Reify view ownership as a message pipe.

services/ui/view_manager/view_registry.cc

Index: services/ui/view_manager/view_registry.cc
diff --git a/services/ui/view_manager/view_registry.cc b/services/ui/view_manager/view_registry.cc
index a92c7f1d8f5a7bbc6e75ec28c4178c0690321cd4..c86177a399344692bd1b908b1e967a3ec2aed774 100644
--- a/services/ui/view_manager/view_registry.cc
+++ b/services/ui/view_manager/view_registry.cc
@@ -14,8 +14,8 @@
 #include "services/ui/view_manager/view_tree_host_impl.h"
 
 namespace view_manager {
-
-static bool AreViewLayoutParamsValid(const mojo::ui::ViewLayoutParams* params) {
+namespace {
+bool AreViewLayoutParamsValid(const mojo::ui::ViewLayoutParams* params) {
   return params && params->constraints && params->constraints->min_width >= 0 &&
          params->constraints->max_width >= params->constraints->min_width &&
          params->constraints->min_height >= 0 &&
@@ -23,6 +23,14 @@ static bool AreViewLayoutParamsValid(const mojo::ui::ViewLayoutParams* params) {
          params->device_pixel_ratio > 0;
 }
 
+bool IsSizeInBounds(mojo::ui::BoxConstraints* constraints, mojo::Size* size) {
+  return size && size->width >= constraints->min_width &&
+         size->width <= constraints->max_width &&
+         size->height >= constraints->min_height &&
+         size->height <= constraints->max_height;
+}
+}  // namespace
+
 ViewRegistry::ViewRegistry(mojo::gfx::composition::CompositorPtr compositor)
     : compositor_(compositor.Pass()) {}
 
@@ -36,11 +44,14 @@ void ViewRegistry::ConnectAssociates(
                                      connection_error_callback);
 }
 
-mojo::ui::ViewTokenPtr ViewRegistry::RegisterView(
+void ViewRegistry::RegisterView(
     mojo::ui::ViewPtr view,
     mojo::InterfaceRequest<mojo::ui::ViewHost> view_host_request,
+    mojo::InterfaceRequest<mojo::ui::ViewOwner> view_owner_request,
     const mojo::String& label) {
   DCHECK(view);
+  DCHECK(view_host_request.is_pending());
+  DCHECK(view_owner_request.is_pending());
 
   auto view_token = mojo::ui::ViewToken::New();
   view_token->value = next_view_token_value_++;
@@ -51,26 +62,19 @@ mojo::ui::ViewTokenPtr ViewRegistry::RegisterView(
   std::string sanitized_label =
       label.get().substr(0, mojo::ui::kLabelMaxLength);
   ViewState* view_state =
-      new ViewState(view.Pass(), view_token.Pass(), sanitized_label);
-  ViewHostImpl* view_host =
-      new ViewHostImpl(this, view_state, view_host_request.Pass());
-  view_state->set_view_host(view_host);
-  view_state->set_view_connection_error_handler(
-      base::Bind(&ViewRegistry::OnViewConnectionError, base::Unretained(this),
-                 view_state));
-  view_host->set_view_host_connection_error_handler(
-      base::Bind(&ViewRegistry::OnViewConnectionError, base::Unretained(this),
-                 view_state));
+      new ViewState(this, view.Pass(), view_token.Pass(),
+                    view_host_request.Pass(), sanitized_label);
+  view_state->BindOwner(view_owner_request.Pass());
 
   // Add to registry and return token.
   views_by_token_.insert({view_state->view_token()->value, view_state});
   DVLOG(1) << "RegisterView: view=" << view_state;
-  return view_state->view_token()->Clone();
 }
 
-void ViewRegistry::OnViewConnectionError(ViewState* view_state) {
+void ViewRegistry::OnViewDied(ViewState* view_state,
+                              const std::string& reason) {
   DCHECK(IsViewStateRegisteredDebug(view_state));
-  DVLOG(1) << "OnViewConnectionError: view=" << view_state;
+  DVLOG(1) << "OnViewDied: view=" << view_state << ", reason=" << reason;
 
   UnregisterView(view_state);
 }
@@ -80,18 +84,25 @@ void ViewRegistry::UnregisterView(ViewState* view_state) {
   DVLOG(1) << "UnregisterView: view=" << view_state;
 
   // Remove from parent or roots.
+  // This may send a view unavailable message to the view's parent or tree.
   HijackView(view_state);
 
+  // Recursively unregister all children since they will become unowned
+  // at this point taking care to unlink each one before its unregistration.
+  for (auto& child : view_state->UnlinkAllChildren())
+    UnregisterViewStub(std::move(child));
+
   // Remove from registry.
   views_by_token_.erase(view_state->view_token()->value);
   delete view_state;
 }
 
