手把手教用XNA开发winphone7游戏（五）大结局

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wx634e1c97257c6 2022-11-29 17:54:23 ©著作权

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Alien Game逻辑

在这最有一个部分你将创建game-specific logic和 helper方法和类。胜利就在眼前，你的第一个winphone7程序就要出现了，加油加油！！（感谢http://www.appfsoft.com/）。另外最近imagecup要开始了，帮小美女推一下，学生们有时间都去参加。为了帮大家了解imagecup和winphone7的开发，我可能会针对学生讲一些入门级webcast，有详细计划再通知大家。刚刚翻译了imagecup的winphone7的评判标准，觉得有一条很好玩，就是要让使用了你的程序的人想去买一部winphone，呵呵这个占20%，大家加油了。另外有人要源代码，我把最后成品的源代码放到下一篇“番外篇”了，大家感兴趣可以去下载。

1. 在 GameplayScreen.cs 文件里，创建一个新的helper 类(在GameplayScreen类之外)根据下面的代码:

(Code Snippet – Game Development with XNA – Gameplay Screen – Bullet class)

/// <summary>

/// Represents either an alien or player bullet

/// </summary>

publicclass Bullet

{

public Vector2

public bool

}

2. 在Bullet 类后面添加两个helper类:

(Code Snippet – Game Development with XNA – Gameplay Screen – Player and Alien classes)

/// <summary>

/// The player's state

/// </summary>

publicclass Player

{

public Vector2

public float

public bool

public float

public string

public Texture2D

public int

}

/// <summary>

/// Data for an alien. The only difference between the ships

/// and the badguys are the texture used.

/// </summary>

publicclass Alien

{

public Vector2

public Texture2D

public Vector2

public float

public int

public bool

public float

public int

}

3. 添加如下GameplayScreen类的变量:

(Code Snippet – Game Development with XNA – Gameplay Screen – even more variables)

Player player;

List<Alien> aliens;

List<Bullet> alienBullets;

List<Bullet> playerBullets;

4. 在构造函数中初始化这些变量，如下所示:

(Code Snippet – Game Development with XNA – Gameplay Screen – Player and Alien Initialization)

public

{

...

player = new Player();

playerBullets = new List<Bullet>();

aliens = new List<Alien>();

alienBullets = new List<Bullet>();

Accelerometer = new Accelerometer();

if (AccelerometerSensor.Default.State == SensorState.Ready)

{

...

}

...

}

5. 初始化player变量Width 和 Height在LoadContent 方法中 (初始化ParticleSystem之后):

(Code Snippet – Game Development with XNA – Gameplay Screen – Player Initialization in LoadContent method)

public override void LoadContent()

{

...

particles = new ParticleSystem(ScreenManager.Game.Content, ScreenManager.SpriteBatch);

player.Width = tankTexture.Width;

player.Height = tankTexture.Height;

base.LoadContent();

}

6. 下面的一小段代码将添加游戏的逻辑。他们将根据用户的输入来改变“player1”的移动。导航到 HandleInput 的方法，并找到以下行：

//TODO: Update player Velocity over X axis #1

在它的后面添加如下代码段:

(Code Snippet – Game Development with XNA – Gameplay Screen – Player Movements 1 in HandleInput method)

player.Velocity.X = movement;

7. 找到以下行 (在HandleInput 方法中):

//TODO: Update player velocity over X axis #2

在其后添加如下代码:

(Code Snippet – Game Development with XNA – Gameplay Screen – Player Movements 2 in HandleInput method)

player.Velocity.X = -1.0f;

8. 找到以下行 (在HandleInput 方法中):

//TODO: Update player velocity over X axis #3

在其后添加如下代码:

(Code Snippet – Game Development with XNA – Gameplay Screen – Player Movements 3 in HandleInput method)

player.Velocity.X = 1.0f;

9. 找到以下行 (在HandleInput 方法中):

//TODO: Update player velocity over X axis #4

在其后添加如下代码:

