Horizontal "tab"ish scroll between views

I'm interested in creating a horizontal scroll view that "snaps" to the viewed item, so only one item is ever shown at a time. The user can touch-drag left/right and will see previous/next views, switching to it if there's enough velocity. This interaction is exactly like what the new weather/news widget that comes with the Nexus One does for navigating between its "tabs".

Are there any existing view widgets that do this?

Update: found a copy of the news/weather widget (GenieWidget) and they seem to have implemented their own widget to accomplish this which they call com.google.android.apps.genie.geniewidget.ui.FlingableLinearLayout which is part of their own custom com.google.android.apps.genie.geniewidget.ui.TabView. As that source isn't available, that's not looking too hopeful a direction.

(update 20110905: Official android tools now do this better)

I cloned Eric Taix's http://code.google.com/p/andro-views/ on github

https://github.com/olibye/AndroViews

Then applied the patches from above:

• JonO's patch

• Tom de Waard's patch

• Split into a library and an example, allowing simple inclusion in other projects

I would have made this comment above, however I didn't appear able to comment on JonO's answer

Don't look at the News and weather implementation, it has a couple of flaws. You can however use the source code of the Home app (called Launcher or Launcher2), at android.git.kernel.org. The widget we use to do the scrolling on Home is in Workspace.java.

Eric Taix has done most of the grunt work of stripping the Workspace into a WorkspaceView that can be reused. It can be found here: http://code.google.com/p/andro-views/

The version as of posting does what it's supposed to in the emulator, but on real hardware it sometimes gets stuck between views instead of snapping back--I have emailed him a patch for this (which he is testing before committing, as of the date of posting this) that should make it behave exactly as the Workspace does.

If the patch doesn't appear there shortly, I will post it separately.

As promised, since it hasn't yet appeared, here is my patched version:

    /**
     * Copyright 2010 Eric Taix (eric.taix@gmail.com) 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.
     */

    import android.content.Context;
    import android.graphics.Bitmap;
    import android.graphics.Canvas;
    import android.graphics.Paint;
    import android.graphics.RectF;
    import android.os.Parcel;
    import android.os.Parcelable;
    import android.util.AttributeSet;
    import android.util.Log;
    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.Interpolator;
    import android.widget.Scroller;

    /**
     * The workspace is a wide area with a infinite number of screens. Each screen contains a view. A workspace is meant to
     * be used with a fixed width only.<br/>
     * <br/>
     * This code has been done by using com.android.launcher.Workspace.java
     */
    public class WorkspaceView extends ViewGroup {

        private static final int INVALID_POINTER = -1;

        private int mActivePointerId = INVALID_POINTER;
        private static final int INVALID_SCREEN = -1;

        // The velocity at which a fling gesture will cause us to snap to the next screen
        private static final int SNAP_VELOCITY = 500;

        // the default screen index
        private int defaultScreen;
        // The current screen index
        private int currentScreen;
        // The next screen index
        private int nextScreen = INVALID_SCREEN;
        // Wallpaper properties
        private Bitmap wallpaper;
        private Paint paint;
        private int wallpaperWidth;
        private int wallpaperHeight;
        private float wallpaperOffset;
        private boolean wallpaperLoaded;
        private boolean firstWallpaperLayout = true;
        private static final int TAB_INDICATOR_HEIGHT_PCT = 2;
        private RectF selectedTab;


        // The scroller which scroll each view
        private Scroller scroller;
        // A tracker which to calculate the velocity of a mouvement
        private VelocityTracker mVelocityTracker;

        // Tha last known values of X and Y
        private float lastMotionX;
        private float lastMotionY;

        private final static int TOUCH_STATE_REST = 0;
        private final static int TOUCH_STATE_SCROLLING = 1;

        // The current touch state
        private int touchState = TOUCH_STATE_REST;
        // The minimal distance of a touch slop
        private int touchSlop;

