feat: add method to get field names from templates

Author: avolutions

src/Template/Services/HandlebarsTemplateService.cs

@@ -4,7 +4,12 @@ namespace Avolutions.Baf.Core.Template.Services;
 
 public class HandlebarsTemplateService : TemplateService<string, string>
 {
-    protected override Task<string> ApplyValuesToTemplateAsync(string template, IDictionary<string, string> values, CancellationToken ct)
+    public override IReadOnlyList<string> ExtractFieldNames(Stream template)
+    {
+        throw new NotImplementedException();
+    }
+
+    public override Task<string> ApplyValuesToTemplateAsync(string template, IDictionary<string, string> values, CancellationToken ct)
     {
         var compiledTemplate = Handlebars.Compile(template);
         var result = compiledTemplate(values);

src/Template/Services/PdfTemplateService.cs

@@ -18,8 +18,13 @@ public PdfTemplateService()
             _fontsConfigured = true;
         }
     }
-    
-    protected override Task<byte[]> ApplyValuesToTemplateAsync(Stream template, IDictionary<string, string> values, CancellationToken ct)
+
+    public override IReadOnlyList<string> ExtractFieldNames(Stream template)
+    {
+        throw new NotImplementedException();
+    }
+
+    public override Task<byte[]> ApplyValuesToTemplateAsync(Stream template, IDictionary<string, string> values, CancellationToken ct)
     {
         using var templateBuffer = new MemoryStream();
         template.CopyTo(templateBuffer);

src/Template/Services/TemplateService.cs

@@ -15,18 +15,21 @@ public Task<TResult> ApplyModelToTemplateAsync(TTemplate template, object model,
 
         return ApplyValuesToTemplateAsync(template, values, ct);
     }
-    
-    protected abstract Task<TResult> ApplyValuesToTemplateAsync(
+
+    public abstract IReadOnlyList<string> ExtractFieldNames(Stream template);
+
+    public abstract Task<TResult> ApplyValuesToTemplateAsync(
         TTemplate template,
         IDictionary<string, string> values,
         CancellationToken ct);
 
-    protected Dictionary<string, string> BuildValueDictionary(object model)
+    protected virtual Dictionary<string, string> BuildValueDictionary(object model)
     {
-        var result = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
         var type = model.GetType();
+        var properties = type.GetProperties(BindingFlags.Instance | BindingFlags.Public);
+        var result = new Dictionary<string, string>(properties.Length, StringComparer.OrdinalIgnoreCase);
 
-        foreach (var property in type.GetProperties(BindingFlags.Instance | BindingFlags.Public))
+        foreach (var property in properties)
         {
             if (!property.CanRead)
             {

src/Template/Services/WordTemplateService.cs

@@ -6,10 +6,48 @@ namespace Avolutions.Baf.Core.Template.Services;
 
 public class WordTemplateService : TemplateService<Stream, byte[]>
 {
-    protected override Task<byte[]> ApplyValuesToTemplateAsync(Stream template, IDictionary<string, string> values, CancellationToken ct)
+    public override IReadOnlyList<string> ExtractFieldNames(Stream template)
+    {
+        var fieldNames = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+
+        using var document = WordprocessingDocument.Open(template, false);
+        
+        var body = document.MainDocumentPart?.Document.Body;
+        if (body != null)
+        {
+            ExtractMergeFieldNames(body, fieldNames);
+        }
+
+        return fieldNames.ToList();
+    }
+    
+    private static void ExtractMergeFieldNames(OpenXmlElement root, HashSet<string> fieldNames)
+    {
+        foreach (var fieldCode in root.Descendants<FieldCode>())
+        {
+            var instruction = fieldCode.Text;
+            if (string.IsNullOrWhiteSpace(instruction))
+            {
+                continue;
+            }
+
+            if (!instruction.Contains("MERGEFIELD", StringComparison.OrdinalIgnoreCase))
+            {
+                continue;
+            }
+
+            var fieldName = ExtractMergeFieldName(instruction);
+            if (fieldName != null)
+            {
+                fieldNames.Add(fieldName);
+            }
+        }
+    }
+
+    public override Task<byte[]> ApplyValuesToTemplateAsync(Stream template, IDictionary<string, string> values, CancellationToken ct)
     {
         // Copy to a writable, seekable stream
-        var output = new MemoryStream();
+        using var output = new MemoryStream();
         template.CopyTo(output);
         output.Position = 0;
 
@@ -21,8 +59,6 @@ protected override Task<byte[]> ApplyValuesToTemplateAsync(Stream template, IDic
                 ReplaceMergeFields(body, values);
             }
         }
-
-        output.Position = 0;
 
         return Task.FromResult(output.ToArray());
     }