13 Mar 2018

Design a responsive radar view with libyue & observer pattern

Introduction to libyue usage based on C++

[ libyue  demo  observer  c++  ]


Usually a simple GUI tool can give you direct visual status about your dev system, and can offer great help during development.

This article demonstrates a GUI demo which fetches latest sensor data from radar & sonar sensors to draw.

The final UI is showed as follows. It’s not beautiful actually, but it can helps. You can find the whole sample at github

Introduction to libyue

For detail documents, pls refer to libyue doc

  • libyue is a light-weight gui framework which you can design cross-platform gui painlessly.
  • linux version is based on gtk, and mac version is based on native cocoa framework.
  • It deeply relies on event & delegate design pattern, so you can use c++ lamda function to connect custom reaction to ui event easily.
  • It uses Facebook Yoga as layout engine, and you can change view layout easily through call of SetStyle function.

Observer pattern

Reference to Wikipedia:

The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.

Using observer pattern, we donnot need to check if data was updated. We are notified with latest data instead. Then we can make a data-driven ui more responsive.

In this article, we use observable lib which can be found at github, and you can find detail instructions in it.

Data model

We design a test model which generate radar test data every second.

class TestModel
    // data-driven heartbeat
    observable_property<bool> dataHB { false };

  typedef std::vector<ObjectInfo_77> objects_t;

    std::srand(std::time(nullptr)); // use current time as seed for random generator

    pAmpUpdateTh = new std::thread(&TestModel::gen_amp, this);



  SonarData m_sonar;
  std::unordered_map<int, objects_t> m_objs_map;

  std::thread *pAmpUpdateTh;
  bool _toggle = false;

  void gen_amp(void)
      int cnt = std::rand() % 10;
      objects_t front, rear;
      for (int i = 0; i < cnt; ++i)
          float Range = 0.1f * (std::rand() % 2000);  // cm
          float Azimuth = ((std::rand() % 180) - 90) * pi / 180;        
      m_objs_map[0] = front;

      cnt = std::rand() % 10;
      for (int i = 0; i < cnt; ++i)
          float Range = 0.1f * (std::rand() % 2000);  // cm
          float Azimuth = ((std::rand() % 180) - 90) * pi / 180;        
      m_objs_map[1] = rear;
      m_sonar = {
        0.1f * (std::rand() % 2000),
        0.1f * (std::rand() % 2000)

      _toggle = !_toggle;

      dataHB = _toggle;


Draw with yue

basic 2D drawing

A simple example listed below demonstrates how to draw a radar point.

void drawRadarObstacle(nu::Painter *painter, nu::PointF center, float radius)

  painter->SetStrokeColor(nu::Color("#550000")); //"#550000"
  painter->SetFillColor(nu::Color("#DD0000"));  //"#DD0000"

  painter->Arc(center, radius, 0, 2*pi);



redraw with callback

We use container view to manage radar view redraw procedure.

It consists of three steps:

  1. draw a rectangle car body
  2. draw sonar arc view
  3. draw radar point view

During the every redraw, we get the latest sensor data from model object.

  scoped_refptr<nu::Container> radar_view(new nu::Container);
  radar_view->SetStyle("position", "absolute", "width", window_width, "height", window_height, "top", 0, "right", 0);

  radar_view->on_draw.Connect([&](nu::Container* self, nu::Painter* painter, const nu::RectF& dirty){
    updateSonarArc(painter, &model);  
    updateRadarDetectedObjects(painter, &model);


In this article, we try to use libyue to design a simple GUI and it workes well. It can be applied to large GUI system also. You can have a try.

Hope you a nice journey!