(Code Snippet – Game Development with XNA – Gameplay Screen – Player Movements 4 in HandleInput method)

player.Velocity.X = MathHelper.Min(input.CurrentGamePadStates[0].ThumbSticks.Left.X * 2.0f, 1.0f);

10. 找到以下行:

//TODO: Fire the bullet

更改"if"语句根据下面的代码片断：:

(Code Snippet – Game Development with XNA – Gameplay Screen – HandleInput firing the bullet code)

if (player.FireTimer <= 0.0f && player.IsAlive && !gameOver)

{

Bullet

bullet.Position = new Vector2((int)(player.Position.X + player.Width / 2) - bulletTexture.Width / 2, player.Position.Y - 4);

bullet.Velocity = new Vector2(0, -256.0f);

player.FireTimer = 1.0f;

particles.CreatePlayerFireSmoke(player);

playerFired.Play();

}

elseif (gameOver)

finishCurrentGame();

11. 创建 GameplayScreen 类中的以下方法：

此方法将创建Bullet类的一个实例。此实例使用在前面的代码段。

(Code Snippet – Game Development with XNA – Gameplay Screen – CreatePlayerBullet method)

/// <summary>

/// Returns an instance of a usable player bullet. Prefers reusing an /// existing (dead)

/// bullet over creating a new instance.

/// </summary>

/// <returns>A bullet ready to place into the world.</returns>

Bullet CreatePlayerBullet()

{

Bullet b = null;

for (int

{

if (playerBullets[i].IsAlive == false)

{

b = playerBullets[i];

break;

}

if (b == null)

{

b = new Bullet();

playerBullets.Add(b);

}

b.IsAlive = true;

return

}

12. 更改Update方法。在"base.Update(…)"方法调用之前，添加下面的蓝色突出显示的代码段：

此块的代码实际上提供了游戏的逻辑 – 它移动玩家和调用该方法来更新异型和重新计算子弹的位置。

(Code Snippet – Game Development with XNA – Gameplay Screen – Update method)

public override voidGameTime gameTime,

bool otherScreenHasFocus, bool

{

float elapsed = (float)gameTime.ElapsedGameTime.TotalSeconds;

{

// Move the player

if (player.IsAlive == true)

{

player.Position += player.Velocity * 128.0f * elapsed;

player.FireTimer -= elapsed;

player.Position = new Vector2(0.0f, player.Position.Y);

player.Position = new Vector2(worldBounds.Right - player.Width, player.Position.Y);

}

Respawn(elapsed);

UpdateAliens(elapsed);

UpdateBullets(elapsed);

CheckHits();

particles.CreatePlayerDust(player);

particles.Update(elapsed);

}

base.Update(gameTime, otherScreenHasFocus, coveredByOtherScreen);

}

13. 向GameplayScreen 类中添加helper方法:

Note: 下面的代码段中添加大量的帮助器方法。方法用途如下：

重生：检查是否，玩家已经"死亡"但这场游戏还没有结束，如果这，它完成等待 respawnTimer结束后创建一个新的玩家实例屏幕的中间。

UpdateBullets：检查和更新屏幕上的玩家和异形子弹的位置。

UpdateAliens：移动异形，并计算是否发射子弹和朝哪个方向。

CheckHits：检查所有的子弹和玩家/异形的碰撞。它还将处理游戏逻辑，当命中发生时，进行让死亡，加分或者结束游戏等。

(Code Snippet – Game Development with XNA – Gameplay Screen – Helper update methods)

/// <summary>
/// Handles respawning the player if we are playing a game and the player is dead.
/// </summary>
/// <param name="elapsed">Time elapsed since Respawn was called last.</param>
voidfloat elapsed)
{
if
return;

if
{
player.RespawnTimer -= elapsed;
if
{
// See if there are any bullets close...
int
int

for (int
{
if (alienBullets[i].IsAlive == false)
continue;

if
return;
}

player.IsAlive = true;
player.Position = new Vector2(worldBounds.Width / 2 - player.Width / 2, worldBounds.Bottom - groundTexture.Height + 2 - player.Height);
player.Velocity = Vector2.Zero;
player.Lives--;
}
}
}

