Improve ComponentBase and Container::Tab Focusable implementations (#341)

- Provide better defaults for ComponentBase `Focusable()` and
  `ActiveChild()` methods. This resolves:
  https://github.com/ArthurSonzogni/FTXUI/issues/335

- Implement `Container::Tab` 's  `Focusable()` methods. This prevents
  the users to navigate into a tab with no interactivity.

CHANGELOG.md

@@ -27,6 +27,9 @@ Element gaugeDirection(float ratio, GaugeDirection);
 ### Component
 - Support SIGTSTP. (ctrl+z).
 - Support task posting. `ScreenInteractive::Post(Task)`.
+- **bugfix** Container::Tab implements `Focusable()`.
+- **bugfix** Improved default implementations of ComponentBase `Focusable()` and
+  `ActiveChild()` methods.
 
 2.0.0
 -----

src/ftxui/component/component.cpp

@@ -105,7 +105,11 @@ bool ComponentBase::OnEvent(Event event) {
 /// @return the currently Active child.
 /// @ingroup component
 Component ComponentBase::ActiveChild() {
-  return children_.empty() ? nullptr : children_.front();
+  for (auto& child : children_) {
+    if (child->Focusable())
+      return child;
+  }
+  return nullptr;
 }
 
 /// @brief Return true when the component contains focusable elements.
@@ -128,14 +132,15 @@ bool ComponentBase::Active() const {
 
 /// @brief Returns if the elements if focused by the user.
 /// True when the ComponentBase is focused by the user. An element is Focused
-/// when it is with all its ancestors the ActiveChild() of their parents.
+/// when it is with all its ancestors the ActiveChild() of their parents, and it
+/// Focusable().
 /// @ingroup component
 bool ComponentBase::Focused() const {
   auto current = this;
   while (current && current->Active()) {
     current = current->parent_;
   }
-  return !current;
+  return !current && Focusable();
 }
 
 /// @brief Make the |child| to be the "active" one.

src/ftxui/component/component_test.cpp

@@ -3,6 +3,7 @@
 #include <memory>  // for shared_ptr, __shared_ptr_access, allocator, make_shared
 
 #include "ftxui/component/captured_mouse.hpp"  // for ftxui
+#include "ftxui/component/component.hpp"       // for Make
 #include "ftxui/component/component_base.hpp"  // for ComponentBase, Component
 #include "gtest/gtest_pred_impl.h"  // for EXPECT_EQ, Test, SuiteApiResolver, TEST, TestFactoryImpl
 
@@ -155,6 +156,22 @@ TEST(ContainerTest, ChildAt) {
   EXPECT_EQ(parent->ChildAt(0u), child_2);
 }
 
+TEST(ComponentTest, NonFocusableAreNotFocused) {
+  class NonFocusable : public ComponentBase {
+    bool Focusable() const override { return false; }
+  };
+  auto root = Make<NonFocusable>();
+  EXPECT_FALSE(root->Focused());
+  EXPECT_EQ(root->ActiveChild(), nullptr);
+
+  auto child = Make<NonFocusable>();
+  root->Add(child);
+  EXPECT_FALSE(root->Focused());
+  EXPECT_FALSE(child->Focused());
+  EXPECT_EQ(root->ActiveChild(), nullptr);
+  EXPECT_EQ(child->ActiveChild(), nullptr);
+}
+
 // 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.

src/ftxui/component/container.cpp

@@ -190,6 +190,12 @@ class TabContainer : public ContainerBase {
     return text("Empty container");
   }
 
+  bool Focusable() const override {
+    if (children_.size() == 0)
+      return false;
+    return children_[*selector_ % children_.size()]->Focusable();
+  }
+
   bool OnMouseEvent(Event event) override {
     return ActiveChild()->OnEvent(event);
   }

src/ftxui/component/container_test.cpp

@@ -305,6 +305,34 @@ TEST(ContainerTest, TakeFocus) {
   EXPECT_FALSE(c23->Active());
 }
 
+TEST(ContainerTest, TabFocusable) {
+  int selected = 0;
+  auto c = Container::Tab(
+      {
+          Focusable(),
+          NonFocusable(),
+          Focusable(),
+          NonFocusable(),
+      },
+      &selected);
+
+  selected = 0;
+  EXPECT_TRUE(c->Focusable());
+  EXPECT_TRUE(c->Focused());
+
+  selected = 1;
+  EXPECT_FALSE(c->Focusable());
+  EXPECT_FALSE(c->Focused());
+
+  selected = 2;
+  EXPECT_TRUE(c->Focusable());
+  EXPECT_TRUE(c->Focused());
+
+  selected = 3;
+  EXPECT_FALSE(c->Focusable());
+  EXPECT_FALSE(c->Focused());
+}
+
 // 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.