android: Better scroll of zoomed images in gallery (#1283)

apps/android/app/src/main/java/chat/simplex/app/views/chat/item/ImageFullScreenView.kt

@@ -13,6 +13,7 @@ import androidx.compose.ui.graphics.*
 import androidx.compose.ui.graphics.painter.BitmapPainter
 import androidx.compose.ui.input.pointer.*
 import androidx.compose.ui.layout.ContentScale
+import androidx.compose.ui.layout.onGloballyPositioned
 import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.res.stringResource
 import chat.simplex.app.R
@@ -25,6 +26,7 @@ import coil.request.ImageRequest
 import coil.size.Size
 import com.google.accompanist.pager.*
 import kotlinx.coroutines.launch
+import kotlin.math.absoluteValue
 
 interface ImageGalleryProvider {
   val initialIndex: Int
@@ -88,6 +90,8 @@ fun ImageFullScreenView(imageProvider: () -> ImageGalleryProvider, close: () ->
         var scale by remember { mutableStateOf(1f) }
         var translationX by remember { mutableStateOf(0f) }
         var translationY by remember { mutableStateOf(0f) }
+        var viewWidth by remember { mutableStateOf(0) }
+        var allowTranslate by remember { mutableStateOf(true) }
         LaunchedEffect(settledCurrentPage) {
           scale = 1f
           translationX = 0f
@@ -113,6 +117,9 @@ fun ImageFullScreenView(imageProvider: () -> ImageGalleryProvider, close: () ->
           contentDescription = stringResource(R.string.image_descr),
           contentScale = ContentScale.Fit,
           modifier = Modifier
+            .onGloballyPositioned {
+              viewWidth = it.size.width
+            }
             .graphicsLayer(
               scaleX = scale,
               scaleY = scale,
@@ -121,12 +128,14 @@ fun ImageFullScreenView(imageProvider: () -> ImageGalleryProvider, close: () ->
             )
             .pointerInput(Unit) {
               detectTransformGestures(
+                { allowTranslate },
                 onGesture = { _, pan, gestureZoom, _ ->
                   scale = (scale * gestureZoom).coerceIn(1f, 20f)
-                  if (scale > 1) {
+                  allowTranslate = viewWidth * (scale - 1f) - ((translationX + pan.x * scale).absoluteValue * 2) > 0
+                  if (scale > 1 && allowTranslate) {
                     translationX += pan.x * scale
                     translationY += pan.y * scale
-                  } else {
+                  } else if (allowTranslate) {
                     translationX = 0f
                     translationY = 0f
                   }

apps/android/app/src/main/java/chat/simplex/app/views/helpers/GestureDetector.kt

@@ -214,13 +214,14 @@ fun interactionSourceWithDetection(onClick: () -> Unit, onLongClick: () -> Unit)
 }
 
 suspend fun PointerInputScope.detectTransformGestures(
+  allowIntercept: () -> Boolean,
   panZoomLock: Boolean = false,
   onGesture: (centroid: Offset, pan: Offset, zoom: Float, rotation: Float) -> Unit
 ) {
+  var zoom = 1f
   forEachGesture {
     awaitPointerEventScope {
       var rotation = 0f
-      var zoom = 1f
       var pan = Offset.Zero
       var pastTouchSlop = false
       val touchSlop = viewConfiguration.touchSlop
@@ -264,7 +265,7 @@ suspend fun PointerInputScope.detectTransformGestures(
               onGesture(centroid, panChange, zoomChange, effectiveRotation)
             }
             event.changes.fastForEach {
-              if (it.positionChanged() && zoomChange != 1f) {
+              if (it.positionChanged() && zoom != 1f && allowIntercept()) {
                 it.consume()
               }
             }