MyMagnumRenderer.h

#ifndef MyMagnumRenderer_guard
#define MyMagnumRenderer_guard
#include <Magnum/GL/Buffer.h>
#include <Magnum/GL/DefaultFramebuffer.h>
#include <Magnum/GL/Mesh.h>
#include <Magnum/Platform/GLContext.h>
#include <Magnum/Shaders/VertexColor.h>

using namespace Magnum;
using namespace Math::Literals;

struct TriangleVertex {
    Vector2 position;
    Color3 color;
};

class MyMagnumRenderer
{
public:
    Platform::GLContext* shared_ctx = NULL;
    ~MyMagnumRenderer();
    void activateMagnum();
    void drawEvent();
private:
    const TriangleVertex data[3]{
        { { -0.5f, -0.5f }, 0xff0000_rgbf },    /* Left vertex, red color */
        { { 0.5f, -0.5f }, 0x00ff00_rgbf },    /* Right vertex, green color */
        { { 0.0f,  0.5f }, 0x0000ff_rgbf }     /* Top vertex, blue color */
    };

    GL::Buffer _buffer;
    GL::Mesh _mesh;
    Shaders::VertexColor2D _shader;
};

MyMagnumRenderer::~MyMagnumRenderer()
{
delete shared_ctx;
}

void MyMagnumRenderer::activateMagnum() {
shared_ctx = new Platform::GLContext();

_buffer.setData(data, GL::BufferUsage::StaticDraw);
_mesh.setPrimitive(GL::MeshPrimitive::Triangles)
.setCount(3)
.addVertexBuffer(_buffer, 0,
    Shaders::VertexColor2D::Position{},
    Shaders::VertexColor2D::Color{ Shaders::VertexColor2D::Color::Components::Three });
}

void MyMagnumRenderer::drawEvent() {
    GL::defaultFramebuffer.clear(GL::FramebufferClear::Color);

    _mesh.draw(_shader);
    }
#endif