-mojo::ui::ViewTreeTokenPtr ViewRegistry::RegisterViewTree(
+void ViewRegistry::RegisterViewTree(
     mojo::ui::ViewTreePtr view_tree,
     mojo::InterfaceRequest<mojo::ui::ViewTreeHost> view_tree_host_request,
     const mojo::String& label) {
   DCHECK(view_tree);
+  DCHECK(view_tree_host_request.is_pending());
 
   auto view_tree_token = mojo::ui::ViewTreeToken::New();
   view_tree_token->value = next_view_tree_token_value_++;
@@ -101,28 +112,20 @@ mojo::ui::ViewTreeTokenPtr ViewRegistry::RegisterViewTree(
   // Create the state and bind host to it.
   std::string sanitized_label =
       label.get().substr(0, mojo::ui::kLabelMaxLength);
-  ViewTreeState* tree_state = new ViewTreeState(
-      view_tree.Pass(), view_tree_token.Pass(), sanitized_label);
-  ViewTreeHostImpl* tree_host =
-      new ViewTreeHostImpl(this, tree_state, view_tree_host_request.Pass());
-  tree_state->set_view_tree_host(tree_host);
-  tree_state->set_view_tree_connection_error_handler(
-      base::Bind(&ViewRegistry::OnViewTreeConnectionError,
-                 base::Unretained(this), tree_state));
-  tree_host->set_view_tree_host_connection_error_handler(
-      base::Bind(&ViewRegistry::OnViewTreeConnectionError,
-                 base::Unretained(this), tree_state));
+  ViewTreeState* tree_state =
+      new ViewTreeState(this, view_tree.Pass(), view_tree_token.Pass(),
+                        view_tree_host_request.Pass(), sanitized_label);
 
   // Add to registry.
   view_trees_by_token_.insert(
       {tree_state->view_tree_token()->value, tree_state});
   DVLOG(1) << "RegisterViewTree: tree=" << tree_state;
-  return tree_state->view_tree_token()->Clone();
 }
 
-void ViewRegistry::OnViewTreeConnectionError(ViewTreeState* tree_state) {
+void ViewRegistry::OnViewTreeDied(ViewTreeState* tree_state,
+                                  const std::string& reason) {
   DCHECK(IsViewTreeStateRegisteredDebug(tree_state));
-  DVLOG(1) << "OnViewTreeConnectionError: tree=" << tree_state;
+  DVLOG(1) << "OnViewTreeDied: tree=" << tree_state << ", reason=" << reason;
 
   UnregisterViewTree(tree_state);
 }
@@ -133,7 +136,7 @@ void ViewRegistry::UnregisterViewTree(ViewTreeState* tree_state) {
 
   // Unlink the root if needed.
   if (tree_state->root())
-    UnlinkRoot(tree_state);
+    UnregisterViewStub(tree_state->UnlinkRoot());
 
   // Remove from registry.
   view_trees_by_token_.erase(tree_state->view_tree_token()->value);
@@ -144,6 +147,7 @@ void ViewRegistry::CreateScene(
     ViewState* view_state,
     mojo::InterfaceRequest<mojo::gfx::composition::Scene> scene) {
   DCHECK(IsViewStateRegisteredDebug(view_state));
+  DCHECK(scene.is_pending());
   DVLOG(1) << "CreateScene: view=" << view_state;
 
   compositor_->CreateScene(
@@ -175,46 +179,40 @@ void ViewRegistry::RequestLayout(ViewState* view_state) {
 
 void ViewRegistry::AddChild(ViewState* parent_state,
                             uint32_t child_key,
-                            mojo::ui::ViewTokenPtr child_view_token) {
+                            mojo::ui::ViewOwnerPtr child_view_owner) {
   DCHECK(IsViewStateRegisteredDebug(parent_state));
-  DCHECK(child_view_token);
-  DVLOG(1) << "AddChild: parent=" << parent_state << ", child_key=" << child_key
-           << ", child=" << child_view_token;
+  DCHECK(child_view_owner);
+  DVLOG(1) << "AddChild: parent=" << parent_state
+           << ", child_key=" << child_key;
 
-  // Check for duplicate children.
+  // Ensure there are no other children with the same key.
   if (parent_state->children().find(child_key) !=
       parent_state->children().end()) {
     LOG(ERROR) << "View attempted to add a child with a duplicate key: "
-               << "parent=" << parent_state << ", child_key=" << child_key
-               << ", child=" << child_view_token;
+               << "parent=" << parent_state << ", child_key=" << child_key;
     UnregisterView(parent_state);
     return;
   }
 
