打开Widget蓝图，在Palette(画板)面板中，可以选择Widget最外层的Panel类型，默认已经帮你建了一个Canvas Panel，看了下文档：
https://docs.unrealengine.com/latest/INT/Engine/UMG/UserGuide/WidgetTypeReference/index.html
这个Canvas Panel内部不适合通过代码动态创建widget，除非你想创建的widget位置是绝对路径。Canvas Panel的好处是可以任意摆放widget，Unitform Grid Panle缺点是位置相对固定，优点是可以自动对齐。官方的三消游戏项目，使用了好几个Unitform Grid Panle，没有用到Canvas Panel。两种Panel的解释如下：
Canvas Panel
The canvas panel is a designer friendly panel that allows widgets to be laid out at arbitrary locations, anchored and z-ordered with other children of the canvas. The canvas panel is a great widget for manual layout, but bad when you want to procedurally just generate widgets and place them in a container (unless you want absolute layout). For more information on Anchors, see the Anchors page.
Uniform Grid Panel
A panel that evenly divides up available space between all of its children.
