每一个布局均有一个叫LayoutParams的内部类，如：
LinearLayout.LayoutParams
RelativeLayout.LayoutParams
AbsoluteLayout.LayoutParams
TableLayout.LayoutParams
TableLayout.LayoutParams
FrameLayout.LayoutParams
此内部类用于指明某个view在其父元素中的位置，以linearLayout为例：
Per-child
layout information associated with LinearLayout.
Standard
gravity constant that a child supplies to its parent. Defines how the child view should be positioned, on both the X and Y axes, within its enclosing layout.
在LinearLayout中，只有2个xml属性：layout_gravity, layout_weight
但在RelativeLayout中，由于其涉及较多的位置选项，因此有多个xml属性：
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| android:layout_above | Positions the bottom edge of this view above the given anchor view ID. | |
| android:layout_alignBaseline | Positions the baseline of this view on the baseline of the given anchor view ID. | |
| android:layout_alignBottom | Makes the bottom edge of this view match the bottom edge of the given anchor view ID. | |
| android:layout_alignEnd | Makes the end edge of this view match the end edge of the given anchor view ID. | |
| android:layout_alignLeft | Makes the left edge of this view match the left edge of the given anchor view ID. | |
| android:layout_alignParentBottom | If true, makes the bottom edge of this view match the bottom edge of the parent. | |
| android:layout_alignParentEnd | If true, makes the end edge of this view match the end edge of the parent. | |
| android:layout_alignParentLeft | If true, makes the left edge of this view match the left edge of the parent. | |
| android:layout_alignParentRight | If true, makes the right edge of this view match the right edge of the parent. | |
| android:layout_alignParentStart | If true, makes the start edge of this view match the start edge of the parent. | |
| android:layout_alignParentTop | If true, makes the top edge of this view match the top edge of the parent. | |
| android:layout_alignRight | Makes the right edge of this view match the right edge of the given anchor view ID. | |
| android:layout_alignStart | Makes the start edge of this view match the start edge of the given anchor view ID. | |
| android:layout_alignTop | Makes the top edge of this view match the top edge of the given anchor view ID. | |
| android:layout_alignWithParentIfMissing | If set to true, the parent will be used as the anchor when the anchor cannot be be found for layout_toLeftOf, layout_toRightOf, etc. | |
| android:layout_below | Positions the top edge of this view below the given anchor view ID. | |
| android:layout_centerHorizontal | If true, centers this child horizontally within its parent. | |
| android:layout_centerInParent | If true, centers this child horizontally and vertically within its parent. | |
| android:layout_centerVertical | If true, centers this child vertically within its parent. | |
| android:layout_toEndOf | Positions the start edge of this view to the end of the given anchor view ID. | |
| android:layout_toLeftOf | Positions the right edge of this view to the left of the given anchor view ID. | |
| android:layout_toRightOf | Positions the left edge of this view to the right of the given anchor view ID. | |
| android:layout_toStartOf | Positions the end edge of this view to the start of the given anchor view ID. |
总而言之，LayoutParams子类用于指定某个元素的位置信息。
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