mus: Fix handled status in UI event ack, add MessageLoop::NestingObserver

base/message_loop/message_loop_test.cc

Index: base/message_loop/message_loop_test.cc
diff --git a/base/message_loop/message_loop_test.cc b/base/message_loop/message_loop_test.cc
index c0e6481c4209b785a9bfee60800b63b5f73080d1..0f1e924518cbdcb78d1bc46924d66dcc874d6139 100644
--- a/base/message_loop/message_loop_test.cc
+++ b/base/message_loop/message_loop_test.cc
@@ -378,6 +378,66 @@ void RunTest_Nesting(MessagePumpFactory factory) {
   EXPECT_EQ(depth, 0);
 }
 
+// A NestingObserver that tracks the number of nested message loop starts it
+// has seen.
+class TestNestingObserver : public MessageLoop::NestingObserver {
+ public:
+  TestNestingObserver() {}
+  ~TestNestingObserver() override {}
+
+  int begin_nested_loop_count() const { return begin_nested_loop_count_; }
+
+  // MessageLoop::NestingObserver:
+  void OnBeginNestedMessageLoop() override { begin_nested_loop_count_++; }
+
+ private:
+  int begin_nested_loop_count_ = 0;
+
+  DISALLOW_COPY_AND_ASSIGN(TestNestingObserver);
+};
+
+void ExpectOneBeginNestedLoop(TestNestingObserver* observer) {
+  EXPECT_EQ(1, observer->begin_nested_loop_count());
+}
+
+// Starts a nested message loop.
+void RunNestedLoop(TestNestingObserver* observer,
+                   const Closure& quit_outer_loop) {
+  // The nested loop hasn't started yet.
+  EXPECT_EQ(0, observer->begin_nested_loop_count());
+
+  MessageLoop::ScopedNestableTaskAllower allow(MessageLoop::current());
+  RunLoop nested_loop;
+  // Verify that by the time the first task is run the observer has seen the
+  // message loop begin.
+  MessageLoop::current()->PostTask(FROM_HERE,
+                                   Bind(&ExpectOneBeginNestedLoop, observer));
+  MessageLoop::current()->PostTask(FROM_HERE, nested_loop.QuitClosure());
+  nested_loop.Run();
+
+  // Quitting message loops doesn't change the begin count.
+  EXPECT_EQ(1, observer->begin_nested_loop_count());
+
+  quit_outer_loop.Run();
+}
+
+// Tests that a NestingObserver is notified when a nested message loop begins.
+void RunTest_NestingObserver(MessagePumpFactory factory) {
+  std::unique_ptr<MessagePump> pump(factory());
+  MessageLoop outer_loop(std::move(pump));
+
+  // Observe the outer loop for nested message loops beginning.
+  TestNestingObserver nesting_observer;
+  outer_loop.AddNestingObserver(&nesting_observer);
+
+  // Post a task that runs a nested message loop.
+  outer_loop.PostTask(FROM_HERE, Bind(&RunNestedLoop, &nesting_observer,
+                                      outer_loop.QuitWhenIdleClosure()));
+  outer_loop.Run();
+
+  outer_loop.RemoveNestingObserver(&nesting_observer);
+}
+
 enum TaskType {
   MESSAGEBOX,
   ENDDIALOG,