Android: opt-in removal snapshot (snapshotOnRemoval) + interactive edge-swipe-back

android/src/main/java/com/swmansion/rnscreens/Screen.kt

@@ -2,10 +2,19 @@ package com.swmansion.rnscreens
 
 import android.annotation.SuppressLint
 import android.content.pm.ActivityInfo
+import android.graphics.Bitmap
 import android.graphics.Paint
+import android.graphics.Rect
+import android.graphics.drawable.BitmapDrawable
+import android.os.Build
+import android.os.Handler
+import android.os.HandlerThread
+import android.os.Looper
 import android.os.Parcelable
+import android.util.Log
 import android.util.SparseArray
 import android.view.MotionEvent
+import android.view.PixelCopy
 import android.view.View
 import android.view.ViewGroup
 import android.view.WindowManager
@@ -34,6 +43,9 @@ import com.swmansion.rnscreens.events.SheetDetentChangedEvent
 import com.swmansion.rnscreens.ext.asScreenStackFragment
 import com.swmansion.rnscreens.ext.parentAsViewGroup
 import com.swmansion.rnscreens.gamma.common.FragmentProviding
+import java.util.concurrent.CountDownLatch
+import java.util.concurrent.TimeUnit
+import java.util.concurrent.atomic.AtomicBoolean
 
 @SuppressLint("ViewConstructor") // Only we construct this view, it is never inflated.
 class Screen(
@@ -459,12 +471,136 @@ class Screen(
 
     fun startRemovalTransition() {
         if (!isBeingRemoved) {
+            captureScreenSnapshotForRemoval()
             isBeingRemoved = true
             startTransitionRecursive(this)
         }
     }
 
