android - 如何在 View 上绘制自定义 View (例如圆形)？

Question

对于我的 Pong 克隆，我正在尝试将类 Ball 中的自定义 View 绘制到我在类 GamePanel 中的 View 上。但是，在运行时屏幕上看不到它。

public class Ball extends View {

private float posX;
private float posY;
private float radius;

private Paint paint;

public Ball(Context context, float posX, float posY, float radius){
    super(context);
    this.posX = posX;
    this.posY = posY;
    this.radius = radius;
    setWillNotDraw(false);
    init();

}

private void init(){

    paint = new Paint();
    paint.setColor(Color.WHITE);
}

@Override
protected void onDraw(Canvas canvas) {

    canvas.drawCircle(posX, posY, radius, paint);

}

现在这是我的类 GamePanel。

public class GamePanel extends View implements GestureDetector.OnGestureListener{

private Context context;

//Screen height and width in pixels
private static int width;
private static int height;

private MainThread thread;

private Rect paddle1;
private Rect paddle2;

private Ball ball;

private Paint paint;

//Starting position paddle1
private int xPos1;
private int yPos1;

// Starting position paddle2
private int xPos2;
private int yPos2;

//Width and height of paddles
private int pWidth;
private int pHeight;

private GestureDetectorCompat gestureDetector;

public GamePanel(Context context) {

    super(context);

    this.context = context;

    //Information we will need for positioning our objects inside the panel
    DisplayMetrics displayMetrics = Resources.getSystem().getDisplayMetrics();

    width = displayMetrics.widthPixels;
    height = displayMetrics.heightPixels;

    //Make the GamePanel focusable
    setFocusable(true);

    // Thread needs information about the game panel
    thread = new MainThread(this);

    pWidth = 100;
    pHeight = height/2;

    xPos1 = width/15;
    yPos1 = 0;

    xPos2 = width - xPos1 - pWidth;
    yPos1 = 0;

    paddle1 = new Rect(xPos1, yPos1, xPos1 + pWidth, yPos1 + pHeight);
    paddle2 = new Rect(xPos2, yPos2, xPos2 + pWidth, yPos2 + pHeight);

    paint = new Paint();
    paint.setColor(Color.WHITE);

    ball = new Ball(context, 300, 300, 300);

    setBackgroundColor(Color.BLACK);

    this.gestureDetector = new GestureDetectorCompat(context, this);

    thread.setRunning(true);
    thread.start();
}


@Override
protected void onDraw(Canvas canvas) {

    canvas.drawRect(paddle1, paint);
    canvas.drawRect(paddle2, paint);

}

我尝试在我的 MainThread 线程、GamePanel 的构造函数和 Ball 的构造函数中调用 invalidate() 和 postInvalidate()。球永远不会被绘制到屏幕上。

正如您所看到的，我已经在 Pong 中遇到了这个问题，因此我在 GamePanel 中创建了两个非常难看的矩形。

如何封装矩形和圆形(球)，以便它仍然可以在 Canvas 上绘制？

Answer 1

您正在 GamePanel View 中创建 Ball(circle) View 。仅仅因为您正在创建一个 View 并不意味着它会被绘制在屏幕上。您需要将 View 附加到布局。简单的解决方案是将您的 GamePanel 扩展到 Ball

public class GamePanel extends Ball {

}

然后在GamePanel的onDraw()函数中添加super.onDraw();

@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(); //Will draw the circle before drawing rectangle
    canvas.drawRect(paddle1, paint);
    canvas.drawRect(paddle2, paint);    
}

如果将 super.onDraw() 放在 drawRect 下方，则在绘制矩形后绘制圆。

You also need to understand the basic concept of adding the view to layout.