关于LayoutParams

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：

Attribute Name	Related Method	Description
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。