Add examples (#43)

- How to run the animation between 2 specifics frames
- How to export the animation to a file

example/lib/examples/save_frames.dart

@@ -0,0 +1,88 @@
+import 'dart:io';
+import 'dart:ui';
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:lottie/lottie.dart';
+import 'package:path/path.dart' as p;
+import 'package:path_provider/path_provider.dart';
+
+void main() => runApp(MyApp());
+
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      home: Scaffold(
+        body: ListView(
+          children: [
+            RaisedButton(
+              child: Text('Save Film Strip'),
+              onPressed: () async {
+                var data = await rootBundle.load('assets/HamburgerArrow.json');
+                var result = await exportFilmStrip(data);
+
+                var outputDir = await getDownloadsDirectory();
+                await File(p.join(outputDir.path, 'hamburger_film_strip.png'))
+                    .writeAsBytes(result.buffer.asUint8List());
+              },
+            ),
+            RaisedButton(
+              child: Text('Save all frames'),
+              onPressed: () async {
+                var data = await rootBundle.load('assets/HamburgerArrow.json');
+                var outputDir = await getDownloadsDirectory();
+
+                await saveAllFrames(data, p.join(outputDir.path, 'hamburger'));
+              },
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+Future<ByteData> exportFilmStrip(ByteData data) async {
+  var composition = await LottieComposition.fromByteData(data);
+  var drawable = LottieDrawable(composition);
+
+  var pictureRecorder = PictureRecorder();
+  var canvas = Canvas(pictureRecorder);
+
+  var size = Size(500, 500);
+  var columns = 10;
+  for (var i = composition.startFrame; i < composition.endFrame; i += 1) {
+    drawable.setProgress(i / composition.durationFrames);
+
+    var destRect = Offset(i % columns * 50.0, i ~/ 10 * 80.0) & (size / 5);
+    drawable.draw(canvas, destRect);
+  }
+
+  var picture = pictureRecorder.endRecording();
+  var image = await picture.toImage(size.width.toInt(), size.height.toInt());
+  var bytes = await image.toByteData(format: ImageByteFormat.png);
+  return bytes;
+}
+
+Future<void> saveAllFrames(ByteData data, String destination) async {
+  var composition = await LottieComposition.fromByteData(data);
+  var drawable = LottieDrawable(composition);
+
+  var size = Size(composition.bounds.width.toDouble(),
+      composition.bounds.height.toDouble());
+  for (var i = composition.startFrame; i < composition.endFrame; i += 1) {
+    drawable.setProgress(i / composition.durationFrames);
+
+    var pictureRecorder = PictureRecorder();
+    var canvas = Canvas(pictureRecorder);
+
+    drawable.draw(canvas, Offset.zero & size);
+
+    var picture = pictureRecorder.endRecording();
+    var image = await picture.toImage(size.width.toInt(), size.height.toInt());
+    var bytes = await image.toByteData(format: ImageByteFormat.png);
+    var fileName = i.toInt().toString().padLeft(3, '0');
+    await File(p.join(destination, '$fileName.png'))
+        .writeAsBytes(bytes.buffer.asUint8List());
+  }
+}