流程：

• 创建MyView继承View/ViewGroup,需要三个构造方法
• 重写onDraw（）方法
• 在布局xml中引用

如果需要添加自定义view的自定义属性：

• 在values包下的attrs文件中添加属性id
• 在 MyView的构造方法中解析这个属性值, 在onDraw（）中设置这个值
• 在布局xml中引用

自定义View需要注意的坑

• 支持特殊属性，warp_content，padding & margin
  如果不在onMeasur()中设置warp_content属性，那么设置wrap_content则会失效，至于为什么以后再补吧
• 多线程直接使用post（），view本生提供了post，不需要自己创建handler
• 注意内存泄漏，动画需要及时停止
• 处理好事件冲突

demo:

创建MyView，继承View/ViewGroup

public class CircularCoverView extends View

在values下的attrs文件创建CircularCoverView的自定义属性

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <!--RoundImageView-->
    <declare-styleable name="roundedimageview">
        <attr name="border_thickness" format="dimension" />
        <attr name="border_inside_color" format="color" />
        <attr name="border_outside_color" format="color"/>
        <attr name="border_radius" format="dimension"/>
    </declare-styleable>
</resources>

重写三个构造方法，在构造方法中解析自定义属性

 public CircularCoverView(Context context) {
        this(context, null, 0);
    }

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

    public CircularCoverView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        TypedArray typedArray = context.obtainStyledAttributes(attrs, R.styleable.CircularCoverView);
        leftTopRadians = typedArray.getDimensionPixelSize(R.styleable.CircularCoverView_left_top_radius, leftTopRadians);
        leftBottomRadians = typedArray.getDimensionPixelSize(R.styleable.CircularCoverView_left_bottom_radius, leftBottomRadians);
        rightTopRadians = typedArray.getDimensionPixelSize(R.styleable.CircularCoverView_right_top_radius, rightTopRadians);
        rightBottomRadians = typedArray.getDimensionPixelSize(R.styleable.CircularCoverView_right_bottom_radius, rightBottomRadians);
        coverColor = typedArray.getColor(R.styleable.CircularCoverView_cover_color, coverColor);
    }

重写onDraw（）

protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        try {
            Paint paint = new Paint();
            paint.setFilterBitmap(false);
            paint.setStyle(Paint.Style.FILL);
            //create a canvas layer to show the mix-result
            @SuppressLint("WrongConstant") int sc = canvas.saveLayer(0, 0, getWidth(), getHeight(), null);
            //draw sector-dst-bitmap at first.
            canvas.drawBitmap(drawSector(getWidth(), getHeight()), 0, 0, paint);
            //set Xfermode of paint.
            paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_OUT));
            //then draw rect-src-bitmap
            canvas.drawBitmap(drawRect(getWidth(), getHeight()), 0, 0, paint);
            paint.setXfermode(null);
            //restore the canvas
            canvas.restoreToCount(sc);
        }catch (Error e){

        }
    }

在xml引用并且设置自定义属性值

<com.jszc.mobile.videostream.ui.CircularCoverView
            android:id="@+id/head_cover"
            android:layout_width="56dp"
            android:layout_height="56dp"
            android:layout_centerInParent="true"
            app:left_top_radius="28dp"/>

完整的代码

public class CircularCoverView extends View {

  private int leftTopRadians = 30;        //leftTopRadians
  private int leftBottomRadians = 30;     //leftBottomRadians
  private int rightTopRadians = 30;       //rightTopRadians
  private int rightBottomRadians = 30;    //rightBottomRadians
  private int border = 0;

  private int coverColor = 0xffeaeaea;    //color of cover.

  public CircularCoverView(Context context) {
      this(context, null, 0);
  }

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

  public CircularCoverView(Context context, AttributeSet attrs, int defStyleAttr) {
      super(context, attrs, defStyleAttr);
      TypedArray typedArray = context.obtainStyledAttributes(attrs, R.styleable.CircularCoverView);
      leftTopRadians = typedArray.getDimensionPixelSize(R.styleable.CircularCoverView_left_top_radius, leftTopRadians);
      leftBottomRadians = typedArray.getDimensionPixelSize(R.styleable.CircularCoverView_left_bottom_radius, leftBottomRadians);
      rightTopRadians = typedArray.getDimensionPixelSize(R.styleable.CircularCoverView_right_top_radius, rightTopRadians);
      rightBottomRadians = typedArray.getDimensionPixelSize(R.styleable.CircularCoverView_right_bottom_radius, rightBottomRadians);
      coverColor = typedArray.getColor(R.styleable.CircularCoverView_cover_color, coverColor);
  }


