Recently, while developing mobile applications for the Android platform, we were pleasantly surprised to see how much the internal display classes work like Java’s Swing components.  The online Android documentation was good, and there were plenty of available example apps to help speed us along.

However there is a glaring limitation:  there are base classes for horizontal scrollviews and vertical scrollviews, but not one where one can scroll in two dimensions at the same time.

It would be a challenge to write a two-dimensional scrollview class from scratch.  Luckily for us, the entire Android platform is open-source, so we had access to both the vertical and horizontal scrollview source code.  After a few hours of work we had created a new TwoDScrollView class.

First, here is a disclaimer: this class has been “munged” together and is not bullet-proof for a true generalized solution.  For example the methods that handle sub-view focusing have not been tested, and there are some nuances about how to prioritize focused sub-views in two-dimensions.  I have a fully-stripped version of this class without sub-view focusing or key events that I’d be happy to share upon request.

Without further ado, here is the new TwoDScrollView class:

/*
 * Copyright (C) 2006 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
/*
 * Revised 5/19/2010 by GORGES
 * Now supports two-dimensional view scrolling
 * http://GORGES.us
 */

package us.gorges.my_package;

import java.util.List;

import android.content.Context;
import android.graphics.Rect;
import android.util.AttributeSet;
import android.view.FocusFinder;
import android.view.KeyEvent;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.View;
import android.view.ViewConfiguration;
import android.view.ViewGroup;
import android.view.ViewParent;
import android.view.animation.AnimationUtils;
import android.widget.FrameLayout;
import android.widget.LinearLayout;
import android.widget.Scroller;
import android.widget.TextView;

/**
 * Layout container for a view hierarchy that can be scrolled by the user,
 * allowing it to be larger than the physical display.  A TwoDScrollView
 * is a {@link FrameLayout}, meaning you should place one child in it
 * containing the entire contents to scroll; this child may itself be a layout
 * manager with a complex hierarchy of objects.  A child that is often used
 * is a {@link LinearLayout} in a vertical orientation, presenting a vertical
 * array of top-level items that the user can scroll through.
 *
 * <p>The {@link TextView} class also
 * takes care of its own scrolling, so does not require a TwoDScrollView, but
 * using the two together is possible to achieve the effect of a text view
 * within a larger container.
 */
public class TwoDScrollView extends FrameLayout {
 static final int ANIMATED_SCROLL_GAP = 250;
 static final float MAX_SCROLL_FACTOR = 0.5f;

 private long mLastScroll;

 private final Rect mTempRect = new Rect();
 private Scroller mScroller;

 /**
 * Flag to indicate that we are moving focus ourselves. This is so the
 * code that watches for focus changes initiated outside this TwoDScrollView
 * knows that it does not have to do anything.
 */
 private boolean mTwoDScrollViewMovedFocus;

 /**
 * Position of the last motion event.
 */
 private float mLastMotionY;
 private float mLastMotionX;

 /**
 * True when the layout has changed but the traversal has not come through yet.
 * Ideally the view hierarchy would keep track of this for us.
 */
 private boolean mIsLayoutDirty = true;

 /**
 * The child to give focus to in the event that a child has requested focus while the
 * layout is dirty. This prevents the scroll from being wrong if the child has not been
 * laid out before requesting focus.
 */
 private View mChildToScrollTo = null;

 /**
 * True if the user is currently dragging this TwoDScrollView around. This is
 * not the same as 'is being flinged', which can be checked by
 * mScroller.isFinished() (flinging begins when the user lifts his finger).
 */
 private boolean mIsBeingDragged = false;

 /**
 * Determines speed during touch scrolling
 */
 private VelocityTracker mVelocityTracker;

 /**
 * Whether arrow scrolling is animated.
 */
 private int mTouchSlop;
 private int mMinimumVelocity;
 private int mMaximumVelocity;

 public TwoDScrollView(Context context) {
   super(context);
   initTwoDScrollView();
 }

 public TwoDScrollView(Context context, AttributeSet attrs) {
   super(context, attrs);
   initTwoDScrollView();
 }

 public TwoDScrollView(Context context, AttributeSet attrs, int defStyle) {
   super(context, attrs, defStyle);
   initTwoDScrollView();
 }

 @Override
 protected float getTopFadingEdgeStrength() {
   if (getChildCount() == 0) {
     return 0.0f;
   }
   final int length = getVerticalFadingEdgeLength();
   if (getScrollY() < length) {
     return getScrollY() / (float) length;
   }
   return 1.0f;
 }

 @Override
 protected float getBottomFadingEdgeStrength() {
   if (getChildCount() == 0) {
     return 0.0f;
   }
   final int length = getVerticalFadingEdgeLength();
   final int bottomEdge = getHeight() - getPaddingBottom();
   final int span = getChildAt(0).getBottom() - getScrollY() - bottomEdge;
   if (span < length) {
     return span / (float) length;
   }
   return 1.0f;
 }

