views: Make Button default constructible

This CL is part of the effort to make Views default
constructible, allowing UI to be built in a declarative fashion.

* Default button listener to nullptr
* Add button listener setter
* Add unittest for setter

Bug: 1108460
Change-Id: I694acd0422b78951b1a7d17a90dcee3f8c8195d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2327150Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792960}

ui/views/controls/button/button.h

@@ -106,6 +106,8 @@ class VIEWS_EXPORT Button : public InkDropHostView,
   int tag() const { return tag_; }
   void set_tag(int tag) { tag_ = tag; }
 
+  void set_listener(ButtonListener* listener) { listener_ = listener; }
+
   void SetAccessibleName(const base::string16& name);
   const base::string16& GetAccessibleName() const;
 
@@ -233,7 +235,7 @@ class VIEWS_EXPORT Button : public InkDropHostView,
   // Construct the Button with a Listener. The listener can be null. This can be
   // true of buttons that don't have a listener - e.g. menubuttons where there's
   // no default action and checkboxes.
-  explicit Button(ButtonListener* listener);
+  explicit Button(ButtonListener* listener = nullptr);
 
   // Called when the button has been clicked or tapped and should request focus
   // if necessary.

ui/views/controls/button/button_unittest.cc

@@ -105,6 +105,7 @@ class TestButton : public Button, public ButtonListener {
   bool canceled() { return canceled_; }
   int ink_drop_layer_add_count() { return ink_drop_layer_add_count_; }
   int ink_drop_layer_remove_count() { return ink_drop_layer_remove_count_; }
+  ButtonListener* listener() const { return listener_; }
 
   void set_custom_key_click_action(KeyClickAction custom_key_click_action) {
     custom_key_click_action_ = custom_key_click_action;
@@ -179,6 +180,21 @@ class TestButtonObserver : public ButtonObserver {
   DISALLOW_COPY_AND_ASSIGN(TestButtonObserver);
 };
 
+class TestButtonListener : public ButtonListener {
+ public:
+  void ButtonPressed(Button* sender, const ui::Event& event) override {
+    pressed_ = true;
+    sender_ = sender;
+  }
+
+  bool pressed() const { return pressed_; }
+  Button* sender() const { return sender_; }
+
+ private:
+  bool pressed_ = false;
+  Button* sender_ = nullptr;
+};
+
 TestInkDrop* AddTestInkDrop(TestButton* button) {
   auto owned_ink_drop = std::make_unique<TestInkDrop>();
   TestInkDrop* ink_drop = owned_ink_drop.get();
@@ -747,6 +763,28 @@ TEST_F(ButtonTest, InkDropStaysHiddenWhileDragging) {
   SetDraggedView(nullptr);
 }
 
+// Ensure ButtonListener is dynamically settable.
+TEST_F(ButtonTest, SetListener) {
+  gfx::Point center(10, 10);
+  ButtonListener* old_listener = button()->listener();
+  auto listener = std::make_unique<TestButtonListener>();
+
+  button()->set_listener(listener.get());
+  EXPECT_EQ(listener.get(), button()->listener());
+
+  button()->OnMousePressed(ui::MouseEvent(
+      ui::ET_MOUSE_PRESSED, center, center, ui::EventTimeForNow(),
+      ui::EF_LEFT_MOUSE_BUTTON, ui::EF_LEFT_MOUSE_BUTTON));
+  // Default button controller notifies listener at mouse release.
+  button()->OnMouseReleased(ui::MouseEvent(
+      ui::ET_MOUSE_RELEASED, center, center, ui::EventTimeForNow(),
+      ui::EF_LEFT_MOUSE_BUTTON, ui::EF_LEFT_MOUSE_BUTTON));
+  EXPECT_TRUE(listener->pressed());
+  EXPECT_EQ(button(), listener->sender());
+
+  button()->set_listener(old_listener);
+}
+
 // VisibilityTestButton tests to see if an ink drop or a layer has been added to
 // the button at any point during the visibility state changes of its Widget.
 class VisibilityTestButton : public TestButton {