gecko-dev/mobile/android/base/gfx/GeckoLayerClient.java

/* -*- Mode: Java; c-basic-offset: 4; tab-width: 20; indent-tabs-mode: nil; -*-
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

package org.mozilla.gecko.gfx;

import org.mozilla.gecko.BrowserApp;
import org.mozilla.gecko.GeckoAppShell;
import org.mozilla.gecko.GeckoEvent;
import org.mozilla.gecko.Tab;
import org.mozilla.gecko.Tabs;
import org.mozilla.gecko.ZoomConstraints;
import org.mozilla.gecko.util.EventDispatcher;
import org.mozilla.gecko.util.FloatUtils;
import org.mozilla.gecko.util.ThreadUtils;

import android.content.Context;
import android.graphics.PointF;
import android.graphics.RectF;
import android.os.SystemClock;
import android.util.DisplayMetrics;
import android.util.Log;

public class GeckoLayerClient implements LayerView.Listener, PanZoomTarget
{
    private static final String LOGTAG = "GeckoLayerClient";

    private LayerRenderer mLayerRenderer;
    private boolean mLayerRendererInitialized;

    private Context mContext;
    private IntSize mScreenSize;
    private IntSize mWindowSize;
    private DisplayPortMetrics mDisplayPort;

    private boolean mRecordDrawTimes;
    private final DrawTimingQueue mDrawTimingQueue;

    private VirtualLayer mRootLayer;

    /* The Gecko viewport as per the UI thread. Must be touched only on the UI thread.
     * If any events being sent to Gecko that are relative to the Gecko viewport position,
     * they must (a) be relative to this viewport, and (b) be sent on the UI thread to
     * avoid races. As long as these two conditions are satisfied, and the events being
     * sent to Gecko are processed in FIFO order, the events will properly be relative
     * to the Gecko viewport position. Note that if Gecko updates its viewport independently,
     * we get notified synchronously and also update this on the UI thread.
     */
    private ImmutableViewportMetrics mGeckoViewport;

    /*
     * The viewport metrics being used to draw the current frame. This is only
     * accessed by the compositor thread, and so needs no synchronisation.
     */
    private ImmutableViewportMetrics mFrameMetrics;

    /* Used by robocop for testing purposes */
    private DrawListener mDrawListener;

    /* Used as a temporary ViewTransform by syncViewportInfo */
    private final ViewTransform mCurrentViewTransform;

    /* Used as the return value of progressiveUpdateCallback */
    private final ProgressiveUpdateData mProgressiveUpdateData;
    private DisplayPortMetrics mProgressiveUpdateDisplayPort;
    private boolean mLastProgressiveUpdateWasLowPrecision;
    private boolean mProgressiveUpdateWasInDanger;

    private boolean mForceRedraw;

    /* The current viewport metrics.
     * This is volatile so that we can read and write to it from different threads.
     * We avoid synchronization to make getting the viewport metrics from
     * the compositor as cheap as possible. The viewport is immutable so
     * we don't need to worry about anyone mutating it while we're reading from it.
     * Specifically:
     * 1) reading mViewportMetrics from any thread is fine without synchronization
     * 2) writing to mViewportMetrics requires synchronizing on the layer controller object
     * 3) whenver reading multiple fields from mViewportMetrics without synchronization (i.e. in
     *    case 1 above) you should always frist grab a local copy of the reference, and then use
     *    that because mViewportMetrics might get reassigned in between reading the different
     *    fields. */
    private volatile ImmutableViewportMetrics mViewportMetrics;

    private ZoomConstraints mZoomConstraints;

    private boolean mGeckoIsReady;

    private final PanZoomController mPanZoomController;
    private LayerView mView;

    private boolean mClampOnMarginChange;

    public GeckoLayerClient(Context context, LayerView view, EventDispatcher eventDispatcher) {
        // we can fill these in with dummy values because they are always written
        // to before being read
        mContext = context;
        mScreenSize = new IntSize(0, 0);
        mWindowSize = new IntSize(0, 0);
        mDisplayPort = new DisplayPortMetrics();
        mRecordDrawTimes = true;
        mDrawTimingQueue = new DrawTimingQueue();
        mCurrentViewTransform = new ViewTransform(0, 0, 1);
        mProgressiveUpdateData = new ProgressiveUpdateData();
        mProgressiveUpdateDisplayPort = new DisplayPortMetrics();
        mLastProgressiveUpdateWasLowPrecision = false;
        mProgressiveUpdateWasInDanger = false;
        mClampOnMarginChange = true;