        // An internal flag to reset long press when user is scrolling
        private boolean allowLongPress;
        // A flag to know if touch event have to be ignored. Used also in internal
        private boolean locked;

        private WorkspaceOvershootInterpolator mScrollInterpolator;

        private int mMaximumVelocity;

        private Paint selectedTabPaint;
        private Canvas canvas;

        private RectF bar;

        private Paint tabIndicatorBackgroundPaint;

        private static class WorkspaceOvershootInterpolator implements Interpolator {
            private static final float DEFAULT_TENSION = 1.3f;
            private float mTension;

            public WorkspaceOvershootInterpolator() {
                mTension = DEFAULT_TENSION;
            }

            public void setDistance(int distance) {
                mTension = distance > 0 ? DEFAULT_TENSION / distance : DEFAULT_TENSION;
            }

            public void disableSettle() {
                mTension = 0.f;
            }

            public float getInterpolation(float t) {
                // _o(t) = t * t * ((tension + 1) * t + tension)
                // o(t) = _o(t - 1) + 1
                t -= 1.0f;
                return t * t * ((mTension + 1) * t + mTension) + 1.0f;
            }
        }

        /**
         * Used to inflate the Workspace from XML.
         * 
         * @param context The application's context.
         * @param attrs The attribtues set containing the Workspace's customization values.
         */
        public WorkspaceView(Context context, AttributeSet attrs) {
            this(context, attrs, 0);
        }

        /**
         * Used to inflate the Workspace from XML.
         * 
         * @param context The application's context.
         * @param attrs The attribtues set containing the Workspace's customization values.
         * @param defStyle Unused.
         */
        public WorkspaceView(Context context, AttributeSet attrs, int defStyle) {
            super(context, attrs, defStyle);
            defaultScreen = 0;
            initWorkspace();
        }

        /**
         * Initializes various states for this workspace.
         */
        private void initWorkspace() {
            mScrollInterpolator = new WorkspaceOvershootInterpolator();
            scroller = new Scroller(getContext(),mScrollInterpolator);
            currentScreen = defaultScreen;

            paint = new Paint();
            paint.setDither(false);

            // Does this do anything for me?
            final ViewConfiguration configuration = ViewConfiguration.get(getContext());
            touchSlop = configuration.getScaledTouchSlop();
            mMaximumVelocity = configuration.getScaledMaximumFlingVelocity();

            selectedTabPaint = new Paint();
            selectedTabPaint.setColor(getResources().getColor(R.color.RED));
            selectedTabPaint.setStyle(Paint.Style.FILL_AND_STROKE);

            tabIndicatorBackgroundPaint = new Paint();
            tabIndicatorBackgroundPaint.setColor(getResources().getColor(R.color.GRAY));
            tabIndicatorBackgroundPaint.setStyle(Paint.Style.FILL);
        }

        /**
         * Set a new distance that a touch can wander before we think the user is scrolling in pixels slop<br/>
         * 
         * @param touchSlopP
         */
        public void setTouchSlop(int touchSlopP) {
            touchSlop = touchSlopP;
        }

        /**
         * Set the background's wallpaper.
         */
        public void loadWallpaper(Bitmap bitmap) {
            wallpaper = bitmap;
            wallpaperLoaded = true;
            requestLayout();
            invalidate();
        }

        boolean isDefaultScreenShowing() {
            return currentScreen == defaultScreen;
        }

        /**
         * Returns the index of the currently displayed screen.
         * 
         * @return The index of the currently displayed screen.
         */
        int getCurrentScreen() {
            return currentScreen;
        }

        /**
         * Sets the current screen.
         * 
         * @param currentScreen
         */
        public void setCurrentScreen(int currentScreen) {

            if (!scroller.isFinished()) scroller.abortAnimation();
            currentScreen = Math.max(0, Math.min(currentScreen, getChildCount()));
            scrollTo(currentScreen * getWidth(), 0);
            Log.d("workspace", "setCurrentScreen: width is " + getWidth());
            invalidate();
        }

        /**
         * Shows the default screen (defined by the firstScreen attribute in XML.)
         */
        void showDefaultScreen() {
            setCurrentScreen(defaultScreen);
        }

