＊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
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