android布局文件的可视化面板不能显示解决

不显示原因：
       在布局文件中加入了自定义控件，并在自定义控件的构造函数或者其他绘制相关地方使用系统依赖的代码，会导致可视化编辑器无法工作报错，一般会在下方提示：Use View.isInEditMode() in your custom views to skip code when shown in Eclipse
解决方法：
在自定义控件的构造函数中赋值语句后加如下判断：if (isInEditMode()) { return; }

isInEditMode:

Indicates whether this View is currently in edit mode. A View is usually in edit mode when displayed within a developer tool. For instance, if this View is being drawn by a visual user interface builder, this method should return true. Subclasses should check the return value of this method to provide different behaviors if their normal behavior might interfere with the host environment. For instance: the class spawns a thread in its constructor, the drawing code relies on device-specific features, etc. This method is usually checked in the drawing code of custom widgets.

例子：

public class MyRelativeLayout extends RelativeLayout {
	private Handler mainHandler = null; //与主Activity通信的Handler对象

	public MyRelativeLayout(Context context, AttributeSet attrs) {
		super(context, attrs, 0);
		mContext = context;
		if (isInEditMode()) { return; }
		mainHandler = ((MainActivity)mContext).getHandler();
	}
}