  /**
   * set radians of cover.
   */
  public void setRadians(int leftTopRadians, int rightTopRadians, int leftBottomRadians, int rightBottomRadians,int border) {
      this.leftTopRadians = leftTopRadians;
      this.rightTopRadians = rightTopRadians;
      this.leftBottomRadians = leftBottomRadians;
      this.rightBottomRadians = rightBottomRadians;
      this.border = border;
  }

  /**
   * set color of cover.
   *
   * @param coverColor cover's color
   */
  public void setCoverColor(@ColorInt int coverColor) {
      this.coverColor = coverColor;
  }

  /**
   * create a sector-bitmap as the dst.
   *
   * @param w width of bitmap
   * @param h height of bitmap
   * @return bitmap
   */
  private Bitmap drawSector(int w, int h) {
      Bitmap bm = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
      Canvas c = new Canvas(bm);
      Paint p = new Paint(Paint.ANTI_ALIAS_FLAG);
      p.setColor(0xFFFFCC44);//notice:cannot set transparent color here.otherwise cannot clip at final.

      c.drawArc(new RectF(border, border, leftTopRadians * 2+border, leftTopRadians * 2+border), 180, 90, true, p);
      c.drawArc(new RectF(border, getHeight() - leftBottomRadians * 2-border, leftBottomRadians * 2+border, getHeight()-border), 90, 90, true, p);
      c.drawArc(new RectF(getWidth() - rightTopRadians * 2-border, border, getWidth()-border, rightTopRadians * 2+border), 270, 90, true, p);
      c.drawArc(new RectF(getWidth() - rightBottomRadians * 2-border, getHeight() - rightBottomRadians * 2-border, getWidth()-border, getHeight()-border), 0, 90, true, p);
      return bm;
  }

  /**
   * create a rect-bitmap as the src.
   *
   * @param w width of bitmap
   * @param h height of bitmap
   * @return bitmap
   */
  private Bitmap drawRect(int w, int h) {
      Bitmap bm = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
      Canvas c = new Canvas(bm);
      Paint p = new Paint(Paint.ANTI_ALIAS_FLAG);
      p.setColor(coverColor);

      c.drawRect(new RectF(border, border, leftTopRadians+border, leftTopRadians+border), p);
      c.drawRect(new RectF(border, getHeight() - leftBottomRadians-border, leftBottomRadians+border, getHeight()-border), p);
      c.drawRect(new RectF(getWidth() - rightTopRadians-border, border, getWidth()-border, rightTopRadians+border), p);
      c.drawRect(new RectF(getWidth() - rightBottomRadians-border, getHeight() - rightBottomRadians-border, getWidth()-border, getHeight()-border), p);

      c.drawRect(new RectF(0, 0, getWidth(), border), p);
      c.drawRect(new RectF(0, 0, border, getHeight()), p);
      c.drawRect(new RectF(getWidth()-border, 0, getWidth(), getHeight()), p);
      c.drawRect(new RectF(0, getHeight()-border, getWidth(), getHeight()), p);

      return bm;
  }

  @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
  @Override
  protected void onDraw(Canvas canvas) {
      super.onDraw(canvas);
      try {
          Paint paint = new Paint();
          paint.setFilterBitmap(false);
          paint.setStyle(Paint.Style.FILL);

          //create a canvas layer to show the mix-result
          @SuppressLint("WrongConstant") int sc = canvas.saveLayer(0, 0, getWidth(), getHeight(), null);
          //draw sector-dst-bitmap at first.
          canvas.drawBitmap(drawSector(getWidth(), getHeight()), 0, 0, paint);
          //set Xfermode of paint.
          paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_OUT));
          //then draw rect-src-bitmap
          canvas.drawBitmap(drawRect(getWidth(), getHeight()), 0, 0, paint);
          paint.setXfermode(null);
          //restore the canvas
          canvas.restoreToCount(sc);
      }catch (Error e){

      }
  }
}