Hey there, let's break down these two issues you're facing after upgrading to Android Studio 3.1—they're almost certainly linked, so let's unpack what's going on and how to fix it.

1. Why "Cannot resolve symbol" for Theme in styles.xml? #

This is the core issue that's likely causing your layout preview to fail. Here are the most common reasons:

• Mismatched Gradle Plugin Version: Android Studio 3.1 requires the Gradle plugin version 3.1.0 (or a minor update like 3.1.4). Even if you tried changing the version, incomplete syncs can leave resource indexing broken. Double-check your project-level build.gradle has:

  classpath 'com.android.tools.build:gradle:3.1.0'
  

  Then click "Sync Now" and wait for the process to finish completely.
• Broken Theme Inheritance or Missing Dependencies: If your theme inherits from a support library theme (like Theme.AppCompat.Light), make sure your appcompat-v7 dependency matches the plugin version. For AS 3.1, that's usually 27.1.1:

  implementation 'com.android.support:appcompat-v7:27.1.1'
  

  Also, check if the theme you're referencing was renamed or removed in the updated support library—some older theme names get deprecated over time.
• Corrupted Cache (Beyond Invalidate Cache): The built-in "Invalidate Cache/Restart" doesn't always clear all corrupted files. Try closing Android Studio, then delete:
  • The .gradle folder in your project root
  • The build folder inside your app module
  • Android Studio's system cache (Windows: C:\Users\<YourName>\.AndroidStudio3.1\system\caches; Mac: ~/Library/Caches/AndroidStudio3.1)
    Reopen the project and sync again.

2. Why Layout Preview Times Out & Shows Blank? #

This is a direct side effect of the Theme resolution failure. When the layout editor can't find the specified theme, it gets stuck trying to render the layout without valid styling, eventually timing out. That said, if fixing the Theme issue doesn't resolve it, check these:

• Complex Custom Views: If your layout uses custom Views, make sure they have a proper constructor that handles AttributeSet (required for preview rendering). Avoid running heavy logic (like network calls) in the constructor or onMeasure/onDraw—these can block the preview renderer.
• Mismatched Preview Device Config: Ensure the preview device's API level matches your project's minSdkVersion or targetSdkVersion. Sometimes selecting an API level that's too low/high for your theme can cause rendering issues.

Quick Test to Confirm the Link #

Create a new, empty layout file and set its theme to a default AppCompat theme (e.g., @style/AppTheme if your styles define it correctly). If this preview loads without timing out, you know the original issue is tied to the Theme resolution in your main styles/layouts.

内容的提问来源于stack exchange，提问作者hsteffano