        mForceRedraw = true;
        DisplayMetrics displayMetrics = context.getResources().getDisplayMetrics();
        mViewportMetrics = new ImmutableViewportMetrics(displayMetrics)
                           .setViewportSize(view.getWidth(), view.getHeight());
        mZoomConstraints = new ZoomConstraints(false);

        mPanZoomController = PanZoomController.Factory.create(this, view, eventDispatcher);
        mView = view;
        mView.setListener(this);
    }

    /** Attaches to root layer so that Gecko appears. */
    public void notifyGeckoReady() {
        mGeckoIsReady = true;

        mRootLayer = new VirtualLayer(new IntSize(mView.getWidth(), mView.getHeight()));
        mLayerRenderer = mView.getRenderer();

        sendResizeEventIfNecessary(true);

        DisplayPortCalculator.initPrefs();

        // Gecko being ready is one of the two conditions (along with having an available
        // surface) that cause us to create the compositor. So here, now that we know gecko
        // is ready, call createCompositor() to see if we can actually do the creation.
        // This needs to run on the UI thread so that the surface validity can't change on
        // us while we're in the middle of creating the compositor.
        mView.post(new Runnable() {
            @Override
            public void run() {
                mView.getGLController().createCompositor();
            }
        });
    }

    public void destroy() {
        mPanZoomController.destroy();
    }

    /**
     * Returns true if this client is fine with performing a redraw operation or false if it
     * would prefer that the action didn't take place.
     */
    private boolean getRedrawHint() {
        if (mForceRedraw) {
            mForceRedraw = false;
            return true;
        }

        if (!mPanZoomController.getRedrawHint()) {
            return false;
        }

        return DisplayPortCalculator.aboutToCheckerboard(mViewportMetrics,
                mPanZoomController.getVelocityVector(), mDisplayPort);
    }

    Layer getRoot() {
        return mGeckoIsReady ? mRootLayer : null;
    }

    public LayerView getView() {
        return mView;
    }

    public FloatSize getViewportSize() {
        return mViewportMetrics.getSize();
    }

    /**
     * The view calls this function to indicate that the viewport changed size. It must hold the
     * monitor while calling it.
     *
     * TODO: Refactor this to use an interface. Expose that interface only to the view and not
     * to the layer client. That way, the layer client won't be tempted to call this, which might
     * result in an infinite loop.
     */
    void setViewportSize(int width, int height) {
        mViewportMetrics = mViewportMetrics.setViewportSize(width, height);

        if (mGeckoIsReady) {
            // here we send gecko a resize message. The code in browser.js is responsible for
            // picking up on that resize event, modifying the viewport as necessary, and informing
            // us of the new viewport.
            sendResizeEventIfNecessary(true);
            // the following call also sends gecko a message, which will be processed after the resize
            // message above has updated the viewport. this message ensures that if we have just put
            // focus in a text field, we scroll the content so that the text field is in view.
            GeckoAppShell.viewSizeChanged();
        }
    }

    PanZoomController getPanZoomController() {
        return mPanZoomController;
    }

    /* Informs Gecko that the screen size has changed. */
    private void sendResizeEventIfNecessary(boolean force) {
        DisplayMetrics metrics = mContext.getResources().getDisplayMetrics();

        IntSize newScreenSize = new IntSize(metrics.widthPixels, metrics.heightPixels);
        IntSize newWindowSize = new IntSize(mView.getWidth(), mView.getHeight());

        boolean screenSizeChanged = !mScreenSize.equals(newScreenSize);
        boolean windowSizeChanged = !mWindowSize.equals(newWindowSize);

        if (!force && !screenSizeChanged && !windowSizeChanged) {
            return;
        }

        mScreenSize = newScreenSize;
        mWindowSize = newWindowSize;

        if (screenSizeChanged) {
            Log.d(LOGTAG, "Screen-size changed to " + mScreenSize);
        }

        if (windowSizeChanged) {
            Log.d(LOGTAG, "Window-size changed to " + mWindowSize);
        }