        /**
         * Registers the specified listener on each screen contained in this workspace.
         * 
         * @param l The listener used to respond to long clicks.
         */
        @Override
        public void setOnLongClickListener(OnLongClickListener l) {
            final int count = getChildCount();
            for (int i = 0; i < count; i++) {
                getChildAt(i).setOnLongClickListener(l);
            }
        }


        @Override
        public void computeScroll() {
            if (scroller.computeScrollOffset()) {
                scrollTo(scroller.getCurrX(), scroller.getCurrY());
                postInvalidate();
            } else if (nextScreen != INVALID_SCREEN) {
                currentScreen = Math.max(0, Math.min(nextScreen, getChildCount() - 1));
                nextScreen = INVALID_SCREEN;
            }
        }

        /**
         * ViewGroup.dispatchDraw() supports many features we don't need: clip to padding, layout animation, animation
         * listener, disappearing children, etc. The following implementation attempts to fast-track the drawing dispatch by
         * drawing only what we know needs to be drawn.
         */
        @Override
        protected void dispatchDraw(Canvas canvas) {
            // First draw the wallpaper if needed

            if (wallpaper != null) {
                float x = getScrollX() * wallpaperOffset;
                if (x + wallpaperWidth < getRight() - getLeft()) {
                    x = getRight() - getLeft() - wallpaperWidth;
                }
                canvas.drawBitmap(wallpaper, x, (getBottom() - getTop() - wallpaperHeight) / 2, paint);
            }

            // Determine if we need to draw every child or only the current screen
            boolean fastDraw = touchState != TOUCH_STATE_SCROLLING && nextScreen == INVALID_SCREEN;
            // If we are not scrolling or flinging, draw only the current screen
            if (fastDraw) {
                View v = getChildAt(currentScreen);
                drawChild(canvas, v, getDrawingTime());
            }
            else {
                final long drawingTime = getDrawingTime();
                // If we are flinging, draw only the current screen and the target screen
                if (nextScreen >= 0 && nextScreen < getChildCount() && Math.abs(currentScreen - nextScreen) == 1) {
                    drawChild(canvas, getChildAt(currentScreen), drawingTime);
                    drawChild(canvas, getChildAt(nextScreen), drawingTime);
                }
                else {
                    // If we are scrolling, draw all of our children
                    final int count = getChildCount();
                    for (int i = 0; i < count; i++) {
                        drawChild(canvas, getChildAt(i), drawingTime);
                    }
                }
            }
            updateTabIndicator();
            canvas.drawBitmap(bitmap, getScrollX(), getMeasuredHeight()*(100-TAB_INDICATOR_HEIGHT_PCT)/100, paint);

        }


        /**
         * Measure the workspace AND also children
         */
        @Override
        protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
            super.onMeasure(widthMeasureSpec, heightMeasureSpec);

            final int width = MeasureSpec.getSize(widthMeasureSpec);
            final int height = MeasureSpec.getSize(heightMeasureSpec);
    //      Log.d("workspace","Height is " + height);
            final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
            if (widthMode != MeasureSpec.EXACTLY) {
                throw new IllegalStateException("Workspace can only be used in EXACTLY mode.");
            }

            final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
            if (heightMode != MeasureSpec.EXACTLY) {
                throw new IllegalStateException("Workspace can only be used in EXACTLY mode.");
            }

            // The children are given the same width and height as the workspace
            final int count = getChildCount();
            for (int i = 0; i < count; i++) {
                int adjustedHeightMeasureSpec = MeasureSpec.makeMeasureSpec(height*(100-TAB_INDICATOR_HEIGHT_PCT)/100, heightMode);
                getChildAt(i).measure(widthMeasureSpec,adjustedHeightMeasureSpec);

            }

            // Compute wallpaper
            if (wallpaperLoaded) {
                wallpaperLoaded = false;
                wallpaper = centerToFit(wallpaper, width, height, getContext());
                wallpaperWidth = wallpaper.getWidth();
                wallpaperHeight = wallpaper.getHeight();
            }
            wallpaperOffset = wallpaperWidth > width ? (count * width - wallpaperWidth) / ((count - 1) * (float) width) : 1.0f;
            if (firstWallpaperLayout) {
                scrollTo(currentScreen * width, 0);
                firstWallpaperLayout = false;
            }

