Implement vertical page panning in paged reader mode when using hardware keys (#716)

* Implement vertical page panning in paged reader mode when using hardware keys

* Pan to top or bottom of the page instead of directly moving to the previous/next page.
* Add `canPanUp`, `canPanDown`, `panUp`, `panDown` to `ReaderPageImageView`.
* Modify `moveUp` and `moveDown` in `PagerViewer` to utilize the new panning functions.

* add annotation

app/src/main/java/eu/kanade/tachiyomi/ui/reader/viewer/ReaderPageImageView.kt

@@ -187,6 +187,18 @@ open class ReaderPageImageView @JvmOverloads constructor(
      */
     fun canPanRight(): Boolean = canPan { it.right }
 
+    // KMK -->
+    /**
+     * Check if the image can be panned up
+     */
+    fun canPanUp(): Boolean = canPan { it.top }
+
+    /**
+     * Check if the image can be panned down
+     */
+    fun canPanDown(): Boolean = canPan { it.bottom }
+    // KMK <--
+
     /**
      * Check whether the image can be panned.
      * @param fn a function that returns the direction to check for
@@ -215,6 +227,22 @@ open class ReaderPageImageView @JvmOverloads constructor(
         pan { center, view -> center.also { it.x += view.width / view.scale } }
     }
 
+    // KMK -->
+    /**
+     * Pans the image down by a screen's height worth.
+     */
+    fun panDown() {
+        pan { center, view -> center.also { it.y += view.height / view.scale } }
+    }
+
+    /**
+     * Pans the image up by a screen's height worth.
+     */
+    fun panUp() {
+        pan { center, view -> center.also { it.y -= view.height / view.scale } }
+    }
+    // KMK <--
+
     /**
      * Pans the image.
      * @param fn a function that computes the new center of the image

app/src/main/java/eu/kanade/tachiyomi/ui/reader/viewer/pager/PagerViewer.kt

@@ -384,17 +384,31 @@ abstract class PagerViewer(
     }
 
     /**
-     * Moves to the page at the top (or previous).
+     * Pans to the top of the page or if already on the top moves to the previous page.
      */
     protected open fun moveUp() {
-        moveToPrevious()
+        // KMK -->
+        val holder = (currentPage as? ReaderPage)?.let(::getPageHolder)
+        if (holder != null && holder.canPanUp()) {
+            holder.panUp()
+        } else {
+            // KMK <--
+            moveToPrevious()
+        }
     }
 
     /**
-     * Moves to the page at the bottom (or next).
+     * Pans to the bottom of the page or if already on the bottom moves to the next page.
      */
     protected open fun moveDown() {
-        moveToNext()
+        // KMK -->
+        val holder = (currentPage as? ReaderPage)?.let(::getPageHolder)
+        if (holder != null && holder.canPanDown()) {
+            holder.panDown()
+        } else {
+            // KMK <--
+            moveToNext()
+        }
     }
 
     /**