MonoGame.Extended/Source/Demos/StarWarrior/Systems/EnemySpawnSystem.cs

// Original code dervied from:
// https://github.com/thelinuxlich/starwarrior_CSharp/blob/master/StarWarrior/StarWarrior/Systems/EnemySpawnSystem.cs

// --------------------------------------------------------------------------------------------------------------------
// <copyright file="EnemySpawnSystem.cs" company="GAMADU.COM">
//     Copyright © 2013 GAMADU.COM. All rights reserved.
//
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//        2. Redistributions in binary form must reproduce the above copyright notice, this list
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// </copyright>
// <summary>
//   The enemy spawn system.
// </summary>
// --------------------------------------------------------------------------------------------------------------------

using System;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using MonoGame.Extended;
using MonoGame.Extended.Entities.Systems;
using StarWarrior.Components;

namespace StarWarrior.Systems
{
    public class EnemySpawnSystem : UpdateSystem
    {
        private readonly GraphicsDevice _graphicsDevice;
        private readonly EntityFactory _entityFactory;
        private readonly Random _random = new Random();

        public EnemySpawnSystem(GraphicsDevice graphicsDevice, EntityFactory entityFactory)
        {
            _graphicsDevice = graphicsDevice;
            _entityFactory = entityFactory;
        }

        public override void Update(GameTime gameTime)
        {
            var viewport = _graphicsDevice.Viewport;
            var entity = _entityFactory.CreateEnemyShip();
            var transform = entity.Get<Transform2>();

            Vector2 position;
            position.X = _random.Next(viewport.Width);
            position.Y = _random.Next(400) + 50;
            transform.Position = position;

            var physics = entity.Get<PhysicsComponent>();
            physics.Speed = 0.05f;
            physics.Angle = _random.Next() % 2 == 0 ? 0 : 180;
        }
    }
}