 @Override
 protected float getLeftFadingEdgeStrength() {
   if (getChildCount() == 0) {
     return 0.0f;
   }
   final int length = getHorizontalFadingEdgeLength();
   if (getScrollX() < length) {
     return getScrollX() / (float) length;
   }
   return 1.0f;
 }

 @Override
 protected float getRightFadingEdgeStrength() {
   if (getChildCount() == 0) {
     return 0.0f;
   }
   final int length = getHorizontalFadingEdgeLength();
   final int rightEdge = getWidth() - getPaddingRight();
   final int span = getChildAt(0).getRight() - getScrollX() - rightEdge;
   if (span < length) {
     return span / (float) length;
   }
   return 1.0f;
 }

 /**
 * @return The maximum amount this scroll view will scroll in response to
 *   an arrow event.
 */
 public int getMaxScrollAmountVertical() {
   return (int) (MAX_SCROLL_FACTOR * getHeight());
 }
 public int getMaxScrollAmountHorizontal() {
   return (int) (MAX_SCROLL_FACTOR * getWidth());
 }

 private void initTwoDScrollView() {
   mScroller = new Scroller(getContext());
   setFocusable(true);
   setDescendantFocusability(FOCUS_AFTER_DESCENDANTS);
   setWillNotDraw(false);
   final ViewConfiguration configuration = ViewConfiguration.get(getContext());
   mTouchSlop = configuration.getScaledTouchSlop();
   mMinimumVelocity = configuration.getScaledMinimumFlingVelocity();
   mMaximumVelocity = configuration.getScaledMaximumFlingVelocity();
 }

 @Override
 public void addView(View child) {
   if (getChildCount() > 0) {
     throw new IllegalStateException("TwoDScrollView can host only one direct child");
   }
   super.addView(child);
 }

 @Override
 public void addView(View child, int index) {
   if (getChildCount() > 0) {
     throw new IllegalStateException("TwoDScrollView can host only one direct child");
   }
   super.addView(child, index);
 }

 @Override
 public void addView(View child, ViewGroup.LayoutParams params) {
   if (getChildCount() > 0) {
     throw new IllegalStateException("TwoDScrollView can host only one direct child");
   }
   super.addView(child, params);
 }

 @Override
 public void addView(View child, int index, ViewGroup.LayoutParams params) {
   if (getChildCount() > 0) {
     throw new IllegalStateException("TwoDScrollView can host only one direct child");
   }
   super.addView(child, index, params);
 }

 /**
 * @return Returns true this TwoDScrollView can be scrolled
 */
 private boolean canScroll() {
   View child = getChildAt(0);
   if (child != null) {
     int childHeight = child.getHeight();
     int childWidth = child.getWidth();
     return (getHeight() < childHeight + getPaddingTop() + getPaddingBottom()) ||
            (getWidth() < childWidth + getPaddingLeft() + getPaddingRight());
   }
   return false;
 }

 @Override
 public boolean dispatchKeyEvent(KeyEvent event) {
   // Let the focused view and/or our descendants get the key first
   boolean handled = super.dispatchKeyEvent(event);
   if (handled) {
     return true;
   }
   return executeKeyEvent(event);
 }

 /**
 * You can call this function yourself to have the scroll view perform
 * scrolling from a key event, just as if the event had been dispatched to
 * it by the view hierarchy.
 *
 * @param event The key event to execute.
 * @return Return true if the event was handled, else false.
 */
 public boolean executeKeyEvent(KeyEvent event) {
   mTempRect.setEmpty();
   if (!canScroll()) {
     if (isFocused()) {
       View currentFocused = findFocus();
       if (currentFocused == this) currentFocused = null;
       View nextFocused = FocusFinder.getInstance().findNextFocus(this, currentFocused, View.FOCUS_DOWN);
       return nextFocused != null && nextFocused != this && nextFocused.requestFocus(View.FOCUS_DOWN);
     }
     return false;
   }
   boolean handled = false;
   if (event.getAction() == KeyEvent.ACTION_DOWN) {
     switch (event.getKeyCode()) {
       case KeyEvent.KEYCODE_DPAD_UP:
         if (!event.isAltPressed()) {
           handled = arrowScroll(View.FOCUS_UP, false);
         } else {
           handled = fullScroll(View.FOCUS_UP, false);
         }
         break;
       case KeyEvent.KEYCODE_DPAD_DOWN:
         if (!event.isAltPressed()) {
           handled = arrowScroll(View.FOCUS_DOWN, false);
         } else {
           handled = fullScroll(View.FOCUS_DOWN, false);
         }
         break;
       case KeyEvent.KEYCODE_DPAD_LEFT:
         if (!event.isAltPressed()) {
           handled = arrowScroll(View.FOCUS_LEFT, true);
         } else {
           handled = fullScroll(View.FOCUS_LEFT, true);
         }
         break;
       case KeyEvent.KEYCODE_DPAD_RIGHT:
         if (!event.isAltPressed()) {
           handled = arrowScroll(View.FOCUS_RIGHT, true);
         } else {
           handled = fullScroll(View.FOCUS_RIGHT, true);
         }
         break;
     }
   }
   return handled;
 }