-  // Check whether the desired child view still exists.
-  // Adding a non-existent child still succeeds but the view manager will
-  // immediately report it as being unavailable.
-  ViewState* child_state = FindView(child_view_token->value);
-  if (!child_state) {
-    LinkChildAsUnavailable(parent_state, child_key);
-    return;
-  }
-
-  // Check whether the child needs to be reparented.
-  // The old parent will receive an unavailable event.  For interface symmetry,
-  // we deliberately do this even if the old and new parents are the same.
-  HijackView(child_state);
+  // Add a stub, pending resolution of the view owner.
+  parent_state->LinkChild(child_key, std::unique_ptr<ViewStub>(new ViewStub(
+                                         this, child_view_owner.Pass())));
 
-  // Link the child into its new parent.
-  LinkChild(parent_state, child_key, child_state);
+  // Schedule layout of the parent on behalf of its newly added child.
+  // We don't need to schedule layout of the child until the parent provides
+  // new layout parameters.
+  InvalidateLayoutForChild(parent_state, child_key);
 }
 
-void ViewRegistry::RemoveChild(ViewState* parent_state, uint32_t child_key) {
+void ViewRegistry::RemoveChild(ViewState* parent_state,
+                               uint32_t child_key,
+                               mojo::InterfaceRequest<mojo::ui::ViewOwner>
+                                   transferred_view_owner_request) {
   DCHECK(IsViewStateRegisteredDebug(parent_state));
   DVLOG(1) << "RemoveChild: parent=" << parent_state
            << ", child_key=" << child_key;
 
-  // Check whether the child key exists in the parent.
+  // Ensure the child key exists in the parent.
   auto child_it = parent_state->children().find(child_key);
   if (child_it == parent_state->children().end()) {
     LOG(ERROR) << "View attempted to remove a child with an invalid key: "
@@ -224,7 +222,13 @@ void ViewRegistry::RemoveChild(ViewState* parent_state, uint32_t child_key) {
   }
 
   // Unlink the child from its parent.
-  UnlinkChild(parent_state, child_it);
+  TransferOrUnregisterViewStub(parent_state->UnlinkChild(child_key),
+                               transferred_view_owner_request.Pass());
+
+  // Schedule layout for the parent now that it has lost its child.
+  // We don't need to schedule layout for the child itself since it will
+  // retain its old layout parameters until it is reparented.
+  InvalidateLayout(parent_state);
 }
 
 void ViewRegistry::LayoutChild(
@@ -260,7 +264,7 @@ void ViewRegistry::LayoutChild(
     return;
   }
 
-  SetLayout(child_it->second, child_layout_params.Pass(), callback);
+  SetLayout(child_it->second.get(), child_layout_params.Pass(), callback);
 }
 
 void ViewRegistry::ConnectToViewService(
@@ -282,32 +286,52 @@ void ViewRegistry::RequestLayout(ViewTreeState* tree_state) {
 
 void ViewRegistry::SetRoot(ViewTreeState* tree_state,
                            uint32_t root_key,
-                           mojo::ui::ViewTokenPtr root_view_token) {
+                           mojo::ui::ViewOwnerPtr root_view_owner) {
   DCHECK(IsViewTreeStateRegisteredDebug(tree_state));
-  DCHECK(root_view_token);
-  DVLOG(1) << "SetRoot: tree=" << tree_state << ", root_key=" << root_key
-           << ", root=" << root_view_token;
-
-  // Check whether the desired root view still exists.
-  // Using a non-existent root view still succeeds but the view manager will
-  // immediately report it as being unavailable.
-  ViewState* root_state = FindView(root_view_token->value);
-  if (root_state) {
-    HijackView(root_state);
-    LinkRoot(tree_state, root_state, root_key);
-  } else {
-    SendRootUnavailable(tree_state, root_key);
+  DCHECK(root_view_owner);
+  DVLOG(1) << "SetRoot: tree=" << tree_state << ", root_key=" << root_key;
+
+  // Ensure there isn't already a root.
+  if (tree_state->root()) {
+    LOG(ERROR)
+        << "View tree attempted to set the root while one is already set: tree="
+        << tree_state << ", root_key=" << root_key;
+    UnregisterViewTree(tree_state);
+    return;
   }
-  tree_state->set_explicit_root(true);
+
+  // Set the root to a stub, pending resolution of the view owner.
+  tree_state->LinkRoot(root_key, std::unique_ptr<ViewStub>(new ViewStub(
+                                     this, root_view_owner.Pass())));
+
+  // Schedule layout of the tree on behalf of its newly added root.
+  // We don't need to schedule layout of the root until the tree provides
+  // new layout parameters.
+  InvalidateLayoutForRoot(tree_state);
 }
 