    //      Log.d("workspace","Top is "+getTop()+", bottom is "+getBottom()+", left is "+getLeft()+", right is "+getRight());

            updateTabIndicator();
            invalidate();
        }

        Bitmap bitmap;

        private OnLoadListener load;


    private int lastEvHashCode;

        private void updateTabIndicator(){
            int width = getMeasuredWidth();
            int height = getMeasuredHeight();

            //For drawing in its own bitmap:
            bar = new RectF(0, 0, width, (TAB_INDICATOR_HEIGHT_PCT*height/100));

            int startPos = getScrollX()/(getChildCount());
            selectedTab = new RectF(startPos, 0, startPos+width/getChildCount(), (TAB_INDICATOR_HEIGHT_PCT*height/100));

            bitmap = Bitmap.createBitmap(width, (TAB_INDICATOR_HEIGHT_PCT*height/100), Bitmap.Config.ARGB_8888);
            canvas = new Canvas(bitmap);
            canvas.drawRoundRect(bar,0,0, tabIndicatorBackgroundPaint);
            canvas.drawRoundRect(selectedTab, 5,5, selectedTabPaint);
        }

        /**
         * Overrided method to layout child
         */
        @Override
        protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
            int childLeft = 0;
            final int count = getChildCount();
            for (int i = 0; i < count; i++) {
                final View child = getChildAt(i);
                if (child.getVisibility() != View.GONE) {
                    final int childWidth = child.getMeasuredWidth();
                    child.layout(childLeft, 0, childLeft + childWidth, child.getMeasuredHeight());
                    childLeft += childWidth;
                }
            }
            load.onLoad();
        }

        @Override
        public boolean dispatchUnhandledMove(View focused, int direction) {
            if (direction == View.FOCUS_LEFT) {
                if (getCurrentScreen() > 0) {
                    scrollToScreen(getCurrentScreen() - 1);
                    return true;
                }
            }
            else if (direction == View.FOCUS_RIGHT) {
                if (getCurrentScreen() < getChildCount() - 1) {
                    scrollToScreen(getCurrentScreen() + 1);
                    return true;
                }
            }
            return super.dispatchUnhandledMove(focused, direction);
        }

        /**
         * This method JUST determines whether we want to intercept the motion. If we return true, onTouchEvent will be called
         * and we do the actual scrolling there.
         */
        @Override
        public boolean onInterceptTouchEvent(MotionEvent ev) {
            Log.d("workspace","Intercepted a touch event");
            if (locked) {
                return true;
            }

            /*
             * 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) && (touchState != TOUCH_STATE_REST)) {
                return true;
            }

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

    //      switch (action & MotionEvent.ACTION_MASK) {
            switch (action) {
            case MotionEvent.ACTION_MOVE:

    //          Log.d("workspace","Intercepted a move event");
                /*
                 * Locally do absolute value. mLastMotionX is set to the y value of the down event.
                 */
                handleInterceptMove(ev);
                break;

            case MotionEvent.ACTION_DOWN:
                // Remember location of down touch
                final float x1 = ev.getX();
                final float y1 = ev.getY();
                lastMotionX = x1;
                lastMotionY = y1;
                allowLongPress = true;
                mActivePointerId = ev.getPointerId(0);

                /*
                 * If being flinged and user touches the screen, initiate drag; otherwise don't. mScroller.isFinished should be
                 * false when being flinged.
                 */
                touchState = scroller.isFinished() ? TOUCH_STATE_REST : TOUCH_STATE_SCROLLING;
                break;

            case MotionEvent.ACTION_CANCEL:
            case MotionEvent.ACTION_UP:
                mActivePointerId = INVALID_POINTER;
                allowLongPress = false;

                if (mVelocityTracker != null) {
                    mVelocityTracker.recycle();
                    mVelocityTracker = null;
                }
                touchState = TOUCH_STATE_REST;
                break;

            case MotionEvent.ACTION_POINTER_UP:
                onSecondaryPointerUp(ev);
                break;
            }