 @Override
 public boolean onInterceptTouchEvent(MotionEvent ev) {
   /*
   * This method JUST determines whether we want to intercept the motion.
   * If we return true, onMotionEvent will be called and we do the actual
   * scrolling there.
   *
   * Shortcut the most recurring case: the user is in the dragging
   * state and he is moving his finger.  We want to intercept this
   * motion.
   */
   final int action = ev.getAction();
   if ((action == MotionEvent.ACTION_MOVE) && (mIsBeingDragged)) {
     return true;
   }
   if (!canScroll()) {
     mIsBeingDragged = false;
     return false;
   }
   final float y = ev.getY();
   final float x = ev.getX();
   switch (action) {
     case MotionEvent.ACTION_MOVE:
       /*
       * mIsBeingDragged == false, otherwise the shortcut would have caught it. Check
       * whether the user has moved far enough from his original down touch.
       */
       /*
       * Locally do absolute value. mLastMotionY is set to the y value
       * of the down event.
       */
       final int yDiff = (int) Math.abs(y - mLastMotionY);
       final int xDiff = (int) Math.abs(x - mLastMotionX);
       if (yDiff > mTouchSlop || xDiff > mTouchSlop) {
         mIsBeingDragged = true;
       }
       break;

     case MotionEvent.ACTION_DOWN:
       /* Remember location of down touch */
       mLastMotionY = y;
       mLastMotionX = x;

       /*
       * If being flinged and user touches the screen, initiate drag;
       * otherwise don't.  mScroller.isFinished should be false when
       * being flinged.
       */
       mIsBeingDragged = !mScroller.isFinished();
       break;

     case MotionEvent.ACTION_CANCEL:
     case MotionEvent.ACTION_UP:
       /* Release the drag */
       mIsBeingDragged = false;
       break;
   }

   /*
   * The only time we want to intercept motion events is if we are in the
   * drag mode.
   */
   return mIsBeingDragged;
 }

 @Override
 public boolean onTouchEvent(MotionEvent ev) {

   if (ev.getAction() == MotionEvent.ACTION_DOWN && ev.getEdgeFlags() != 0) {
     // Don't handle edge touches immediately -- they may actually belong to one of our
     // descendants.
     return false;
   }

   if (!canScroll()) {
     return false;
   }

   if (mVelocityTracker == null) {
     mVelocityTracker = VelocityTracker.obtain();
   }
   mVelocityTracker.addMovement(ev);

   final int action = ev.getAction();
   final float y = ev.getY();
   final float x = ev.getX();

   switch (action) {
     case MotionEvent.ACTION_DOWN:
       /*
       * If being flinged and user touches, stop the fling. isFinished
       * will be false if being flinged.
       */
       if (!mScroller.isFinished()) {
         mScroller.abortAnimation();
       }

       // Remember where the motion event started
       mLastMotionY = y;
       mLastMotionX = x;
       break;
     case MotionEvent.ACTION_MOVE:
       // Scroll to follow the motion event
       int deltaX = (int) (mLastMotionX - x);
       int deltaY = (int) (mLastMotionY - y);
       mLastMotionX = x;
       mLastMotionY = y;

       if (deltaX < 0) {
         if (getScrollX() < 0) {
           deltaX = 0;
         }
       } else if (deltaX > 0) {
         final int rightEdge = getWidth() - getPaddingRight();
         final int availableToScroll = getChildAt(0).getRight() - getScrollX() - rightEdge;
         if (availableToScroll > 0) {
           deltaX = Math.min(availableToScroll, deltaX);
         } else {
           deltaX = 0;
         }
       }
       if (deltaY < 0) {
         if (getScrollY() < 0) {
           deltaY = 0;
         }
       } else if (deltaY > 0) {
         final int bottomEdge = getHeight() - getPaddingBottom();
         final int availableToScroll = getChildAt(0).getBottom() - getScrollY() - bottomEdge;
         if (availableToScroll > 0) {
           deltaY = Math.min(availableToScroll, deltaY);
         } else {
           deltaY = 0;
         }
       }
       if (deltaY != 0 || deltaX != 0)
         scrollBy(deltaX, deltaY);
       break;
     case MotionEvent.ACTION_UP:
         final VelocityTracker velocityTracker = mVelocityTracker;
         velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
         int initialXVelocity = (int) velocityTracker.getXVelocity();
         int initialYVelocity = (int) velocityTracker.getYVelocity();
         if ((Math.abs(initialXVelocity) + Math.abs(initialYVelocity) > mMinimumVelocity) && getChildCount() > 0) {
           fling(-initialXVelocity, -initialYVelocity);
         }
         if (mVelocityTracker != null) {
           mVelocityTracker.recycle();
           mVelocityTracker = null;
         }
   }
   return true;
 }