        GeckoEvent event = GeckoEvent.createSizeChangedEvent(mWindowSize.width, mWindowSize.height,
                                                             mScreenSize.width, mScreenSize.height);
        GeckoAppShell.sendEventToGecko(event);
        GeckoAppShell.sendEventToGecko(GeckoEvent.createBroadcastEvent("Window:Resize", ""));
    }

    /** Sets the current page rect. You must hold the monitor while calling this. */
    private void setPageRect(RectF rect, RectF cssRect) {
        // Since the "rect" is always just a multiple of "cssRect" we don't need to
        // check both; this function assumes that both "rect" and "cssRect" are relative
        // the zoom factor in mViewportMetrics.
        if (mViewportMetrics.getCssPageRect().equals(cssRect))
            return;

        mViewportMetrics = mViewportMetrics.setPageRect(rect, cssRect);

        // Page size is owned by the layer client, so no need to notify it of
        // this change.

        post(new Runnable() {
            @Override
            public void run() {
                mPanZoomController.pageRectUpdated();
                mView.requestRender();
            }
        });
    }

    private void adjustViewport(DisplayPortMetrics displayPort) {
        ImmutableViewportMetrics metrics = getViewportMetrics();
        ImmutableViewportMetrics clampedMetrics = metrics.clamp();

        // See if we need to alter the fixed margins - Fixed margins would
        // otherwise be set even when the document is in overscroll and they're
        // unnecessary.
        if ((metrics.fixedLayerMarginLeft > 0 && metrics.viewportRectLeft < metrics.pageRectLeft) ||
            (metrics.fixedLayerMarginTop > 0 && metrics.viewportRectTop < metrics.pageRectTop) ||
            (metrics.fixedLayerMarginRight > 0 && metrics.viewportRectRight > metrics.pageRectRight) ||
            (metrics.fixedLayerMarginBottom > 0 && metrics.viewportRectBottom > metrics.pageRectBottom)) {
            clampedMetrics = clampedMetrics.setFixedLayerMargins(
              Math.max(0, metrics.fixedLayerMarginLeft + Math.min(0, metrics.viewportRectLeft - metrics.pageRectLeft)),
              Math.max(0, metrics.fixedLayerMarginTop + Math.min(0, metrics.viewportRectTop - metrics.pageRectTop)),
              Math.max(0, metrics.fixedLayerMarginRight + Math.min(0, (metrics.pageRectRight - metrics.viewportRectRight))),
              Math.max(0, metrics.fixedLayerMarginBottom + Math.min(0, (metrics.pageRectBottom - metrics.viewportRectBottom))));
        }

        if (displayPort == null) {
            displayPort = DisplayPortCalculator.calculate(metrics, mPanZoomController.getVelocityVector());
        }

        mDisplayPort = displayPort;
        mGeckoViewport = clampedMetrics;

        if (mRecordDrawTimes) {
            mDrawTimingQueue.add(displayPort);
        }

        GeckoAppShell.sendEventToGecko(GeckoEvent.createViewportEvent(clampedMetrics, displayPort));
    }

    /** Aborts any pan/zoom animation that is currently in progress. */
    private void abortPanZoomAnimation() {
        if (mPanZoomController != null) {
            post(new Runnable() {
                @Override
                public void run() {
                    mPanZoomController.abortAnimation();
                }
            });
        }
    }

    /**
     * The different types of Viewport messages handled. All viewport events
     * expect a display-port to be returned, but can handle one not being
     * returned.
     */
    private enum ViewportMessageType {
        UPDATE,       // The viewport has changed and should be entirely updated
        PAGE_SIZE     // The viewport's page-size has changed
    }

