[MacViews] Enable dragging a window by its caption/draggable areas.

ui/views/cocoa/bridged_native_widget.mm

Index: ui/views/cocoa/bridged_native_widget.mm
diff --git a/ui/views/cocoa/bridged_native_widget.mm b/ui/views/cocoa/bridged_native_widget.mm
index 78abdb837c97ba5c25f94525c20d25606e7990a9..67003cad41d75e90ff7c5b16956a70683057c252 100644
--- a/ui/views/cocoa/bridged_native_widget.mm
+++ b/ui/views/cocoa/bridged_native_widget.mm
@@ -7,9 +7,11 @@
 #import <objc/runtime.h>
 
 #include "base/logging.h"
+#import "base/mac/foundation_util.h"
 #include "base/mac/mac_util.h"
 #import "base/mac/sdk_forward_declarations.h"
 #include "base/thread_task_runner_handle.h"
+#include "ui/base/hit_test.h"
 #include "ui/base/ime/input_method.h"
 #include "ui/base/ime/input_method_factory.h"
 #include "ui/base/ui_base_switches_util.h"
@@ -77,6 +79,92 @@ gfx::Size GetClientSizeForWindowSize(NSWindow* window,
   return gfx::Size([window contentRectForFrameRect:frame_rect].size);
 }
 
+BOOL WindowWantsMouseDownReposted(NSEvent* ns_event) {
+  id delegate = [[ns_event window] delegate];
+  return
+      [delegate
+          respondsToSelector:@selector(shouldRepostPendingLeftMouseDown:)] &&
+      [delegate shouldRepostPendingLeftMouseDown:[ns_event locationInWindow]];
+}
+
+// Check if a mouse-down event should drag the window. If so, repost the event.
+NSEvent* RepostEventIfHandledByWindow(NSEvent* ns_event) {
+  // Which repost we're expecting to receive.
+  static bool expecting_repost = false;
+  // The event number of the reposted event. This let's us track whether an
+  // event is actually a repost since user-generated events have increasing
+  // event numbers.
+  static NSInteger reposted_event_number = -1;

[tapted, 2015/06/03 00:28:37]

did we want to try this out with 0 instead of -1? (maybe kNoEventNumber since
it's reused below). Or perhaps we need a separate static bool for
`reposting_cancelled`. Or enum
RepostState {
  NONE,  // Nothing reposted: hit-test new mousedowns to see if they need to be
ignored and reposted after changing draggability.
  EXPECTING_REPOST,  // Expecting the next event to be the reposted event: let
it go through.
  REPOST_CANCELLED,  // If, while reposting, another mousedown was received:
when the reposted event is seen, ignore it.
};


That might make the logic easier to follow, and make it neater for the most
common case of `NONE` to be dealt with first. Then the flow in the function is
more "linear" (i.e. reading from top-to-bottom is then the order that things
happen in.)

[jackhou1, 2015/06/03 04:04:44]

Done.

+
+  CGEventRef cg_event = [ns_event CGEvent];
+  NSInteger event_number = [ns_event eventNumber];
+
+  // The logic here is a bit convoluted because we want to mitigate race
+  // conditions if somehow a different mouse-down occurs between reposts.
+  // Specifically, we want to avoid:
+  // - BridgedNativeWidget's draggability getting out of sync (e.g. it's

[tapted, 2015/06/03 00:28:37]

nit: it's -> its

[jackhou1, 2015/06/03 04:04:44]

This is actually an it's. Changed to "if it is".

[tapted, 2015/06/03 07:04:48]

whoops! I must have read as "its draggability outside.." :p. 

