＊gravity的取值详表

android有 android:layout_gravity 和 android:gravity，前者设置相对父控件布局，后者是设置自己内部的控件的布局。

Value	Description
top	Put the object at the top of its container, not changing its size.  将对象放在其容器的顶部，不改变其大小.
bottom	Put the object at the bottom of its container, not changing its size.  将对象放在其容器的底部，不改变其大小.
left	Put the object at the left edge of its container, not changing its size.  将对象放在其容器的左侧，不改变其大小.
right	Put the object at the right edge of its container, not changing its size.  将对象放在其容器的右侧，不改变其大小.
center_vertical	Place object in the vertical center of its container, not changing its size.  将对象纵向居中，不改变其大小.  垂直对齐方式：垂直方向上居中对齐。
fill_vertical	Grow the vertical size of the object if needed so it completely fills its container.  必要的时候增加对象的纵向大小，以完全充满其容器.  垂直方向填充
center_horizontal	Place object in the horizontal center of its container, not changing its size.  将对象横向居中，不改变其大小.  水平对齐方式：水平方向上居中对齐
fill_horizontal	Grow the horizontal size of the object if needed so it completely fills its container.  必要的时候增加对象的横向大小，以完全充满其容器.  水平方向填充
center	Place the object in the center of its container in both the vertical and horizontal axis, not changing its size.  将对象横纵居中，不改变其大小.
fill	Grow the horizontal and vertical size of the object if needed so it completely fills its container. This is the default.  必要的时候增加对象的横纵向大小，以完全充满其容器.
clip_vertical	Additional option that can be set to have the top and/or bottom edges of the child clipped to its container's bounds. The clip is based on the vertical gravity: a top gravity clips the bottom edge, a bottom gravity clips the top edge, and neither clips both edges. 附加选项，用于按照容器的边来剪切对象的顶部和/或底部的内容. 剪切基于其纵向对齐设置：顶部对齐时，剪切底部；底部对齐时剪切顶部；除此之外剪切顶部和底部. 垂直方向裁剪
clip_horizontal	Additional option that can be set to have the left and/or right edges of the child clipped to its container's bounds. The clip is based on the horizontal gravity: a left gravity clips the right edge, a right gravity clips the left edge, and neither clips both edges. 附加选项，用于按照容器的边来剪切对象的左侧和/或右侧的内容. 剪切基于其横向对齐设置：左侧对齐时，剪切右侧；右侧对齐时剪切左侧；除此之外剪切左侧和右侧. 水平方向裁剪