摘要:取得鍵盤輸入。
上個範例「我的第一個xna程式！ 」已經讓娜歐可以左右移動了，但是這樣有點無趣，接著讓我們用鍵盤控制他的動向。
xna要取得鍵盤狀態非常容易，呼叫Keyboard.GetState() 即可獲得鍵盤狀態，此函式會回傳KeyboardState的struct，我們在從回傳的鍵盤狀態檢查我們需要的資訊即可。
先在上個範例新增一個int變數，用來控制娜歐的y方向，另外也在Update函式加入控制程式碼，如下
01 
private int stepY = 0;
02
03
protected override void Update(GameTime gameTime) {
04
05 if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
06 this.Exit();
07
if (position.X + myImage.Width > Window.ClientBounds.Width || position.X < 0) {
08 stepX *= -1;
09
}
10 if (position.Y + myImage.Height > Window.ClientBounds.Height || position.Y < 0) {
11 stepY *= -1;
12 }
13
14 KeyboardState keyboardState = Keyboard.GetState();
15 if (keyboardState.IsKeyDown(Keys.Left)) {
16 stepX--;
17 } else if (keyboardState.IsKeyDown(Keys.Right)) {
18 stepX++;
19 } else if (keyboardState.IsKeyDown(Keys.Up)) {
20 stepY--;
21 } else if (keyboardState.IsKeyDown(Keys.Down)) {
22 stepY++;
23 }
24
25 position.X += stepX;
26 position.Y += stepY;
27 base.Update(gameTime);
28
}
private int stepY = 0; 02
03
protected override void Update(GameTime gameTime) { 
protected override void Update(GameTime gameTime) 04
05
if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed) 06
this.Exit(); 07
if (position.X + myImage.Width > Window.ClientBounds.Width || position.X < 0) { 
if (position.X + myImage.Width > Window.ClientBounds.Width || position.X < 0) 08
stepX *= -1; 09
} 10
if (position.Y + myImage.Height > Window.ClientBounds.Height || position.Y < 0) { 11
stepY *= -1; 12
} 13
14
KeyboardState keyboardState = Keyboard.GetState(); 15
if (keyboardState.IsKeyDown(Keys.Left)) { 16
stepX--; 17
} else if (keyboardState.IsKeyDown(Keys.Right)) { 18
stepX++; 19
} else if (keyboardState.IsKeyDown(Keys.Up)) { 20
stepY--; 21
} else if (keyboardState.IsKeyDown(Keys.Down)) { 22
stepY++; 23
} 24
25
position.X += stepX; 26
position.Y += stepY; 27
base.Update(gameTime); 28
}其中14行就是取得鍵盤狀態用的，而15、17、19、21行則是檢查鍵盤按鍵有沒有被按下。這應該很容易理解。
我們只要按右就增加x軸方向的增量，按左就減少。上下則對應y軸的增量。
執行此程式之後，讓我們隨便按個幾下方向鍵…娜歐的速度可以飛上天啦！！我並不想要他飛這麼快阿！
這是因為每次執行update的時候，我們都會去檢查一次鍵盤的按鍵，而預設呼叫的時間間隔是1/60秒，所以你按著鍵盤一秒，娜歐就被你加速了60次，難怪可以飛上天啊！
我們該如何一次只增加一點點速度呢？
只要多記錄前一次鍵盤狀態就可以避免發生這種狀況，修改後的程式碼如下
01 KeyboardState preKeyboardState = Keyboard.GetState();
02protected override void Update(GameTime gameTime) {
03
04 if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
05 this.Exit();
06 if (position.X + myImage.Width > Window.ClientBounds.Width || position.X < 0) {
07 stepX *= -1;
08 }
09 if (position.Y + myImage.Height > Window.ClientBounds.Height || position.Y < 0) {
10 stepY *= -1;
11 }
12
13 KeyboardState keyboardState = Keyboard.GetState();
14 if (preKeyboardState.IsKeyDown(Keys.Left) && keyboardState.IsKeyUp(Keys.Left)) {
15 stepX--;
16 } else if (preKeyboardState.IsKeyDown(Keys.Right) && keyboardState.IsKeyUp(Keys.Right)) {
17 stepX++;
18 } else if (preKeyboardState.IsKeyDown(Keys.Up) && keyboardState.IsKeyUp(Keys.Up)) {
19 stepY--;
20 } else if (preKeyboardState.IsKeyDown(Keys.Down) && keyboardState.IsKeyUp(Keys.Down)) {
21 stepY++;
22 }
23 preKeyboardState = keyboardState;
24
25 position.X += stepX;
26 position.Y += stepY;
27 base.Update(gameTime);
28}
KeyboardState preKeyboardState = Keyboard.GetState(); 02
protected override void Update(GameTime gameTime) { 03
04
if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed) 05
this.Exit(); 06
if (position.X + myImage.Width > Window.ClientBounds.Width || position.X < 0) { 07
stepX *= -1; 08
} 09
if (position.Y + myImage.Height > Window.ClientBounds.Height || position.Y < 0) { 10
stepY *= -1; 11
} 12
13
KeyboardState keyboardState = Keyboard.GetState(); 14
if (preKeyboardState.IsKeyDown(Keys.Left) && keyboardState.IsKeyUp(Keys.Left)) { 15
stepX--; 16
} else if (preKeyboardState.IsKeyDown(Keys.Right) && keyboardState.IsKeyUp(Keys.Right)) { 17
stepX++; 18
} else if (preKeyboardState.IsKeyDown(Keys.Up) && keyboardState.IsKeyUp(Keys.Up)) { 19
stepY--; 20
} else if (preKeyboardState.IsKeyDown(Keys.Down) && keyboardState.IsKeyUp(Keys.Down)) { 21
stepY++; 22
} 23
preKeyboardState = keyboardState; 24
25
position.X += stepX; 26
position.Y += stepY; 27
base.Update(gameTime); 28
}第一行就宣告一個用來存前次狀態的KeyboardState，只有當前次狀態是壓下去而現在放開了，我們才增加速度，最後記得像23行一樣，把這次的狀態設為前次狀態。
這樣子的娜歐好控制多了！