 /**
  * Finds the next focusable component that fits in this View's bounds
  * (excluding fading edges) pretending that this View's top is located at
  * the parameter top.
  *
  * @param topFocus           look for a candidate is the one at the top of the bounds
  *                           if topFocus is true, or at the bottom of the bounds if topFocus is
  *                           false
  * @param top                the top offset of the bounds in which a focusable must be
  *                           found (the fading edge is assumed to start at this position)
  * @param preferredFocusable the View that has highest priority and will be
  *                           returned if it is within my bounds (null is valid)
  * @return the next focusable component in the bounds or null if none can be
  *         found
  */
 private View findFocusableViewInMyBounds(final boolean topFocus, final int top, final boolean leftFocus, final int left, View preferredFocusable) {
   /*
   * The fading edge's transparent side should be considered for focus
   * since it's mostly visible, so we divide the actual fading edge length
   * by 2.
   */
   final int verticalFadingEdgeLength = getVerticalFadingEdgeLength() / 2;
   final int topWithoutFadingEdge = top + verticalFadingEdgeLength;
   final int bottomWithoutFadingEdge = top + getHeight() - verticalFadingEdgeLength;
   final int horizontalFadingEdgeLength = getHorizontalFadingEdgeLength() / 2;
   final int leftWithoutFadingEdge = left + horizontalFadingEdgeLength;
   final int rightWithoutFadingEdge = left + getWidth() - horizontalFadingEdgeLength;

   if ((preferredFocusable != null)
     && (preferredFocusable.getTop() < bottomWithoutFadingEdge)
     && (preferredFocusable.getBottom() > topWithoutFadingEdge)
     && (preferredFocusable.getLeft() < rightWithoutFadingEdge)
     && (preferredFocusable.getRight() > leftWithoutFadingEdge)) {
     return preferredFocusable;
   }
   return findFocusableViewInBounds(topFocus, topWithoutFadingEdge, bottomWithoutFadingEdge, leftFocus, leftWithoutFadingEdge, rightWithoutFadingEdge);
 }

 /**
 * Finds the next focusable component that fits in the specified bounds.
 * </p>
 *
 * @param topFocus look for a candidate is the one at the top of the bounds
 *                 if topFocus is true, or at the bottom of the bounds if topFocus is
 *                 false
 * @param top      the top offset of the bounds in which a focusable must be
 *                 found
 * @param bottom   the bottom offset of the bounds in which a focusable must
 *                 be found
 * @return the next focusable component in the bounds or null if none can
 *         be found
 */
 private View findFocusableViewInBounds(boolean topFocus, int top, int bottom, boolean leftFocus, int left, int right) {
   List<View> focusables = getFocusables(View.FOCUS_FORWARD);
   View focusCandidate = null;

   /*
   * A fully contained focusable is one where its top is below the bound's
   * top, and its bottom is above the bound's bottom. A partially
   * contained focusable is one where some part of it is within the
   * bounds, but it also has some part that is not within bounds.  A fully contained
   * focusable is preferred to a partially contained focusable.
   */
   boolean foundFullyContainedFocusable = false;

   int count = focusables.size();
   for (int i = 0; i < count; i++) {
     View view = focusables.get(i);
     int viewTop = view.getTop();
     int viewBottom = view.getBottom();
     int viewLeft = view.getLeft();
     int viewRight = view.getRight();

     if (top < viewBottom && viewTop < bottom && left < viewRight && viewLeft < right) {
       /*
       * the focusable is in the target area, it is a candidate for
       * focusing
       */
       final boolean viewIsFullyContained = (top < viewTop) && (viewBottom < bottom) && (left < viewLeft) && (viewRight < right);
       if (focusCandidate == null) {
         /* No candidate, take this one */
         focusCandidate = view;
         foundFullyContainedFocusable = viewIsFullyContained;
       } else {
         final boolean viewIsCloserToVerticalBoundary =
           (topFocus && viewTop < focusCandidate.getTop()) ||
           (!topFocus && viewBottom > focusCandidate.getBottom());
         final boolean viewIsCloserToHorizontalBoundary =
           (leftFocus && viewLeft < focusCandidate.getLeft()) ||
           (!leftFocus && viewRight > focusCandidate.getRight());
         if (foundFullyContainedFocusable) {
           if (viewIsFullyContained && viewIsCloserToVerticalBoundary && viewIsCloserToHorizontalBoundary) {
             /*
              * We're dealing with only fully contained views, so
              * it has to be closer to the boundary to beat our
              * candidate
              */
             focusCandidate = view;
           }
         } else {
           if (viewIsFullyContained) {
             /* Any fully contained view beats a partially contained view */
             focusCandidate = view;
             foundFullyContainedFocusable = true;
           } else if (viewIsCloserToVerticalBoundary && viewIsCloserToHorizontalBoundary) {
             /*
              * Partially contained view beats another partially
              * contained view if it's closer
              */
             focusCandidate = view;
           }
         }
       }
     }
   }
   return focusCandidate;
 }

