import android.view.ViewTreeObserver.OnGlobalFocusChangeListener; //导入依赖的package包/类
public MenuView(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
    mLayoutInflater = LayoutInflater.from(context);
    // Set hardware layer type for smooth animation of lots of views.
    setLayerType(LAYER_TYPE_HARDWARE, null);
    getViewTreeObserver().addOnGlobalFocusChangeListener(new OnGlobalFocusChangeListener() {
        @Override
        public void onGlobalFocusChanged(View oldFocus, View newFocus) {
            MenuRowView newParent = getParentMenuRowView(newFocus);
            if (newParent != null) {
                if (DEBUG) Log.d(TAG, "Focus changed to " + newParent);
                // When the row is selected, the row view itself has the focus because the row
                // is collapsed. To make the child of the row have the focus, requestFocus()
                // should be called again after the row is expanded. It's done in
                // setSelectedPosition().
                setSelectedPositionSmooth(mMenuRowViews.indexOf(newParent));
            }
        }
    });
    mLayoutManager = new MenuLayoutManager(context, this);
}