gravity 這個英文單詞是重心的意思，在這裏就表示停靠位置的意思。
android:layout_gravity 和 android:gravity 的區別
從名字上可以看到，android:gravity是對元素本身說的，元素本身的文本顯示在什麼地方靠着換個屬性設置，不過不設置默認是在左側的。
android:layout_gravity是相對與它的父元素說的，說明元素顯示在父元素的什麼位置。
比如說button： android:layout_gravity 表示按鈕在界面上的位置。 android:gravity表示button上的字在button上的位置。
可選值
這兩個屬性可選的值有：top、bottom、left、right、center_vertical、fill_vertical、center_horizontal、fill_horizontal、center、fill、clip_vertical。
而且這些屬性是可以多選的，用“|”分開。
默認這個的值是：Gravity.LEFT
對這些屬性的描述：
出自：
http://androidmirror.com/guide/topics/resources/drawable-resource.html
http://android.toolib.net/reference/android/graphics/drawable/ClipDrawable.html
| Value | Description |
| top | Put the object at the top of its container, not changing its size. 將對象放在其容器的頂部，不改變其大小. |
| bottom | Put the object at the bottom of its container, not changing its size. 將對象放在其容器的底部，不改變其大小. |
| left | Put the object at the left edge of its container, not changing its size. 將對象放在其容器的左側，不改變其大小. |
| right | Put the object at the right edge of its container, not changing its size. 將對象放在其容器的右側，不改變其大小. |
| center_vertical | Place object in the vertical center of its container, not changing its size. 將對象縱向居中，不改變其大小. 垂直對齊方式：垂直方向上居中對齊。 |
| fill_vertical | Grow the vertical size of the object if needed so it completely fills its container. 必要的時候增加對象的縱向大小，以完全充滿其容器. 垂直方向填充 |
| center_horizontal | Place object in the horizontal center of its container, not changing its size. 將對象橫向居中，不改變其大小. 水平對齊方式：水平方向上居中對齊 |
| fill_horizontal | Grow the horizontal size of the object if needed so it completely fills its container. 必要的時候增加對象的橫向大小，以完全充滿其容器. 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. 將對象橫縱居中，不改變其大小. |
| fill | Grow
the horizontal and vertical size of the object if needed so it
completely fills its container. This is the default. 必要的時候增加對象的橫縱向大小，以完全充滿其容器. |
| 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. 附加選項，用於按照容器的邊來剪切對象的頂部和/或底部的內容. 剪切基於其縱向對齊設置：頂部對齊時，剪切底部；底部對齊時剪切頂部；除此之外剪切頂部和底部. 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. 附加選項，用於按照容器的邊來剪切對象的左側和/或右側的內容. 剪切基於其橫向對齊設置：左側對齊時，剪切右側；右側對齊時剪切左側；除此之外剪切左側和右側. 水平方向裁剪 |