/// <summary>
/// Moves all of the bullets (player and alien) and prunes "dead" bullets.
/// </summary>
/// <param name="elapsed"></param>
voidfloat elapsed)
{
for (int
{
if (playerBullets[i].IsAlive == false)
continue;

playerBullets[i].Position += playerBullets[i].Velocity * elapsed;

if
{
playerBullets[i].IsAlive = false;
hitStreak = 0;
}
}

for (int
{
if (alienBullets[i].IsAlive == false)
continue;

alienBullets[i].Position += alienBullets[i].Velocity * elapsed;

if
alienBullets[i].IsAlive = false;
}
}

/// <summary>
/// Moves the aliens and performs their "thinking" by determining if they
/// should shoot and where.
/// </summary>
/// <param name="elapsed">The elapsed time since UpdateAliens was called last.</param>
privatevoid UpdateAliens(float elapsed)
{
// See if it's time to spawn an alien;
alienSpawnTimer -= elapsed;
if
{
SpawnAlien();
alienSpawnTimer += alienSpawnRate;
}

for (int
{
if (aliens[i].IsAlive == false)
continue;

aliens[i].Position += aliens[i].Velocity * elapsed;
if
(aliens[i].Position.X > worldBounds.Width + 64 && aliens[i].Velocity.X > 0.0f))
{
aliens[i].IsAlive = false;
continue;
}

aliens[i].FireTimer -= elapsed;

if
{
if
{
Bullet
bullet.Position.X = aliens[i].Position.X + aliens[i].Width / 2 - laserTexture.Width / 2;
bullet.Position.Y = aliens[i].Position.Y + aliens[i].Height;
if ((float)random.NextDouble() <= aliens[i].Accuracy)
{
bullet.Velocity = Vector2.Normalize(player.Position - aliens[i].Position) * 64.0f;
}
else
{
bullet.Velocity = new Vector2(-8.0f + 16.0f * (float)random.NextDouble(), 64.0f);
}

alienFired.Play();
}

aliens[i].FireCount--;
}
}
}

/// <summary>
/// Performs all bullet and player/alien collision detection. Also handles game logic
/// when a hit occurs, such as killing something, adding score, ending the game, etc.
/// </summary>
void CheckHits()
{
if
return;

for (int
{
if (playerBullets[i].IsAlive == false)
continue;

for (int
{
if (aliens[a].IsAlive == false)
continue;

if
{
playerBullets[i].IsAlive = false;
aliens[a].IsAlive = false;

hitStreak++;

player.Score += aliens[a].Score * (hitStreak / 5 + 1);

if (player.Score > highScore)
highScore = player.Score;

if
{
player.Lives++;
nextLife += nextLife;
}

levelKillCount--;
if
AdvanceLevel();

particles.CreateAlienExplosion(new Vector2(aliens[a].Position.X + aliens[a].Width / 2, aliens[a].Position.Y + aliens[a].Height / 2));

alienDied.Play();
}
}
}

if (player.IsAlive == false)
return;

for (int
{
if (alienBullets[i].IsAlive == false)
continue;

if
{
alienBullets[i].IsAlive = false;

player.IsAlive = false;

hitStreak = 0;

player.RespawnTimer = 3.0f;
particles.CreatePlayerExplosion(new Vector2(player.Position.X + player.Width / 2, player.Position.Y + player.Height / 2));

playerDied.Play();

if
{
gameOver = true;
}
}

}
}
/// <summary>
/// Advances the difficulty of the game one level.
/// </summary>
void AdvanceLevel()
{
baseLevelKillCount += 5;
levelKillCount = baseLevelKillCount;
alienScore += 25;
alienSpawnRate -= 0.3f;
alienMaxAccuracy += 0.1f;
if
alienMaxAccuracy = 0.75f;

alienSpeedMin *= 1.35f;
alienSpeedMax *= 1.35f;

if
alienSpawnRate = 0.33f;

if
{
transitionRate = -0.5f;
}
else
{
transitionRate = 0.5f;
}
}

14. 在GameplayScreen 类中添加如下代码:

Note: 下面的代码片断还增加了大量的帮助器方法。他们的目的是，如下所示： CreateAlienBullet：创建的异形的子弹，将用于异形向玩家开火。

SpawnAlien：初始化一个新的异形实例，设置了的初始位置、速度，选择颜色纹理等。

CreateAlien：创建新异形的实例，并将其添加到异形集合

(Code Snippet – Game Development with XNA – Gameplay Screen – Helper aliens methods)
 
   
C#
 
   
/// <summary>
/// Returns an instance of a usable alien bullet. Prefers reusing an existing (dead)
/// bullet over creating a new instance.
/// </summary>
/// <returns>A bullet ready to place into the world.</returns>
Bullet CreateAlienBullet()
{
Bullet b = null;

for (int
{
if (alienBullets[i].IsAlive == false)
{
b = alienBullets[i];
break;
}
}

if (b == null)
{
b = new Bullet();
alienBullets.Add(b);
}

b.IsAlive = true;

return
}

/// <summary>
/// Creates an instance of an alien, sets the initial state, and places it into the world.
/// </summary>
privatevoid SpawnAlien()
{
Alien

if
{
newAlien.Position.X = -64.0f;
newAlien.Velocity.X = random.Next((int)alienSpeedMin, (int)alienSpeedMax);
}
else
{
newAlien.Position.X = worldBounds.Width + 32;
newAlien.Velocity.X = -random.Next((int)alienSpeedMin, (int)alienSpeedMax);
}

newAlien.Position.Y = 24.0f + 80.0f * (float)random.NextDouble();

// Aliens
if
{
switch
{
case
newAlien.Texture = badguy_blue;
break;
case
newAlien.Texture = badguy_red;
break;
case
newAlien.Texture = badguy_green;
break;
case
newAlien.Texture = badguy_orange;
break;
}
}
else
{
newAlien.Texture = alienTexture;
}

newAlien.Width = newAlien.Texture.Width;
newAlien.Height = newAlien.Texture.Height;
newAlien.IsAlive = true;
newAlien.Score = alienScore;

float

newAlien.FireTimer = duration * (float)random.NextDouble();
newAlien.FireCount = 1;

newAlien.Accuracy = alienMaxAccuracy;
}

/// <summary>
/// Returns an instance of a usable alien instance. Prefers reusing an existing (dead)
/// alien over creating a new instance.
/// </summary>
/// <returns>An alien ready to place into the world.</returns>
Alien CreateAlien()
{
Alien b = null;

for (int
{
if (aliens[i].IsAlive == false)
{
b = aliens[i];
break;
}
}

if (b == null)
{
b = new Alien();
aliens.Add(b);
}

b.IsAlive = true;

return
}

15. .导航到Draw方法，并添加下面的蓝色突出显示的代码段，在Screen.SpriteBatch.Begin() 和 Screen.SpriteBatch.End() 之间：

这样更改方法方法将呼调用helper方法以在屏幕上绘制变化后的内容。

(Code Snippet – Game Development with XNA – Gameplay Screen – Draw method update)

public override voidGameTime gameTime)

{

float elapsedTime = (float)gameTime.ElapsedGameTime.TotalSeconds;

ScreenManager.SpriteBatch.Begin();

DrawBackground(elapsedTime);

DrawAliens();

DrawPlayer();

DrawBullets();

particles.Draw();

DrawForeground(elapsedTime);

DrawHud();

ScreenManager.SpriteBatch.End();

}

16. 在Draw 方法后面添加如下方法:

Note: 下面的代码段中添加大量的绘图相关的Helper方法。方法用途如下： DrawPlayer：绘制玩家的坦克。

DrawAliens：绘制所有异形。

DrawBullets：绘制所有的子弹（玩家的和异形的）。

DrawForeground：作为前台绘制云，并将其移动。

DrawBackground：绘制草、丘陵、山地和太阳/月亮。此外处理白天和黑夜之间转换。

DrawHud：绘制分数、剩余的生命值和" GAME OVER "需要的时候。

drawString：绘制隐藏的文本的通用方法。

(Code Snippet – Game Development with XNA – Gameplay Screen – Draw methods)
 