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

        private void handleInterceptMove(MotionEvent ev) {
            final int pointerIndex = ev.findPointerIndex(mActivePointerId);
            final float x = ev.getX(pointerIndex);
            final float y = ev.getY(pointerIndex);
            final int xDiff = (int) Math.abs(x - lastMotionX);
            final int yDiff = (int) Math.abs(y - lastMotionY);
            boolean xMoved = xDiff > touchSlop;
            boolean yMoved = yDiff > touchSlop;

            if (xMoved || yMoved) {
                //Log.d("workspace","Detected move.  Checking to scroll.");
                if (xMoved && !yMoved) {
                    //Log.d("workspace","Detected X move.  Scrolling.");
                    // Scroll if the user moved far enough along the X axis
                    touchState = TOUCH_STATE_SCROLLING;
                    lastMotionX = x;
                }
                // Either way, cancel any pending longpress
                if (allowLongPress) {
                    allowLongPress = false;
                    // Try canceling the long press. It could also have been scheduled
                    // by a distant descendant, so use the mAllowLongPress flag to block
                    // everything
                    final View currentView = getChildAt(currentScreen);
                    currentView.cancelLongPress();
                }
            }
        }

        private void onSecondaryPointerUp(MotionEvent ev) {
            final int pointerIndex = (ev.getAction() & MotionEvent.ACTION_POINTER_ID_MASK) >>
                    MotionEvent.ACTION_POINTER_ID_SHIFT;
            final int pointerId = ev.getPointerId(pointerIndex);
            if (pointerId == mActivePointerId) {
                // This was our active pointer going up. Choose a new
                // active pointer and adjust accordingly.
                // TODO: Make this decision more intelligent.
                final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
                lastMotionX = ev.getX(newPointerIndex);
                lastMotionY = ev.getY(newPointerIndex);
                mActivePointerId = ev.getPointerId(newPointerIndex);
                if (mVelocityTracker != null) {
                    mVelocityTracker.clear();
                }
            }
        }

        /**
         * Track the touch event
         */
        @Override
        public boolean onTouchEvent(MotionEvent ev) {
    //      Log.d("workspace","caught a touch event");
            if (locked) {
                return true;
            }
            if (mVelocityTracker == null) {
                mVelocityTracker = VelocityTracker.obtain();
            }
            mVelocityTracker.addMovement(ev);

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

            switch (action) {
            case MotionEvent.ACTION_DOWN:

                //We can still get here even if we returned false from the intercept function.
                //That's the only way we can get a TOUCH_STATE_REST (0) here.
                //That means that our child hasn't handled the event, so we need to 
    //          Log.d("workspace","caught a down touch event and touchstate =" + touchState);

                if(touchState != TOUCH_STATE_REST){
                    /*
                     * If being flinged and user touches, stop the fling. isFinished will be false if being flinged.
                     */
                    if (!scroller.isFinished()) {
                        scroller.abortAnimation();
                    }

                    // Remember where the motion event started
                    lastMotionX = x;
                    mActivePointerId = ev.getPointerId(0);
                } 
                break;
            case MotionEvent.ACTION_MOVE:

                if (touchState == TOUCH_STATE_SCROLLING) {
                    handleScrollMove(ev);
                } else {
    //              Log.d("workspace","caught a move touch event but not scrolling");
                    //NOTE:  We will never hit this case in Android 2.2.  This is to fix a 2.1 bug.
                    //We need to do the work of interceptTouchEvent here because we don't intercept the move
                    //on children who don't scroll.

