Add page snapping and single page fling (DImuthuUpe#557)

* Add auto spacing feature
* Add page snapping
* Add single page flinging
* Disable snap and page fling when zoomed
* Update README.md

Author: Flamedek

README.md

@@ -100,8 +100,11 @@ pdfView.fromAsset(String)
     .enableAntialiasing(true) // improve rendering a little bit on low-res screens
     // spacing between pages in dp. To define spacing color, set view background
     .spacing(0)
+    .autoSpacing(false) // add dynamic spacing to fit each page on its own on the screen
     .linkHandler(DefaultLinkHandler)
     .pageFitPolicy(FitPolicy.WIDTH)
+    .pageSnap(true) // snap pages to screen boundaries
+    .pageFling(false) // make a fling change only a single page like ViewPager
     .load();
 ```
 
@@ -206,6 +209,15 @@ Configurator.onRender(new OnRenderListener() {
 });
 ```
 
+### How can I scroll through single pages like a ViewPager?
+You can use a combination of the following settings to get scroll and fling behaviour similar to a ViewPager:
+``` java
+    .swipeHorizontal(true)
+    .pageSnap(true)
+    .autoSpacing(true)
+    .pageFling(true)
+```
+
 ## One more thing
 If you have any suggestions on making this lib better, write me, create issue or write some code and send pull request.
 

android-pdf-viewer/src/main/java/com/github/barteksc/pdfviewer/AnimationManager.java

@@ -41,6 +41,8 @@ class AnimationManager {
 
     private boolean flinging = false;
 
+    private boolean pageFlinging = false;
+
     public AnimationManager(PDFView pdfView) {
         this.pdfView = pdfView;
         scroller = new OverScroller(pdfView.getContext());
@@ -85,6 +87,15 @@ public void startFlingAnimation(int startX, int startY, int velocityX, int veloc
         scroller.fling(startX, startY, velocityX, velocityY, minX, maxX, minY, maxY);
     }
 
+    public void startPageFlingAnimation(float targetOffset) {
+        if (pdfView.isSwipeVertical()) {
+            startYAnimation(pdfView.getCurrentYOffset(), targetOffset);
+        } else {
+            startXAnimation(pdfView.getCurrentXOffset(), targetOffset);
+        }
+        pageFlinging = true;
+    }
+
     void computeFling() {
         if (scroller.computeScrollOffset()) {
             pdfView.moveTo(scroller.getCurrX(), scroller.getCurrY());
@@ -93,6 +104,7 @@ void computeFling() {
             flinging = false;
             pdfView.loadPages();
             hideHandle();
+            pdfView.performPageSnap();
         }
     }
 
@@ -109,6 +121,10 @@ public void stopFling() {
         scroller.forceFinished(true);
     }
 
+    public boolean isFlinging() {
+        return flinging || pageFlinging;
+    }
+
     class XAnimation extends AnimatorListenerAdapter implements AnimatorUpdateListener {
 
         @Override
@@ -121,11 +137,13 @@ public void onAnimationUpdate(ValueAnimator animation) {
         @Override
         public void onAnimationCancel(Animator animation) {
             pdfView.loadPages();
+            pageFlinging = false;
         }
 
         @Override
         public void onAnimationEnd(Animator animation) {
             pdfView.loadPages();
+            pageFlinging = false;
         }
     }
 
@@ -141,11 +159,13 @@ public void onAnimationUpdate(ValueAnimator animation) {
         @Override
         public void onAnimationCancel(Animator animation) {
             pdfView.loadPages();
+            pageFlinging = false;
         }
 
         @Override
         public void onAnimationEnd(Animator animation) {
             pdfView.loadPages();
+            pageFlinging = false;
         }
     }
 
@@ -173,6 +193,7 @@ public void onAnimationCancel(Animator animation) {
         public void onAnimationEnd(Animator animation) {
             pdfView.loadPages();
             hideHandle();
+            pdfView.performPageSnap();
         }
 
         @Override

android-pdf-viewer/src/main/java/com/github/barteksc/pdfviewer/DecodingAsyncTask.java

@@ -48,7 +48,7 @@ protected Throwable doInBackground(Void... params) {
         try {
             PdfDocument pdfDocument = docSource.createDocument(pdfView.getContext(), pdfiumCore, password);
             pdfFile = new PdfFile(pdfiumCore, pdfDocument, pdfView.getPageFitPolicy(), getViewSize(),
-                    userPages, pdfView.isSwipeVertical(), pdfView.getSpacingPx());
+                    userPages, pdfView.isSwipeVertical(), pdfView.getSpacingPx(), pdfView.doAutoSpacing());
             return null;
         } catch (Throwable t) {
             return t;

android-pdf-viewer/src/main/java/com/github/barteksc/pdfviewer/DragPinchManager.java

@@ -24,6 +24,7 @@
 
 import com.github.barteksc.pdfviewer.model.LinkTapEvent;
 import com.github.barteksc.pdfviewer.scroll.ScrollHandle;
+import com.github.barteksc.pdfviewer.util.SnapEdge;
 import com.shockwave.pdfium.PdfDocument;
 import com.shockwave.pdfium.util.SizeF;
 