   
C#
 
   
/// <summary>
/// Draws the player's tank
/// </summary>
void DrawPlayer()
{
if
{
ScreenManager.SpriteBatch.Draw(tankTexture, player.Position, Color.White);
}
}

/// <summary>
/// Draws all of the aliens.
/// </summary>
void DrawAliens()
{
for (int
{
if (aliens[i].IsAlive) ScreenManager.SpriteBatch.Draw(aliens[i].Texture, new Rectangle((int)aliens[i].Position.X, (int)aliens[i].Position.Y, (int)aliens[i].Width, (int)aliens[i].Height), Color.White);
}
}

/// <summary>
/// Draw both the player and alien bullets.
/// </summary>
privatevoid DrawBullets()
{
for (int
{
if
ScreenManager.SpriteBatch.Draw(bulletTexture, playerBullets[i].Position, Color.White);
}

for (int
{
if
ScreenManager.SpriteBatch.Draw(laserTexture, alienBullets[i].Position, Color.White);
}
}

/// <summary>
/// Draw the foreground, which is basically the clouds. I think I had planned on one point
/// having foreground grass that was drawn in front of the tank.
/// </summary>
/// <param name="elapsedTime">The elapsed time since last Draw</param>
privatevoid DrawForeground(float elapsedTime)
{
// Move the clouds. Movement seems like an Update thing to do, but this animations
// have no impact over gameplay.
cloud1Position += new Vector2(24.0f, 0.0f) * elapsedTime;
if
cloud1Position.X = -cloud1Texture.Width * 2.0f;

cloud2Position += new Vector2(16.0f, 0.0f) * elapsedTime;
if
cloud2Position.X = -cloud1Texture.Width * 2.0f;

ScreenManager.SpriteBatch.Draw(cloud1Texture, cloud1Position, Color.White);
ScreenManager.SpriteBatch.Draw(cloud2Texture, cloud2Position, Color.White);
}

/// <summary>
/// Draw the grass, hills, mountains, and sun/moon. Handle transitioning
/// between day and night as well.
/// </summary>
/// <param name="elapsedTime">The elapsed time since last Draw</param>
privatevoid DrawBackground(float elapsedTime)
{
transitionFactor += transitionRate * elapsedTime;
if
{
transitionFactor = 0.0f;
transitionRate = 0.0f;
}
if
{
transitionFactor = 1.0f;
transitionRate = 0.0f;
}

Vector3 day = Color.White.ToVector3();
Vector3 night = new Color(80, 80, 180).ToVector3();
Vector3 dayClear = Color.CornflowerBlue.ToVector3();
Vector3

Color clear = new Color(Vector3.Lerp(dayClear, nightClear, transitionFactor));
Color tint = new Color(Vector3.Lerp(day, night, transitionFactor));

// Clear the background, using the day/night color
ScreenManager.Game.GraphicsDevice.Clear(clear);

// Draw the mountains
ScreenManager.SpriteBatch.Draw(mountainsTexture, new Vector2(0, screenHeight - mountainsTexture.Height), tint);

// Draw the hills
ScreenManager.SpriteBatch.Draw(hillsTexture, new Vector2(0, screenHeight - hillsTexture.Height), tint);

// Draw the ground
ScreenManager.SpriteBatch.Draw(groundTexture, new Vector2(0, screenHeight - groundTexture.Height), tint);

// Draw the sun or moon (based on time)
ScreenManager.SpriteBatch.Draw(sunTexture, sunPosition, new Color(255, 255, 255, (byte)(255.0f * (1.0f - transitionFactor))));
ScreenManager.SpriteBatch.Draw(moonTexture, sunPosition, new Color(255, 255, 255, (byte)(255.0f * transitionFactor)));
}

/// <summary>
/// Draw the hud, which consists of the score elements and the GAME OVER tag.
/// </summary>
void DrawHud()
{
float

if
{
Vector2 size = menuFont.MeasureString("GAME OVER");
DrawString(menuFont, "GAME OVER", new Vector2(ScreenManager.Game.GraphicsDevice.Viewport.Width / 2 - size.X, ScreenManager.Game.GraphicsDevice.Viewport.Height / 2 - size.Y / 2), new Color(255, 64, 64), scale);