                    Log.d("workspace","handling move from onTouch");

                    if(onInterceptTouchEvent(ev) && touchState == TOUCH_STATE_SCROLLING){
                        handleScrollMove(ev);
                    }

                }

                break;
            case MotionEvent.ACTION_UP:
    //          Log.d("workspace","caught an up touch event");
                if (touchState == TOUCH_STATE_SCROLLING) {
                    final VelocityTracker velocityTracker = mVelocityTracker;
                    velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
                    int velocityX = (int) velocityTracker.getXVelocity();

                    if (velocityX > SNAP_VELOCITY && currentScreen > 0) {
                        // Fling hard enough to move left
                        scrollToScreen(currentScreen - 1);
                    }
                    else if (velocityX < -SNAP_VELOCITY && currentScreen < getChildCount() - 1) {
                        // Fling hard enough to move right
                        scrollToScreen(currentScreen + 1);
                    }
                    else {
                        snapToDestination();
                    }

                    if (mVelocityTracker != null) {
                        mVelocityTracker.recycle();
                        mVelocityTracker = null;
                    }
                }
                touchState = TOUCH_STATE_REST;
                mActivePointerId = INVALID_POINTER;
                break;
            case MotionEvent.ACTION_CANCEL:
                Log.d("workspace","caught a cancel touch event");
                touchState = TOUCH_STATE_REST;
                mActivePointerId = INVALID_POINTER;
                break;
            case MotionEvent.ACTION_POINTER_UP:
                Log.d("workspace","caught a pointer up touch event");
                onSecondaryPointerUp(ev);
                break;
            }

            return true;
        }

        private void handleScrollMove(MotionEvent ev){
            // Scroll to follow the motion event
            final int pointerIndex = ev.findPointerIndex(mActivePointerId);
            final float x1 = ev.getX(pointerIndex);
            final int deltaX = (int) (lastMotionX - x1);
            lastMotionX = x1;

            if (deltaX < 0) {
                if (getScrollX() > 0) {
                    //Scrollby invalidates automatically
                    scrollBy(Math.max(-getScrollX(), deltaX), 0);
                }
            }
            else if (deltaX > 0) {
                final int availableToScroll = getChildAt(getChildCount() - 1).getRight() - getScrollX() - getWidth();
                if (availableToScroll > 0) {
                    //Scrollby invalidates automatically
                    scrollBy(Math.min(availableToScroll, deltaX), 0);
                }
            } else {
                awakenScrollBars();
            }
        }

        /**
         * Scroll to the appropriated screen depending of the current position
         */
        private void snapToDestination() {
            final int screenWidth = getWidth();
            final int whichScreen = (getScrollX() + (screenWidth / 2)) / screenWidth;
            Log.d("workspace", "snapToDestination");
            scrollToScreen(whichScreen);
        }

        /**
         * Scroll to a specific screen
         * 
         * @param whichScreen
         */
        public void scrollToScreen(int whichScreen) {
            scrollToScreen(whichScreen, false);
        }

        private void scrollToScreen(int whichScreen, boolean immediate){
            Log.d("workspace", "snapToScreen=" + whichScreen);

            boolean changingScreens = whichScreen != currentScreen;

            nextScreen = whichScreen;

            View focusedChild = getFocusedChild();
            if (focusedChild != null && changingScreens && focusedChild == getChildAt(currentScreen)) {
                focusedChild.clearFocus();
            }

            final int newX = whichScreen * getWidth();
            final int delta = newX - getScrollX();
            Log.d("workspace", "newX=" + newX + " scrollX=" + getScrollX() + " delta=" + delta);
            scroller.startScroll(getScrollX(), 0, delta, 0, immediate ? 0 : Math.abs(delta) * 2);
            invalidate();
        }

        public void scrollToScreenImmediate(int whichScreen){
            scrollToScreen(whichScreen, true);
        }

        /**
         * Return the parceable instance to be saved
         */
        @Override
        protected Parcelable onSaveInstanceState() {
            final SavedState state = new SavedState(super.onSaveInstanceState());
            state.currentScreen = currentScreen;
            return state;
        }

        /**
         * Restore the previous saved current screen
         */
        @Override
        protected void onRestoreInstanceState(Parcelable state) {
            SavedState savedState = (SavedState) state;
            super.onRestoreInstanceState(savedState.getSuperState());
            if (savedState.currentScreen != -1) {
                currentScreen = savedState.currentScreen;
            }
        }

        /**
         * Scroll to the left right screen
         */
        public void scrollLeft() {
            if (nextScreen == INVALID_SCREEN && currentScreen > 0 && scroller.isFinished()) {
                scrollToScreen(currentScreen - 1);
            }
        }