    /** Viewport message handler. */
    private DisplayPortMetrics handleViewportMessage(ImmutableViewportMetrics messageMetrics, ViewportMessageType type) {
        synchronized (this) {
            ImmutableViewportMetrics metrics;
            ImmutableViewportMetrics oldMetrics = getViewportMetrics();

            switch (type) {
            default:
            case UPDATE:
                // Keep the old viewport size
                metrics = messageMetrics.setViewportSize(oldMetrics.getWidth(), oldMetrics.getHeight())
                                        .setFixedLayerMarginsFrom(oldMetrics);
                if (!oldMetrics.fuzzyEquals(metrics)) {
                    abortPanZoomAnimation();
                }
                break;
            case PAGE_SIZE:
                // adjust the page dimensions to account for differences in zoom
                // between the rendered content (which is what Gecko tells us)
                // and our zoom level (which may have diverged).
                float scaleFactor = oldMetrics.zoomFactor / messageMetrics.zoomFactor;
                metrics = oldMetrics.setPageRect(RectUtils.scale(messageMetrics.getPageRect(), scaleFactor), messageMetrics.getCssPageRect());
                break;
            }

            final ImmutableViewportMetrics newMetrics = metrics;
            post(new Runnable() {
                @Override
                public void run() {
                    mGeckoViewport = newMetrics;
                }
            });
            setViewportMetrics(newMetrics, type == ViewportMessageType.UPDATE);
            mDisplayPort = DisplayPortCalculator.calculate(getViewportMetrics(), null);
        }
        return mDisplayPort;
    }

    public DisplayPortMetrics getDisplayPort(boolean pageSizeUpdate, boolean isBrowserContentDisplayed, int tabId, ImmutableViewportMetrics metrics) {
        Tabs tabs = Tabs.getInstance();
        if (tabs.isSelectedTab(tabs.getTab(tabId)) && isBrowserContentDisplayed) {
            // for foreground tabs, send the viewport update unless the document
            // displayed is different from the content document. In that case, just
            // calculate the display port.
            return handleViewportMessage(metrics, pageSizeUpdate ? ViewportMessageType.PAGE_SIZE : ViewportMessageType.UPDATE);
        } else {
            // for background tabs, request a new display port calculation, so that
            // when we do switch to that tab, we have the correct display port and
            // don't need to draw twice (once to allow the first-paint viewport to
            // get to java, and again once java figures out the display port).
            return DisplayPortCalculator.calculate(metrics, null);
        }
    }

    /**
     * Sets margins on fixed-position layers, to be used when compositing.
     * Must be called on the UI thread!
     */
    public void setFixedLayerMargins(float left, float top, float right, float bottom) {
        ImmutableViewportMetrics oldMetrics = getViewportMetrics();
        if (oldMetrics.fixedLayerMarginLeft == left &&
            oldMetrics.fixedLayerMarginTop == top &&
            oldMetrics.fixedLayerMarginRight == right &&
            oldMetrics.fixedLayerMarginBottom == bottom) {
            // Do nothing if the margins haven't changed.
            return;
        }

        ImmutableViewportMetrics newMetrics = oldMetrics.setFixedLayerMargins(left, top, right, bottom);

        if (mClampOnMarginChange) {
            newMetrics = newMetrics.clampWithMargins();
        }

        mForceRedraw = true;
        setViewportMetrics(newMetrics, true);
    }

    public void setClampOnFixedLayerMarginsChange(boolean aClamp) {
        mClampOnMarginChange = aClamp;
    }

    // This is called on the Gecko thread to determine if we're still interested
    // in the update of this display-port to continue. We can return true here
    // to abort the current update and continue with any subsequent ones. This
    // is useful for slow-to-render pages when the display-port starts lagging
    // behind enough that continuing to draw it is wasted effort.
    public ProgressiveUpdateData progressiveUpdateCallback(boolean aHasPendingNewThebesContent,
                                                           float x, float y, float width, float height,
                                                           float resolution, boolean lowPrecision) {
        // Skip all low precision draws until we're at risk of checkerboarding
        if (lowPrecision && !mProgressiveUpdateWasInDanger) {
            mProgressiveUpdateData.abort = true;
            return mProgressiveUpdateData;
        }

        // Reset the checkerboard risk flag
        if (!lowPrecision && mLastProgressiveUpdateWasLowPrecision) {
            mProgressiveUpdateWasInDanger = false;
        }
        mLastProgressiveUpdateWasLowPrecision = lowPrecision;

        // Grab a local copy of the last display-port sent to Gecko and the
        // current viewport metrics to avoid races when accessing them.
        DisplayPortMetrics displayPort = mDisplayPort;
        ImmutableViewportMetrics viewportMetrics = mViewportMetrics;
        mProgressiveUpdateData.setViewport(viewportMetrics);
        mProgressiveUpdateData.abort = false;