 /**
  * <p>Handles scrolling in response to a "home/end" shortcut press. This
  * method will scroll the view to the top or bottom and give the focus
  * to the topmost/bottommost component in the new visible area. If no
  * component is a good candidate for focus, this scrollview reclaims the
  * focus.</p>
  *
  * @param direction the scroll direction: {@link android.view.View#FOCUS_UP}
  *                  to go the top of the view or
  *                  {@link android.view.View#FOCUS_DOWN} to go the bottom
  * @return true if the key event is consumed by this method, false otherwise
  */
 public boolean fullScroll(int direction, boolean horizontal) {
   if (!horizontal) {
     boolean down = direction == View.FOCUS_DOWN;
     int height = getHeight();
     mTempRect.top = 0;
     mTempRect.bottom = height;
     if (down) {
       int count = getChildCount();
       if (count > 0) {
         View view = getChildAt(count - 1);
         mTempRect.bottom = view.getBottom();
         mTempRect.top = mTempRect.bottom - height;
       }
     }
     return scrollAndFocus(direction, mTempRect.top, mTempRect.bottom, 0, 0, 0);
   } else {
     boolean right = direction == View.FOCUS_DOWN;
     int width = getWidth();
     mTempRect.left = 0;
     mTempRect.right = width;
     if (right) {
       int count = getChildCount();
       if (count > 0) {
         View view = getChildAt(count - 1);
         mTempRect.right = view.getBottom();
         mTempRect.left = mTempRect.right - width;
       }
     }
     return scrollAndFocus(0, 0, 0, direction, mTempRect.top, mTempRect.bottom);
   }
 }

 /**
  * <p>Scrolls the view to make the area defined by <code>top</code> and
  * <code>bottom</code> visible. This method attempts to give the focus
  * to a component visible in this area. If no component can be focused in
  * the new visible area, the focus is reclaimed by this scrollview.</p>
  *
  * @param direction the scroll direction: {@link android.view.View#FOCUS_UP}
  *                  to go upward
  *                  {@link android.view.View#FOCUS_DOWN} to downward
  * @param top       the top offset of the new area to be made visible
  * @param bottom    the bottom offset of the new area to be made visible
  * @return true if the key event is consumed by this method, false otherwise
  */
 private boolean scrollAndFocus(int directionY, int top, int bottom, int directionX, int left, int right) {
   boolean handled = true;
   int height = getHeight();
   int containerTop = getScrollY();
   int containerBottom = containerTop + height;
   boolean up = directionY == View.FOCUS_UP;
   int width = getWidth();
   int containerLeft = getScrollX();
   int containerRight = containerLeft + width;
   boolean leftwards = directionX == View.FOCUS_UP;
   View newFocused = findFocusableViewInBounds(up, top, bottom, leftwards, left, right);
   if (newFocused == null) {
     newFocused = this;
   }
   if ((top >= containerTop && bottom <= containerBottom) || (left >= containerLeft && right <= containerRight)) {
     handled = false;
   } else {
     int deltaY = up ? (top - containerTop) : (bottom - containerBottom);
     int deltaX = leftwards ? (left - containerLeft) : (right - containerRight);
     doScroll(deltaX, deltaY);
   }
   if (newFocused != findFocus() && newFocused.requestFocus(directionY)) {
     mTwoDScrollViewMovedFocus = true;
     mTwoDScrollViewMovedFocus = false;
   }
   return handled;
 }

 /**
  * Handle scrolling in response to an up or down arrow click.
  *
  * @param direction The direction corresponding to the arrow key that was
  *                  pressed
  * @return True if we consumed the event, false otherwise
  */
 public boolean arrowScroll(int direction, boolean horizontal) {
   View currentFocused = findFocus();
   if (currentFocused == this) currentFocused = null;
   View nextFocused = FocusFinder.getInstance().findNextFocus(this, currentFocused, direction);
   final int maxJump = horizontal ? getMaxScrollAmountHorizontal() : getMaxScrollAmountVertical();

   if (!horizontal) {
     if (nextFocused != null) {
       nextFocused.getDrawingRect(mTempRect);
       offsetDescendantRectToMyCoords(nextFocused, mTempRect);
       int scrollDelta = computeScrollDeltaToGetChildRectOnScreen(mTempRect);
       doScroll(0, scrollDelta);
       nextFocused.requestFocus(direction);
     } else {
       // no new focus
       int scrollDelta = maxJump;
       if (direction == View.FOCUS_UP && getScrollY() < scrollDelta) {
         scrollDelta = getScrollY();
       } else if (direction == View.FOCUS_DOWN) {
         if (getChildCount() > 0) {
           int daBottom = getChildAt(0).getBottom();
           int screenBottom = getScrollY() + getHeight();
           if (daBottom - screenBottom < maxJump) {
             scrollDelta = daBottom - screenBottom;
           }
         }
       }
       if (scrollDelta == 0) {
         return false;
       }
       doScroll(0, direction == View.FOCUS_DOWN ? scrollDelta : -scrollDelta);
     }
   } else {
     if (nextFocused != null) {
       nextFocused.getDrawingRect(mTempRect);
       offsetDescendantRectToMyCoords(nextFocused, mTempRect);
       int scrollDelta = computeScrollDeltaToGetChildRectOnScreen(mTempRect);
       doScroll(scrollDelta, 0);
       nextFocused.requestFocus(direction);
     } else {
       // no new focus
       int scrollDelta = maxJump;
       if (direction == View.FOCUS_UP && getScrollY() < scrollDelta) {
         scrollDelta = getScrollY();
       } else if (direction == View.FOCUS_DOWN) {
         if (getChildCount() > 0) {
           int daBottom = getChildAt(0).getBottom();
           int screenBottom = getScrollY() + getHeight();
           if (daBottom - screenBottom < maxJump) {
             scrollDelta = daBottom - screenBottom;
           }
         }
       }
       if (scrollDelta == 0) {
         return false;
       }
       doScroll(direction == View.FOCUS_DOWN ? scrollDelta : -scrollDelta, 0);
     }
   }
   return true;
 }

