saveLayer：https://blog.csdn.net/u010852160/article/details/72918551

知识点：saveLayer 建立新的图层 进行绘制
代码段

        int layer;
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            layer = canvas.saveLayer(0, 0, getWidth(), getHeight(), null);
        } else {
            layer = canvas.saveLayer(0, 0, getWidth(), getHeight(), null, Canvas.ALL_SAVE_FLAG);
        }

        canvas.drawBitmap(mBackground, 0, 0, null);
        canvas.drawCircle(mStartX, mStartY, mCurrentRadius, mPaint);
        canvas.restoreToCount(layer);

save : https://blog.csdn.net/lijiuche/article/details/53467844
save 是在相同的图层进行绘制