Making an object appear to move across the screen randomly - xna

Question

I'm currently doing an assignment, on which one of the requirements is for a random object to appear on screen and move across. Being new to XNA, i do not know where to even begin implementing such behaviours to the game, thus would really appreciate if someone could give me a nudge towards the right direction.
I'm only really accustomed to invoking something when a key is pressed, however with something completely random, this can't be done. as far as i am aware of.
Thank you.

Answer 1

You need to create and set up a sprite for the UFO first. In your protected override void Update(GameTime gameTime) code you simply need to get the current time and compare it to the rules in which you wish to apply. Then update if you wish to draw and "move" the sprite. Here is an example:
#region Using Statements
using System;
using System.Collections.Generic;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using Microsoft.Xna.Framework.Storage;
#endregion
public class Game : Microsoft.Xna.Framework.Game {
#region Game Settings
GraphicsDeviceManager graphics;
SpriteBatch spriteBatch;
GraphicsDevice device;
int screenWidth = 800;
int screenHeight = 600;
bool fullscreen = false;
string title = "MyGame";
string origionaltitle;
float frames = 0;
float framesPerSecond = 0;
int startTime;
int currentTime;
int nextTime;
#endregion
struct sprite {
public string TextureName;
public Texture2D Texture;
public Vector2 Position;
public Vector2 Speed;
public Color[] TextureData;
};
bool DrawUFO = false;
sprite ufo = new sprite();
public Game() {
graphics = new GraphicsDeviceManager(this);
Content.RootDirectory = "Content";
}
protected override void LoadContent() {
// Create a new SpriteBatch, which can be used to draw textures.
spriteBatch = new SpriteBatch(GraphicsDevice);
device = graphics.GraphicsDevice;
// TODO: use this.Content to load your game content here
ufo.TextureName = "ufo";
ufo.Texture = Content.Load<Texture2D>(ball.TextureName);
ufo.TextureData = new Color[ufo.Texture.Width *ufo.Texture.Height];
ufo.Texture.GetData(ball.TextureData);
}
protected override void Initialize() {
// TODO: Add your initialization logic here
ufo.Position = new Vector2(10f, 10.0f);
ufo.Speed = new Vector2(0.0f, 10.0f);
//
// Set up game window
graphics.PreferredBackBufferWidth = screenWidth;
graphics.PreferredBackBufferHeight = screenHeight;
graphics.IsFullScreen = fullscreen;
graphics.ApplyChanges();
origionaltitle = title;
Window.Title = title;
//
// Set the initial time
startTime = DateTime.Now.Second;
//
// Set "random"/next time for ufo to be rendered
nextTime = startTime + rand.Next(2);
//
base.Initialize();
}
protected override void Update(GameTime gameTime) {
//
// Set the current time
currentTime = DateTime.Now.Second;
//
// if not drawing ufo then
if(!DrawURO) {
//
// check current time and compare it with the next time
if( currentTime == nextTime ) {
DrawURO = true;
}
} else {
//
// Update UFO position (aka move it)
ufo.Posistion += ball.Speed *(float)gameTime.ElapsedGameTime.TotalSeconds;
//
// if ufo goes of the screen then
if(ufo.Position.Y > screenHeight) {
//
// Reset ufo
DrawURO = false;
ufo.Position.X = 10.0f;
ufo.Position.Y = 10.0f;
//
// set next time to render
nextTime = currentTime + rand.Next(2);
}
}
}
protected override void Draw(GameTime gameTime) {
graphics.GraphicsDevice.Clear(Color.Black);
// TODO: Add your drawing code here
spriteBatch.Begin(SpriteBlendMode.AlphaBlend);
if(DrawUFO == true) {
spriteBatch.Draw(ufo.Texture, ufo.Position, Color.White);
}
spriteBatch.End();
//
base.Draw(gameTime);
}
}
I copied this from some code i did at college a few years a ago, so apologies for any bugs.