-void ViewRegistry::ResetRoot(ViewTreeState* tree_state) {
+void ViewRegistry::ResetRoot(ViewTreeState* tree_state,
+                             mojo::InterfaceRequest<mojo::ui::ViewOwner>
+                                 transferred_view_owner_request) {
   DCHECK(IsViewTreeStateRegisteredDebug(tree_state));
   DVLOG(1) << "ResetRoot: tree=" << tree_state;
 
-  if (tree_state->root())
-    UnlinkRoot(tree_state);
-  tree_state->set_explicit_root(false);
+  // Ensure there is a root.
+  if (!tree_state->root()) {
+    LOG(ERROR)
+        << "View tree attempted to reset the root but there is none: tree="
+        << tree_state;
+    UnregisterViewTree(tree_state);
+    return;
+  }
+
+  // Unlink the root from its tree.
+  TransferOrUnregisterViewStub(tree_state->UnlinkRoot(),
+                               transferred_view_owner_request.Pass());
+
+  // Note: We don't need to schedule layout for the root since it will retain
+  // its old layout parameters.  And there's no need to tell the tree
+  // either since it won't have any work to do.  So we're done.
 }
 
 void ViewRegistry::LayoutRoot(ViewTreeState* tree_state,
@@ -331,8 +355,8 @@ void ViewRegistry::LayoutRoot(ViewTreeState* tree_state,
 
   // Check whether the client called LayoutRoot without first having actually
   // set a root.
-  if (!tree_state->explicit_root()) {
-    LOG(ERROR) << "View tree attempted to layout the rout without having "
+  if (!tree_state->root()) {
+    LOG(ERROR) << "View tree attempted to layout the root without having "
                   "set one first: tree="
                << tree_state << ", root_layout_params=" << root_layout_params;
     UnregisterViewTree(tree_state);
@@ -340,13 +364,6 @@ void ViewRegistry::LayoutRoot(ViewTreeState* tree_state,
     return;
   }
 
-  // Check whether the root is unavailable and therefore cannot be laid out.
-  // This is not an error.
-  if (!tree_state->root()) {
-    callback.Run(nullptr);
-    return;
-  }
-
   SetLayout(tree_state->root(), root_layout_params.Pass(), callback);
 }
 
@@ -366,176 +383,112 @@ ViewState* ViewRegistry::FindView(uint32_t view_token_value) {
   return it != views_by_token_.end() ? it->second : nullptr;
 }
 
-void ViewRegistry::LinkChild(ViewState* parent_state,
-                             uint32_t child_key,
-                             ViewState* child_state) {
-  DCHECK(IsViewStateRegisteredDebug(parent_state));
-  DCHECK(parent_state->children().find(child_key) ==
-         parent_state->children().end());
-  DCHECK(IsViewStateRegisteredDebug(child_state));
-
-  DVLOG(2) << "Added child " << child_key << " {" << child_state->label()
-           << "} to parent {" << parent_state->label() << "}";
-
-  parent_state->children().insert({child_key, child_state});
-  child_state->SetParent(parent_state, child_key);
-
-  // Schedule layout of the parent on behalf of its newly added child.
-  // We don't need to schedule layout of the child until the parent provides
-  // new layout parameters.
-  InvalidateLayoutForChild(parent_state, child_key);
+ViewTreeState* ViewRegistry::FindViewTree(uint32_t view_tree_token_value) {
+  auto it = view_trees_by_token_.find(view_tree_token_value);
+  return it != view_trees_by_token_.end() ? it->second : nullptr;
 }
 
-void ViewRegistry::LinkChildAsUnavailable(ViewState* parent_state,
-                                          uint32_t child_key) {
-  DCHECK(IsViewStateRegisteredDebug(parent_state));
-  DCHECK(parent_state->children().find(child_key) ==
-         parent_state->children().end());
-
-  DVLOG(2) << "Added unavailable child " << child_key << " to parent {"
-           << parent_state->label() << "}";
-
-  parent_state->children().insert({child_key, nullptr});
-  SendChildUnavailable(parent_state, child_key);
+void ViewRegistry::HijackView(ViewState* view_state) {
+  DCHECK(IsViewStateRegisteredDebug(view_state));
 
-  // Don't schedule layout for the parent just yet.  Wait for it to
-  // remove its child in response to the OnChildUnavailable notification.
+  ViewStub* view_stub = view_state->view_stub();
+  if (view_stub)
+    ReleaseViewStubAndNotify(view_stub);
 }
 
