Reify view ownership as a message pipe.

mojo/services/ui/views/interfaces/view_trees.mojom

 // 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.
 
 [DartPackage="mojo_services"]
 module mojo.ui;
 
 import "mojo/public/interfaces/application/service_provider.mojom";
 import "mojo/services/ui/views/interfaces/layouts.mojom";
 import "mojo/services/ui/views/interfaces/views.mojom";
@@ skipping 44 matching lines @@
   OnRootUnavailable(uint32 root_key) => ();
 };
 
 // The view tree host provides an interface for a view tree to configure itself
 // and interact with its views.
 //
 // Each view tree obtains its own view tree host when registered with the
 // ViewManager.  To unregister the view tree, close its view tree
 // and/or view tree host message pipes.
 interface ViewTreeHost {
+  // Gets the view tree's token.
+  GetToken() => (ViewTreeToken token);
+
   // Gets a service provider to access services which are associated with
   // the view tree such as input, accessibility and editing capabilities.
   // The view tree service provider is private to the view tree and should
   // not be shared with anyone else.
   //
   // See |mojo.ui.InputDispatcher|.
   GetServiceProvider(mojo.ServiceProvider& service_provider);
 
   // Requests that the view tree's OnLayout() method be called to compute a
   // new layout due to a change in the view tree's layout information.
   RequestLayout();
 
   // Sets the root of the view tree and assigns it the provided |root_key|
   // to distinguish it from any other roots this view tree has had.
   //
   // It is a good idea to provide a distinct |root_key| each time a new root
   // is set so that callbacks related to the root can be clearly distinguished
   // across these changes.
   //
-  // If |root_view_token| refers to a view which is already unavailable
+  // If |root_view_owner| refers to a view which is already unavailable
   // then the call proceeds as if it succeeded but an OnChildUnavailable()
   // message will be sent.
   //
-  // If |root_view_token| refers to a view which already has a parent or is
+  // If |root_view_owner| refers to a view which already has a parent or is
   // the root of a view tree then an OnChildUnavailable() or OnRootUnavailable()
   // message will be sent to its old parent or root and the the view will be
   // used as the root of the new view tree as usual.  This special case also
   // applies when the specified view is already the root of this view tree, in
   // which case the behavior is similar to the view having been transferred to
   // some other view tree and then back again.
-  SetRoot(uint32 root_key, mojo.ui.ViewToken root_view_token);
+  //
+  // It is an error to call this function if the root has already been set;
+  // the connection will be closed.
+  SetRoot(uint32 root_key, mojo.ui.ViewOwner root_view_owner);
 
   // Removes the root of the view tree.
   //
-  // Does nothing if the view tree currently does not have a root.
-  ResetRoot();
+  // If |transferred_view_owner| is not null, associates it with the previously
+  // configured view or closes the |transferred_view_owner| message pipe
+  // if there was none.
+  //
+  // It is an error to call this function if the root was not previously set;
+  // the connection will be closed.
+  ResetRoot(mojo.ui.ViewOwner&? transferred_view_owner);
 
   // Sets the layout parameters of the root of the view tree and retrieves
   // its layout information.
   //
   // The returned |info| is null if this layout request was canceled either
   // because it has been superceded by a subsequently issued layout request
   // or because the root has become unavailable.
   //
   // It is an error to call this function if the view tree does not currently
   // have a root; the connection will be closed.
   //
   // It is an error to specify malformed |root_layout_params| such
   // as invalid size constraints; the connection will be closed.
   LayoutRoot(mojo.ui.ViewLayoutParams root_layout_params) =>
       (mojo.ui.ViewLayoutInfo? info);
 };