使用 Android 个图形在 Paint 中绘制图形

Question

我正在尝试使用 canvas 在视图上绘制图表。到目前为止，它绘制正确，如下图所示：

Desired Output

一旦您点击屏幕，预期的结果应该会清除屏幕并绘制一个新图形，而是在它上面绘制，从而导致：

Actual Output

private List<Float> xPosList, yPosList;
    private List<Path> pathList;
    private Path path;
    private Paint paint;

    private final Paint clearPaint = new Paint();



    public Plotter(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);

        this.xPosList = new ArrayList<>();
        this.yPosList = new ArrayList<>();
        this.pathList = new ArrayList<>();
        this.paint = new Paint();
        this.paint.setStrokeWidth(20);
        this.paint.setColor(Color.GREEN);
        this.clearPaint.setColor(Color.BLACK);
        generateData();

    }

    private void generateData() {

        int min = 5;
        int max = 100;
        double random = 0;

        float xPos = 0;
        float yPos = 0;

        for (int i = 1; i <= 20; i++) {
            random = min + Math.random() * (max - min);
            xPos = 50 * i;                   //50 pixels
            yPos = (float)random;

            this.xPosList.add(xPos);
            this.yPosList.add(yPos);

            path = new Path();         //Create path
            path.moveTo(xPos, yPos);   //Add values to path
            this.pathList.add(path);   //Add path to pathList
        }
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                generateData();
                break;
            case MotionEvent.ACTION_UP:
                //invalidate();
                break;
        }

        //invalidate();                               //Refresh canvas
        return true;                                //Activate event
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);


        Paint p = new Paint();
        canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR);


        p.setColor(Color.GREEN);
        p.setStrokeWidth(10);
        p.setStyle(Paint.Style.FILL);
        /***
         * Better use 50 by 50 pixels
         */

        float startX = 0;
        float startY = canvas.getHeight();                            //Start at the bottom (max height)

        float nextX = 0;
        float nextY = 0;

        for (int i = 0; i < this.xPosList.size(); i++){

            nextX = this.xPosList.get(i);
            nextY = canvas.getHeight() - (this.yPosList.get(i) * 10);                    //Add 50 offset


            canvas.drawLine(startX, startY, nextX, nextY, p);

            startX = nextX;
            startY = nextY;
        }
    }
}

如何正确清除屏幕？

我试过了

canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR);

从其他问题上看到的，但是不行。

Answer 1

实际上它与您的清除颜色没有任何关系 - 屏幕被正确清除。问题是 xPosList 和 yPosList ArrayList.

只要用户点击屏幕，就会调用 generateData() 函数，该函数会不断向两个 ArrayList 添加元素。但是：你永远不会清除那些！因此，只要您点击屏幕两次，ArrayList 就会包含来自第一次和第二次点击的随机数据。

您需要在 for-loop:

之前清除那些

this.xPosList.clear();
this.yPosList.clear();
for (int i = 1; i <= 20; i++) {
    random = min + Math.random() * (max - min);
    xPos = 50 * i;                   //50 pixels
    yPos = (float)random;

    this.xPosList.add(xPos);
    this.yPosList.add(yPos);

    path = new Path();         //Create path
    path.moveTo(xPos, yPos);   //Add values to path
    this.pathList.add(path);   //Add path to pathList
}

使用 Android 个图形在 Paint 中绘制图形

Drawing a graph in Paint with Android graphics

java

graphics

android

canvas

android-graphics