        // Always abort updates if the resolution has changed. There's no use
        // in drawing at the incorrect resolution.
        if (!FloatUtils.fuzzyEquals(resolution, viewportMetrics.zoomFactor)) {
            Log.d(LOGTAG, "Aborting draw due to resolution change");
            mProgressiveUpdateData.abort = true;
            return mProgressiveUpdateData;
        }

        // Store the high precision displayport for comparison when doing low
        // precision updates.
        if (!lowPrecision) {
            if (!FloatUtils.fuzzyEquals(resolution, mProgressiveUpdateDisplayPort.resolution) ||
                !FloatUtils.fuzzyEquals(x, mProgressiveUpdateDisplayPort.getLeft()) ||
                !FloatUtils.fuzzyEquals(y, mProgressiveUpdateDisplayPort.getTop()) ||
                !FloatUtils.fuzzyEquals(x + width, mProgressiveUpdateDisplayPort.getRight()) ||
                !FloatUtils.fuzzyEquals(y + height, mProgressiveUpdateDisplayPort.getBottom())) {
                mProgressiveUpdateDisplayPort =
                    new DisplayPortMetrics(x, y, x+width, y+height, resolution);
            }
        }

        // XXX All sorts of rounding happens inside Gecko that becomes hard to
        //     account exactly for. Given we align the display-port to tile
        //     boundaries (and so they rarely vary by sub-pixel amounts), just
        //     check that values are within a couple of pixels of the
        //     display-port bounds.

        // Never abort drawing if we can't be sure we've sent a more recent
        // display-port. If we abort updating when we shouldn't, we can end up
        // with blank regions on the screen and we open up the risk of entering
        // an endless updating cycle.
        if (Math.abs(displayPort.getLeft() - mProgressiveUpdateDisplayPort.getLeft()) <= 2 &&
            Math.abs(displayPort.getTop() - mProgressiveUpdateDisplayPort.getTop()) <= 2 &&
            Math.abs(displayPort.getBottom() - mProgressiveUpdateDisplayPort.getBottom()) <= 2 &&
            Math.abs(displayPort.getRight() - mProgressiveUpdateDisplayPort.getRight()) <= 2) {
            return mProgressiveUpdateData;
        }

        if (!lowPrecision && !mProgressiveUpdateWasInDanger) {
            // If we're not doing low precision draws and we're about to
            // checkerboard, give up and move onto low precision drawing.
            if (DisplayPortCalculator.aboutToCheckerboard(viewportMetrics,
                  mPanZoomController.getVelocityVector(), mProgressiveUpdateDisplayPort)) {
                mProgressiveUpdateWasInDanger = true;
            }
        }

        // Abort updates when the display-port no longer contains the visible
        // area of the page (that is, the viewport cropped by the page
        // boundaries).
        // XXX This makes the assumption that we never let the visible area of
        //     the page fall outside of the display-port.
        if (Math.max(viewportMetrics.viewportRectLeft, viewportMetrics.pageRectLeft) + 1 < x ||
            Math.max(viewportMetrics.viewportRectTop, viewportMetrics.pageRectTop) + 1 < y ||
            Math.min(viewportMetrics.viewportRectRight, viewportMetrics.pageRectRight) - 1 > x + width ||
            Math.min(viewportMetrics.viewportRectBottom, viewportMetrics.pageRectBottom) - 1 > y + height) {
            Log.d(LOGTAG, "Aborting update due to viewport not in display-port");
            mProgressiveUpdateData.abort = true;
            return mProgressiveUpdateData;
        }

        // Abort drawing stale low-precision content if there's a more recent
        // display-port in the pipeline.
        if (lowPrecision && !aHasPendingNewThebesContent) {
          mProgressiveUpdateData.abort = true;
        }
        return mProgressiveUpdateData;
    }

    void setZoomConstraints(ZoomConstraints constraints) {
        mZoomConstraints = constraints;
    }

