Session 12.1 Adding a Bread Bat - Andrews Universitygreenley/cs2/Slides12.1AddingaBreadBat… · Chapter 12.1: Adding a Bread Bat 4 Texture2D breadTexture; Rectangle breadRectangle;

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Adding a Bread BatSession 12.1

Session Overview

We have created a cheese sprite that bounces around the display

We now need to create a bread bat sprite that the player will control and can be used to hit the cheese

In this session, we find out the best way to manage multiple sprites in a game, how a player can control the position of a sprite, and how to detect and respond to collisions between the two objects

At the end, we will have the first game play element of “Bread and Cheese”

Chapter 12.1: Adding a Bread Bat 2

Cheese Sprite Information

This is all the information that is required to represent the cheese on the screen

It includes the texture, size, and speed information for the sprite


Texture2D cheeseTexture;Rectangle cheeseRectangle;float cheeseWidthFactor = 0.05f;float cheeseX;float cheeseXSpeed;float cheeseY;float cheeseYSpeed;float cheeseTicksToCrossScreen = 200.0f;

Creating a Bread Sprite

To create a bread sprite we could declare a set of variables with information about the bread

Each variable matches the purpose of the corresponding cheese variable


Texture2D breadTexture;Rectangle breadRectangle;float breadWidthFactor = 0.05f;float breadX;float breadXSpeed;float breadY;float breadYSpeed;float breadTicksToCrossScreen = 200.0f;

Variables and Design

From a design point of view, it is not sensible to just declare a whole set of extra variables Although from a technical point of view it would work,

from a programming point of view a large number of variables with similar names is not a good play

Instead of using a convention that the word at the start of the variable name sets out which sprite it is part of, it would be nice if we had a way of grouping sprite information together

C# provides a mechanism called a structure to collect together related elements


The C# Struct

A struct is a C# construction that brings together methods and data

Structures are very good for holding “lumps of data”

The XNA Framework and C# system library use structures a lot A DateTime value is a struct type

A Color value is a struct type

We are going to create a structure to hold a sprite


Cheese Sprite Information

The GameSpriteStruct type is one we have invented to hold all the data fields that this type needs


struct GameSpriteStruct{

public Texture2D SpriteTexture;public Rectangle SpriteRectangle;public float X;public float Y;public float XSpeed;public float YSpeed;public float WidthFactor;public float TicksToCrossScreen;

}

Creating GameSpriteStruct variables

We have added a new type to the ones available to the program

Once we have declared the structure GameSpriteStruct,we can now create variables of this type

Each variable that we declare contains all the fields of the struct that we declared


GameSpriteStruct cheese;GameSpriteStruct bread;

Accessing Structure Fields

A program can access fields in a structure by giving the variable name followed by the name of the required field

The statements above set the texture fields for the bread and the cheese

This works because these fields have been made public in the GameSpriteStruct declaration


cheese.SpriteTexture = Content.Load<Texture2D>("Images/Cheese");

bread.SpriteTexture = Content.Load<Texture2D>("Images/Bread");

Public Access Modifiers in GameSpriteStruct

The fields in GameSpriteStruct are all public

This makes them useable in code outside it


struct GameSpriteStruct{

public Texture2D SpriteTexture;public Rectangle SpriteRectangle;public float X;public float Y;public float XSpeed;public float YSpeed;public float WidthFactor;public float TicksToCrossScreen;

}

Public and Private Fields

Fields that should not be used by code outside a structure can be made private

They will be accessed by calls to public methods


struct BankAccount{

private float bankBalance;public void PayInFunds ( float amount){}public void WithDrawFunds (float amount){}

}

Scaling the Bread and Cheese

We can now add statements to the ScaleSpritesmethod to set up the bread as well as the cheese

The bread is made larger than the cheese and is able to move slightly faster

The relative speed of the bread and the cheese is something we might want to return to later


bread.WidthFactor = 0.15f;bread.TicksToCrossScreen = 120.0f;// rest of calculations here

1. Bread and Cheese


This shows the bread and the cheese together on the screen

At the moment only the cheese moves, however

Controlling the Bread Bat with a Thumbstick

We are going to use the left thumbstick to control the movement of the bread around the screen

The thumbstick provides float values in the range -1 to 1 for the X and Y directions

The values are obtained from a GamepadState variable


Moving the Bread Bat

The bread speed is multiplied by the position of the thumbstick

The further the thumbstick is moved, the faster the bread will move


GamePadState gamePad1 = GamePad.GetState(PlayerIndex.One);

bread.X = bread.X + (bread.XSpeed * pad1.ThumbSticks.Left.X);

bread.Y = bread.Y –(bread.YSpeed * gamePad1.ThumbSticks.Left.Y);

bread.SpriteRectangle.X = (int)bread.X;bread.SpriteRectangle.Y = (int)bread.Y;

2. Moving the Bread


With this version of the game we can move the bread around

The level of control is very good, for such simple code

Improving the Program Design with Methods

At the moment, the program works OK, but some of the code still looks a bit messy

