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

package com.simpleplugin;

import com.intellij.lang.ASTNode;
import com.intellij.lang.folding.FoldingBuilderEx;
import com.intellij.lang.folding.FoldingDescriptor;
import com.intellij.openapi.editor.Document;
import com.intellij.openapi.editor.FoldingGroup;
import com.intellij.openapi.project.Project;
import com.intellij.openapi.util.TextRange;
import com.intellij.psi.PsiElement;
import com.intellij.psi.PsiLiteralExpression;
import com.intellij.psi.util.PsiTreeUtil;
import com.simpleplugin.psi.SimpleProperty;
import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;

public class SimpleFoldingBuilder extends FoldingBuilderEx {
    @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) {
        return "...";
    }

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