A file view provider (FileViewProvider) manages access to multiple PSI trees within a single file.

For example, a JSPX page has a separate PSI tree for the Java code in it (PsiJavaFile), a separate tree for the XML code (XmlFile), and a separate tree for JSP as a whole (JspFile).

Each of the PSI trees covers the entire contents of the file and contains special "outer language elements" in the places where contents in a different language can be found.

A FileViewProvider instance corresponds to a single VirtualFile, a single Document, and can retrieve multiple PsiFile instances.

How do I get a FileViewProvider?

• From a VirtualFile: PsiManager.getInstance(project).findViewProvider()
• From a PsiFile: psiFile.getViewProvider()

What can I do with a FileViewProvider?

• To get the set of all languages for which PSI trees exist in a file: fileViewProvider.getLanguages()
• To get the PSI tree for a particular language: fileViewProvider.getPsi(language). For example, to get the PSI tree for XML, use fileViewProvider.getPsi(XMLLanguage.INSTANCE).
• To find an element of a particular language at the specified offset in the file: fileViewProvider.findElementAt(offset, language)

How do I extend the FileViewProvider?

To create a file type that has multiple interspersing trees for different languages, a plugin must implement FileViewProviderFactory and return your FileViewProvider implementation from createFileViewProvider() method.

Register the factory by annotating the implementation class with @ExtensionImpl:

@ExtensionImpl
public class MyFileViewProviderFactory implements FileViewProviderFactory {
    @Override
    public FileViewProvider createFileViewProvider(VirtualFile file, Language language, PsiManager manager, boolean eventSystemEnabled) {
        return new MyFileViewProvider(manager, file, eventSystemEnabled);
    }
}