Layout1.2

Relative Layout

If you find yourself using several nested LinearLayout groups, you may be able to replace them with a single RelativeLayout.

RelativeLayout is a view group that displays child views in relative positions

android:layout_alignParentTopIf "true", makes the top edge of this view match the top edge of the parent.android:layout_centerVerticalIf "true", centers this child vertically within its parent.android:layout_belowPositions the top edge of this view below the view specified with a resource ID.android:layout_toRightOfPositions the left edge of this view to the right of the view specified with a resource ID.

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:layout_marginLeft="29dp"
        android:layout_marginTop="42dp"
        android:text="Button1" />

    <Button
        android:id="@+id/button4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/button3"
        android:layout_alignBottom="@+id/button3"
        android:layout_toRightOf="@+id/button3"
        android:text="Button4" />

    <Button
        android:id="@+id/button3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/button1"
        android:layout_below="@+id/button1"
        android:text="Button3" />

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_above="@+id/button4"
        android:layout_alignRight="@+id/button4"
        android:text="Button2" />
    
</RelativeLayout>

 android:layout_alignBaseline 对齐android:layout_alignBottom 底部对齐android:layout_toRightOf：某个组件右边android:layout_alignLeft：左边对齐android:layout_below：某个组件下面android:layout_above：某个组件上面android:layout_alignRight：右边对齐