@@ -97,7 +98,7 @@ private boolean checkLinkTapped(float x, float y) {
         for (PdfDocument.Link link : pdfFile.getPageLinks(page)) {
             RectF mapped = pdfFile.mapRectToDevice(page, pageX, pageY, (int) pageSize.getWidth(),
                     (int) pageSize.getHeight(), link.getBounds());
-            fixCoords(mapped);
+            mapped.sort();
             if (mapped.contains(mappedX, mappedY)) {
                 pdfView.callbacks.callLinkHandler(new LinkTapEvent(x, y, mappedX, mappedY, mapped, link));
                 return true;
@@ -106,26 +107,27 @@ private boolean checkLinkTapped(float x, float y) {
         return false;
     }
 
-    /** Fix different coordinate axis */
-    private void fixCoords(RectF rect) {
-        if (rect.top > rect.bottom) {
-            swapTopBottom(rect);
+    private void startPageFling(MotionEvent downEvent, MotionEvent ev, float velocityX, float velocityY) {
+        if (!checkDoPageFling(velocityX, velocityY)) {
+            return;
         }
-        if (rect.left > rect.right) {
-            swapLeftRight(rect);
-        }
-    }
-
-    private void swapTopBottom(RectF rect) {
-        float tmp = rect.top;
-        rect.top = rect.bottom;
-        rect.bottom = tmp;
-    }
 
-    private void swapLeftRight(RectF rect) {
-        float tmp = rect.left;
-        rect.left = rect.right;
-        rect.right = tmp;
+        int direction;
+        if (pdfView.isSwipeVertical()) {
+            direction = velocityY > 0 ? -1 : 1;
+        } else {
+            direction = velocityX > 0 ? -1 : 1;
+        }
+        // get the focused page during the down event to ensure only a single page is changed
+        float delta = pdfView.isSwipeVertical() ? ev.getY() - downEvent.getY() : ev.getX() - downEvent.getX();
+        float offsetX = pdfView.getCurrentXOffset() - delta * pdfView.getZoom();
+        float offsetY = pdfView.getCurrentYOffset() - delta * pdfView.getZoom();
+        int startingPage = pdfView.findFocusPage(offsetX, offsetY);
+        int targetPage = Math.max(0, Math.min(pdfView.getPageCount() - 1, startingPage + direction));
+
+        SnapEdge edge = pdfView.findSnapEdge(targetPage);
+        float offset = pdfView.snapOffsetForPage(targetPage, edge);
+        animationManager.startPageFlingAnimation(-offset);
     }
 
     @Override
@@ -180,6 +182,9 @@ public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float d
     private void onScrollEnd(MotionEvent event) {
         pdfView.loadPages();
         hideHandle();
+        if (!animationManager.isFlinging()) {
+            pdfView.performPageSnap();
+        }
     }
 
     @Override
@@ -192,6 +197,15 @@ public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float ve
         if (!pdfView.isSwipeEnabled()) {
             return false;
         }
+        if (pdfView.doPageFling()) {
+            if (pdfView.pageFillsScreen()) {
+                onBoundedFling(velocityX, velocityY);
+            } else {
+                startPageFling(e1, e2, velocityX, velocityY);
+            }
+            return true;
+        }
+
         int xOffset = (int) pdfView.getCurrentXOffset();
         int yOffset = (int) pdfView.getCurrentYOffset();
 
@@ -207,10 +221,34 @@ public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float ve
 
         animationManager.startFlingAnimation(xOffset, yOffset, (int) (velocityX), (int) (velocityY),
                 (int) minX, 0, (int) minY, 0);
-
         return true;
     }
 
+    private void onBoundedFling(float velocityX, float velocityY) {
+        int xOffset = (int) pdfView.getCurrentXOffset();
+        int yOffset = (int) pdfView.getCurrentYOffset();
+
+        PdfFile pdfFile = pdfView.pdfFile;
+
+        float pageStart = -pdfFile.getPageOffset(pdfView.getCurrentPage(), pdfView.getZoom());
+        float pageEnd = pageStart - pdfFile.getPageLength(pdfView.getCurrentPage(), pdfView.getZoom());
+        float minX, minY, maxX, maxY;
+        if (pdfView.isSwipeVertical()) {
+            minX = -(pdfView.toCurrentScale(pdfFile.getMaxPageWidth()) - pdfView.getWidth());
+            minY = pageEnd + pdfView.getHeight();
+            maxX = 0;
+            maxY = pageStart;
+        } else {
+            minX = pageEnd + pdfView.getWidth();
+            minY = -(pdfView.toCurrentScale(pdfFile.getMaxPageHeight()) - pdfView.getHeight());
+            maxX = pageStart;
+            maxY = 0;
+        }
+
+        animationManager.startFlingAnimation(xOffset, yOffset, (int) (velocityX), (int) (velocityY),
+                (int) minX, (int) maxX, (int) minY, (int) maxY);
+    }
+
     @Override
     public boolean onScale(ScaleGestureDetector detector) {
         float dr = detector.getScaleFactor();
@@ -261,4 +299,10 @@ private void hideHandle() {
             scrollHandle.hideDelayed();
         }
     }
+
+    private boolean checkDoPageFling(float velocityX, float velocityY) {
+        float absX = Math.abs(velocityX);
+        float absY = Math.abs(velocityY);
+        return pdfView.isSwipeVertical() ? absY > absX : absX > absY;
+    }
 }