}
else
{
int
string bonusString = (bonus > 0 ? " (" + bonus.ToString(System.Globalization.CultureInfo.CurrentCulture) + "%)" : "");
// Score
DrawString(scoreFont, "SCORE: " + player.Score.ToString(System.Globalization.CultureInfo .CurrentCulture) + bonusString, new Vector2(leftOffset, topOffset), Color.Yellow, scale);

string text = "LIVES: " + player.Lives.ToString(System.Globalization.CultureInfo .CurrentCulture);
Vector2
size *= scale;

// Lives
DrawString(scoreFont, text, new Vector2(screenWidth - leftOffset - (int)size.X, topOffset), Color.Yellow, scale);

DrawString(scoreFont, "LEVEL: " + (((baseLevelKillCount - 5) / 5) + 1).ToString(System.Globalization.CultureInfo.CurrentCulture), new Vector2(leftOffset, screenHeight - bottomOffset), Color.Yellow, scale);

text = "HIGH SCORE: " + highScore.ToString(System.Globalization.CultureInfo
.CurrentCulture);

size = scoreFont.MeasureString(text);

DrawString(scoreFont, text, new Vector2(screenWidth - leftOffset - (int)size.X * 2, screenHeight - bottomOffset), Color.Yellow, scale);
}
}

/// <summary>
/// A simple helper to draw shadowed text.
/// </summary>
voidSpriteFont font, string
Vector2Color color)
{
ScreenManager.SpriteBatch.DrawString(font, text, new Vector2(position.X + 1, position.Y + 1), Color.Black);
ScreenManager.SpriteBatch.DrawString(font, text, position, color);
}

/// <summary>
/// A simple helper to draw shadowed text.
/// </summary>
voidSpriteFont font, string
Vector2Color color, float
{
ScreenManager.SpriteBatch.DrawString(font, text, new Vector2(position.X + 1, position.Y + 1), Color.Black, 0, new Vector2(0, font.LineSpacing / 2), fontScale, SpriteEffects.None, 0);
ScreenManager.SpriteBatch.DrawString(font, text, position, color, 0, new Vector2(0, font.LineSpacing / 2), fontScale, SpriteEffects.None, 0);
}
 
   
17. 找到 LoadContent 方法，在base.LoadContent() 调用后面体检如下代码：
 
   
C#
 
   
public override void LoadContent()
{
...
player.Width = tankTexture.Width;
player.Height = tankTexture.Height;
base.LoadContent();
LoadHighscore();
Start();
}
 
   
18. 找到 UnloadContent 方法并在“particles = null;”之前的位置添加下面的代码段:
 
   
C#
 
   
public override void UnloadContent()
{
SaveHighscore();
particles = null;
base.UnloadContent();
}
 
   
19. 创建一个区域loading/unloading high scores logic。我们需要通过Isolated Storage来读取Windows Phone 文件系统中的数据。用如下代码在GameplayScreen类中创建logic:
(Code Snippet – Game Development with XNA – Gameplay Screen – Highscore storage methods)
 
   
C#
 
   
#region Highscore loading/saving logic
/// <summary>
/// Saves the current highscore to a text file. The StorageDevice was selected during screen loading.
/// </summary>
privatevoid SaveHighscore()
{
using (IsolatedStorageFile isf = IsolatedStorageFile.GetUserStoreForApplication())
{
using (IsolatedStorageFileStream isfs = new IsolatedStorageFileStream("highscores.txt", FileMode.Create, isf))
{
using (StreamWriter writer = new StreamWriter(isfs))
{
writer.Write(highScore.ToString(System.Globalization.CultureInfo.InvariantCulture));
writer.Flush();
writer.Close();
}
}
}
}

