android button 居中

可以看到Button 與Gallery的對齊方式是居中對齊，也即Button 與Parent居中對齊。
另外，
：Place object in the vertical center of its container, not changing its size. 這個是垂直居中對齊
”：Push object to the bottom of its container, not changing its size.放在容器的底部
android:gravity="CENTER“ ：Place the object in the center of its container in both the vertical and horizontal axis, not changing its size.放在容器的中心