The scaleSprites method is a bit of a mess in that it contains a lot of statements that set different parts of the sprites

It would be easy to miss out one of the settings and find that a sprite did not behave correctly

A way to improve this would be to write a method that is given a set of parameters to set up a sprite


The setupSprite Method Header

The parameters to the method give the initial sprite settings

The setup values are calculated and used to set the sprite up


void setupSprite( GameSpriteStruct sprite,float widthFactor,float ticksToCrossScreen,float initialX,float initialY)

{// sets the values of the sprite in here

}

The setupSprite Method Body


{sprite.WidthFactor = widthFactor;sprite.TicksToCrossScreen = ticksToCrossScreen;sprite.SpriteRectangle.Width =

(int)((displayWidth * widthFactor) + 0.5f);float aspectRatio = (float)sprite.SpriteTexture.Width /

sprite.SpriteTexture.Height;sprite.SpriteRectangle.Height =

(int)((sprite.SpriteRectangle.Width / aspectRatio) + 0.5f);

sprite.X = initialX;sprite.Y = initialY;sprite.XSpeed = displayWidth / ticksToCrossScreen;sprite.YSpeed = sprite.Xspeed;

}

Calling setupSprite

The setupSprite method is called twice, to set up the cheese and the bread

The values are the ones that have used before


void setupSprites(){

setupSprite(cheese, 0.05f, 200.0f , minDisplayX, minDisplayY);

setupSprite(bread, 0.15f, 120.0f, displayWidth / 2, displayHeight / 2);

}

A Clearer Call of setupSprite

This call works in exactly the same way

However, I have added some comments to make it clear what each parameter does

I find this format much clearer


setupSprite(cheese, // sprite to set up0.05f, // width factor (a 20th)200.0f, // ticks to cross the screenminDisplayX, // starting X positionminDisplayY); // starting Y position

3. Using Methods to Break Programs


The code is now much clearer and well designed

Unfortunately it doesn’t work

We need to find out why

The Broken Method Call

There is nothing wrong with the method

There is nothing wrong with the values we are using to call it

Unfortunately, the problem is that we are not using parameters correctly


setupSprite(cheese, // sprite to set up0.05f, // width factor (a 20th)200.0f, // ticks to cross the screenminDisplayX, // starting X positionminDisplayY); // starting Y position

Passing Parameters by Value

These are the parameters that are passed into the setupSprite method

They are all passed by value

This is what is causing our problem


void setupSprite( GameSpriteStruct sprite,float widthFactor,float ticksToCrossScreen,float initialX,float initialY)


}

Passing Parameters by Value

When a parameter is passed by value this tells C# to send a copy of the value into the method that is called

The method then works on the copy

This means that setupSprite will work on a copy of the GameSpriteStruct it was given, not the original

So it will not change the fields in the cheesevariable, it will change fields in the copy, which means the cheese variable will not be set up


Passing Parameters by Reference

We can tell the compiler we want to pass the sprite as a reference by using the keyword ref

Now the method is given a reference to the parameter variable, not a copy of it


void setupSprite( ref GameSpriteStruct sprite,float widthFactor,float ticksToCrossScreen,float initialX,float initialY)


}

Supplying a Reference in the Call

If you tell C# a parameter is being passed by reference you have to give a reference in the call too

This is done by putting the ref keyword in front of the variable in the method call

You can pass other types by reference too


setupSprite(ref cheese, // reference to the sprite to set up0.05f, // width factor (a 20th)200.0f, // ticks to cross the screenminDisplayX, // starting X positionminDisplayY); // starting Y position

4. Mending Methods


Passing references into the methods means that they are set up correctly, and the game will now work

Making the Bread Hit the Cheese

When the bat hits the cheese it should “bounce” back up the screen We can make the cheese bounce, just by reversing

the direction of the Y speed, as we do when the cheese hits the top or bottom

But we have to know when the two sprites collide

The program must detect when the player hits the cheese with the bread


Collisions and Rectangles

A simple collision detection can be implemented by detecting when the bread and cheese rectangles intersect

The Rectangle type provides a very easy way to do this


Using the Intersects Method

A Rectangle value provides an Intersectsmethod

This is given another Rectangle to test against

The method returns true if the two rectangles intersect

We can use this to bounce the cheese off the bread


if (cheese.SpriteRectangle.Intersects(bread.SpriteRectangle)) {

cheese.YSpeed = cheese.YSpeed * -1;}

5. Bouncing Cheese


Adding just that one condition to the Update method turns the program into a game of sorts

Summary A C# struct is a way to bring together related

fields

Members of a struct can be private (only usable by methods in the struct) or public (useable by all)

Variables of struct type are managed by value

Parameters are usually passed by value

If a method is to change the value of a parameter the parameter must be passed by reference

The Rectangle class has an Intersects method that can be used to detect when game objects collide


Documents

Session 12.1 Adding a Bread Bat - Andrews Universitygreenley/cs2/Slides12.1AddingaBreadBat… · Chapter 12.1: Adding a Bread Bat 4 Texture2D breadTexture; Rectangle breadRectangle;