/// <summary>
/// Loads the high score from a text file. The StorageDevice was selected during the loading screen.
/// </summary>
privatevoid LoadHighscore()
{
using (IsolatedStorageFile isf = IsolatedStorageFile.GetUserStoreForApplication())
{
if (isf.FileExists("highscores.txt"))
{
using (IsolatedStorageFileStream isfs = new IsolatedStorageFileStream("highscores.txt", FileMode.Open, isf))
{
using (StreamReader reader = new StreamReader(isfs))
{
try
{
highScore = Int32.Parse(reader.ReadToEnd(), System.Globalization.CultureInfo.InvariantCulture);
}
catch (FormatException)
{
highScore = 10000;
}
finally
{
if (reader != null)
reader.Close();
}
}
}
}
}
}
#endregion
 
   
20. 在GameplayScreen 类中创建Start方法来开始一个新游戏:
(Code Snippet – Game Development with XNA – Gameplay Screen – Start method)
 
   
C#
 
   
/// <summary>
/// Starts a new game session, setting all game states to initial values.
/// </summary>
void Start()
{
if
{
player.Score = 0;
player.Lives = 3;
player.RespawnTimer = 0.0f;

gameOver = false;

aliens.Clear();
alienBullets.Clear();
playerBullets.Clear();

Respawn(0.0f);
}

transitionRate = 0.0f;
transitionFactor = 0.0f;
levelKillCount = 5;
baseLevelKillCount = 5;
alienScore = 25;
alienSpawnRate = 1.0f;

alienMaxAccuracy = 0.25f;

alienSpeedMin = 24.0f;
alienSpeedMax = 32.0f;

alienSpawnRate = 2.0f;
alienSpawnTimer = alienSpawnRate;

nextLife = 5000;
}
 
   
21. 打开 ParticleSystem.cs文件
22. 添加如下命名申明:
(Code Snippet – Game Development with XNA – ParticleSystem – using statement)
 
   
C#
 
   
using AlienGame;
 
   
23. 在这个步骤，你将添加两个方法。一个用来创建玩家控制坦克时泥土/灰尘的效果。一个用来创建玩家发射子弹时，火焰的效果。添加如下方法到ParticleSystem 类中:
(Code Snippet – Game Development with XNA – ParticleSystem – Helper player effects methods)
 
   
C#
 
   
/// <summary>
/// Creates the mud/dust effect when the player moves.
/// </summary>
/// <param name="position">Where on the screen to create the effect.</param> 
publicvoidPlayer player)
{
forint i = 0; i < 2; ++i)
{
Particle p = CreateParticle();
p.Texture = smoke;
p.Color = new Color(125, 108, 43);
p.Position.X = player.Position.X + player.Width * (float)random.NextDouble();
p.Position.Y = player.Position.Y + player.Height - 3.0f * (float)random.NextDouble();
p.Alpha = 1.0f;
p.AlphaRate = -2.0f;
p.Life = 0.5f;
p.Rotation = 0.0f;
p.RotationRate = -2.0f + 4.0f * (float)random.NextDouble();
p.Scale = 0.25f;
p.ScaleRate = 0.5f;
p.Velocity.X = -4 + 8.0f * (float)random.NextDouble();
p.Velocity.Y = -8 + 4.0f * (float)random.NextDouble();
}
}

/// <summary>
/// Creates the effect for when the player fires a bullet.
/// </summary>
/// <param name="position">Where on the screen to create the effect.</param> 
publicvoidPlayer player)
{
forint i = 0; i < 8; ++i)
{
Particle p = CreateParticle();
p.Texture = smoke;
p.Color = Color.White;
p.Position.X = player.Position.X + player.Width / 2;
p.Position.Y = player.Position.Y;
p.Alpha = 1.0f;
p.AlphaRate = -1.0f;
p.Life = 1.0f;
p.Rotation = 0.0f;
p.RotationRate = -2.0f + 4.0f * (float)random.NextDouble();
p.Scale = 0.25f;
p.ScaleRate = 0.25f;
p.Velocity.X = -4 + 8.0f * (float)random.NextDouble();
p.Velocity.Y = -16.0f + -32.0f * (float)random.NextDouble();
}
}