+  //   draggable outside of a repost cycle),
+  // - any repost loop.
+  if (reposted_event_number == event_number) {
+    // This is a reposted event.
+    if (expecting_repost) {
+      expecting_repost = false;
+      reposted_event_number = -1;
+      // This is the final repost, call through to make the window
+      // non-draggable.
+      WindowWantsMouseDownReposted(ns_event);
+    } else {
+      // Reposting was cancelled, now that we've received it, we don't expect
+      // to see it again.
+      reposted_event_number = -1;
+    }
+    return nil;  // Ignore the event.
+  }
+
+  // This is a new event.
+  if (expecting_repost) {
+    // We were expecting a repost, but since this is a new mouse-down, cancel
+    // reposting and allow event to continue as usual.
+    expecting_repost = false;
+    // Call through so that the window is made non-draggable again.
+    WindowWantsMouseDownReposted(ns_event);
+    return ns_event;
+  }
+
+  // We're not in the middle of reposting, process this new event.
+  if (WindowWantsMouseDownReposted(ns_event)) {
+    expecting_repost = true;
+    reposted_event_number = [ns_event eventNumber];
+    CGEventPost(kCGSessionEventTap, cg_event);
+    return nil;
+  }
+
+  return ns_event;
+}
+
+// Support window caption/draggable regions.
+// In AppKit, non-client regions are set by overriding
+// -[NSView mouseDownCanMoveWindow]. NSApplication caches this area as views are
+// installed and performs window moving when mouse-downs land in the area.
+// In Views, non-client regions are determined via hit-tests when the event
+// occurs.
+// To bridge the two models, we monitor mouse-downs with
+// +[NSEvent addLocalMonitorForEventsMatchingMask:handler:]. This receives
+// events after window dragging is handled, so for mouse-downs that land on a
+// draggable point, we cancel the event and repost it at the CGSessionEventTap
+// level so that window dragging will be handled again.
+void SetupDragEventMonitor() {
+  static id monitor = nil;
+  if (monitor)
+    return;
+
+  monitor = [NSEvent
+      addLocalMonitorForEventsMatchingMask:NSLeftMouseDownMask
+      handler:^NSEvent*(NSEvent* ns_event) {
+        return RepostEventIfHandledByWindow(ns_event);
+      }];
+}
+
 }  // namespace
 
 namespace views {
@@ -100,6 +188,7 @@ BridgedNativeWidget::BridgedNativeWidget(NativeWidgetMac* parent)
       in_fullscreen_transition_(false),
       window_visible_(false),
       wants_to_be_visible_(false) {
+  SetupDragEventMonitor();
   DCHECK(parent);
   window_delegate_.reset(
       [[ViewsNSWindowDelegate alloc] initWithBridgedNativeWidget:this]);
@@ -532,6 +621,32 @@ void BridgedNativeWidget::OnWindowKeyStatusChangedTo(bool is_key) {
   }
 }
 
+bool BridgedNativeWidget::ShouldRepostPendingLeftMouseDown(
+    NSPoint location_in_window) {
+  if (!bridged_view_)
+    return false;
+
+  if ([bridged_view_ mouseDownCanMoveWindow]) {
+    // This is a re-post, the movement has already started, so we can make the
+    // window non-draggable again.
+    SetDraggable(false);
+    return false;
+  }
+
+  gfx::Point point(location_in_window.x,
+                   NSHeight([window_ frame]) - location_in_window.y);
+  bool should_move_window =
+      native_widget_mac()->GetWidget()->GetNonClientComponent(point) ==
+      HTCAPTION;
+
+  if (!should_move_window)
+    return false;
+
+  // Make the window draggable, then return true to repost the event.
+  SetDraggable(true);
+  return true;
+}
+
 void BridgedNativeWidget::OnSizeConstraintsChanged() {
   // Don't modify the size constraints or fullscreen collection behavior while
   // in fullscreen or during a transition. OnFullscreenTransitionComplete will
@@ -870,4 +985,14 @@ NSMutableDictionary* BridgedNativeWidget::GetWindowProperties() const {
   return properties;
 }
 
+void BridgedNativeWidget::SetDraggable(bool draggable) {
+  [bridged_view_ setMouseDownCanMoveWindow:draggable];
+  // AppKit will not update its cache of mouseDownCanMoveWindow unless something
+  // changes. Previously we tried adding an NSView and removing it, but for some
+  // reason it required reposting the mouse-down event, and didn't always work.
+  // Calling the below seems to be an effective solution.
+  [window_ setMovableByWindowBackground:NO];

[jackhou1, 2015/06/02 06:33:09]

I'm not sure exactly why it wasn't working for browser windows, but this seems
to make it work with only one repost.

[tapted, 2015/06/03 00:28:37]

Nice!

+  [window_ setMovableByWindowBackground:YES];
+}
+
 }  // namespace views