java - Android中如何用canvas绘制不同的点

Question

我正在开发一个 Android 应用程序，我必须实现一个功能，允许我在 Activity 中绘制不同的点。

这是我的代码:

public class MainActivity extends AppCompatActivity {
    public Paint paint;
    public List<Point> coords;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(new DrawingView(this));

        paint = new Paint();
        coords = new ArrayList();

        ImageView iv = new ImageView(getApplicationContext());
        iv.setImageResource(R.drawable.car);
        iv.setScaleType(ImageView.ScaleType.CENTER_INSIDE);
        LinearLayout.LayoutParams parms = new LinearLayout.LayoutParams(LinearLayout.LayoutParams.MATCH_PARENT,LinearLayout.LayoutParams.MATCH_PARENT);
        iv.setLayoutParams(parms);
    }

    class DrawingView extends SurfaceView {

        private final SurfaceHolder surfaceHolder;
        private final Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);

        public DrawingView(Context context) {
            super(context);
            surfaceHolder = getHolder();
            paint.setColor(Color.BLACK);
            paint.setStyle(Paint.Style.FILL);
        }

        @Override
        public boolean onTouchEvent(MotionEvent event) {
            if(event.getAction() == MotionEvent.ACTION_DOWN) {
              addpoint(event.getX(), event.getY());
            }
            return false;
        }

        public void addpoint(float x, float y){

            Point point = new Point();
            point.x = Math.round(x);
            point.y = Math.round(y);
            coords.add(point);

            for(int i = 0; i< coords.size(); i++) {
                Canvas canvas = surfaceHolder.lockCanvas();
                canvas.drawColor(Color.WHITE);
                canvas.drawCircle(coords.get(i).x, coords.get(i).y, 20, paint);
                surfaceHolder.unlockCanvasAndPost(canvas);
            }
        }
    }
}

每次我触摸屏幕时，我都会得到坐标并将其保存在列表中，然后我尝试绘制点列表，但屏幕上只剩下一个点，我不明白为什么。

那么我如何在屏幕上绘制不同的点(并保存它)？

另一个问题:如何显示背景图像？

Answer 1

来自官方 Javadocs:

drawColor(int color) Fill the entire canvas' bitmap (restricted to the current clip) with the specified color, using srcover porterduff mode.

所以每次你画一个圆圈时，首先要用白色清除整个 Canvas 。所以画完之后，之前画的圆就被清除了。

因此，一种选择是保存背景的当前状态，并始终将其绘制在白色之上，或者尝试不使用drawColor