Dart: Wait to handle events on a Stub until it makes sense to do it.

mojo/dart/packages/mojo_services/lib/mojo/ui/views.mojom.dart

Index: mojo/dart/packages/mojo_services/lib/mojo/ui/views.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/ui/views.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/ui/views.mojom.dart
index 57bf9d0273f55cae19c53c49cb5a3dcd3bff4de8..0c52d2d82d3b7befb8f03a5ef11573b17ecc9040 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/ui/views.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/ui/views.mojom.dart
@@ -684,14 +684,19 @@ class ViewProxy implements bindings.ProxyBase {
 
 
 class ViewStub extends bindings.Stub {
-  View _impl = null;
+  View _impl;
 
   ViewStub.fromEndpoint(
-      core.MojoMessagePipeEndpoint endpoint, [this._impl])
-      : super.fromEndpoint(endpoint);
+      core.MojoMessagePipeEndpoint endpoint, [View impl])
+      : super.fromEndpoint(endpoint, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
-  ViewStub.fromHandle(core.MojoHandle handle, [this._impl])
-      : super.fromHandle(handle);
+  ViewStub.fromHandle(
+      core.MojoHandle handle, [View impl])
+      : super.fromHandle(handle, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
   ViewStub.unbound() : super.unbound();
 
@@ -714,7 +719,9 @@ class ViewStub extends bindings.Stub {
                                                           0,
                                                           message);
     }
-    assert(_impl != null);
+    if (_impl == null) {
+      throw new core.MojoApiError("$this has no implementation set");
+    }
     switch (message.header.type) {
       case _viewMethodGetTokenName:
         var response = _impl.getToken(_viewGetTokenResponseParamsFactory);
@@ -760,10 +767,23 @@ class ViewStub extends bindings.Stub {
 
   View get impl => _impl;
   set impl(View d) {
-    assert(_impl == null);
+    if (d == null) {
+      throw new core.MojoApiError("$this: Cannot set a null implementation");
+    }
+    if (isBound && (_impl == null)) {
+      beginHandlingEvents();
+    }
     _impl = d;
   }
 
+  @override
+  void bind(core.MojoMessagePipeEndpoint endpoint) {
+    super.bind(endpoint);
+    if (!isOpen && (_impl != null)) {
+      beginHandlingEvents();
+    }
+  }
+
   String toString() {
     var superString = super.toString();
     return "ViewStub($superString)";
@@ -930,14 +950,19 @@ class ViewListenerProxy implements bindings.ProxyBase {
 
 
 class ViewListenerStub extends bindings.Stub {
-  ViewListener _impl = null;
+  ViewListener _impl;
 
   ViewListenerStub.fromEndpoint(
-      core.MojoMessagePipeEndpoint endpoint, [this._impl])
-      : super.fromEndpoint(endpoint);
+      core.MojoMessagePipeEndpoint endpoint, [ViewListener impl])
+      : super.fromEndpoint(endpoint, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
-  ViewListenerStub.fromHandle(core.MojoHandle handle, [this._impl])
-      : super.fromHandle(handle);
+  ViewListenerStub.fromHandle(
+      core.MojoHandle handle, [ViewListener impl])
+      : super.fromHandle(handle, autoBegin: impl != null) {
+    _impl = impl;
+  }
 
   ViewListenerStub.unbound() : super.unbound();
 
@@ -959,7 +984,9 @@ class ViewListenerStub extends bindings.Stub {
                                                           0,
                                                           message);
     }
-    assert(_impl != null);
+    if (_impl == null) {
+      throw new core.MojoApiError("$this has no implementation set");
+    }
     switch (message.header.type) {
       case _viewListenerMethodOnPropertiesChangedName:
         var params = _ViewListenerOnPropertiesChangedParams.deserialize(
@@ -992,10 +1019,23 @@ class ViewListenerStub extends bindings.Stub {
 
   ViewListener get impl => _impl;
   set impl(ViewListener d) {
-    assert(_impl == null);
+    if (d == null) {
+      throw new core.MojoApiError("$this: Cannot set a null implementation");
+    }
+    if (isBound && (_impl == null)) {
+      beginHandlingEvents();
+    }
     _impl = d;
   }
 
+  @override
+  void bind(core.MojoMessagePipeEndpoint endpoint) {
+    super.bind(endpoint);
+    if (!isOpen && (_impl != null)) {
+      beginHandlingEvents();
+    }
+  }
+
   String toString() {
     var superString = super.toString();
     return "ViewListenerStub($superString)";