-void ViewRegistry::MarkChildAsUnavailable(ViewState* parent_state,
-                                          uint32_t child_key) {
-  DCHECK(IsViewStateRegisteredDebug(parent_state));
-  auto child_it = parent_state->children().find(child_key);
-  DCHECK(child_it != parent_state->children().end());
-  DCHECK(child_it->second);
+void ViewRegistry::OnViewResolved(ViewStub* view_stub,
+                                  mojo::ui::ViewTokenPtr view_token) {
+  DCHECK(view_stub);
 
-  DVLOG(2) << "Marked unavailable child " << child_key << " {"
-           << child_it->second->label() << "} from parent {"
-           << parent_state->label() << "}";
-
-  ResetStateWhenUnlinking(child_it->second);
-  child_it->second->ResetContainer();
-  child_it->second = nullptr;
-  SendChildUnavailable(parent_state, child_key);
-
-  // Don't schedule layout for the parent just yet.  Wait for it to
-  // remove its child in response to the OnChildUnavailable notification.
-  // We don't need to schedule layout for the child either since it will
-  // retain its old layout parameters.
+  ViewState* view_state = view_token ? FindView(view_token->value) : nullptr;
+  if (view_state)
+    AttachViewStubAndNotify(view_stub, view_state);
+  else
+    ReleaseViewStubAndNotify(view_stub);
 }
 
-void ViewRegistry::UnlinkChild(ViewState* parent_state,
-                               ViewState::ChildrenMap::iterator child_it) {
-  DCHECK(IsViewStateRegisteredDebug(parent_state));
-  DCHECK(child_it != parent_state->children().end());
-
-  ViewState* child_state = child_it->second;
-  if (child_state) {
-    DVLOG(2) << "Removed child " << child_state->key() << " {"
-             << child_state->label() << "} from parent {"
-             << parent_state->label() << "}";
-    ResetStateWhenUnlinking(child_it->second);
-    child_state->ResetContainer();
-  } else {
-    DVLOG(2) << "Removed unavailable child " << child_it->first
-             << "} from parent {" << parent_state->label() << "}";
-  }
-  parent_state->children().erase(child_it);
+void ViewRegistry::AttachViewStubAndNotify(ViewStub* view_stub,
+                                           ViewState* view_state) {
+  DCHECK(view_stub);
 
-  // Schedule layout for the parent now that it has lost its child.
-  // We don't need to schedule layout for the child itself since it will
-  // retain its old layout parameters.
-  InvalidateLayout(parent_state);
-}
+  view_state->ReleaseOwner();  // don't need the ViewOwner pipe anymore
+  view_stub->AttachView(view_state);
 
-ViewTreeState* ViewRegistry::FindViewTree(uint32_t view_tree_token_value) {
-  auto it = view_trees_by_token_.find(view_tree_token_value);
-  return it != view_trees_by_token_.end() ? it->second : nullptr;
+  if (view_stub->pending_layout_request()) {
+    view_state->pending_layout_requests().push_back(
+        std::move(view_stub->pending_layout_request()));
+    IssueNextViewLayoutRequest(view_state);
+  }
 }
 
-void ViewRegistry::LinkRoot(ViewTreeState* tree_state,
-                            ViewState* root_state,
-                            uint32_t root_key) {
-  DCHECK(IsViewTreeStateRegisteredDebug(tree_state));
-  DCHECK(IsViewStateRegisteredDebug(root_state));
-  DCHECK(!tree_state->root());
-  DCHECK(!root_state->parent());
+void ViewRegistry::ReleaseViewStubAndNotify(ViewStub* view_stub) {
+  DCHECK(view_stub);
 
-  DVLOG(2) << "Linked view tree root " << root_key << " {"
-           << root_state->label() << "}";
+  view_stub->ReleaseView();
 
-  tree_state->SetRoot(root_state, root_key);
+  if (view_stub->parent())
+    SendChildUnavailable(view_stub->parent(), view_stub->key());
+  else if (view_stub->tree())
+    SendRootUnavailable(view_stub->tree(), view_stub->key());
 
-  // Schedule layout of the tree on behalf of its newly added root.
-  // We don't need to schedule layout of the root until the tree provides
-  // new layout parameters.
-  InvalidateLayoutForRoot(tree_state);
+  // Note: We don't need to schedule layout for the previous owner.
+  // We can simply wait for it to remove its unavailable child or root in
+  // response to the notification at which point layout will occur.
+  // We don't need to schedule layout for the child either since it will
+  // retain its old layout parameters.
 }
 
