Allow jumping to previous/next page with horizontal swipe gesture

app/src/main/java/app/grapheneos/pdfviewer/GestureHelper.java

@@ -7,28 +7,24 @@
 import android.view.ScaleGestureDetector;
 import android.view.View;
 
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+
 /*
     The GestureHelper present a simple gesture api for the PdfViewer
 */
 
 class GestureHelper {
     public interface GestureListener {
         boolean onTapUp();
+        boolean onFling(@Nullable MotionEvent e1, @NonNull MotionEvent e2, float velocityX, float velocityY);
         void onZoom(float scaleFactor, float focusX, float focusY);
         void onZoomEnd();
     }
 
     @SuppressLint("ClickableViewAccessibility")
     static void attach(Context context, View gestureView, GestureListener listener) {
 
-        final GestureDetector detector = new GestureDetector(context,
-                new GestureDetector.SimpleOnGestureListener() {
-                    @Override
-                    public boolean onSingleTapUp(MotionEvent motionEvent) {
-                        return listener.onTapUp();
-                    }
-                });
-
         final ScaleGestureDetector scaleDetector = new ScaleGestureDetector(context,
                 new ScaleGestureDetector.SimpleOnScaleGestureListener() {
                     @Override
@@ -45,6 +41,29 @@ public void onScaleEnd(ScaleGestureDetector detector) {
                     }
                 });
 
+        final GestureDetector detector = new GestureDetector(context,
+                new GestureDetector.SimpleOnGestureListener() {
+                    @Override
+                    public boolean onSingleTapUp(@NonNull MotionEvent motionEvent) {
+                        return listener.onTapUp();
+                    }
+
+                    @Override
+                    public boolean onFling(@Nullable MotionEvent e1, @NonNull MotionEvent e2,
+                                           float velocityX, float velocityY) {
+                        if (scaleDetector.isInProgress()) {
+                            return false;
+                        }
+
+                        // Ignore multi-touch
+                        if (e1 != null && (e1.getPointerCount() > 1 || e2.getPointerCount() > 1)) {
+                            return false;
+                        }
+
+                        return listener.onFling(e1, e2, velocityX, velocityY);
+                    }
+                });
+
         gestureView.setOnTouchListener((view, motionEvent) -> {
             detector.onTouchEvent(motionEvent);
             scaleDetector.onTouchEvent(motionEvent);

app/src/main/java/app/grapheneos/pdfviewer/PdfViewer.java

@@ -13,7 +13,9 @@
 import android.view.Menu;
 import android.view.MenuInflater;
 import android.view.MenuItem;
+import android.view.MotionEvent;
 import android.view.View;
+import android.view.ViewConfiguration;
 import android.webkit.CookieManager;
 import android.webkit.JavascriptInterface;
 import android.webkit.RenderProcessGoneDetail;
@@ -28,6 +30,7 @@
 import androidx.activity.result.ActivityResultLauncher;
 import androidx.activity.result.contract.ActivityResultContracts;
 import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
 import androidx.appcompat.app.AppCompatActivity;
 import androidx.core.view.WindowCompat;
 import androidx.fragment.app.Fragment;
@@ -125,6 +128,8 @@ public class PdfViewer extends AppCompatActivity implements LoaderManager.Loader
     private float mZoomRatio = 1f;
     private float mZoomFocusX = 0f;
     private float mZoomFocusY = 0f;
+    private int swipeThreshold;
+    private int swipeVelocityThreshold;
     private int mDocumentOrientationDegrees;
     private int mDocumentState;
     private String mEncryptedDocumentPassword;
@@ -431,6 +436,8 @@ public boolean onRenderProcessGone(WebView view, RenderProcessGoneDetail detail)
             }
         });
 
+        initializeGestures();
+
         GestureHelper.attach(PdfViewer.this, binding.webview,
                 new GestureHelper.GestureListener() {
                     @Override
@@ -450,6 +457,39 @@ public boolean onTapUp() {
                         return false;
                     }
 
+                    @Override
+                    public boolean onFling(@Nullable MotionEvent e1, @NonNull MotionEvent e2, float velocityX, float velocityY) {
+                        if (e1 == null) return false;
+
+                        float deltaX = e2.getX() - e1.getX();
+                        float deltaY = e2.getY() - e1.getY();
+                        float absDeltaX = Math.abs(deltaX);
+                        float absDeltaY = Math.abs(deltaY);
+
+                        // Check primarily horizontal
+                        if (absDeltaX > absDeltaY &&
+                                absDeltaX > swipeThreshold &&
+                                Math.abs(velocityX) > swipeVelocityThreshold) {
+
+                            boolean swipeLeft = deltaX < 0;
+                            boolean swipeRight = deltaX > 0;
+
+                            // Edge detection
+                            boolean atLeftEdge = !binding.webview.canScrollHorizontally(-1);
+                            boolean atRightEdge = !binding.webview.canScrollHorizontally(1);
+
+                            if (swipeLeft && atRightEdge) {
+                                onJumpToPageInDocument(mPage + 1);
+                                return true;
+                            } else if (swipeRight && atLeftEdge) {
+                                onJumpToPageInDocument(mPage - 1);
+                                return true;
+                            }
+                        }
+
+                        return false;
+                    }
+
                     @Override
                     public void onZoom(float scaleFactor, float focusX, float focusY) {
                         zoom(scaleFactor, focusX, focusY, false);
@@ -520,6 +560,12 @@ public void onZoomEnd() {
         }
     }
 
+    private void initializeGestures() {
+        ViewConfiguration vc = ViewConfiguration.get(this);
+        swipeThreshold = vc.getScaledTouchSlop() * 4;
+        swipeVelocityThreshold = vc.getScaledMinimumFlingVelocity();
+    }
+
     private void purgeWebView() {
         binding.webview.removeJavascriptInterface("channel");
         binding.getRoot().removeView(binding.webview);