Introduce Loop. (#476)

It can be used to give developers a better control on the loop. Users
can use it not to take full control of the thread, and poll FTXUI from
time to time as part of an external loop.

This resolves: https://github.com/ArthurSonzogni/FTXUI/issues/474

src/ftxui/component/screen_interactive.cpp

@@ -20,6 +20,7 @@
 #include "ftxui/component/captured_mouse.hpp"  // for CapturedMouse, CapturedMouseInterface
 #include "ftxui/component/component_base.hpp"  // for ComponentBase
 #include "ftxui/component/event.hpp"           // for Event
+#include "ftxui/component/loop.hpp"            // for Loop
 #include "ftxui/component/receiver.hpp"  // for Sender, ReceiverImpl, MakeReceiver, SenderImpl, Receiver
 #include "ftxui/component/screen_interactive.hpp"
 #include "ftxui/component/terminal_input_parser.hpp"  // for TerminalInputParser
@@ -350,8 +351,8 @@ void ScreenInteractive::RequestAnimationFrame() {
   animation_requested_ = true;
   auto now = animation::Clock::now();
   const auto time_histeresis = std::chrono::milliseconds(33);
-  if (now - previous_animation_time >= time_histeresis) {
-    previous_animation_time = now;
+  if (now - previous_animation_time_ >= time_histeresis) {
+    previous_animation_time_ = now;
   }
 }
 
@@ -365,6 +366,15 @@ CapturedMouse ScreenInteractive::CaptureMouse() {
 }
 
 void ScreenInteractive::Loop(Component component) {  // NOLINT
+  class Loop loop(this, component);
+  loop.Run();
+}
+
+bool ScreenInteractive::HasQuitted() {
+  return task_receiver_->HasQuitted();
+}
+
+void ScreenInteractive::PreMain() {
   // Suspend previously active screen:
   if (g_active_screen) {
     std::swap(suspended_screen_, g_active_screen);
@@ -378,7 +388,11 @@ void ScreenInteractive::Loop(Component component) {  // NOLINT
   // This screen is now active:
   g_active_screen = this;
   g_active_screen->Install();
-  g_active_screen->Main(std::move(component));
+
+  previous_animation_time_ = animation::Clock::now();
+}
+
+void ScreenInteractive::PostMain() {
   g_active_screen->Uninstall();
   g_active_screen = nullptr;
 
@@ -531,81 +545,78 @@ void ScreenInteractive::Uninstall() {
 }
 
 // NOLINTNEXTLINE
-void ScreenInteractive::Main(Component component) {
-  previous_animation_time = animation::Clock::now();
-
-  auto draw = [&] {
-    Draw(component);
-    std::cout << ToString() << set_cursor_position;
-    Flush();
-    Clear();
-  };
-
-  bool attempt_draw = true;
-  while (!quit_) {
-    if (attempt_draw && !task_receiver_->HasPending()) {
-      draw();
-      attempt_draw = false;
-    }
-
-    Task task;
-    if (!task_receiver_->Receive(&task)) {
-      break;
-    }
-
-    // clang-format off
-    std::visit([&](auto&& arg) {
-      using T = std::decay_t<decltype(arg)>;
-
-      // Handle Event.
-      if constexpr (std::is_same_v<T, Event>) {
-        if (arg.is_cursor_reporting()) {
-          cursor_x_ = arg.cursor_x();
-          cursor_y_ = arg.cursor_y();
-          return;
-        }
-
-        if (arg.is_mouse()) {
-          arg.mouse().x -= cursor_x_;
-          arg.mouse().y -= cursor_y_;
-        }
-
-        arg.screen_ = this;
-        component->OnEvent(arg);
-        attempt_draw = true;
-        return;
-      }
-
-      // Handle callback
-      if constexpr (std::is_same_v<T, Closure>) {
-        arg();
-        return;
-      }
-
-      // Handle Animation
-      if constexpr (std::is_same_v<T, AnimationTask>) {
-        if (!animation_requested_) {
-          return;
-        }
-
-        animation_requested_ = false;
-        animation::TimePoint now = animation::Clock::now();
-        animation::Duration delta = now - previous_animation_time;
-        previous_animation_time = now;
-
-        animation::Params params(delta);
-        component->OnAnimation(params);
-        attempt_draw = true;
-        return;
-      }
-    },
-    task);
-    // clang-format on
+void ScreenInteractive::RunOnceBlocking(Component component) {
+  Task task;
+  if (task_receiver_->Receive(&task)) {
+    HandleTask(component, task);
   }
+
+  RunOnce(component);
+}
+
+void ScreenInteractive::RunOnce(Component component) {
+  Task task;
+  while (task_receiver_->ReceiveNonBlocking(&task)) {
+    HandleTask(component, task);
+  }
+  Draw(component);
+}
+
+void ScreenInteractive::HandleTask(Component component, Task& task) {
+  // clang-format off
+  std::visit([&](auto&& arg) {
+    using T = std::decay_t<decltype(arg)>;
+
+    // Handle Event.
+    if constexpr (std::is_same_v<T, Event>) {
+      if (arg.is_cursor_reporting()) {
+        cursor_x_ = arg.cursor_x();
+        cursor_y_ = arg.cursor_y();
+        return;
+      }
+
+      if (arg.is_mouse()) {
+        arg.mouse().x -= cursor_x_;
+        arg.mouse().y -= cursor_y_;
+      }
+
+      arg.screen_ = this;
+      component->OnEvent(arg);
+      frame_valid_ = false;
+      return;
+    }
+
+    // Handle callback
+    if constexpr (std::is_same_v<T, Closure>) {
+      arg();
+      return;
+    }
+
+    // Handle Animation
+    if constexpr (std::is_same_v<T, AnimationTask>) {
+      if (!animation_requested_) {
+        return;
+      }
+
+      animation_requested_ = false;
+      animation::TimePoint now = animation::Clock::now();
+      animation::Duration delta = now - previous_animation_time_;
+      previous_animation_time_ = now;
+
+      animation::Params params(delta);
+      component->OnAnimation(params);
+      frame_valid_ = false;
+      return;
+    }
+  },
+  task);
+  // clang-format on
 }
 
 // NOLINTNEXTLINE
 void ScreenInteractive::Draw(Component component) {
+  if (frame_valid_)
+    return;
   auto document = component->Render();
   int dimx = 0;
   int dimy = 0;
@@ -685,13 +696,22 @@ void ScreenInteractive::Draw(Component component) {
     set_cursor_position += "\x1B[" + std::to_string(dy) + "A";
     reset_cursor_position += "\x1B[" + std::to_string(dy) + "B";
   }
+
+  std::cout << ToString() << set_cursor_position;
+  Flush();
+  Clear();
+  frame_valid_ = true;
 }
 
 Closure ScreenInteractive::ExitLoopClosure() {
-  return [this] {
+  return [this] { Exit(); };
+}
+
+void ScreenInteractive::Exit() {
+  Post([this] {
     quit_ = true;
     task_sender_.reset();
-  };
+  });
 }
 
 void ScreenInteractive::SigStop() {