<?xml version="1.0" encoding="utf-8"?>
<resources>

  <declare-styleable name="WritingBoardView">
    <attr name="boardBackground" format="color"></attr> <!--画板颜色-->
    <attr name="paintColor" format="color"></attr> <!--画笔颜色-->
    <attr name="paintWidth" format="dimension"></attr> <!--画笔宽度-->
  </declare-styleable>
</resources>

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  xmlns:custom="http://schemas.android.com/apk/res-auto"
  android:paddingBottom="@dimen/activity_vertical_margin"
  android:paddingLeft="@dimen/activity_horizontal_margin"
  android:paddingRight="@dimen/activity_horizontal_margin"
  android:paddingTop="@dimen/activity_vertical_margin"
  tools:context="com.qiangyu.test.writingboardview.MainActivity">

  <com.qiangyu.test.writingboardview.view.WritingBoardView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    custom:paintColor="@color/colorAccent"
    custom:boardBackground="@color/colorPrimary"
    custom:paintWidth="3dp"/>
</RelativeLayout>

  private int mBoardBackground;//画板颜色
  private int mPaintColor;//画笔颜色
  private int mPaintWidth;//画笔宽度
  private Path mPath;
  private Paint mPaint;//画笔

  public WritingBoardView(Context context) {
    this(context,null);
  }

  public WritingBoardView(Context context, AttributeSet attrs) {
    this(context, attrs,0);
  }

  public WritingBoardView(Context context, AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);
    init(context,attrs);
  }


 private void init(Context context,AttributeSet attrs) {
    TypedArray a = context.obtainStyledAttributes(attrs,R.styleable.WritingBoardView);
    mBoardBackground =  a.getColor(R.styleable.WritingBoardView_boardBackground,Color.WHITE);
    mPaintColor =  a.getColor(R.styleable.WritingBoardView_paintColor,Color.BLUE);
    mPaintWidth = a.getDimensionPixelSize(R.styleable.WritingBoardView_paintWidth,
        (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP,5,getResources().getDisplayMetrics()));
    a.recycle();
    mPaint = new Paint();
    mPath = new Path();
    setBackgroundColor(mBoardBackground);
    mPaint.setColor(mPaintColor);
    mPaint.setStrokeWidth(mPaintWidth);
    mPaint.setStyle(Paint.Style.STROKE);
    mPaint.setAntiAlias(true);
  }

/**
 * The Path class encapsulates compound (multiple contour) geometric paths
 * consisting of straight line segments, quadratic curves, and cubic curves.
 * It can be drawn with canvas.drawPath(path, paint), either filled or stroked
 * (based on the paint's Style), or it can be used for clipping or to draw
 * text on a path.
 */
public class Path {

  ...

} 

 /**
   * Set the beginning of the next contour to the point (x,y).
   *
   * @param x The x-coordinate of the start of a new contour
   * @param y The y-coordinate of the start of a new contour
   */
  public void moveTo(float x, float y) {
    native_moveTo(mNativePath, x, y);
  } 

/**
   * Add a line from the last point to the specified point (x,y).
   * If no moveTo() call has been made for this contour, the first point is
   * automatically set to (0,0).
   *
   * @param x The x-coordinate of the end of a line
   * @param y The y-coordinate of the end of a line
   */
  public void lineTo(float x, float y) {
    isSimplePath = false;
    native_lineTo(mNativePath, x, y);
  } 

 @Override
  public boolean onTouchEvent(MotionEvent event) {
    float touchX = event.getX();
    float touchY = event.getY();
    switch (event.getAction()){
      case MotionEvent.ACTION_DOWN:
        mPath.moveTo(touchX,touchY);//重新设置即将出现的线的起点
        break;
      case MotionEvent.ACTION_MOVE:
        mPath.lineTo(touchX,touchY);//连线
        break;
      case MotionEvent.ACTION_UP:
        break;
    }
    invalidate();//通知系统重绘
    return true;//要处理当前事件
  }

 @Override
  protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    canvas.drawPath(mPath,mPaint);
  }