android 设置activity为对话框，且背景为透明

1.在drawable/目录下建立文件：bg_transparent.xml
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android" >

    <solid android:color="#88000000" /> <!-- 实体设置为透明 -->

    <stroke
        android:width="1dp"
        color="#FFFFFF" />

    <corners android:radius="20dp" />

    <padding
        android:bottom="1dp"
        android:left="1dp"
        android:right="1dp"
        android:top="1dp" />

</shape>

2.在values/ 目录下，建立风格文件style.xml
<?xml version="1.0" encoding="utf-8"?>
<resources>
 <!-- Theme  -->
 <style name="mytheme" parent="android:style/Theme.Dialog">

        <!-- <item name="android:paddingBottom">100px</item> -->
        <item name="android:windowBackground">@drawable/upload_dialog</item>
        <item name="android:windowIsTranslucent">true</item>
        <item                  name="android:windowAnimationStyle">@+android:style/Animation.Translucent</item>
    </style>
</resources>

3.在Manifest.xml中：
<activity
 android:exported="true"
        android:label="@string/app_name"
        android:name=".ShowChatMessageActivity" 
        android: