Add TakeFocus and SetActiveChild.

This allows developers to set child children component must be the
currently active/focused one.

This can be used to "control" where the focus is, without user
interactions.

src/ftxui/component/component.cpp

@@ -58,9 +58,16 @@ Component* Component::ActiveChild() {
   return children_.empty() ? nullptr : children_.front();
 }
 
+/// @brief Returns if the element if the currently active child of its parent.
+/// @ingroup component
+bool Component::Active() {
+  return !parent_ || parent_->ActiveChild() == this;
+}
+
 /// @brief Returns if the elements if focused by the user.
 /// True when the Component is focused by the user. An element is Focused when
 /// it is with all its ancestors the ActiveChild() of their parents.
+/// @ingroup component
 bool Component::Focused() {
   Component* current = this;
   for (;;) {
@@ -73,6 +80,23 @@ bool Component::Focused() {
   }
 }
 
+/// @brief Make the |child| to be the "active" one.
+/// @argument child the child to become active.
+/// @ingroup component
+void Component::SetActiveChild(Component*) {}
+
+/// @brief Configure all the ancestors to give focus to this component.
+/// @ingroup component
+void Component::TakeFocus() {
+  Component* child = this;
+  Component* parent = parent_;
+  while (parent) {
+    parent->SetActiveChild(child);
+    child = parent;
+    parent = parent->parent_;
+  }
+}
+
 /// @brief Detach this children from its parent.
 /// @see Attach
 /// @see Detach

src/ftxui/component/container.cpp

@@ -47,6 +47,15 @@ Component* Container::ActiveChild() {
   return children_[selected % children_.size()];
 }
 
+void Container::SetActiveChild(Component* child) {
+  for(size_t i = 0; i < children_.size(); ++i) {
+    if (children_[i] == child) {
+      (selector_ ? *selector_ : selected_) = i;
+      return;
+    }
+  }
+}
+
 bool Container::VerticalEvent(Event event) {
   int old_selected = selected_;
   if (event == Event::ArrowUp || event == Event::Character('k'))

src/ftxui/component/container_test.cpp

@@ -75,6 +75,211 @@ TEST(ContainerTest, HorizontalEvent) {
   container.OnEvent(Event::TabReverse);
 }
 
+TEST(ContainerTest, VerticalEvent) {
+  auto container = Container::Vertical();
+  Component c0, c1, c2;
+  container.Add(&c0);
+  container.Add(&c1);
+  container.Add(&c2);
+
+  // With arrow key.
+  EXPECT_EQ(container.ActiveChild(), &c0);
+  container.OnEvent(Event::ArrowDown);
+  EXPECT_EQ(container.ActiveChild(), &c1);
+  container.OnEvent(Event::ArrowDown);
+  EXPECT_EQ(container.ActiveChild(), &c2);
+  container.OnEvent(Event::ArrowDown);
+  EXPECT_EQ(container.ActiveChild(), &c2);
+  container.OnEvent(Event::ArrowUp);
+  EXPECT_EQ(container.ActiveChild(), &c1);
+  container.OnEvent(Event::ArrowUp);
+  EXPECT_EQ(container.ActiveChild(), &c0);
+  container.OnEvent(Event::ArrowUp);
+  EXPECT_EQ(container.ActiveChild(), &c0);
+
+  // With arrow key in the wrong dimension.
+  container.OnEvent(Event::ArrowLeft);
+  EXPECT_EQ(container.ActiveChild(), &c0);
+  container.OnEvent(Event::ArrowRight);
+  EXPECT_EQ(container.ActiveChild(), &c0);
+
+  // With vim like characters.
+  EXPECT_EQ(container.ActiveChild(), &c0);
+  container.OnEvent(Event::Character('j'));
+  EXPECT_EQ(container.ActiveChild(), &c1);
+  container.OnEvent(Event::Character('j'));
+  EXPECT_EQ(container.ActiveChild(), &c2);
+  container.OnEvent(Event::Character('j'));
+  EXPECT_EQ(container.ActiveChild(), &c2);
+  container.OnEvent(Event::Character('k'));
+  EXPECT_EQ(container.ActiveChild(), &c1);
+  container.OnEvent(Event::Character('k'));
+  EXPECT_EQ(container.ActiveChild(), &c0);
+  container.OnEvent(Event::Character('k'));
+  EXPECT_EQ(container.ActiveChild(), &c0);
+
+  // With vim like characters in the wrong direction.
+  container.OnEvent(Event::Character('h'));
+  EXPECT_EQ(container.ActiveChild(), &c0);
+  container.OnEvent(Event::Character('l'));
+  EXPECT_EQ(container.ActiveChild(), &c0);
+
+  // With tab characters.
+  container.OnEvent(Event::Tab);
+  EXPECT_EQ(container.ActiveChild(), &c1);
+  container.OnEvent(Event::Tab);
+  EXPECT_EQ(container.ActiveChild(), &c2);
+  container.OnEvent(Event::Tab);
+  EXPECT_EQ(container.ActiveChild(), &c0);
+  container.OnEvent(Event::Tab);
+  EXPECT_EQ(container.ActiveChild(), &c1);
+  container.OnEvent(Event::Tab);
+  EXPECT_EQ(container.ActiveChild(), &c2);
+  container.OnEvent(Event::TabReverse);
+  EXPECT_EQ(container.ActiveChild(), &c1);
+  container.OnEvent(Event::TabReverse);
+  EXPECT_EQ(container.ActiveChild(), &c0);
+  container.OnEvent(Event::TabReverse);
+  EXPECT_EQ(container.ActiveChild(), &c2);
+  container.OnEvent(Event::TabReverse);
+  EXPECT_EQ(container.ActiveChild(), &c1);
+  container.OnEvent(Event::TabReverse);
+}
+
+TEST(ContainerTest, SetActiveChild) {
+  auto container = Container::Horizontal();
+  Component c0, c1, c2;
+  container.Add(&c0);
+  container.Add(&c1);
+  container.Add(&c2);
+
+  EXPECT_EQ(container.ActiveChild(), &c0);
+  EXPECT_TRUE(c0.Focused());
+  EXPECT_TRUE(c0.Active());
+  EXPECT_FALSE(c1.Focused());
+  EXPECT_FALSE(c1.Active());
+  EXPECT_FALSE(c2.Focused());
+  EXPECT_FALSE(c2.Active());
+
+  container.SetActiveChild(&c0);
+  EXPECT_EQ(container.ActiveChild(), &c0);
+  EXPECT_TRUE(c0.Focused());
+  EXPECT_TRUE(c0.Active());
+  EXPECT_FALSE(c1.Focused());
+  EXPECT_FALSE(c1.Active());
+  EXPECT_FALSE(c2.Focused());