-void ViewRegistry::UnlinkRoot(ViewTreeState* tree_state) {
-  DCHECK(IsViewTreeStateRegisteredDebug(tree_state));
-  DCHECK(tree_state->root());
-
-  DVLOG(2) << "Unlinked view tree root " << tree_state->root()->key() << " {"
-           << tree_state->root()->label() << "}";
-
-  ResetStateWhenUnlinking(tree_state->root());
-  tree_state->ResetRoot();
-
-  // We don't need to schedule layout for the root since it will retain
-  // its old layout parameters.
-}
+void ViewRegistry::TransferOrUnregisterViewStub(
+    std::unique_ptr<ViewStub> view_stub,
+    mojo::InterfaceRequest<mojo::ui::ViewOwner>
+        transferred_view_owner_request) {
+  DCHECK(view_stub);
 
-void ViewRegistry::HijackView(ViewState* view_state) {
-  if (view_state->parent()) {
-    MarkChildAsUnavailable(view_state->parent(), view_state->key());
-  } else if (view_state->tree()) {
-    ViewTreeState* tree_state = view_state->tree();
-    uint32_t root_key = tree_state->root()->key();
-    UnlinkRoot(tree_state);
-    SendRootUnavailable(tree_state, root_key);
+  if (transferred_view_owner_request.is_pending()) {
+    if (view_stub->state())
+      view_stub->state()->BindOwner(transferred_view_owner_request.Pass());
+    else if (view_stub->is_pending())
+      CHECK(false);  // TODO(jeffbrown): Handle transfer of pending view
+  } else {
+    UnregisterViewStub(std::move(view_stub));
   }
 }
 
-void ViewRegistry::InvalidateLayout(ViewState* view_state) {
-  DCHECK(IsViewStateRegisteredDebug(view_state));
+void ViewRegistry::UnregisterViewStub(std::unique_ptr<ViewStub> view_stub) {
+  DCHECK(view_stub);
 
-  // We can consider the layout request to have been satisfied if
-  // there is already a pending layout request in the queue that has not
-  // yet been issued (this is coalescing).  Otherwise we must manufacture
-  // a new one based on the current layout parameters.
-  if (view_state->layout_params() &&
-      (view_state->pending_layout_requests().empty() ||
-       view_state->pending_layout_requests().back()->issued())) {
-    EnqueueLayoutRequest(view_state, view_state->layout_params()->Clone());
-    IssueNextViewLayoutRequest(view_state);
-  }
+  if (view_stub->state())
+    UnregisterView(view_stub->state());
 }
 
-void ViewRegistry::InvalidateLayoutForChild(ViewState* parent_state,
-                                            uint32_t child_key) {
-  DCHECK(IsViewStateRegisteredDebug(parent_state));
-  DCHECK(parent_state->children().find(child_key) !=
-         parent_state->children().end());
-
-  parent_state->children_needing_layout().insert(child_key);
-  InvalidateLayout(parent_state);
-}
+void ViewRegistry::SetLayout(ViewStub* view_stub,
+                             mojo::ui::ViewLayoutParamsPtr layout_params,
+                             const ViewLayoutCallback& callback) {
+  DCHECK(view_stub);
+  DCHECK(AreViewLayoutParamsValid(layout_params.get()));
 
-void ViewRegistry::InvalidateLayoutForRoot(ViewTreeState* tree_state) {
-  DCHECK(IsViewTreeStateRegisteredDebug(tree_state));
+  // Immediately discard layout requests on unavailable views.
+  if (view_stub->is_unavailable()) {
+    callback.Run(nullptr);
+    return;
+  }
 
-  if (!tree_state->layout_request_pending()) {
-    tree_state->set_layout_request_pending(true);
-    IssueNextViewTreeLayoutRequest(tree_state);
+  // For pending views, only remember the most recent distinct layout request.
+  if (view_stub->is_pending()) {
+    if (!view_stub->pending_layout_request() ||
+        !view_stub->pending_layout_request()->layout_params()->Equals(
+            *layout_params)) {
+      view_stub->pending_layout_request().reset(
+          new ViewLayoutRequest(layout_params.Pass()));
+    }
+    view_stub->pending_layout_request()->AddCallback(callback);
+    return;
   }
-}
 
-void ViewRegistry::SetLayout(ViewState* view_state,
-                             mojo::ui::ViewLayoutParamsPtr layout_params,
-                             const ViewLayoutCallback& callback) {
-  DCHECK(IsViewStateRegisteredDebug(view_state));
-  DCHECK(AreViewLayoutParamsValid(layout_params.get()));
+  // For actual views, maintain a queue of pending layout requests.
+  ViewState* view_state = view_stub->state();
+  DCHECK(view_state);
+  DCHECK(!view_stub->pending_layout_request());
 