 /**
  * Smooth scroll by a Y delta
  *
  * @param delta the number of pixels to scroll by on the Y axis
  */
 private void doScroll(int deltaX, int deltaY) {
   if (deltaX != 0 || deltaY != 0) {
     smoothScrollBy(deltaX, deltaY);
   }
 }

 /**
  * Like {@link View#scrollBy}, but scroll smoothly instead of immediately.
  *
  * @param dx the number of pixels to scroll by on the X axis
  * @param dy the number of pixels to scroll by on the Y axis
  */
 public final void smoothScrollBy(int dx, int dy) {
   long duration = AnimationUtils.currentAnimationTimeMillis() - mLastScroll;
   if (duration > ANIMATED_SCROLL_GAP) {
     mScroller.startScroll(getScrollX(), getScrollY(), dx, dy);
     awakenScrollBars(mScroller.getDuration());
     invalidate();
   } else {
     if (!mScroller.isFinished()) {
       mScroller.abortAnimation();
     }
     scrollBy(dx, dy);
   }
   mLastScroll = AnimationUtils.currentAnimationTimeMillis();
 }

 /**
  * Like {@link #scrollTo}, but scroll smoothly instead of immediately.
  *
  * @param x the position where to scroll on the X axis
  * @param y the position where to scroll on the Y axis
  */
 public final void smoothScrollTo(int x, int y) {
   smoothScrollBy(x - getScrollX(), y - getScrollY());
 }

 /**
  * <p>The scroll range of a scroll view is the overall height of all of its
  * children.</p>
  */
 @Override
 protected int computeVerticalScrollRange() {
   int count = getChildCount();
   return count == 0 ? getHeight() : (getChildAt(0)).getBottom();
 }
 @Override
 protected int computeHorizontalScrollRange() {
   int count = getChildCount();
   return count == 0 ? getWidth() : (getChildAt(0)).getRight();
 }

 @Override
 protected void measureChild(View child, int parentWidthMeasureSpec, int parentHeightMeasureSpec) {
   ViewGroup.LayoutParams lp = child.getLayoutParams();
   int childWidthMeasureSpec;
   int childHeightMeasureSpec;

   childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec, getPaddingLeft() + getPaddingRight(), lp.width);
   childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);

   child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
 }

 @Override
 protected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed, int parentHeightMeasureSpec, int heightUsed) {
   final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
   final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
   getPaddingLeft() + getPaddingRight() + lp.leftMargin + lp.rightMargin + widthUsed, lp.width);
   final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(lp.topMargin + lp.bottomMargin, MeasureSpec.UNSPECIFIED);

   child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
 }

 @Override
 public void computeScroll() {
   if (mScroller.computeScrollOffset()) {
     // This is called at drawing time by ViewGroup.  We don't want to
     // re-show the scrollbars at this point, which scrollTo will do,
     // so we replicate most of scrollTo here.
     //
     //         It's a little odd to call onScrollChanged from inside the drawing.
     //
     //         It is, except when you remember that computeScroll() is used to
     //         animate scrolling. So unless we want to defer the onScrollChanged()
     //         until the end of the animated scrolling, we don't really have a
     //         choice here.
     //
     //         I agree.  The alternative, which I think would be worse, is to post
     //         something and tell the subclasses later.  This is bad because there
     //         will be a window where mScrollX/Y is different from what the app
     //         thinks it is.
     //
     int oldX = getScrollX();
     int oldY = getScrollY();
     int x = mScroller.getCurrX();
     int y = mScroller.getCurrY();
     if (getChildCount() > 0) {
       View child = getChildAt(0);
       scrollTo(clamp(x, getWidth() - getPaddingRight() - getPaddingLeft(), child.getWidth()),
       clamp(y, getHeight() - getPaddingBottom() - getPaddingTop(), child.getHeight()));
     } else {
       scrollTo(x, y);
     }
     if (oldX != getScrollX() || oldY != getScrollY()) {
       onScrollChanged(getScrollX(), getScrollY(), oldX, oldY);
     }

     // Keep on drawing until the animation has finished.
     postInvalidate();
   }
 }