+  EXPECT_FALSE(c2.Active());
+
+  container.SetActiveChild(&c1);
+  EXPECT_EQ(container.ActiveChild(), &c1);
+  EXPECT_FALSE(c0.Focused());
+  EXPECT_FALSE(c0.Active());
+  EXPECT_TRUE(c1.Focused());
+  EXPECT_TRUE(c1.Active());
+  EXPECT_FALSE(c2.Focused());
+  EXPECT_FALSE(c2.Active());
+
+  container.SetActiveChild(&c2);
+  EXPECT_EQ(container.ActiveChild(), &c2);
+  EXPECT_FALSE(c0.Focused());
+  EXPECT_FALSE(c0.Active());
+  EXPECT_FALSE(c1.Focused());
+  EXPECT_FALSE(c1.Active());
+  EXPECT_TRUE(c2.Focused());
+  EXPECT_TRUE(c2.Active());
+
+  container.SetActiveChild(&c0);
+  EXPECT_EQ(container.ActiveChild(), &c0);
+  EXPECT_TRUE(c0.Focused());
+  EXPECT_TRUE(c0.Active());
+  EXPECT_FALSE(c1.Focused());
+  EXPECT_FALSE(c1.Active());
+  EXPECT_FALSE(c2.Focused());
+  EXPECT_FALSE(c2.Active());
+}
+
+TEST(ContainerTest, TakeFocus) {
+  auto c= Container::Horizontal();
+  auto c1 = Container::Vertical();
+  auto c2 = Container::Vertical();
+  auto c3 = Container::Vertical();
+  auto c11 = Container::Horizontal();
+  auto c12 = Container::Horizontal();
+  auto c13 = Container::Horizontal();
+  auto c21 = Container::Horizontal();
+  auto c22 = Container::Horizontal();
+  auto c23 = Container::Horizontal();
+
+  c.Add(&c1);
+  c.Add(&c2);
+  c.Add(&c3);
+  c1.Add(&c11);
+  c1.Add(&c12);
+  c1.Add(&c13);
+  c2.Add(&c21);
+  c2.Add(&c22);
+  c2.Add(&c23);
+
+  EXPECT_TRUE(c.Focused());
+  EXPECT_TRUE(c1.Focused());
+  EXPECT_FALSE(c2.Focused());
+  EXPECT_TRUE(c11.Focused());
+  EXPECT_FALSE(c12.Focused());
+  EXPECT_FALSE(c13.Focused());
+  EXPECT_FALSE(c21.Focused());
+  EXPECT_FALSE(c22.Focused());
+  EXPECT_FALSE(c23.Focused());
+  EXPECT_TRUE(c.Active());
+  EXPECT_TRUE(c1.Active());
+  EXPECT_FALSE(c2.Active());
+  EXPECT_TRUE(c11.Active());
+  EXPECT_FALSE(c12.Active());
+  EXPECT_FALSE(c13.Active());
+  EXPECT_TRUE(c21.Active());
+  EXPECT_FALSE(c22.Active());
+  EXPECT_FALSE(c23.Active());
+
+  c22.TakeFocus();
+  EXPECT_TRUE(c.Focused());
+  EXPECT_FALSE(c1.Focused());
+  EXPECT_TRUE(c2.Focused());
+  EXPECT_FALSE(c11.Focused());
+  EXPECT_FALSE(c12.Focused());
+  EXPECT_FALSE(c13.Focused());
+  EXPECT_FALSE(c21.Focused());
+  EXPECT_TRUE(c22.Focused());
+  EXPECT_FALSE(c23.Focused());
+  EXPECT_TRUE(c.Active());
+  EXPECT_FALSE(c1.Active());
+  EXPECT_TRUE(c2.Active());
+  EXPECT_TRUE(c11.Active());
+  EXPECT_FALSE(c12.Active());
+  EXPECT_FALSE(c13.Active());
+  EXPECT_FALSE(c21.Active());
+  EXPECT_TRUE(c22.Active());
+  EXPECT_FALSE(c23.Active());
+
+  c1.TakeFocus();
+  EXPECT_TRUE(c.Focused());
+  EXPECT_TRUE(c1.Focused());
+  EXPECT_FALSE(c2.Focused());
+  EXPECT_TRUE(c11.Focused());
+  EXPECT_FALSE(c12.Focused());
+  EXPECT_FALSE(c13.Focused());
+  EXPECT_FALSE(c21.Focused());
+  EXPECT_FALSE(c22.Focused());
+  EXPECT_FALSE(c23.Focused());
+  EXPECT_TRUE(c.Active());
+  EXPECT_TRUE(c1.Active());
+  EXPECT_FALSE(c2.Active());
+  EXPECT_TRUE(c11.Active());
+  EXPECT_FALSE(c12.Active());
+  EXPECT_FALSE(c13.Active());
+  EXPECT_FALSE(c21.Active());
+  EXPECT_TRUE(c22.Active());
+  EXPECT_FALSE(c23.Active());
+}
+
 // Copyright 2020 Arthur Sonzogni. All rights reserved.
 // Use of this source code is governed by the MIT license that can be found in
 // the LICENSE file.