How do I create a transparent Activity on Android?

It goes like this:

<activity android:name=".usual.activity.Declaration" android:theme="@android:style/Theme.Translucent.NoTitleBar" />

2021 facts

Just add

<item name="android:windowBackground">@android:color/transparent</item>

You're done.

windowIsFloating wrong, this makes an INSET floating window.

windowContentOverlay only relates to shadows.

windowIsTranslucent is WRONG, it DOES NOT make it so you can see the activity behind. windowIsTranslucent is only relevant if animating transitions.

backgroundDimEnabled dims the activity below, BUT, it is completely buggy on different devices. (In some cases, it does nothing unless you are using windowIsFloating; in general the behavior is totally buggy/indeterminate.)

colorBackgroundCacheHint is irrelevant except on extremely old devices, the default is null anyway.

in addition to the above answers:

to avoid android Oreo related crash on activity

<style name="AppTheme.Transparent" parent="@style/Theme.AppCompat.Dialog">
    <item name="windowNoTitle">true</item>
    <item name="android:windowCloseOnTouchOutside">false</item>
</style>

<activity
     android:name="xActivity"
     android:theme="@style/AppTheme.Transparent" />

If you are using AppCompatActivity then add this in styles.xml

<style name="TransparentCompat" parent="@style/Theme.AppCompat.Light.DarkActionBar">
    <item name="android:windowNoTitle">true</item>
    <item name="android:windowBackground">@android:color/transparent</item>
    <item name="android:colorBackgroundCacheHint">@null</item>
    <item name="android:windowIsTranslucent">true</item>
    <item name="android:windowAnimationStyle">@android:style/Animation</item>
</style>

In manifest file you can add this theme to activity tag like this

android:theme="@style/TransparentCompat"

for more details read this article

There're two ways:

1. Using Theme.NoDisplay
2. Using Theme.Translucent.NoTitleBar

Using Theme.NoDisplay will still work… but only on older Android devices. On Android 6.0 and higher, using Theme.NoDisplay without calling finish() in onCreate() (or, technically, before onResume()) will crash your app. This is why the recommendation is to use Theme.Translucent.NoTitleBar, which does not suffer from this limitation.”

All those answers might be confusing, there is a difference between Transparent activity and None UI activity.

Using this:

android:theme="@android:style/Theme.Translucent.NoTitleBar"

Will make the activity transparent but will block the UI.

If you want a None UI activity than use this:

android:theme="@android:style/Theme.NoDisplay"

You can remove setContentView(R.layout.mLayout) from your activity and set theme as android:theme="@style/AppTheme.Transparent". Check this link for more details.

just put this in style.xml

<item name="android:windowBackground">@android:color/transparent</item>

oR Add in Manifest

<activity android:name=".usual.activity.Declaration" 
 android:theme="@android:style/Theme.Translucent.NoTitleBar" />

Along with the gnobal's above solution, I had to set alpha to 0 in the layout file of that particular activity, because on certain phone (Redmi Narzo 20 pro running on Android 10) a dialog portion of the screen was showing with the screen that was supposed to be transparent. For some reason the windowIsFloating was causing this issue, but on removing it I wasn't getting the desired output.

Steps:

1. Add the following in the style.xml located under res > values > styles.xml

    <style name="Theme.Transparent" parent="android:Theme">
      <item name="android:windowIsTranslucent">true</item>
      <item name="android:windowBackground">@android:color/transparent</item>
      <item name="android:windowContentOverlay">@null</item>
      <item name="android:windowNoTitle">true</item>
      <item name="android:windowIsFloating">true</item>
      <item name="android:backgroundDimEnabled">false</item>
      <item name="android:colorBackgroundCacheHint">@null</item>
    </style>
   

   
   

2. Set the theme of the activity with the above style in AndroidManifest.xml

   <activity
         android:name=".activityName"
         android:theme="@style/Theme.Transparent"/>
   

   
   

3. Open your layout file of the activity on which you applied the above style and set it's alpha value to 0 (android:alpha="0") for the parent layout element.

   <?xml version="1.0" encoding="utf-8"?>
   <androidx.constraintlayout.widget.ConstraintLayout 
       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:alpha="0">
   
     <WebView        
           android:layout_width="match_parent"
           android:layout_height="match_parent"
           android:alpha="0"/>
   
   </androidx.constraintlayout.widget.ConstraintLayout>