android:alpha="0.5"

参考 View.html#attr_android:alpha

alpha property of the view, as a value between 0 (completely transparent) and 1 (completely opaque).
Related Methods: setAlpha(float)

请善用developer.android.com

<TextView
...
background="#99000000"
style="@style/Transparent"
...
</TextView>

<style name="Transparent" parent="AppTheme">

    <item name="android:windowBackground">@android:color/transparent</item>
    <item name="android:colorBackgroundCacheHint">@null</item>
    <item name="android:windowIsTranslucent">true</item>
    <item name="android:windowAnimationStyle">@null</item>
    <item name="android:windowNoTitle">true</item>

</style>