Handle mac trackpad zoom via GesturePinch events

content/browser/web_contents/web_contents_impl.cc

Index: content/browser/web_contents/web_contents_impl.cc
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index 195edc388d7e714b3c4772930b2b099f260654fa..330f935985a3c02af7247d37fbbc601b03c5e734 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -332,6 +332,8 @@ WebContentsImpl::WebContentsImpl(
       minimum_zoom_percent_(static_cast<int>(kMinimumZoomFactor * 100)),
       maximum_zoom_percent_(static_cast<int>(kMaximumZoomFactor * 100)),
       temporary_zoom_settings_(false),
+      totalPinchGestureAmount_(0),
+      currentPinchZoomStepDelta_(0),
       color_chooser_identifier_(0),
       render_view_message_source_(NULL),
       fullscreen_widget_routing_id_(MSG_ROUTING_NONE),
@@ -1215,6 +1217,44 @@ bool WebContentsImpl::PreHandleGestureEvent(
   return delegate_ && delegate_->PreHandleGestureEvent(this, event);
 }
 
+bool WebContentsImpl::HandleGestureEvent(
+    const blink::WebGestureEvent& event) {
+  // Some platforms (eg. Mac) send GesturePinch events for trackpad pinch-zoom.
+  // Use them to implement browser zoom, as for HandleWheelEvent above.
+  if (event.type == blink::WebInputEvent::GesturePinchUpdate &&
+      event.sourceDevice == blink::WebGestureEvent::Touchpad) {
+    // The scale difference necessary to trigger a zoom action. Derived from
+    // experimentation to find a value that feels reasonable.
+    const float kZoomStepValue = 0.6;
+
+    // Find the (absolute) thresholds on either side of the current zoom factor,
+    // then convert those to actual numbers to trigger a zoom in or out.
+    // This logic deliberately makes the range around the starting zoom value
+    // for the gesture twice as large as the other ranges (i.e., the notches are
+    // at ..., -3*step, -2*step, -step, step, 2*step, 3*step, ... but not at 0)
+    // so that it's easier to get back to your starting point than it is to
+    // overshoot.
+    float nextStep = (abs(currentPinchZoomStepDelta_) + 1) * kZoomStepValue;
+    float backStep = abs(currentPinchZoomStepDelta_) * kZoomStepValue;
+    float zoomInThreshold = (currentPinchZoomStepDelta_ >= 0) ? nextStep
+        : -backStep;
+    float zoomOutThreshold = (currentPinchZoomStepDelta_ <= 0) ? -nextStep
+        : backStep;
+
+    totalPinchGestureAmount_ += event.data.pinchUpdate.scale;
+    if (totalPinchGestureAmount_ > zoomInThreshold) {
+      currentPinchZoomStepDelta_++;
+      delegate_->ContentsZoomChange(true);
+    } else if (totalPinchGestureAmount_ < zoomOutThreshold) {
+      currentPinchZoomStepDelta_--;
+      delegate_->ContentsZoomChange(false);
+    }
+    return true;
+  }
+
+  return false;
+}
+
 #if defined(OS_WIN)
 gfx::NativeViewAccessible WebContentsImpl::GetParentNativeViewAccessible() {
   return accessible_parent_;