16. Formatter

The IntelliJ Platform includes a powerful framework for implementing formatting for custom languages. A formatter enables reformatting code automatically based on code style settings. The formatter controls spaces, indents, wrap, and alignment.

Define a Block

The formatting model represents the formatting structure of a file as a tree of Block objects, with associated indent, wrap, alignment, and spacing settings. The goal is to cover each PSI element with such a block. Since each block builds its children's blocks, it can generate extra blocks or skip any PSI elements. Define SimpleBlock based on AbstractBlock.

public class SimpleBlock extends AbstractBlock {

  private final SpacingBuilder spacingBuilder;

  protected SimpleBlock(@NotNull ASTNode node, @Nullable Wrap wrap, @Nullable Alignment alignment,
                        SpacingBuilder spacingBuilder) {
    super(node, wrap, alignment);
    this.spacingBuilder = spacingBuilder;
  }

  @Override
  protected List<Block> buildChildren() {
    List<Block> blocks = new ArrayList<>();
    ASTNode child = myNode.getFirstChildNode();
    while (child != null) {
      if (child.getElementType() != TokenType.WHITE_SPACE) {
        Block block = new SimpleBlock(child, Wrap.createWrap(WrapType.NONE, false), Alignment.createAlignment(),
            spacingBuilder);
        blocks.add(block);
      }
      child = child.getTreeNext();
    }
    return blocks;
  }

  @Override
  public Indent getIndent() {
    return Indent.getNoneIndent();
  }

  @Nullable
  @Override
  public Spacing getSpacing(@Nullable Block child1, @NotNull Block child2) {
    return spacingBuilder.getSpacing(this, child1, child2);
  }

  @Override
  public boolean isLeaf() {
    return myNode.getFirstChildNode() == null;
  }

}

Define a Formatting Model Builder

Define a formatter that removes extra spaces except for the single spaces around the property separator:

foo = bar

Create SimpleFormattingModelBuilder by implementing FormattingModelBuilder.

final class SimpleFormattingModelBuilder implements FormattingModelBuilder {

  private static SpacingBuilder createSpaceBuilder(CodeStyleSettings settings) {
    return new SpacingBuilder(settings, SimpleLanguage.INSTANCE)
        .around(SimpleTypes.SEPARATOR)
        .spaceIf(settings.getCommonSettings(SimpleLanguage.INSTANCE.getID()).SPACE_AROUND_ASSIGNMENT_OPERATORS)
        .before(SimpleTypes.PROPERTY)
        .none();
  }

  @Override
  public @NotNull FormattingModel createModel(@NotNull FormattingContext formattingContext) {
    final CodeStyleSettings codeStyleSettings = formattingContext.getCodeStyleSettings();
    return FormattingModelProvider
        .createFormattingModelForPsiFile(formattingContext.getContainingFile(),
            new SimpleBlock(formattingContext.getNode(),
                Wrap.createWrap(WrapType.NONE, false),
                Alignment.createAlignment(),
                createSpaceBuilder(codeStyleSettings)),
            codeStyleSettings);
  }

}

Register the Formatter

The ``SimpleFormattingModelBuilder` implementation is registered with the IntelliJ Platform in the plugin configuration file using the com.intellij.lang.formatter extension point.

<extensions defaultExtensionNs="com.intellij">
  <lang.formatter
      language="Simple"
      implementationClass="org.intellij.sdk.language.SimpleFormattingModelBuilder"/>
</extensions>

Run the Project

Run the plugin by using the Gradle runIde task.

Open the example Simple Language properties file in the IDE Development Instance. Add some extra spaces around the = separator between language and English. Reformat the code by invoking Code | Reformat File... dialog and choose Run.

19 March 2025