Implement To be signed flieds

 # Changelog
 
+## 3.3.0
+
+- Implement To be signed flieds
+
 ## 3.2.0
 
 - Fix documentation

@@ -417,9 +417,10 @@ class PdfAnnotSign extends PdfAnnotWidget {
   @override
   void build(PdfPage page, PdfObject object, PdfDict params) {
     super.build(page, object, params);
-    assert(page.pdfDocument.sign != null);
+    if (page.pdfDocument.sign != null) {
       params['/V'] = page.pdfDocument.sign!.ref();
     }
+  }
 }
 
 enum PdfFieldFlags {

@@ -25,16 +25,17 @@ enum PdfSigFlags { signaturesExist, appendOnly }
 class PdfSignature extends PdfObjectDict {
   PdfSignature(
     PdfDocument pdfDocument, {
-    required this.crypto,
-    Set<PdfSigFlags>? flags,
-  })  : flags = flags ?? const <PdfSigFlags>{PdfSigFlags.signaturesExist},
-        super(pdfDocument, type: '/Sig');
+    required this.value,
+    required this.flags,
+  }) : super(pdfDocument, type: '/Sig');
 
   final Set<PdfSigFlags> flags;
 
-  final PdfSignatureBase crypto;
+  final PdfSignatureBase value;
 
-  int get flagsValue => flags
+  int get flagsValue => flags.isEmpty
+      ? 0
+      : flags
           .map<int>((PdfSigFlags e) => 1 >> e.index)
           .reduce((int a, int b) => a | b);
 
@@ -43,7 +44,7 @@ class PdfSignature extends PdfObjectDict {
 
   @override
   void write(PdfStream os) {
-    crypto.preSign(this, params);
+    value.preSign(this, params);
 
     _offsetStart = os.offset + '$objser $objgen obj\n'.length;
     super.write(os);
@@ -54,7 +55,7 @@ class PdfSignature extends PdfObjectDict {
     assert(_offsetStart != null && _offsetEnd != null,
         'Must reserve the object space before signing the document');
 
-    await crypto.sign(this, os, params, _offsetStart, _offsetEnd);
+    await value.sign(this, os, params, _offsetStart, _offsetEnd);
   }
 }
 

@@ -100,57 +100,6 @@ class AnnotationUrl extends AnnotationBuilder {
   }
 }
 
-class AnnotationSignature extends AnnotationBuilder {
-  AnnotationSignature(
-    this.crypto, {
-    this.name,
-    this.signFlags,
-    this.border,
-    this.flags,
-    this.date,
-    this.color,
-    this.highlighting,
-  });
-
-  final Set<PdfSigFlags>? signFlags;
-
-  final PdfSignatureBase crypto;
-
-  final String? name;
-
-  final PdfBorder? border;
-
-  final Set<PdfAnnotFlags>? flags;
-
-  final DateTime? date;
-
-  final PdfColor? color;
-
-  final PdfAnnotHighlighting? highlighting;
-
-  @override
-  void build(Context context, PdfRect? box) {
-    context.document.sign ??= PdfSignature(
-      context.document,
-      crypto: crypto,
-      flags: signFlags,
-    );
-
-    PdfAnnot(
-      context.page,
-      PdfAnnotSign(
-        rect: context.localToGlobal(box!),
-        fieldName: name,
-        border: border,
-        flags: flags,
-        date: date,
-        color: color,
-        highlighting: highlighting,
-      ),
-    );
-  }
-}
-
 class AnnotationTextField extends AnnotationBuilder {
   AnnotationTextField({
     this.name,
@@ -259,31 +208,6 @@ class UrlLink extends Annotation {
   }) : super(child: child, builder: AnnotationUrl(destination));
 }
 
-class Signature extends Annotation {
-  Signature({
-    required Widget child,
-    required PdfSignatureBase crypto,
-    required String name,
-    Set<PdfSigFlags>? signFlags,
-    PdfBorder? border,
-    Set<PdfAnnotFlags>? flags,
-    DateTime? date,
-    PdfColor? color,
-    PdfAnnotHighlighting? highlighting,
-  }) : super(
-            child: child,
-            builder: AnnotationSignature(
-              crypto,
-              signFlags: signFlags,
-              name: name,
-              border: border,
-              flags: flags,
-              date: date,
-              color: color,
-              highlighting: highlighting,
-            ));
-}
-
 class Outline extends Anchor {
   Outline({
     Widget? child,

@@ -307,3 +307,94 @@ class TextField extends StatelessWidget {
     PdfAnnot(context.page, tf);
   }
 }
+
+class Signature extends SingleChildWidget {
+  Signature({
+    Widget? child,
+    @Deprecated('Use value instead') PdfSignatureBase? crypto,
+    PdfSignatureBase? value,
+    required this.name,
+    this.appendOnly = false,
+    this.border,
+    this.flags,
+    this.date,
+    this.color,
+    this.highlighting,
+  })  : value = value ?? crypto,
+        super(child: child);
+
+  /// Field name
+  final String name;
+
+  /// Digital signature
+  final PdfSignatureBase? value;
+
+  /// Append
+  final bool appendOnly;
+
+  final PdfBorder? border;
+
+  /// Flags for this field
+  final Set<PdfAnnotFlags>? flags;
+
+  /// Date metadata
+  final DateTime? date;
+
+  /// Field color
+  final PdfColor? color;
+
+  /// Field highlighting
+  final PdfAnnotHighlighting? highlighting;
+
+  @override
+  void paint(Context context) {
+    super.paint(context);
+
+    if (value != null) {
+      context.document.sign ??= PdfSignature(
+        context.document,
+        value: value!,
+        flags: {
+          PdfSigFlags.signaturesExist,
+          if (appendOnly) PdfSigFlags.appendOnly,
+        },
+      );
+    } else {
+      paintChild(context);
+    }
+
+    final bf = PdfAnnotSign(
+      rect: context.localToGlobal(box!),
+      fieldName: name,
+      border: border,
+      flags: flags,
+      date: date,
+      color: color,
+      highlighting: highlighting,
+    );
+
+    if (child != null && value != null) {
+      final mat = context.canvas.getTransform();
+      final translation = Vector3(0, 0, 0);
+      final rotation = Quaternion(0, 0, 0, 0);
+      final scale = Vector3(0, 0, 0);
+      mat
+        ..decompose(translation, rotation, scale)
+        ..leftTranslate(-translation.x, -translation.y)
+        ..translate(box!.x, box!.y);
+
+      final canvas = bf.appearance(context.document, PdfAnnotAppearance.normal,
+          matrix: mat);
+      Widget.draw(
+        child!,
+        offset: PdfPoint.zero,
+        canvas: canvas,
+        page: context.page,
+        constraints:
+            BoxConstraints.tightFor(width: box!.width, height: box!.height),
+      );
+    }
+
+    PdfAnnot(context.page, bf);
+  }
+}

@@ -4,7 +4,7 @@ description: A pdf producer for Dart. It can create pdf files for both web or fl
 homepage: https://github.com/DavBfr/dart_pdf/tree/master/pdf
 repository: https://github.com/DavBfr/dart_pdf
 issue_tracker: https://github.com/DavBfr/dart_pdf/issues
-version: 3.2.0
+version: 3.3.0
 
 environment:
   sdk: ">=2.12.0-0 <3.0.0"