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

ui/views/cocoa/cocoa_non_client_drag.mm

Index: ui/views/cocoa/cocoa_non_client_drag.mm
diff --git a/ui/views/cocoa/cocoa_non_client_drag.mm b/ui/views/cocoa/cocoa_non_client_drag.mm
new file mode 100644
index 0000000000000000000000000000000000000000..f36e69e8819c1130f2349a439f20b74150ca76e7
--- /dev/null
+++ b/ui/views/cocoa/cocoa_non_client_drag.mm
@@ -0,0 +1,77 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#import "ui/views/cocoa/cocoa_non_client_drag.h"
+
+#import <Cocoa/Cocoa.h>
+
+#include "base/logging.h"
+#import "base/mac/foundation_util.h"
+#include "base/process/process_handle.h"
+
+namespace views {
+
+namespace {
+
+int g_ref_count = 0;

[jackhou1, 2015/05/20 05:50:37]

Do we already have something that implements this behavior? I.e. singleton, but
reference counted so it is disabled when nobody needs the behavior? Or maybe we
could get away with having it always enabled.

[tapted, 2015/05/20 07:18:17]

Yeah I would just create it lazily, once, and not worry about discarding it.
(see comment in BridgedNativeWidget constructor)

[jackhou1, 2015/05/22 02:49:17]

Done.

+id g_event_monitor = nil;
+bool is_first_repost = false;

[tapted, 2015/05/20 07:18:18]

make this a local static of DoubleRepostEventIfHandledByWindow() ?

Also, there may be some racy stuff here... events can sometimes get queued (e.g.
try putting a sleep(5) and sending a bunch of mouse events).

I actually don't know if CGEventPost(kCGSessionEventTap, event) puts the event
at the start or the end of the queue. If it's the start, I don't think there
will be problems.

If it's at the end, maybe you need to track something like kCGMouseEventNumber
to ensure it's tracking the correct mouse event. (but then I have no idea what
to do with ones that don't match..

Ideal would be if we could annotate the CGEventRef with some user data like
'repost count' but I don't think that's a thing

[jackhou1, 2015/05/22 02:49:16]

Fortunately, the eventNumber does remain unchanged, so we can use it to track
reposted events.
I've improved the logic to identify intermediate mouse-downs and cancel reposts.

+
+bool DoubleRepostEventIfHandledByWindow(NSWindow* window,
+                                        NSPoint location,
+                                        CGEventRef event) {
+  // If the event was just reposted, repost it again, otherwise it won't work.
+  if (is_first_repost) {
+    LOG(INFO) << "double reposting " << event;
+    is_first_repost = false;
+    CGEventPost(kCGSessionEventTap, event);
+    return true;
+  }
+  if ([window respondsToSelector:@selector(willReceiveLeftMouseDown:)] &&

[tapted, 2015/05/20 07:18:17]

maybe just check if [[window delegate]
respondsToSelector:@selector(onWindowWillReceiveLeftMouseDown:) and send
straight to the delegate?

i.e. no need for the NativeWidgetMacNSWindow (CocoaNonClientDrag) category

[jackhou1, 2015/05/22 02:49:17]

Done.

+      [base::mac::ObjCCast<NativeWidgetMacNSWindow>(window)
+          willReceiveLeftMouseDown:location]) {
+    is_first_repost = true;
+    CGEventPost(kCGSessionEventTap, event);
+    return true;
+  }
+
+  return false;
+}
+
+}  // namespace
+
+CocoaNonClientDrag::CocoaNonClientDrag() {
+  if (g_ref_count)
+    return;
+
+  NSEvent* (^monitor_callback)(NSEvent* ns_event);
+  monitor_callback = ^NSEvent*(NSEvent* ns_event) {
+    CGEventRef cg_event = [ns_event CGEvent];
+    LOG(INFO) << "MonitorCallback " << cg_event;
+    if (DoubleRepostEventIfHandledByWindow(

[tapted, 2015/05/20 07:18:18]

just pass ns_event?

[jackhou1, 2015/05/22 02:49:16]

Done.

+            [ns_event window], [ns_event locationInWindow], cg_event)) {
+      return nil;
+    }
+    return ns_event;
+  };
+
+  g_event_monitor =

[tapted, 2015/05/20 07:18:18]

I guess this can be a static local too.

[jackhou1, 2015/05/22 02:49:17]

Done.

+      [NSEvent addLocalMonitorForEventsMatchingMask:NSLeftMouseDownMask
+                                            handler:monitor_callback];
+
+  ++g_ref_count;
+}
+
+CocoaNonClientDrag::~CocoaNonClientDrag() {
+  --g_ref_count;

[tapted, 2015/05/20 07:18:18]

I guess this would just become a NOTREACHED() with all the other advice.

[jackhou1, 2015/05/22 02:49:16]

Done.

+  if (g_ref_count)
+    return;
+
+  if (g_event_monitor) {
+    [NSEvent removeMonitor:g_event_monitor];
+    g_event_monitor = nil;
+  }
+}
+
+}  // namespace views