    /** This function is invoked by Gecko via JNI; be careful when modifying signature.
      * The compositor invokes this function just before compositing a frame where the document
      * is different from the document composited on the last frame. In these cases, the viewport
      * information we have in Java is no longer valid and needs to be replaced with the new
      * viewport information provided. setPageRect will never be invoked on the same frame that
      * this function is invoked on; and this function will always be called prior to syncViewportInfo.
      */
    public void setFirstPaintViewport(float offsetX, float offsetY, float zoom,
            float pageLeft, float pageTop, float pageRight, float pageBottom,
            float cssPageLeft, float cssPageTop, float cssPageRight, float cssPageBottom) {
        synchronized (this) {
            ImmutableViewportMetrics currentMetrics = getViewportMetrics();

            // If we're meant to be scrolled to the top, take into account any
            // margin set on the pan zoom controller.
            if (offsetY == 0) {
                offsetY = -currentMetrics.fixedLayerMarginTop;
            }

            final ImmutableViewportMetrics newMetrics = currentMetrics
                .setViewportOrigin(offsetX, offsetY)
                .setZoomFactor(zoom)
                .setPageRect(new RectF(pageLeft, pageTop, pageRight, pageBottom),
                             new RectF(cssPageLeft, cssPageTop, cssPageRight, cssPageBottom));
            // Since we have switched to displaying a different document, we need to update any
            // viewport-related state we have lying around. This includes mGeckoViewport and
            // mViewportMetrics. Usually this information is updated via handleViewportMessage
            // while we remain on the same document.
            post(new Runnable() {
                @Override
                public void run() {
                    mGeckoViewport = newMetrics;
                }
            });
            setViewportMetrics(newMetrics);

            Tab tab = Tabs.getInstance().getSelectedTab();
            mView.setBackgroundColor(tab.getBackgroundColor());
            setZoomConstraints(tab.getZoomConstraints());

            // At this point, we have just switched to displaying a different document than we
            // we previously displaying. This means we need to abort any panning/zooming animations
            // that are in progress and send an updated display port request to browser.js as soon
            // as possible. The call to PanZoomController.abortAnimation accomplishes this by calling the
            // forceRedraw function, which sends the viewport to gecko. The display port request is
            // actually a full viewport update, which is fine because if browser.js has somehow moved to
            // be out of sync with this first-paint viewport, then we force them back in sync.
            abortPanZoomAnimation();

            // Indicate that the document is about to be composited so the
            // LayerView background can be removed.
            if (mView.getPaintState() == LayerView.PAINT_START) {
                mView.setPaintState(LayerView.PAINT_BEFORE_FIRST);
            }
        }
        DisplayPortCalculator.resetPageState();
        mDrawTimingQueue.reset();
    }

    /** This function is invoked by Gecko via JNI; be careful when modifying signature.
      * The compositor invokes this function whenever it determines that the page rect
      * has changed (based on the information it gets from layout). If setFirstPaintViewport
      * is invoked on a frame, then this function will not be. For any given frame, this
      * function will be invoked before syncViewportInfo.
      */
    public void setPageRect(float cssPageLeft, float cssPageTop, float cssPageRight, float cssPageBottom) {
        synchronized (this) {
            RectF cssPageRect = new RectF(cssPageLeft, cssPageTop, cssPageRight, cssPageBottom);
            float ourZoom = getViewportMetrics().zoomFactor;
            setPageRect(RectUtils.scale(cssPageRect, ourZoom), cssPageRect);
            // Here the page size of the document has changed, but the document being displayed
            // is still the same. Therefore, we don't need to send anything to browser.js; any
            // changes we need to make to the display port will get sent the next time we call
            // adjustViewport().
        }
    }

    /** This function is invoked by Gecko via JNI; be careful when modifying signature.
      * The compositor invokes this function on every frame to figure out what part of the
      * page to display, and to inform Java of the current display port. Since it is called
      * on every frame, it needs to be ultra-fast.
      * It avoids taking any locks or allocating any objects. We keep around a
      * mCurrentViewTransform so we don't need to allocate a new ViewTransform
      * everytime we're called. NOTE: we might be able to return a ImmutableViewportMetrics
      * which would avoid the copy into mCurrentViewTransform.
      */
    public ViewTransform syncViewportInfo(int x, int y, int width, int height, float resolution, boolean layersUpdated) {
        // getViewportMetrics is thread safe so we don't need to synchronize.
        // We save the viewport metrics here, so we later use it later in
        // createFrame (which will be called by nsWindow::DrawWindowUnderlay on
        // the native side, by the compositor). The viewport
        // metrics can change between here and there, as it's accessed outside
        // of the compositor thread.
        mFrameMetrics = getViewportMetrics();

        mCurrentViewTransform.x = mFrameMetrics.viewportRectLeft;
        mCurrentViewTransform.y = mFrameMetrics.viewportRectTop;
        mCurrentViewTransform.scale = mFrameMetrics.zoomFactor;