 /**
  * Scrolls the view to the given child.
  *
  * @param child the View to scroll to
  */
 private void scrollToChild(View child) {
   child.getDrawingRect(mTempRect);
   /* Offset from child's local coordinates to TwoDScrollView coordinates */
   offsetDescendantRectToMyCoords(child, mTempRect);
   int scrollDelta = computeScrollDeltaToGetChildRectOnScreen(mTempRect);
   if (scrollDelta != 0) {
     scrollBy(0, scrollDelta);
   }
 }

 /**
  * If rect is off screen, scroll just enough to get it (or at least the
  * first screen size chunk of it) on screen.
  *
  * @param rect      The rectangle.
  * @param immediate True to scroll immediately without animation
  * @return true if scrolling was performed
  */
 private boolean scrollToChildRect(Rect rect, boolean immediate) {
   final int delta = computeScrollDeltaToGetChildRectOnScreen(rect);
   final boolean scroll = delta != 0;
   if (scroll) {
     if (immediate) {
       scrollBy(0, delta);
     } else {
       smoothScrollBy(0, delta);
     }
   }
   return scroll;
 }

 /**
  * Compute the amount to scroll in the Y direction in order to get
  * a rectangle completely on the screen (or, if taller than the screen,
  * at least the first screen size chunk of it).
  *
  * @param rect The rect.
  * @return The scroll delta.
  */
 protected int computeScrollDeltaToGetChildRectOnScreen(Rect rect) {
   if (getChildCount() == 0) return 0;
   int height = getHeight();
   int screenTop = getScrollY();
   int screenBottom = screenTop + height;
   int fadingEdge = getVerticalFadingEdgeLength();
   // leave room for top fading edge as long as rect isn't at very top
   if (rect.top > 0) {
     screenTop += fadingEdge;
   }

   // leave room for bottom fading edge as long as rect isn't at very bottom
   if (rect.bottom < getChildAt(0).getHeight()) {
     screenBottom -= fadingEdge;
   }
   int scrollYDelta = 0;
   if (rect.bottom > screenBottom && rect.top > screenTop) {
     // need to move down to get it in view: move down just enough so
     // that the entire rectangle is in view (or at least the first
     // screen size chunk).
     if (rect.height() > height) {
       // just enough to get screen size chunk on
       scrollYDelta += (rect.top - screenTop);
     } else {
       // get entire rect at bottom of screen
       scrollYDelta += (rect.bottom - screenBottom);
     }

     // make sure we aren't scrolling beyond the end of our content
     int bottom = getChildAt(0).getBottom();
     int distanceToBottom = bottom - screenBottom;
     scrollYDelta = Math.min(scrollYDelta, distanceToBottom);

   } else if (rect.top < screenTop && rect.bottom < screenBottom) {
     // need to move up to get it in view: move up just enough so that
     // entire rectangle is in view (or at least the first screen
     // size chunk of it).

     if (rect.height() > height) {
       // screen size chunk
       scrollYDelta -= (screenBottom - rect.bottom);
     } else {
       // entire rect at top
       scrollYDelta -= (screenTop - rect.top);
     }

     // make sure we aren't scrolling any further than the top our content
     scrollYDelta = Math.max(scrollYDelta, -getScrollY());
   }
   return scrollYDelta;
 }

 @Override
 public void requestChildFocus(View child, View focused) {
   if (!mTwoDScrollViewMovedFocus) {
     if (!mIsLayoutDirty) {
       scrollToChild(focused);
     } else {
       // The child may not be laid out yet, we can't compute the scroll yet
       mChildToScrollTo = focused;
     }
   }
   super.requestChildFocus(child, focused);
 }

 /**
  * When looking for focus in children of a scroll view, need to be a little
  * more careful not to give focus to something that is scrolled off screen.
  *
  * This is more expensive than the default {@link android.view.ViewGroup}
  * implementation, otherwise this behavior might have been made the default.
  */
 @Override
 protected boolean onRequestFocusInDescendants(int direction, Rect previouslyFocusedRect) {
   // convert from forward / backward notation to up / down / left / right
   // (ugh).
   if (direction == View.FOCUS_FORWARD) {
     direction = View.FOCUS_DOWN;
   } else if (direction == View.FOCUS_BACKWARD) {
     direction = View.FOCUS_UP;
   }

   final View nextFocus = previouslyFocusedRect == null ?
   FocusFinder.getInstance().findNextFocus(this, null, direction) :
   FocusFinder.getInstance().findNextFocusFromRect(this,
   previouslyFocusedRect, direction);

   if (nextFocus == null) {
     return false;
   }

   return nextFocus.requestFocus(direction, previouslyFocusedRect);
 }

 @Override
 public boolean requestChildRectangleOnScreen(View child, Rect rectangle, boolean immediate) {
   // offset into coordinate space of this scroll view
   rectangle.offset(child.getLeft() - child.getScrollX(), child.getTop() - child.getScrollY());
   return scrollToChildRect(rectangle, immediate);
 }

 @Override
 public void requestLayout() {
   mIsLayoutDirty = true;
   super.requestLayout();
 }

