1 files changed, 78 insertions, 0 deletions
diff --git a/android/source/src/java/org/mozilla/gecko/gfx/SubdocumentScrollHelper.java b/android/source/src/java/org/mozilla/gecko/gfx/SubdocumentScrollHelper.java
new file mode 100644
index 000000000..5a752e3c7
--- /dev/null
+++ b/android/source/src/java/org/mozilla/gecko/gfx/SubdocumentScrollHelper.java
@@ -0,0 +1,78 @@
+/* -*- 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 android.graphics.PointF;
+import android.os.Handler;
+
+class SubdocumentScrollHelper {
+    private static final String LOGTAG = "GeckoSubdocumentScrollHelper";
+
+    private final Handler mUiHandler;
+
+    /* This is the amount of displacement we have accepted but not yet sent to JS; this is
+     * only valid when mOverrideScrollPending is true. */
+    private final PointF mPendingDisplacement;
+
+    /* When this is true, we're sending scroll events to JS to scroll the active subdocument. */
+    private boolean mOverridePanning;
+
+    /* When this is true, we have received an ack for the last scroll event we sent to JS, and
+     * are ready to send the next scroll event. Note we only ever have one scroll event inflight
+     * at a time. */
+    private boolean mOverrideScrollAck;
+
+    /* When this is true, we have a pending scroll that we need to send to JS; we were unable
+     * to send it when it was initially requested because mOverrideScrollAck was not true. */
+    private boolean mOverrideScrollPending;
+
+    /* When this is true, the last scroll event we sent actually did some amount of scrolling on
+     * the subdocument; we use this to decide when we have reached the end of the subdocument. */
+    private boolean mScrollSucceeded;
+
+    SubdocumentScrollHelper() {
+        // mUiHandler will be bound to the UI thread since that's where this constructor runs
+        mUiHandler = new Handler();
+        mPendingDisplacement = new PointF();
+    }
+
+    void destroy() {
+    }
+
+    boolean scrollBy(PointF displacement) {
+        if (! mOverridePanning) {
+            return false;
+        }
+
+        if (! mOverrideScrollAck) {
+            mOverrideScrollPending = true;
+            mPendingDisplacement.x += displacement.x;
+            mPendingDisplacement.y += displacement.y;
+            return true;
+        }
+
+        mOverrideScrollAck = false;
+        mOverrideScrollPending = false;
+        // clear the |mPendingDisplacement| after serializing |displacement| to
+        // JSON because they might be the same object
+        mPendingDisplacement.x = 0;
+        mPendingDisplacement.y = 0;
+
+        return true;
+    }
+
+    void cancel() {
+        mOverridePanning = false;
+    }
+
+    boolean scrolling() {
+        return mOverridePanning;
+    }
+
+    boolean lastScrollSucceeded() {
+        return mScrollSucceeded;
+    }
+}