        // Adjust the fixed layer margins so that overscroll subtracts from them.
        mCurrentViewTransform.fixedLayerMarginLeft =
            Math.max(0, mFrameMetrics.fixedLayerMarginLeft +
                     Math.min(0, mFrameMetrics.viewportRectLeft - mFrameMetrics.pageRectLeft));
        mCurrentViewTransform.fixedLayerMarginTop =
            Math.max(0, mFrameMetrics.fixedLayerMarginTop +
                     Math.min(0, mFrameMetrics.viewportRectTop - mFrameMetrics.pageRectTop));
        mCurrentViewTransform.fixedLayerMarginRight =
            Math.max(0, mFrameMetrics.fixedLayerMarginRight +
                     Math.min(0, (mFrameMetrics.pageRectRight - mFrameMetrics.viewportRectRight)));
        mCurrentViewTransform.fixedLayerMarginBottom =
            Math.max(0, mFrameMetrics.fixedLayerMarginBottom +
                     Math.min(0, (mFrameMetrics.pageRectBottom - mFrameMetrics.viewportRectBottom)));

        mRootLayer.setPositionAndResolution(x, y, x + width, y + height, resolution);

        if (layersUpdated && mRecordDrawTimes) {
            // If we got a layers update, that means a draw finished. Check to see if the area drawn matches
            // one of our requested displayports; if it does calculate the draw time and notify the
            // DisplayPortCalculator
            DisplayPortMetrics drawn = new DisplayPortMetrics(x, y, x + width, y + height, resolution);
            long time = mDrawTimingQueue.findTimeFor(drawn);
            if (time >= 0) {
                long now = SystemClock.uptimeMillis();
                time = now - time;
                mRecordDrawTimes = DisplayPortCalculator.drawTimeUpdate(time, width * height);
            }
        }

        if (layersUpdated && mDrawListener != null) {
            /* Used by robocop for testing purposes */
            mDrawListener.drawFinished();
        }

        return mCurrentViewTransform;
    }

    /** This function is invoked by Gecko via JNI; be careful when modifying signature. */
    public LayerRenderer.Frame createFrame() {
        // Create the shaders and textures if necessary.
        if (!mLayerRendererInitialized) {
            mLayerRenderer.checkMonitoringEnabled();
            mLayerRenderer.createDefaultProgram();
            mLayerRendererInitialized = true;
        }

        return mLayerRenderer.createFrame(mFrameMetrics);
    }

    /** This function is invoked by Gecko via JNI; be careful when modifying signature. */
    public void activateProgram() {
        mLayerRenderer.activateDefaultProgram();
    }

    /** This function is invoked by Gecko via JNI; be careful when modifying signature. */
    public void deactivateProgram() {
        mLayerRenderer.deactivateDefaultProgram();
    }

    private void geometryChanged() {
        /* Let Gecko know if the screensize has changed */
        sendResizeEventIfNecessary(false);
        if (getRedrawHint()) {
            adjustViewport(null);
        }
    }

    /** Implementation of LayerView.Listener */
    @Override
    public void renderRequested() {
        try {
            GeckoAppShell.scheduleComposite();
        } catch (UnsupportedOperationException uoe) {
            // In some very rare cases this gets called before libxul is loaded,
            // so catch and ignore the exception that will throw. See bug 837821
            Log.d(LOGTAG, "Dropping renderRequested call before libxul load.");
        }
    }

    /** Implementation of LayerView.Listener */
    @Override
    public void sizeChanged(int width, int height) {
        // We need to make sure a draw happens synchronously at this point,
        // but resizing the surface before the SurfaceView has resized will
        // cause a visible jump.
        mView.getGLController().resumeCompositor(mWindowSize.width, mWindowSize.height);
    }

    /** Implementation of LayerView.Listener */
    @Override
    public void surfaceChanged(int width, int height) {
        setViewportSize(width, height);
    }

