首先定义主题

		
<resources xmlns:tools="http://schemas.android.com/tools">
    <style name="Theme.Main.Fullscreen" parent="Theme.AppCompat.Light">
        <item name="windowActionBar">false</item>//无ActionBar
        <item name="windowNoTitle">true</item> //无标题
        <item name="android:windowFullscreen">true</item> //全屏
        <item name="android:navigationBarColor" tools:targetApi="lollipop">@android:color/transparent</item>
        <item name="android:windowBackground">@drawable/fo2</item>//背景图
        <item name="android:windowDrawsSystemBarBackgrounds" tools:targetApi="lollipop">true</item>
    </style>
</resources>
		
		

Layout

		
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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:id="@+id/title"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    tools:context="cn.netkiller.demo.MainActivity">

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:scaleType="matrix"
        app:srcCompat="@drawable/fo2"
        tools:ignore="MissingConstraints" />
</androidx.constraintlayout.widget.ConstraintLayout>		
		
		

AndroidManifest.xml 切换主题

		
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android">

    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
    <uses-permission android:name="android.permission.CHANGE_NETWORK_STATE" />
    <uses-permission android:name="android.permission.READ_PHONE_STATE" />
    <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.RECORD_AUDIO" />

    <application
        android:name="cn.netkiller.demo.ContextHolder"
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="Demo"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.Main.Fullscreen"
        android:usesCleartextTraffic="true">
        <activity
            android:name="cn.netkiller.demo.MainActivity"
            android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>


    </application>

</manifest>