11 Adding a Bread Bat Session 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”  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 Bat2

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  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 Chapter 12.1: Adding a Bread Bat3 Texture2D cheeseTexture; Rectangle cheeseRectangle; float cheeseWidthFactor = 0.05f; float cheeseX; float cheeseXSpeed; float cheeseY; float cheeseYSpeed; float cheeseTicksToCrossScreen = 200.0f; 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  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 Chapter 12.1: Adding a Bread Bat4 Texture2D breadTexture; Rectangle breadRectangle; float breadWidthFactor = 0.05f; float breadX; float breadXSpeed; float breadY; float breadYSpeed; float breadTicksToCrossScreen = 200.0f; 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  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 Chapter 12.1: Adding a Bread Bat5

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  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 Chapter 12.1: Adding a Bread Bat6

Cheese Sprite Information  The GameSpriteStruct type is one we have invented to hold all the data fields that this type needs Chapter 12.1: Adding a Bread Bat7 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  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 Chapter 12.1: Adding a Bread Bat8 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  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 Chapter 12.1: Adding a Bread Bat9 cheese.SpriteTexture = Content.Load ("Images/Cheese"); bread.SpriteTexture = Content.Load ("Images/Bread"); cheese.SpriteTexture = Content.Load ("Images/Cheese"); bread.SpriteTexture = Content.Load ("Images/Bread");

Public Access Modifiers in GameSpriteStruct  The fields in GameSpriteStruct are all public  This makes them useable in code outside it  The fields in GameSpriteStruct are all public  This makes them useable in code outside it Chapter 12.1: Adding a Bread Bat10 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  Fields that should not be used by code outside a structure can be made private  They will be accessed by calls to public methods Chapter 12.1: Adding a Bread Bat11 struct BankAccount { private float bankBalance; public void PayInFunds ( float amount) { } public void WithDrawFunds (float amount) { } } 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 ScaleSprites method 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  We can now add statements to the ScaleSprites method 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 Chapter 12.1: Adding a Bread Bat12 bread.WidthFactor = 0.15f; bread.TicksToCrossScreen = 120.0f; // rest of calculations here bread.WidthFactor = 0.15f; bread.TicksToCrossScreen = 120.0f; // rest of calculations here

1. Bread and Cheese Chapter 12.1: Adding a Bread Bat13  This shows the bread and the cheese together on the screen  At the moment only the cheese moves, however  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  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 Chapter 12.1: Adding a Bread Bat14

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  The bread speed is multiplied by the position of the thumbstick  The further the thumbstick is moved, the faster the bread will move Chapter 12.1: Adding a Bread Bat15 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; 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 Chapter 12.1: Adding a Bread Bat16  With this version of the game we can move the bread around  The level of control is very good, for such simple code  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  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 Chapter 12.1: Adding a Bread Bat17

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  The parameters to the method give the initial sprite settings  The setup values are calculated and used to set the sprite up Chapter 12.1: Adding a Bread Bat18 void setupSprite( GameSpriteStruct sprite, float widthFactor, float ticksToCrossScreen, float initialX, float initialY) { // sets the values of the sprite in here } void setupSprite( GameSpriteStruct sprite, float widthFactor, float ticksToCrossScreen, float initialX, float initialY) { // sets the values of the sprite in here }

The setupSprite Method Body Chapter 12.1: Adding a Bread Bat19 { 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; } { 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  The setupSprite method is called twice, to set up the cheese and the bread  The values are the ones that have used before Chapter 12.1: Adding a Bread Bat20 void setupSprites() { setupSprite(cheese, 0.05f, 200.0f, minDisplayX, minDisplayY); setupSprite(bread, 0.15f, 120.0f, displayWidth / 2, displayHeight / 2); } 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  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 Chapter 12.1: Adding a Bread Bat21 setupSprite( cheese, // sprite to set up 0.05f, // width factor (a 20th) 200.0f, // ticks to cross the screen minDisplayX, // starting X position minDisplayY); // starting Y position setupSprite( cheese, // sprite to set up 0.05f, // width factor (a 20th) 200.0f, // ticks to cross the screen minDisplayX, // starting X position minDisplayY); // starting Y position

3. Using Methods to Break Programs Chapter 12.1: Adding a Bread Bat22  The code is now much clearer and well designed  Unfortunately it doesn’t work  We need to find out why  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  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 Chapter 12.1: Adding a Bread Bat23 setupSprite( cheese, // sprite to set up 0.05f, // width factor (a 20th) 200.0f, // ticks to cross the screen minDisplayX, // starting X position minDisplayY); // starting Y position setupSprite( cheese, // sprite to set up 0.05f, // width factor (a 20th) 200.0f, // ticks to cross the screen minDisplayX, // starting X position minDisplayY); // 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  These are the parameters that are passed into the setupSprite method  They are all passed by value  This is what is causing our problem Chapter 12.1: Adding a Bread Bat24 void setupSprite( GameSpriteStruct sprite, float widthFactor, float ticksToCrossScreen, float initialX, float initialY) { // sets the values of the sprite in here } void setupSprite( GameSpriteStruct sprite, float widthFactor, float ticksToCrossScreen, float initialX, float initialY) { // sets the values of the sprite in here }

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 cheese variable, it will change fields in the copy, which means the cheese variable will not be set up  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 cheese variable, it will change fields in the copy, which means the cheese variable will not be set up Chapter 12.1: Adding a Bread Bat25

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  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 Chapter 12.1: Adding a Bread Bat26 void setupSprite( ref GameSpriteStruct sprite, float widthFactor, float ticksToCrossScreen, float initialX, float initialY) { // sets the values of the sprite in here } void setupSprite( ref GameSpriteStruct sprite, float widthFactor, float ticksToCrossScreen, float initialX, float initialY) { // sets the values of the sprite in here }