 @Override
 protected void onLayout(boolean changed, int l, int t, int r, int b) {
   super.onLayout(changed, l, t, r, b);
   mIsLayoutDirty = false;
   // Give a child focus if it needs it
   if (mChildToScrollTo != null && isViewDescendantOf(mChildToScrollTo, this)) {
     scrollToChild(mChildToScrollTo);
   }
   mChildToScrollTo = null;

   // Calling this with the present values causes it to re-clam them
   scrollTo(getScrollX(), getScrollY());
 }

 @Override
 protected void onSizeChanged(int w, int h, int oldw, int oldh) {
   super.onSizeChanged(w, h, oldw, oldh);

   View currentFocused = findFocus();
   if (null == currentFocused || this == currentFocused)
     return;

   // If the currently-focused view was visible on the screen when the
   // screen was at the old height, then scroll the screen to make that
   // view visible with the new screen height.
   currentFocused.getDrawingRect(mTempRect);
   offsetDescendantRectToMyCoords(currentFocused, mTempRect);
   int scrollDeltaX = computeScrollDeltaToGetChildRectOnScreen(mTempRect);
   int scrollDeltaY = computeScrollDeltaToGetChildRectOnScreen(mTempRect);
   doScroll(scrollDeltaX, scrollDeltaY);
 }

 /**
  * Return true if child is an descendant of parent, (or equal to the parent).
  */
 private boolean isViewDescendantOf(View child, View parent) {
   if (child == parent) {
     return true;
   }

   final ViewParent theParent = child.getParent();
   return (theParent instanceof ViewGroup) && isViewDescendantOf((View) theParent, parent);
 }

 /**
  * Fling the scroll view
  *
  * @param velocityY The initial velocity in the Y direction. Positive
  *                  numbers mean that the finger/curor is moving down the screen,
  *                  which means we want to scroll towards the top.
  */
 public void fling(int velocityX, int velocityY) {
   if (getChildCount() > 0) {
     int height = getHeight() - getPaddingBottom() - getPaddingTop();
     int bottom = getChildAt(0).getHeight();
     int width = getWidth() - getPaddingRight() - getPaddingLeft();
     int right = getChildAt(0).getWidth();

     mScroller.fling(getScrollX(), getScrollY(), velocityX, velocityY, 0, right - width, 0, bottom - height);

     final boolean movingDown = velocityY > 0;
     final boolean movingRight = velocityX > 0;

     View newFocused = findFocusableViewInMyBounds(movingRight, mScroller.getFinalX(), movingDown, mScroller.getFinalY(), findFocus());
     if (newFocused == null) {
       newFocused = this;
     }

     if (newFocused != findFocus() && newFocused.requestFocus(movingDown ? View.FOCUS_DOWN : View.FOCUS_UP)) {
       mTwoDScrollViewMovedFocus = true;
       mTwoDScrollViewMovedFocus = false;
     }

     awakenScrollBars(mScroller.getDuration());
     invalidate();
   }
 }

 /**
  * {@inheritDoc}
  *
  * <p>This version also clamps the scrolling to the bounds of our child.
  */
 public void scrollTo(int x, int y) {
   // we rely on the fact the View.scrollBy calls scrollTo.
   if (getChildCount() > 0) {
     View child = getChildAt(0);
     x = clamp(x, getWidth() - getPaddingRight() - getPaddingLeft(), child.getWidth());
     y = clamp(y, getHeight() - getPaddingBottom() - getPaddingTop(), child.getHeight());
     if (x != getScrollX() || y != getScrollY()) {
       super.scrollTo(x, y);
     }
   }
 }

 private int clamp(int n, int my, int child) {
   if (my >= child || n < 0) {
     /* my >= child is this case:
      *                    |--------------- me ---------------|
      *     |------ child ------|
      * or
      *     |--------------- me ---------------|
      *            |------ child ------|
      * or
      *     |--------------- me ---------------|
      *                                  |------ child ------|
      *
      * n < 0 is this case:
      *     |------ me ------|
      *                    |-------- child --------|
      *     |-- mScrollX --|
      */
     return 0;
   }
   if ((my+n) > child) {
     /* this case:
      *                    |------ me ------|
      *     |------ child ------|
      *     |-- mScrollX --|
      */
     return child-my;
   }
   return n;
 }
}
<pre>

In hindsight I think I know why two-dimensioning scrolling is not inherently included:  it is a memory hog.  There is an emphasis for beautiful graphics in the framework to achieve the best user experience; examples of this emphasis are true-color (24-bit) pixels, alpha transparency channel support, smooth scrolling, and display caching.  However the Android system restricts an application to 16MB of heap memory.  This limit is met quickly for a large cached two-dimensional scrolled view – note that 2,000 by 2,000 pixels times 4 bytes/pixel is 16MB.

Tags: Android, google, mobile phone, mobility, scrollview, two-dimensional

This entry was posted on Wednesday, June 2nd, 2010 at 1:32 pm and is filed under Mobile Development, Technology. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.