每一個布局均有一個叫LayoutParams的內(nèi)部類，如：

LinearLayout.LayoutParams ?RelativeLayout.LayoutParams ?AbsoluteLayout.LayoutParams ?TableLayout.LayoutParams ?TableLayout.LayoutParams ?FrameLayout.LayoutParams

此內(nèi)部類用于指明某個view在其父元素中的位置，以linearLayout為例：

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個xml屬性：layout_gravity, layout_weight

但在RelativeLayout中，由于其涉及較多的位置選項，因此有多個xml屬性：

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.?

總而言之，LayoutParams子類用于指定某個元素的位置信息。

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