        /**
         * Scroll to the next right screen
         */
        public void scrollRight() {
            if (nextScreen == INVALID_SCREEN && currentScreen < getChildCount() - 1 && scroller.isFinished()) {
                scrollToScreen(currentScreen + 1);
            }
        }

        /**
         * Return the screen's index where a view has been added to.
         * 
         * @param v
         * @return
         */
        public int getScreenForView(View v) {
            int result = -1;
            if (v != null) {
                ViewParent vp = v.getParent();
                int count = getChildCount();
                for (int i = 0; i < count; i++) {
                    if (vp == getChildAt(i)) {
                        return i;
                    }
                }
            }
            return result;
        }

        /**
         * Return a view instance according to the tag parameter or null if the view could not be found
         * 
         * @param tag
         * @return
         */
        public View getViewForTag(Object tag) {
            int screenCount = getChildCount();
            for (int screen = 0; screen < screenCount; screen++) {
                View child = getChildAt(screen);
                if (child.getTag() == tag) {
                    return child;
                }
            }
            return null;
        }

        /**
         * Unlocks the SlidingDrawer so that touch events are processed.
         * 
         * @see #lock()
         */
        public void unlock() {
            locked = false;
        }

        /**
         * Locks the SlidingDrawer so that touch events are ignores.
         * 
         * @see #unlock()
         */
        public void lock() {
            locked = true;
        }

        /**
         * @return True is long presses are still allowed for the current touch
         */
        public boolean allowLongPress() {
            return allowLongPress;
        }

        /**
         * Move to the default screen
         */
        public void moveToDefaultScreen() {
            scrollToScreen(defaultScreen);
            getChildAt(defaultScreen).requestFocus();
        }

        // ========================= INNER CLASSES ==============================

        /**
         * A SavedState which save and load the current screen
         */
        public static class SavedState extends BaseSavedState {
            int currentScreen = -1;

            /**
             * Internal constructor
             * 
             * @param superState
             */
            SavedState(Parcelable superState) {
                super(superState);
            }

            /**
             * Private constructor
             * 
             * @param in
             */
            private SavedState(Parcel in) {
                super(in);
                currentScreen = in.readInt();
            }

            /**
             * Save the current screen
             */
            @Override
            public void writeToParcel(Parcel out, int flags) {
                super.writeToParcel(out, flags);
                out.writeInt(currentScreen);
            }

            /**
             * Return a Parcelable creator
             */
            public static final Parcelable.Creator<SavedState> CREATOR = new Parcelable.Creator<SavedState>() {
                public SavedState createFromParcel(Parcel in) {
                    return new SavedState(in);
                }

                public SavedState[] newArray(int size) {
                    return new SavedState[size];
                }
            };
        }

        //Added for "flipper" compatibility
        public int getDisplayedChild(){
            return getCurrentScreen();
        }

        public void setDisplayedChild(int i){
            //    setCurrentScreen(i);
            scrollToScreen(i);
            getChildAt(i).requestFocus();
        }

        public void setOnLoadListener(OnLoadListener load){
            this.load = load;
        }

        public void flipLeft(){
            scrollLeft();
        }

        public void flipRight(){
            scrollRight();
        }

        // ======================== UTILITIES METHODS ==========================

        /**
         * Return a centered Bitmap
         * 
         * @param bitmap
         * @param width
         * @param height
         * @param context
         * @return
         */
        static Bitmap centerToFit(Bitmap bitmap, int width, int height, Context context) {
            final int bitmapWidth = bitmap.getWidth();
            final int bitmapHeight = bitmap.getHeight();

            if (bitmapWidth < width || bitmapHeight < height) {
                // Normally should get the window_background color of the context
                int color = Integer.valueOf("FF191919", 16);
                Bitmap centered = Bitmap.createBitmap(bitmapWidth < width ? width : bitmapWidth, bitmapHeight < height ? height
                        : bitmapHeight, Bitmap.Config.RGB_565);
                Canvas canvas = new Canvas(centered);
                canvas.drawColor(color);
                canvas.drawBitmap(bitmap, (width - bitmapWidth) / 2.0f, (height - bitmapHeight) / 2.0f, null);
                bitmap = centered;
            }
            return bitmap;
        }

    }

I am not aware of Nexus one but, i can suggest u the gallery view. It is perfectly apt one for your requirement according to your above explanation.

is this what are you looking for ? http://code.google.com/p/mobyfactory-uiwidgets-android/

I tried many of the code examples we can find here and there online, and even if almost all were good, they were not handling properly what I had in mind.

