+                ZoomControllerView(Modifier, scalableState)
-                    Modifier
-                        .align(Alignment.BottomCenter)
-                        .clip(RoundedCornerShape(topStart = 8.dp, topEnd = 8.dp))
-                        .background(ImageviewerColors.filterButtonsBackground)
-                        .padding(16.dp),
-                    horizontalAlignment = Alignment.CenterHorizontally
-                ) {
-                    FilterButtons(
-                        picture = picture,
-                        filters = availableFilters,
-                        selectedFilters = selectedFilters,
-                        onSelectFilter = {
-                            if (it !in selectedFilters) {
-                                selectedFilters += it
-                            } else {
-                                selectedFilters -= it
-                            }
-                        },
-                    )
-                    ZoomControllerView(Modifier, scalableState)
-                }
             }
         } else {
             LoadingScreen()

experimental/examples/imageviewer/shared/src/commonMain/kotlin/example/imageviewer/view/ScalableImage.common.kt

@@ -2,23 +2,91 @@ package example.imageviewer.view
 
 import androidx.compose.foundation.gestures.detectTapGestures
 import androidx.compose.foundation.gestures.detectTransformGestures
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.BoxWithConstraints
+import androidx.compose.foundation.layout.BoxWithConstraintsScope
+import androidx.compose.runtime.*
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.drawWithContent
+import androidx.compose.ui.geometry.Offset
+import androidx.compose.ui.geometry.Size
+import androidx.compose.ui.graphics.ImageBitmap
+import androidx.compose.ui.graphics.drawscope.drawIntoCanvas
+import androidx.compose.ui.graphics.withSave
+import androidx.compose.ui.input.pointer.PointerEventType
 import androidx.compose.ui.input.pointer.pointerInput
+import androidx.compose.ui.platform.LocalDensity
 import example.imageviewer.model.ScalableState
-import example.imageviewer.model.addDragAmount
+import example.imageviewer.utils.onPointerEvent
-import example.imageviewer.model.addScale
+import kotlin.math.min
-import example.imageviewer.model.setScale
+import kotlin.math.pow
 
-expect fun Modifier.addUserInput(state: ScalableState): Modifier
+@Composable
+internal fun ScalableImage(scalableState: ScalableState, image: ImageBitmap, modifier: Modifier = Modifier) {
+    BoxWithConstraints {
+        val areaSize = areaSize
+        val imageSize = image.size
+        val imageCenter = Offset(image.width / 2f, image.height / 2f)
+        val areaCenter = Offset(areaSize.width / 2f, areaSize.height / 2f)
 
-fun Modifier.addTouchUserInput(state: ScalableState): Modifier =
+        if (areaSize.width > 0 && areaSize.height > 0) {
-    pointerInput(Unit) {
+            DisposableEffect(Unit) {
-        detectTransformGestures { _, pan, zoom, _ ->
+                scalableState.setScale(
-            state.addDragAmount(pan)
+                    min(areaSize.width / imageSize.width, areaSize.height / imageSize.height),
-            state.addScale(zoom - 1f)
+                    Offset.Zero,
+                )
+                onDispose { }
+            }
         }
-    }.pointerInput(Unit) {
+
-        detectTapGestures(
+        Box(
-            onDoubleTap = { state.setScale(1f) }
+            modifier
+                .drawWithContent {
+                    drawIntoCanvas {
+                        it.withSave {
+                            it.translate(areaCenter.x, areaCenter.y)
+                            it.translate(scalableState.offset.x, scalableState.offset.y)
+                            it.scale(scalableState.scale, scalableState.scale)
+                            it.translate(-imageCenter.x, -imageCenter.y)
+                            drawImage(image)
+                        }
+                    }
+                }
+                .pointerInput(Unit) {
+                    detectTransformGestures { centroid, pan, zoom, _ ->
+                        scalableState.addPan(pan)
+                        scalableState.addScale(zoom, centroid - areaCenter)
+                    }
+                }
+                .onPointerEvent(PointerEventType.Scroll) {
+                    val centroid = it.changes[0].position
+                    val delta = it.changes[0].scrollDelta
+                    val zoom = 1.2f.pow(-delta.y)
+                    scalableState.addScale(zoom, centroid - areaCenter)
+                }
+                .pointerInput(Unit) {
+                    detectTapGestures(onDoubleTap = { position ->
+                        scalableState.setScale(
+                            if (scalableState.scale > 2.0) {
+                                scalableState.scaleLimits.start
+                            } else {
+                                scalableState.scaleLimits.endInclusive
+                            },
+                            position - areaCenter
+                        )
+                    }) { }
+                },
         )
+
+        SideEffect {
+            scalableState.limitTargetInsideArea(areaSize, imageSize)
+        }
+    }
+}
+
+private val ImageBitmap.size get() = Size(width.toFloat(), height.toFloat())
+
+private val BoxWithConstraintsScope.areaSize
+    @Composable get() = with(LocalDensity.current) {
+        Size(maxWidth.toPx(), maxHeight.toPx())
     }

experimental/examples/imageviewer/shared/src/desktopMain/kotlin/example/imageviewer/view/ScalableImage.desktop.kt

@@ -1,28 +0,0 @@
-package example.imageviewer.view
-
-import androidx.compose.foundation.gestures.detectDragGestures
-import androidx.compose.ui.Modifier
-import androidx.compose.ui.geometry.Offset
-import androidx.compose.ui.input.pointer.PointerEventType
-import androidx.compose.ui.input.pointer.pointerInput
-import example.imageviewer.model.ScalableState
-import example.imageviewer.model.addDragAmount
-import example.imageviewer.model.addScale
-
-actual fun Modifier.addUserInput(state: ScalableState): Modifier =
-    pointerInput(Unit) {
-        detectDragGestures { change, dragAmount: Offset ->
-            state.addDragAmount(dragAmount)
-            change.consume()
-        }
-    }.pointerInput(Unit) {
-        awaitPointerEventScope {
-            while (true) {
-                val event = awaitPointerEvent()
-                if (event.type == PointerEventType.Scroll) {
-                    val delta = event.changes.getOrNull(0)?.scrollDelta ?: Offset.Zero
-                    state.addScale(delta.y / 100)
-                }
-            }
-        }
-    }

experimental/examples/imageviewer/shared/src/desktopMain/kotlin/example/imageviewer/view/ZoomControllerView.desktop.kt

@@ -8,17 +8,14 @@ import androidx.compose.runtime.Composable
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.unit.dp
-import example.imageviewer.model.MAX_SCALE
-import example.imageviewer.model.MIN_SCALE
 import example.imageviewer.model.ScalableState
-import example.imageviewer.model.setScale
 
 @Composable
 internal actual fun ZoomControllerView(modifier: Modifier, scalableState: ScalableState) {
     Slider(
         modifier = modifier.fillMaxWidth(0.5f).padding(12.dp),
         value = scalableState.scale,
-        valueRange = MIN_SCALE..MAX_SCALE,
+        valueRange = scalableState.scaleLimits.start..scalableState.scaleLimits.endInclusive,
         onValueChange = { scalableState.setScale(it) },
         colors = SliderDefaults.colors(thumbColor = Color.White, activeTrackColor = Color.White)
     )

experimental/examples/imageviewer/shared/src/iosMain/kotlin/example/imageviewer/view/addUserInput.kt

@@ -1,7 +0,0 @@
-package example.imageviewer.view
-
-import androidx.compose.ui.Modifier
-import example.imageviewer.model.ScalableState
-
-actual fun Modifier.addUserInput(state: ScalableState): Modifier =
-    addTouchUserInput(state)