Drag+Drop和MouseClick

项目中的一个树形结节，既要响应拖拽事件、又要响应点击事件。实现的时候没多想，依次实现了tree_MouseClick、tree_MouseDown、tree_MouseMove事件。出现的Bug是，偶尔会点击时不响应。

分析下来，应该是触发了MouseMove里的DoDragDrop拖拽事件，因此MouseClick被忽略了。如何正确实现呢？《Drap+Drop and MouseClick》这个帖子里给出了精确的解释，摘录如下：

Good implemented D'n'D doesn't start the operation on mouse down.
The drag operation has to start if the user presses the mouse button and
moves the mouse on a certain distance from the button-down point. Only if
this happens and all other conditions are met (see Bob's post) the
operation should be considered as starting D'n'D.

SystemInformation class has a property DragSize that should be used for
the size of the rectangle arround the click point in which the drag
operation shouldn't start.

大意就是说，不能一MouseMove就触发拖拽，而要超出一定距离之后，再触发。超出的范围，可以用系统设置SystemInformation.DragSize。于是在MouseMove里添加判断：

private void tree_MouseMove(object sender, MouseEventArgs e)
{
    if (mouseDownPosition == Point.Empty ||
        Math.Abs(e.X - mouseDownPosition.X) <= SystemInformation.DragSize.Width ||
        Math.Abs(e.Y - mouseDownPosition.Y) <= SystemInformation.DragSize.Height)
    {
        return;
    }
    (sender as TreeList).DoDragDrop(data, DragDropEffects.Copy);
}

搞定。