   // Check whether the currently cached layout parameters are the same
   // and we already have a result and we have no pending layout requests.
@@ -568,7 +521,7 @@ void ViewRegistry::SetLayout(ViewState* view_state,
 void ViewRegistry::EnqueueLayoutRequest(
     ViewState* view_state,
     mojo::ui::ViewLayoutParamsPtr layout_params) {
-  DCHECK(IsViewStateRegisteredDebug(view_state));
+  DCHECK(view_state);
   DCHECK(AreViewLayoutParamsValid(layout_params.get()));
 
   // Drop the previous layout request if it hasn't been issued yet.
@@ -583,86 +536,76 @@ void ViewRegistry::EnqueueLayoutRequest(
           new ViewLayoutRequest(layout_params.Pass())));
 }
 
-void ViewRegistry::IssueNextViewLayoutRequest(ViewState* view_state) {
+void ViewRegistry::InvalidateLayout(ViewState* view_state) {
   DCHECK(IsViewStateRegisteredDebug(view_state));
 
-  if (!view_state->pending_layout_requests().empty() &&
-      !view_state->pending_layout_requests().front()->issued()) {
-    view_state->pending_layout_requests().front()->set_issued(true);
-    SendViewLayoutRequest(view_state);
-  }
-}
-
-void ViewRegistry::IssueNextViewTreeLayoutRequest(ViewTreeState* tree_state) {
-  DCHECK(IsViewTreeStateRegisteredDebug(tree_state));
-
-  if (tree_state->layout_request_pending() &&
-      !tree_state->layout_request_issued()) {
-    tree_state->set_layout_request_pending(false);
-    tree_state->set_layout_request_issued(true);
-    SendViewTreeLayoutRequest(tree_state);
-  }
-}
-
-void ViewRegistry::ResetStateWhenUnlinking(ViewState* view_state) {
-  // Clean up parent's recorded state for the child.
-  if (view_state->parent()) {
-    view_state->parent()->children_needing_layout().erase(view_state->key());
+  // We can consider the layout request to have been satisfied if
+  // there is already a pending layout request in the queue that has not
+  // yet been issued (this is coalescing).  Otherwise we must manufacture
+  // a new one based on the current layout parameters.
+  if (view_state->layout_params() &&
+      (view_state->pending_layout_requests().empty() ||
+       view_state->pending_layout_requests().back()->issued())) {
+    EnqueueLayoutRequest(view_state, view_state->layout_params()->Clone());
+    IssueNextViewLayoutRequest(view_state);
   }
 }
 
-void ViewRegistry::SendChildUnavailable(ViewState* parent_state,
-                                        uint32_t child_key) {
+void ViewRegistry::InvalidateLayoutForChild(ViewState* parent_state,
+                                            uint32_t child_key) {
   DCHECK(IsViewStateRegisteredDebug(parent_state));
+  DCHECK(parent_state->children().find(child_key) !=
+         parent_state->children().end());
 
-  // TODO: Detect ANRs
-  DVLOG(1) << "SendChildUnavailable: child_key=" << child_key;
-  parent_state->view()->OnChildUnavailable(child_key,
-                                           base::Bind(&base::DoNothing));
+  parent_state->children_needing_layout().insert(child_key);
+  InvalidateLayout(parent_state);
 }
 
-void ViewRegistry::SendRootUnavailable(ViewTreeState* tree_state,
-                                       uint32_t root_key) {
+void ViewRegistry::InvalidateLayoutForRoot(ViewTreeState* tree_state) {
   DCHECK(IsViewTreeStateRegisteredDebug(tree_state));
 
-  // TODO: Detect ANRs
-  DVLOG(1) << "SendRootUnavailable: root_key=" << root_key;
-  tree_state->view_tree()->OnRootUnavailable(root_key,
-                                             base::Bind(&base::DoNothing));
+  if (!tree_state->layout_request_pending()) {
+    tree_state->set_layout_request_pending(true);
+    IssueNextViewTreeLayoutRequest(tree_state);
+  }
 }
 
-void ViewRegistry::SendViewLayoutRequest(ViewState* view_state) {
+void ViewRegistry::IssueNextViewLayoutRequest(ViewState* view_state) {
   DCHECK(IsViewStateRegisteredDebug(view_state));
-  DCHECK(!view_state->pending_layout_requests().empty());
-  DCHECK(view_state->pending_layout_requests().front()->issued());
+
+  if (view_state->pending_layout_requests().empty())
+    return;
+
+  ViewLayoutRequest* request =
+      view_state->pending_layout_requests().front().get();
+  if (request->issued())
+    return;
 