+    // --- Screen-level removal snapshot -------------------------------------------------------
+    // The exit animation renders this screen while surface-backed content (e.g. a mid-fling
+    // WebView) may stop producing pixels, which flashes the window background through. When a
+    // screen opts in via the `snapshotOnRemoval` prop, freeze its last presented frame into its
+    // ViewOverlay for the duration of the removal transition so the pop animates real pixels.
+    // Opt-in only: the capture reads the composited window, which is only correct for opaque,
+    // full-window screens (e.g. the Reader) — never enable it for translucent/partial screens.
+
+    /** Set from JS via the `snapshotOnRemoval` prop; default off. */
+    var snapshotOnRemoval: Boolean = false
+
+    private var removalSnapshotBitmap: Bitmap? = null
+    private var removalSnapshotDrawable: BitmapDrawable? = null
+
+    // One capture attempt per removal: a snapshot is only valid if taken before the removal
+    // transaction starts animating. If the pre-transaction attempt fails (timeout etc.), a later
+    // entry point must NOT retry — it could capture an already-translated, mid-exit window.
+    // Reset alongside the snapshot itself in releaseRemovalSnapshot().
+    @Volatile
+    private var removalSnapshotAttempted: Boolean = false
+
+    // Ordering note: Fabric may call startRemovalTransition() from its mounting thread, where the
+    // capture is posted to the main looper. The ordering guarantee (capture happens before the
+    // removal transaction's animation) does not rest on that post: ScreenStack.onUpdate() invokes
+    // captureScreenSnapshotForRemoval() synchronously on the main thread immediately before it
+    // creates the removal transaction, and whichever call runs first wins the single attempt.
+    fun captureScreenSnapshotForRemoval() {
+        if (!snapshotOnRemoval) return
+        if (removalSnapshotAttempted) return
+        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.O) return
+        removalSnapshotAttempted = true
+        if (Looper.myLooper() == Looper.getMainLooper()) {
+            captureScreenSnapshotOnMain()
+        } else {
+            post { captureScreenSnapshotOnMain() }
+        }
+    }
+
+    private fun captureScreenSnapshotOnMain() {
+        // Recheck the opt-in: a posted capture can outlive a prop flip on a recycled screen.
+        if (!snapshotOnRemoval) return
+        if (removalSnapshotBitmap != null) return
+        if (!isAttachedToWindow || width <= 0 || height <= 0) return
+        val activity = reactContext.currentActivity ?: return
+        val window = activity.window
+        val decor = window.decorView
+
+        val location = IntArray(2)
+        getLocationInWindow(location)
+        val srcRect = Rect(location[0], location[1], location[0] + width, location[1] + height)
+        // Only capture a fully on-window screen; a clipped rect would be stretched over the
+        // screen's full bounds when displayed.
+        if (!srcRect.intersect(0, 0, decor.width, decor.height)) return
+        if (srcRect.width() != width || srcRect.height() != height) return
+
+        val bitmap =
+            try {
+                Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888)
+            } catch (e: OutOfMemoryError) {
+                Log.w(TAG, "removal snapshot allocation failed", e)
+                return
+            }
+
+        val latch = CountDownLatch(1)
+        val resultHolder = intArrayOf(PixelCopy.ERROR_UNKNOWN)
+        // Bitmap ownership on timeout: PixelCopy cannot be cancelled, so the copy may still be
+        // writing into the bitmap after the bounded wait expires. Whoever finishes second (the
+        // callback, or the timed-out waiter observing the callback already ran) recycles it;
+        // recycling from the waiter while the copy is in flight would be a native UAF.
+        val loserRecycles = AtomicBoolean(false)
+        try {
+            PixelCopy.request(window, srcRect, bitmap, { copyResult ->
+                resultHolder[0] = copyResult
+                latch.countDown()
+                if (loserRecycles.getAndSet(true)) {
+                    // The waiter already timed out and abandoned the bitmap — we own cleanup.
+                    bitmap.recycle()
+                }
+            }, snapshotHandler())
+        } catch (e: IllegalArgumentException) {
+            bitmap.recycle()
+            return
+        }
+
+        val completed =
+            try {
+                latch.await(SNAPSHOT_TIMEOUT_MS, TimeUnit.MILLISECONDS)
+            } catch (e: InterruptedException) {
+                Thread.currentThread().interrupt()
+                false
+            }
+
+        if (!completed) {
+            // Abandon: if the callback already finished in the meantime we recycle, otherwise
+            // the callback sees the flag and recycles when the copy completes.
+            if (loserRecycles.getAndSet(true)) {
+                bitmap.recycle()
+            }
+            return
+        }
+        if (resultHolder[0] != PixelCopy.SUCCESS) {
+            // Copy finished (callback ran) but failed; the bitmap is no longer being written.
+            bitmap.recycle()
+            return
+        }
+
+        val drawable = BitmapDrawable(resources, bitmap)
+        drawable.setBounds(0, 0, width, height)
+        removalSnapshotBitmap = bitmap
+        removalSnapshotDrawable = drawable
+        overlay.add(drawable)
+        invalidate()
+    }
+
+    fun releaseRemovalSnapshot() {
+        removalSnapshotDrawable?.let { overlay.remove(it) }
+        removalSnapshotDrawable = null
+        removalSnapshotBitmap = null
+        removalSnapshotAttempted = false
+    }
+    // --- end screen-level removal snapshot ----------------------------------------------------
+
     fun endRemovalTransition() {
+        releaseRemovalSnapshot()
         if (!isBeingRemoved) {
             return
         }
@@ -563,6 +699,11 @@ class Screen(
         }
     }
 
+    override fun onDetachedFromWindow() {
+        releaseRemovalSnapshot()
+        super.onDetachedFromWindow()
+    }
+
     private fun dispatchSheetDetentChanged(
         detentIndex: Int,
         isStable: Boolean,
@@ -650,6 +791,21 @@ class Screen(
          * This value describes value in sheet detents array that will be treated as `fitToContents` option.
          */
         const val SHEET_FIT_TO_CONTENTS = -1.0
+
+        private const val SNAPSHOT_TIMEOUT_MS = 50L
+
+        private var sSnapshotThread: HandlerThread? = null
+        private var sSnapshotHandler: Handler? = null
+
+        @JvmStatic
+        private fun snapshotHandler(): Handler {
+            sSnapshotHandler?.let { return it }
+            val thread = HandlerThread("RNSScreenSnapshot").also { it.start() }
+            val handler = Handler(thread.looper)
+            sSnapshotThread = thread
+            sSnapshotHandler = handler
+            return handler
+        }
     }
 }
 