    /** Implementation of PanZoomTarget */
    @Override
    public ImmutableViewportMetrics getViewportMetrics() {
        return mViewportMetrics;
    }

    /** Implementation of PanZoomTarget */
    @Override
    public ZoomConstraints getZoomConstraints() {
        return mZoomConstraints;
    }

    /** Implementation of PanZoomTarget */
    @Override
    public boolean isFullScreen() {
        return mView.isFullScreen();
    }

    /** Implementation of PanZoomTarget */
    @Override
    public void setAnimationTarget(ImmutableViewportMetrics metrics) {
        if (mGeckoIsReady) {
            // We know what the final viewport of the animation is going to be, so
            // immediately request a draw of that area by setting the display port
            // accordingly. This way we should have the content pre-rendered by the
            // time the animation is done.
            DisplayPortMetrics displayPort = DisplayPortCalculator.calculate(metrics, null);
            adjustViewport(displayPort);
        }
    }

    /** Implementation of PanZoomTarget
     * You must hold the monitor while calling this.
     */
    @Override
    public void setViewportMetrics(ImmutableViewportMetrics metrics) {
        setViewportMetrics(metrics, true);
    }

    private void setViewportMetrics(ImmutableViewportMetrics metrics, boolean notifyGecko) {
        mViewportMetrics = metrics;
        mView.requestRender();
        if (notifyGecko && mGeckoIsReady) {
            geometryChanged();
        }
        setShadowVisibility();
    }

    private void setShadowVisibility() {
        ThreadUtils.postToUiThread(new Runnable() {
            @Override
            public void run() {
                if (BrowserApp.mBrowserToolbar == null) {
                    return;
                }
                ImmutableViewportMetrics m = mViewportMetrics;
                BrowserApp.mBrowserToolbar.setShadowVisibility(m.viewportRectTop >= m.pageRectTop - m.fixedLayerMarginTop);
            }
        });
    }

    /** Implementation of PanZoomTarget */
    @Override
    public void forceRedraw() {
        mForceRedraw = true;
        if (mGeckoIsReady) {
            geometryChanged();
        }
    }

    /** Implementation of PanZoomTarget */
    @Override
    public boolean post(Runnable action) {
        return mView.post(action);
    }

    /** Implementation of PanZoomTarget */
    @Override
    public Object getLock() {
        return this;
    }

    /** Implementation of PanZoomTarget
     * Converts a point from layer view coordinates to layer coordinates. In other words, given a
     * point measured in pixels from the top left corner of the layer view, returns the point in
     * pixels measured from the last scroll position we sent to Gecko, in CSS pixels. Assuming the
     * events being sent to Gecko are processed in FIFO order, this calculation should always be
     * correct.
     */
    @Override
    public PointF convertViewPointToLayerPoint(PointF viewPoint) {
        if (!mGeckoIsReady) {
            return null;
        }

        ImmutableViewportMetrics viewportMetrics = mViewportMetrics;
        PointF origin = viewportMetrics.getOrigin();
        float zoom = viewportMetrics.zoomFactor;
        ImmutableViewportMetrics geckoViewport = mGeckoViewport;
        PointF geckoOrigin = geckoViewport.getOrigin();
        float geckoZoom = geckoViewport.zoomFactor;

        // viewPoint + origin gives the coordinate in device pixels from the top-left corner of the page.
        // Divided by zoom, this gives us the coordinate in CSS pixels from the top-left corner of the page.
        // geckoOrigin / geckoZoom is where Gecko thinks it is (scrollTo position) in CSS pixels from
        // the top-left corner of the page. Subtracting the two gives us the offset of the viewPoint from
        // the current Gecko coordinate in CSS pixels.
        PointF layerPoint = new PointF(
                ((viewPoint.x + origin.x) / zoom) - (geckoOrigin.x / geckoZoom),
                ((viewPoint.y + origin.y) / zoom) - (geckoOrigin.y / geckoZoom));

        return layerPoint;
    }

    /** Used by robocop for testing purposes. Not for production use! */
    public void setDrawListener(DrawListener listener) {
        mDrawListener = listener;
    }

    /** Used by robocop for testing purposes. Not for production use! */
    public static interface DrawListener {
        public void drawFinished();
    }
}