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Syntax Highlighting and Error Highlighting

The class used to specify how a particular range of text should be highlighted is called TextAttributesKey. An instance of this class is created for every distinct type of item that should be highlighted (keyword, number, string, etc.). The TextAttributesKey defines the default attributes applied to items of the corresponding type (for example, keywords are bold, numbers are blue, strings are bold and green). Highlighting from multiple TextAttributesKey items can be layered - for example, one key may define an item's boldness and another color.

Color Settings

The mapping of the TextAttributesKey to specific attributes used in an editor is defined by the EditorColorsScheme class. It can be configured by the user by providing an implementation of ColorSettingPage registered in com.intellij.colorSettingsPage extension point.

The Export to HTML feature uses the same syntax highlighting mechanism as the editor, so it will work automatically for custom languages, which provide a syntax highlighter.

Examples: - ColorSettingsPage for Properties language plugin - Custom Language Support Tutorial: Color Settings Page

NOTE New functionality about Language Defaults and support for additional color schemes are detailed in Color Scheme Management.

TIP To force re-highlighting, use DaemonCodeAnalyzer.restart().

The syntax and error highlighting are performed on multiple levels: Lexer, Parser, and (External) Annotator.

Lexer

The first syntax highlighting level is based on the lexer output and is provided through the SyntaxHighlighter interface. The syntax highlighter returns the TextAttributesKey instances for each token type, which needs special highlighting. For highlighting lexer errors, the standard TextAttributesKey for bad characters HighlighterColors.BAD_CHARACTER can be used.

Examples: - SyntaxHighlighter implementation for Properties language plugin - Custom Language Support Tutorial: Syntax Highlighter

Parser

The second level of error highlighting happens during parsing. If a particular sequence of tokens is invalid according to the grammar of the language, the PsiBuilder.error() method can highlight the invalid tokens and display an error message showing why they are not valid.

Annotator

The third level of highlighting is performed through the Annotator interface. A plugin can register one or more annotators in the com.intellij.annotator extension point, and these annotators are called during the background highlighting pass to process the elements in the custom language's PSI tree.

Annotators can analyze not only the syntax, but also the semantics using PSI, and thus can provide much more complex syntax and error highlighting logic. The annotator can also provide quick fixes to problems it detects. When the file is changed, the annotator is called incrementally to process only changed elements in the PSI tree.

NOTE See also Code Inspections which offer a more fine-grained control and some additional features.

Errors/Warning

See Inspections topic in Consulo UI Guidelines on how to write message texts for highlighting/quick fixes.

To highlight a region of text as a warning or error (2020.1 and later): 

    holder.newAnnotation(HighlightSeverity.WARNING, "Invalid code") // or HighlightSeverity.ERROR
        .withFix(new MyFix(psiElement))
        .create();

In previous versions, call createWarningAnnotation()/createErrorAnnotation() on the AnnotationHolder, and optionally calls registerFix() on the returned Annotation object to add a quick fix for the error or warning.

Syntax

To apply additional syntax highlighting (2020.1 and later):

    holder.newSilentAnnotation(HighlightSeverity.INFORMATION)
            .range(rangeToHighlight).textAttributes(MyHighlighter.EXTRA_HIGHLIGHT_ATTRIBUTE).create();

In previous versions, call AnnotationHolder.createInfoAnnotation() with an empty message and then Annotation.setTextAttributes().

Examples: - Annotator for Properties language plugin - Custom Language Support Tutorial: Annotator

External Tool

Finally, if the custom language employs external tools for validating files in the language (for example, uses the Xerces library for XML schema validation), it can provide an implementation of the ExternalAnnotator interface and register it in com.intellij.externalAnnotator extension point.

The ExternalAnnotator highlighting has the lowest priority and is invoked only after all other background processing has completed. It uses the same AnnotationHolder interface for converting the output of the external tool into editor highlighting.

To skip running specific ExternalAnnotator for given file, register ExternalAnnotatorsFilter extension in com.intellij.daemon.externalAnnotatorsFilter extension point.