Add screenshot example

@@ -2,29 +2,38 @@ import 'dart:io';
 import 'dart:ui';
 
 import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
 import 'package:pdf/pdf.dart';
 import 'package:printing/printing.dart';
 
 void main() => runApp(new MaterialApp(home: new MyApp()));
 
-class MyApp extends StatelessWidget {
+class MyApp extends StatefulWidget {
+  @override
+  MyAppState createState() {
+    return new MyAppState();
+  }
+}
+
+class MyAppState extends State<MyApp> {
   final shareWidget = new GlobalKey();
+  final previewContainer = new GlobalKey();
 
   PDFDocument _generateDocument() {
     final pdf = new PDFDocument(deflate: zlib.encode);
-    final page = new PDFPage(pdf, pageFormat: PDFPageFormat.A4);
+    final page = new PDFPage(pdf, pageFormat: PDFPageFormat.a4);
     final g = page.getGraphics();
     final font = new PDFFont(pdf);
     final top = page.pageFormat.height;
 
     g.setColor(new PDFColor(0.0, 1.0, 1.0));
-    g.drawRect(50.0 * PDFPageFormat.MM, top - 80.0 * PDFPageFormat.MM,
-        100.0 * PDFPageFormat.MM, 50.0 * PDFPageFormat.MM);
+    g.drawRect(50.0 * PDFPageFormat.mm, top - 80.0 * PDFPageFormat.mm,
+        100.0 * PDFPageFormat.mm, 50.0 * PDFPageFormat.mm);
     g.fillPath();
 
     g.setColor(new PDFColor(0.3, 0.3, 0.3));
-    g.drawString(font, 12.0, "Hello World!", 10.0 * PDFPageFormat.MM,
-        top - 10.0 * PDFPageFormat.MM);
+    g.drawString(font, 12.0, "Hello World!", 10.0 * PDFPageFormat.mm,
+        top - 10.0 * PDFPageFormat.mm);
 
     return pdf;
   }
@@ -51,9 +60,43 @@ class MyApp extends StatelessWidget {
     Printing.sharePdf(document: pdf, bounds: bounds);
   }
 
+  Future<void> _printScreen() async {
+    const margin = 10.0 * PDFPageFormat.mm;
+    final pdf = new PDFDocument(deflate: zlib.encode);
+    final page = new PDFPage(pdf, pageFormat: PDFPageFormat.a4);
+    final g = page.getGraphics();
+
+    RenderRepaintBoundary boundary =
+        previewContainer.currentContext.findRenderObject();
+    final im = await boundary.toImage();
+    final bytes = await im.toByteData(format: ImageByteFormat.rawRgba);
+    print("Print Screen ${im.width}x${im.height} ...");
+
+    // Center the image
+    final w = page.pageFormat.width - margin * 2.0;
+    final h = page.pageFormat.height - margin * 2.0;
+    double iw, ih;
+    if (im.width.toDouble() / im.height.toDouble() < 1.0) {
+      ih = h;
+      iw = im.width.toDouble() * ih / im.height.toDouble();
+    } else {
+      iw = w;
+      ih = im.height.toDouble() * iw / im.width.toDouble();
+    }
+
+    PDFImage image = PDFImage(pdf,
+        image: bytes.buffer.asUint8List(), width: im.width, height: im.height);
+    g.drawImage(image, margin + (w - iw) / 2.0,
+        page.pageFormat.height - margin - ih - (h - ih) / 2.0, iw, ih);
+
+    Printing.printPdf(document: pdf);
+  }
+
   @override
   Widget build(BuildContext context) {
-    return new Scaffold(
+    return new RepaintBoundary(
+        key: previewContainer,
+        child: new Scaffold(
           appBar: new AppBar(
             title: const Text('Pdf Printing Example'),
           ),
@@ -67,9 +110,12 @@ class MyApp extends StatelessWidget {
                     key: shareWidget,
                     child: new Text('Share Document'),
                     onPressed: _sharePdf),
+                new RaisedButton(
+                    child: new Text('Print Screenshot'),
+                    onPressed: _printScreen),
               ],
             ),
           ),
-    );
+        ));
   }
 }