按键及其居中

实现button在屏幕居中

要实现button在LinearLayout布注意局屏幕中间居中需要做到两点在linearLayout中设置
android:orientantion 竖屏或者横屏居中
android:gravity 设置内容在layout中的位置
例如：

image.png

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="fill_parent"
android:gravity="center"
android:orientation="vertical">

<Button
android:id="@+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:text="Anr Demo" />

><Button
    android:id="@+id/button2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="center"
    android:text="Performace" />

</LinearLayout >

android:orientation

| Constant | Value | Description |
|:---------:|:-------------:|: -----|
|horizontal |0 |Defines an horizontal widget. |
| vertical | 1| Defines a vertical widget.|

android:gravity

Constant	Value	Description
bottom	50	Push object to the bottom of its container, not changing its size.
center	11	Place the object in the center of its container in both the vertical and horizontal axis, not changing its size.
center_horizontal	1	Place object in the horizontal center of its container, not changing its size.
center_vertical	10	Place object in the vertical center of its container, not changing its size.
clip_horizontal	8	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 will be based on the horizontal gravity: a left gravity will clip the right edge, a right gravity will clip the left edge, and neither will clip both edges.
clip_vertical	80	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 will be based on the vertical gravity: a top gravity will clip the bottom edge, a bottom gravity will clip the top edge, and neither will clip both edges.
end	800005	Push object to the end of its container, not changing its size.
fill	77	Grow the horizontal and vertical size of the object if needed so it completely fills its container.
fill_horizontal	7	Grow the horizontal size of the object if needed so it completely fills its container.
fill_vertical	70	Grow the vertical size of the object if needed so it completely fills its container.
left	3	Push object to the left of its container, not changing its size.
right	5	Push object to the right of its container, not changing its size.
start	800003	Push object to the beginning of its container, not changing its size.
top	30	Push object to the top of its container, not changing its size.