In fact I have several ScrollViews side by side, the screen displaying only one. When the user scrolls vertically, I want the ScrollView to scroll; when the user scrolls horizontally, I want the ViewFlow (the global layout I'm using) to snap to the previous/next ScrollView.

I got something almost close to what I wanted, but that'll do it for now. I know that this was answered long time ago, but I want to share what I made, so here it is.

It's just a class that derives ScrollView and implements onInterceptTouchEvent and onTouchEvent.

/**
 * Try to know if the move will be an horizontal drag
 * If so, we want to intercept the touch event (return true)
 * otherwise, we don't want to intercept this event
 */
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
    int action = ev.getAction() & MotionEvent.ACTION_MASK;

    // If scrolling (over X or Y), we want to intercept: process
    // it within the ScrollView & send it to the ViewFlow
    if (action == MotionEvent.ACTION_MOVE && mState != NO_SCROLL) {
        return true;
    }

    // Try to detect the motion
    switch (action) {
    case MotionEvent.ACTION_MOVE:
        float deltaX = Math.abs(ev.getX()-lastX);
        float deltaY = Math.abs(ev.getY()-lastY);

        boolean xMoved = deltaX > 0.5* mTouchSlop && deltaX > deltaY;
        boolean yMoved = deltaY > 0.5*mTouchSlop;

        if (xMoved || yMoved) {
            if (xMoved && !yMoved) {
                mState = SCROLL_X;
            } else {
                mState = SCROLL_Y;
            }
        } else {
            mState = NO_SCROLL;
        }

        lastX = ev.getX();
        lastY = ev.getY();
        break;

    case MotionEvent.ACTION_DOWN:
        // Remember location of down touch
        lastX = ev.getX();
        lastY = ev.getY();
        mState = NO_SCROLL;
        sendTouchEvent(ev);
        break;

    case MotionEvent.ACTION_CANCEL:
    case MotionEvent.ACTION_UP:
        if (mState == NO_SCROLL) {
            // Thats a tap!
        }
        mState = NO_SCROLL;
        break;

    }

    if (mState != SCROLL_X) {
        super.onInterceptTouchEvent(ev);
    }

    return mState == SCROLL_X;// Intercept only dragging over X axis
}

/**
 * Handles touch events. Basically only horizontal drag.
 * Such events are handled locally by the scrollview
 * _AND_ sent to the {@link ViewFlow} in order to make it snap
 * horizontally
 * @param ev the MotionEvent 
 */
@Override
public boolean onTouchEvent(MotionEvent ev) {
    int action = ev.getAction();
    super.onTouchEvent(ev);

    switch (action) {
    case MotionEvent.ACTION_CANCEL:
    case MotionEvent.ACTION_UP:
        mState = NO_SCROLL;
        sendTouchEvent(ev);
        super.onTouchEvent(ev);
        break;

    case MotionEvent.ACTION_DOWN:
        super.onTouchEvent(ev);
        break;

    case MotionEvent.ACTION_MOVE:
        lastY = ev.getX();
        lastY = ev.getY();
        if (mState == SCROLL_X) {
            sendTouchEvent(ev);
        } else if (mState == SCROLL_Y) {
            super.onTouchEvent(ev);
        }
        break;
    }

    return false;
}

The code is not perfect and requires a little bit more polish, but it's functional. Almost like Genie Widget. (I wish this guy was open source!) Especially around the scrolling detection, it's not using exactly the touch slop, and it's comparing the move between the X and Y axis. But this is easily tuneable.

And I am using this very good code for my ViewFlow: https://github.com/pakerfeldt/android-viewflow