intellij-sdk-code-samples/simple_language_plugin/src/com/simpleplugin/SimpleFoldingBuilder.java

package com.simpleplugin;

import com.intellij.lang.ASTNode;
import com.intellij.lang.folding.*;
import com.intellij.openapi.editor.*;
import com.intellij.openapi.project.DumbAware;
import com.intellij.openapi.project.Project;
import com.intellij.openapi.util.TextRange;
import com.intellij.psi.*;
import com.intellij.psi.util.PsiTreeUtil;
import com.simpleplugin.psi.SimpleProperty;
import org.jetbrains.annotations.*;

import java.util.*;

public class SimpleFoldingBuilder extends FoldingBuilderEx implements DumbAware {
  @NotNull
  @Override
  public FoldingDescriptor[] buildFoldRegions(@NotNull PsiElement root, @NotNull Document document, boolean quick) {
    FoldingGroup group = FoldingGroup.newGroup("simple");

    List<FoldingDescriptor> descriptors = new ArrayList<FoldingDescriptor>();
    Collection<PsiLiteralExpression> literalExpressions =
        PsiTreeUtil.findChildrenOfType(root, PsiLiteralExpression.class);
    for (final PsiLiteralExpression literalExpression : literalExpressions) {
      String value = literalExpression.getValue() instanceof String ? (String) literalExpression.getValue() : null;

      if (value != null && value.startsWith("simple:")) {
        Project project = literalExpression.getProject();
        String key = value.substring(7);
        final List<SimpleProperty> properties = SimpleUtil.findProperties(project, key);
        if (properties.size() == 1) {
          descriptors.add(new FoldingDescriptor(literalExpression.getNode(),
                                                new TextRange(literalExpression.getTextRange().getStartOffset() + 1,
                                                              literalExpression.getTextRange().getEndOffset() - 1),
                                                group) {
            @Nullable
            @Override
            public String getPlaceholderText() {
              // IMPORTANT: keys can come with no values, so a test for null is needed
              // IMPORTANT: Convert embedded \n to backslash n, so that the string will look like it has LF embedded
              // in it and embedded " to escaped "
              String valueOf = properties.get(0).getValue();
              return valueOf == null ? "" : valueOf.replaceAll("\n", "\\n").replaceAll("\"", "\\\\\"");
            }
          });
        }
      }
    }
    return descriptors.toArray(new FoldingDescriptor[descriptors.size()]);
  }

  @Nullable
  @Override
  public String getPlaceholderText(@NotNull ASTNode node) {
    String retTxt = "...";
    if ( node.getPsi() instanceof PsiLiteralExpression ) {
      PsiLiteralExpression nodeElement = (PsiLiteralExpression) node.getPsi();
      String key = ((String) nodeElement.getValue()).substring("simple:".length());
      final List< SimpleProperty > properties = SimpleUtil.findProperties(nodeElement.getProject(), key);
      String place = properties.get(0).getValue();
      // IMPORTANT: keys can come with no values, so a test for null is needed
      // IMPORTANT: Convert embedded \n to backslash n, so that the string will look
      // like it has LF embedded in it and embedded " to escaped "
      return place == null ? retTxt : place.replaceAll("\n", "\\n").replaceAll("\"", "\\\\\"");
    }
    return retTxt;
  }

  @Override
  public boolean isCollapsedByDefault(@NotNull ASTNode node) {
    return true;
  }
}