<?xml version="1.0" encoding="utf-8"?>
<resources>
  <declare-styleable name="CircleImageView">
    <attr name="borderWidth" format="dimension" />
    <attr name="borderColor" format="color" />
    <attr name="cornersRadius" format="dimension" />
    </declare-styleable>
</resources>

reference   //引用资源
color       //颜色值
boolean     //布尔值
dimension   //尺寸值
float       //浮点型
integer     //整型
string      //字符串
fraction    //百分数
enum        //枚举类型
flag        //位或运算

<?xml version="1.0" encoding="utf-8"?>
<cn.apphack.test.CircleImageView 
  xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:apphack="http://schemas.android.com/apk/res-auto"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  apphack:borderWidth="1dp"
  apphack:borderColor="#ff0000"
  apphack:cornersRadius="20dp" />

class CircleImageView extends ImageView{
  ...
  public CircleImageView(Context context, AttributeSet attrs) {
    super(context, attrs);
    TypedArray a =
        context.obtainStyledAttributes(attrs,R.styleable.CircleImageView);
    mBorderWidth = a.getDimensionPixelSize(
        R.styleable.CircleImageView_borderWidth, 1);
    mBorderColor = a.getColor(
        R.styleable.CircleImageView_borderColor, Color.BLACK);
    mCornersRadius = a.getDimensionPixelSize(
        R.styleable.CircleImageView_cornersRadius, 10);
    a.recycle();
  }
}