   // TODO: Detect ANRs
-  DVLOG(1) << "SendViewLayoutRequest: view.token=" << view_state->label();
+  DVLOG(1) << "IssueNextViewLayoutRequest: view_state=" << view_state;
   view_state->view()->OnLayout(
-      view_state->pending_layout_requests().front()->layout_params()->Clone(),
+      request->layout_params()->Clone(),
       mojo::Array<uint32_t>::From(view_state->children_needing_layout()),
       base::Bind(&ViewRegistry::OnViewLayoutResult, base::Unretained(this),
                  view_state->GetWeakPtr()));
   view_state->children_needing_layout().clear();
+  request->set_issued(true);
 }
 
-void ViewRegistry::SendViewTreeLayoutRequest(ViewTreeState* tree_state) {
+void ViewRegistry::IssueNextViewTreeLayoutRequest(ViewTreeState* tree_state) {
   DCHECK(IsViewTreeStateRegisteredDebug(tree_state));
-  DCHECK(tree_state->layout_request_issued());
+
+  if (!tree_state->layout_request_pending() ||
+      tree_state->layout_request_issued())
+    return;
 
   // TODO: Detect ANRs
   DVLOG(1) << "SendViewTreeLayoutRequest";
   tree_state->view_tree()->OnLayout(
       base::Bind(&ViewRegistry::OnViewTreeLayoutResult, base::Unretained(this),
                  tree_state->GetWeakPtr()));
-}
-
-static bool IsSizeInBounds(mojo::ui::BoxConstraints* constraints,
-                           mojo::Size* size) {
-  return size && size->width >= constraints->min_width &&
-         size->width <= constraints->max_width &&
-         size->height >= constraints->min_height &&
-         size->height <= constraints->max_height;
+  tree_state->set_layout_request_pending(false);
+  tree_state->set_layout_request_issued(true);
 }
 
 void ViewRegistry::OnViewLayoutResult(base::WeakPtr<ViewState> view_state_weak,
@@ -682,14 +625,14 @@ void ViewRegistry::OnViewLayoutResult(base::WeakPtr<ViewState> view_state_weak,
       view_state->pending_layout_requests().begin());
 
   DVLOG(1) << "OnViewLayoutResult: view=" << view_state
-           << ", params=" << request->layout_params() << ", result=" << result;
+           << ", params=" << *request->layout_params() << ", result=" << result;
 
   // Validate the layout info.
   if (!IsSizeInBounds(request->layout_params()->constraints.get(),
                       result->size.get())) {
     LOG(ERROR) << "View returned invalid size in its layout info: "
                << "view=" << view_state
-               << ", params=" << request->layout_params()
+               << ", params=" << *request->layout_params()
                << ", result=" << result;
     UnregisterView(view_state);
     return;
@@ -713,11 +656,12 @@ void ViewRegistry::OnViewLayoutResult(base::WeakPtr<ViewState> view_state_weak,
     request->DispatchLayoutInfo(info.Pass());
   }
 
-  if (recurse) {
-    if (view_state->parent()) {
-      InvalidateLayoutForChild(view_state->parent(), view_state->key());
-    } else if (view_state->tree()) {
-      InvalidateLayoutForRoot(view_state->tree());
+  if (recurse && view_state->view_stub()) {
+    if (view_state->view_stub()->parent()) {
+      InvalidateLayoutForChild(view_state->view_stub()->parent(),
+                               view_state->view_stub()->key());
+    } else if (view_state->view_stub()->tree()) {
+      InvalidateLayoutForRoot(view_state->view_stub()->tree());
     }
   }
 
@@ -737,4 +681,26 @@ void ViewRegistry::OnViewTreeLayoutResult(
   }
 }
 
+void ViewRegistry::SendChildUnavailable(ViewState* parent_state,
+                                        uint32_t child_key) {
+  DCHECK(IsViewStateRegisteredDebug(parent_state));
+
+  // TODO: Detect ANRs
+  DVLOG(1) << "SendChildUnavailable: parent_state=" << parent_state
+           << ", child_key=" << child_key;
+  parent_state->view()->OnChildUnavailable(child_key,
+                                           base::Bind(&base::DoNothing));
+}
+
+void ViewRegistry::SendRootUnavailable(ViewTreeState* tree_state,
+                                       uint32_t root_key) {
+  DCHECK(IsViewTreeStateRegisteredDebug(tree_state));
+
+  // TODO: Detect ANRs
+  DVLOG(1) << "SendRootUnavailable: tree_state=" << tree_state
+           << ", root_key=" << root_key;
+  tree_state->view_tree()->OnRootUnavailable(root_key,
+                                             base::Bind(&base::DoNothing));
+}
+
 }  // namespace view_manager