项目当中的UI有这样的一个控件，显示U盘的总容量和已经使用的容量
自己根据自定义控件的开发流程做了一个简单的控件
注：控件还有一些小问题，右边的半圆不知道怎么绘制，只能在超过矩形框的时候一次性全部绘制了，有没有大神指导一下，右边的半圆按容量增加一点点绘制

/**
 * Created by Orange on 2017/5/24.
 * <p>
 * 显示U盘容量的控件
 */

public class CapacityView extends View {

    //总容量100
    private static final int TOTAL_CAPACITY = 100;

    //总容量画笔
    private Paint greyPaint;

    //已使用容量画笔
    private Paint greenPaint;

    //控件高
    private int mHeight;

    //控件宽
    private int mWidth;

    //已使用容量的宽
    private int cWidth = 0;


    public CapacityView(Context context) {
        super(context);
    }

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

    }

    private void init() {
        greenPaint = new Paint();
        greenPaint.setAntiAlias(true);
        greenPaint.setColor(getResources().getColor(R.color.colorAccent));

        greyPaint = new Paint();
        greenPaint.setAntiAlias(true);
        greyPaint.setColor(getResources().getColor(R.color.colorPrimary));
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        mWidth = w;
        mHeight = h;
    }

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

    private void drawBack(Canvas canvas) {
        RectF rectF1 = new RectF(0, 0, mHeight, mHeight);
        canvas.drawArc(rectF1, 90, 180, true, greyPaint);

        RectF rectF2 = new RectF(mWidth - mHeight, 0, mWidth, mHeight);
        canvas.drawArc(rectF2, -90, 180, true, greyPaint);

        RectF rectF3 = new RectF(mHeight / 2, 0, mWidth - mHeight / 2, mHeight);
        canvas.drawRect(rectF3, greyPaint);
    }

    private void drawReal(Canvas canvas) {
        if (cWidth <= mHeight / 2) {
            float x = ((float) mHeight - cWidth * 2) / mHeight;
            float a = (float) (Math.acos(x) * 180 / Math.PI);
            RectF rectF1 = new RectF(0, 0, mHeight, mHeight);
            canvas.drawArc(rectF1, 180 - a, 2 * a, false, greenPaint);
            Log.d("********", a + "");
        } else if (cWidth > mHeight / 2 && cWidth <= (mWidth - mHeight / 2)) {
            RectF rectF1 = new RectF(0, 0, mHeight, mHeight);
            canvas.drawArc(rectF1, 90, 180, true, greenPaint);

            RectF rectF3 = new RectF(mHeight / 2, 0, cWidth, mHeight);
            canvas.drawRect(rectF3, greenPaint);

        } else {
            RectF rectF1 = new RectF(0, 0, mHeight, mHeight);
            canvas.drawArc(rectF1, 90, 180, true, greenPaint);

            RectF rectF2 = new RectF(mWidth - mHeight, 0, mWidth, mHeight);
            canvas.drawArc(rectF2, -90, 180, true, greenPaint);

            RectF rectF3 = new RectF(mHeight / 2, 0, mWidth - mHeight / 2, mHeight);
            canvas.drawRect(rectF3, greenPaint);
        }
    }


    /**
     * 在你的activity内调用，设置u盘已使用的容量
     * @param w 0~100的值，即使用的百分比
     */
    public void setcWidth(int w) {
        this.cWidth = w;
        cWidth = (int) (((float) cWidth) / TOTAL_CAPACITY * mWidth);
        invalidate();
    }