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  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 Chapter 12.1: Adding a Bread Bat27 setupSprite( ref cheese, // reference to the sprite to set up 0.05f, // width factor (a 20th) 200.0f, // ticks to cross the screen minDisplayX, // starting X position minDisplayY); // starting Y position setupSprite( ref cheese, // reference to the sprite to set up 0.05f, // width factor (a 20th) 200.0f, // ticks to cross the screen minDisplayX, // starting X position minDisplayY); // starting Y position

4. Mending Methods Chapter 12.1: Adding a Bread Bat28  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  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 Chapter 12.1: Adding a Bread Bat29

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  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 Chapter 12.1: Adding a Bread Bat30

Using the Intersects Method  A Rectangle value provides an Intersects method  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  A Rectangle value provides an Intersects method  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 Chapter 12.1: Adding a Bread Bat31 if (cheese.SpriteRectangle.Intersects(bread.SpriteRectangle)) { cheese.YSpeed = cheese.YSpeed * -1; }

5. Bouncing Cheese Chapter 12.1: Adding a Bread Bat32  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  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 Chapter 12.1: Adding a Bread Bat33

True/False Revision Quiz  A C# struct can contain methods and data fields.  A C# struct is managed by value.  Private fields in a struct cannot be changed.  Parameters are usually passed into methods by value.  If a method is to change the value of a parameter, the method must be passed by reference.  The gamepad thumbsticks return a value between 0 and 100.  A C# struct can contain methods and data fields.  A C# struct is managed by value.  Private fields in a struct cannot be changed.  Parameters are usually passed into methods by value.  If a method is to change the value of a parameter, the method must be passed by reference.  The gamepad thumbsticks return a value between 0 and 100. Chapter 12.1: Adding a Bread Bat34

True/False Revision Quiz  A C# struct can contain methods and data fields.  A C# struct is managed by value.  Private fields in a struct cannot be changed.  Parameters are usually passed into methods by value.  If a method is to change the value of a parameter, the method must be passed by reference.  The gamepad thumbsticks return a value between 0 and 100.  A C# struct can contain methods and data fields.  A C# struct is managed by value.  Private fields in a struct cannot be changed.  Parameters are usually passed into methods by value.  If a method is to change the value of a parameter, the method must be passed by reference.  The gamepad thumbsticks return a value between 0 and 100. Chapter 12.1: Adding a Bread Bat35

True/False Revision Quiz  A C# struct can contain methods and data fields.  A C# struct is managed by value.  Private fields in a struct cannot be changed.  Parameters are usually passed into methods by value.  If a method is to change the value of a parameter, the method must be passed by reference.  The gamepad thumbsticks return a value between 0 and 100.  A C# struct can contain methods and data fields.  A C# struct is managed by value.  Private fields in a struct cannot be changed.  Parameters are usually passed into methods by value.  If a method is to change the value of a parameter, the method must be passed by reference.  The gamepad thumbsticks return a value between 0 and 100. Chapter 12.1: Adding a Bread Bat36

True/False Revision Quiz  A C# struct can contain methods and data fields.  A C# struct is managed by value.  Private fields in a struct cannot be changed.  Parameters are usually passed into methods by value.  If a method is to change the value of a parameter, the method must be passed by reference.  The gamepad thumbsticks return a value between 0 and 100.  A C# struct can contain methods and data fields.  A C# struct is managed by value.  Private fields in a struct cannot be changed.  Parameters are usually passed into methods by value.  If a method is to change the value of a parameter, the method must be passed by reference.  The gamepad thumbsticks return a value between 0 and 100. Chapter 12.1: Adding a Bread Bat37

True/False Revision Quiz  A C# struct can contain methods and data fields.  A C# struct is managed by value.  Private fields in a struct cannot be changed.  Parameters are usually passed into methods by value.  If a method is to change the value of a parameter, the method must be passed by reference.  The gamepad thumbsticks return a value between 0 and 100.  A C# struct can contain methods and data fields.  A C# struct is managed by value.  Private fields in a struct cannot be changed.  Parameters are usually passed into methods by value.  If a method is to change the value of a parameter, the method must be passed by reference.  The gamepad thumbsticks return a value between 0 and 100. Chapter 12.1: Adding a Bread Bat38

True/False Revision Quiz  A C# struct can contain methods and data fields.  A C# struct is managed by value.  Private fields in a struct cannot be changed.  Parameters are usually passed into methods by value.  If a method is to change the value of a parameter, the method must be passed by reference.  The gamepad thumbsticks return a value between 0 and 100.  A C# struct can contain methods and data fields.  A C# struct is managed by value.  Private fields in a struct cannot be changed.  Parameters are usually passed into methods by value.  If a method is to change the value of a parameter, the method must be passed by reference.  The gamepad thumbsticks return a value between 0 and 100. Chapter 12.1: Adding a Bread Bat39

True/False Revision Quiz  A C# struct can contain methods and data fields.  A C# struct is managed by value.  Private fields in a struct cannot be changed.  Parameters are usually passed into methods by value.  If a method is to change the value of a parameter, the method must be passed by reference.  The gamepad thumbsticks return a value between 0 and 100.  A C# struct can contain methods and data fields.  A C# struct is managed by value.  Private fields in a struct cannot be changed.  Parameters are usually passed into methods by value.  If a method is to change the value of a parameter, the method must be passed by reference.  The gamepad thumbsticks return a value between 0 and 100. Chapter 12.1: Adding a Bread Bat40