android/src/main/java/com/swmansion/rnscreens/ScreenStack.kt

@@ -220,6 +220,13 @@ class ScreenStack(
             }
         }
 
+        if (!shouldUseOpenAnimation && topScreenWillChange) {
+            // Close animation: the current top screen is outgoing. Freeze its last presented
+            // frame before the removal transaction starts animating it (covers dismissal paths
+            // where Fabric's removeViewAt — and thus startRemovalTransition — runs later).
+            topScreenWrapper?.screen?.captureScreenSnapshotForRemoval()
+        }
+
         createTransaction().let { transaction ->
             if (stackAnimation != null) {
                 transaction.setTweenAnimations(stackAnimation, shouldUseOpenAnimation)

android/src/main/java/com/swmansion/rnscreens/ScreenViewManager.kt

@@ -162,6 +162,14 @@ open class ScreenViewManager :
         view.isGestureEnabled = gestureEnabled
     }
 
+    @ReactProp(name = "snapshotOnRemoval", defaultBoolean = false)
+    override fun setSnapshotOnRemoval(
+        view: Screen,
+        snapshotOnRemoval: Boolean,
+    ) {
+        view.snapshotOnRemoval = snapshotOnRemoval
+    }
+
     @ReactProp(name = "replaceAnimation")
     override fun setReplaceAnimation(
         view: Screen,

android/src/main/java/com/swmansion/rnscreens/events/ScreenAnimationDelegate.kt

@@ -71,7 +71,11 @@ class ScreenAnimationDelegate(
         }
     }
 
-    override fun onAnimationCancel(animation: Animator) = Unit
+    override fun onAnimationCancel(animation: Animator) {
+        // A cancelled exit can leave the screen attached and live; drop the frozen removal
+        // snapshot so the user never returns to a stale frame covering real content.
+        wrapper.screen.releaseRemovalSnapshot()
+    }
 
     override fun onAnimationRepeat(animation: Animator) = Unit
 

android/src/paper/java/com/facebook/react/viewmanagers/RNSScreenManagerDelegate.java

@@ -73,6 +73,9 @@ public void setProperty(T view, String propName, @Nullable Object value) {
       case "gestureEnabled":
         mViewManager.setGestureEnabled(view, value == null ? true : (boolean) value);
         break;
+      case "snapshotOnRemoval":
+        mViewManager.setSnapshotOnRemoval(view, value == null ? false : (boolean) value);
+        break;
       case "statusBarColor":
         mViewManager.setStatusBarColor(view, ColorPropConverter.getColor(value, view.getContext()));
         break;

android/src/paper/java/com/facebook/react/viewmanagers/RNSScreenManagerInterface.java

@@ -32,6 +32,7 @@ public interface RNSScreenManagerInterface<T extends View>  {
   void setHomeIndicatorHidden(T view, boolean value);
   void setPreventNativeDismiss(T view, boolean value);
   void setGestureEnabled(T view, boolean value);
+  void setSnapshotOnRemoval(T view, boolean value);
   void setStatusBarColor(T view, @Nullable Integer value);
   void setStatusBarHidden(T view, boolean value);
   void setScreenOrientation(T view, @Nullable String value);

src/fabric/ScreenNativeComponent.ts

@@ -103,6 +103,7 @@ export interface NativeProps extends ViewProps {
   homeIndicatorHidden?: boolean;
   preventNativeDismiss?: boolean;
   gestureEnabled?: WithDefault<boolean, true>;
+  snapshotOnRemoval?: WithDefault<boolean, false>;
   statusBarColor?: ColorValue;
   statusBarHidden?: boolean;
   screenOrientation?: string;

src/types.tsx

@@ -169,6 +169,15 @@ export interface ScreenProps extends ViewProps {
    * @platform ios
    */
   gestureEnabled?: boolean;
+  /**
+   * When `true`, the screen freezes its last presented frame into a native overlay at the start
+   * of its removal transition, so the exit animation shows real pixels even if surface-backed
+   * content (e.g. a WebView) stops rendering mid-transition. Only enable for opaque, full-window
+   * screens — the capture reads the composited window. Defaults to `false`.
+   *
+   * @platform android
+   */
+  snapshotOnRemoval?: boolean;
   /**
    * Use it to restrict the distance from the edges of screen in which the gesture should be recognized. To be used alongside `fullScreenSwipeEnabled`.
    *