Add touchpad pinch zoom support to Blink

Source/web/WebViewImpl.cpp

Index: Source/web/WebViewImpl.cpp
diff --git a/Source/web/WebViewImpl.cpp b/Source/web/WebViewImpl.cpp
index 3d9f83084ba3f24961e679bb124edea2fa77f6b1..838de04bcc7bfefb9aa63c6bbddf5f38c0b1249f 100644
--- a/Source/web/WebViewImpl.cpp
+++ b/Source/web/WebViewImpl.cpp
@@ -2144,8 +2144,29 @@ bool WebViewImpl::handleInputEvent(const WebInputEvent& inputEvent)
 
     // Unhandled touchpad gesture pinch events synthesize mouse wheel events.
     if (inputEvent.type == WebInputEvent::GesturePinchUpdate) {
-        if (handleSyntheticWheelFromTouchpadPinchEvent(static_cast<const WebGestureEvent&>(inputEvent)))
+        const WebGestureEvent& pinchEvent = static_cast<const WebGestureEvent&>(inputEvent);
+        // First, synthesize a Windows-like wheel event to send to any handlers that may exist.
+        if (handleSyntheticWheelFromTouchpadPinchEvent(pinchEvent))
             return true;
+
+        // If the event remains unhandled, use it pinch the viewport.
+        const float magnifyDelta = pinchEvent.data.pinchUpdate.scale;
+        const float oldPageScale = pageScaleFactor();
+        setPageScaleFactor(oldPageScale * magnifyDelta);

[aelias_OOO_until_Jul13, 2015/01/30 21:21:40]

Please avoid this call to setPageScaleFactor, since setting scale and scroll in
separate calls can sometimes cause subtle clamping bugs.  Instead, you can do
"float newPageScale = clampPageScaleFactorToLimits(oldPageScale *
magnifyDelta())", then call pinchViewport.setScaleAndLocation() instead of
move().

[ccameron, 2015/02/02 23:59:07]

Done.

+        const float newPageScale = pageScaleFactor();
+
+        // Keep the center-of-pinch anchor specified by (x, y) in a stable
+        // position over the course of the magnify.
+        const FloatPoint anchor(pinchEvent.x, pinchEvent.y);
+        FloatPoint anchorAtOldScale = anchor.scaledBy(1.f / oldPageScale);
+        FloatPoint anchorAtNewScale = anchor.scaledBy(1.f / newPageScale);
+        FloatSize anchorDelta = anchorAtOldScale - anchorAtNewScale;
+        page()->frameHost().pinchViewport().move(FloatPoint(anchorDelta.width(), anchorDelta.height()));
+
+        // TODO(ccameron): Always return that the event was handled. Once the pinch-zoom content scale
+        // handlers have been remove from content, this function may return false if the page scale does
+        // not change.
+        return true;
     }
 
     return false;