feat(widget): add utility functions to capture widget as image

lib/utils/widget_util.dart

@@ -0,0 +1,82 @@
+import 'dart:typed_data';
+import 'dart:ui';
+
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:moodiary/utils/theme_util.dart';
+
+Future<Uint8List?> captureWidgetOffScreen({
+  required BuildContext context,
+  required Widget widget,
+  required Size size,
+}) async {
+  final view = WidgetsBinding.instance.platformDispatcher.views.first;
+  final devicePixelRatio = view.devicePixelRatio;
+  final RenderRepaintBoundary repaintBoundary = RenderRepaintBoundary();
+  final RenderView renderView = RenderView(
+    child: RenderPositionedBox(
+      alignment: Alignment.center,
+      child: repaintBoundary,
+    ),
+    configuration: ViewConfiguration(
+      logicalConstraints: BoxConstraints.tight(size),
+      devicePixelRatio: devicePixelRatio,
+    ),
+    view: view,
+  );
+
+  final PipelineOwner pipelineOwner = PipelineOwner();
+  final BuildOwner buildOwner = BuildOwner(focusManager: FocusManager());
+  final ThemeData theme;
+  final platformBrightness = MediaQuery.platformBrightnessOf(context);
+  if (platformBrightness == Brightness.dark) {
+    theme = ThemeUtil().darkTheme;
+  } else {
+    theme = ThemeUtil().lightTheme;
+  }
+  pipelineOwner.rootNode = renderView;
+  renderView.prepareInitialFrame();
+  final RenderObjectToWidgetElement<RenderBox> rootElement =
+      RenderObjectToWidgetAdapter<RenderBox>(
+        container: repaintBoundary,
+        child: InheritedTheme.captureAll(
+          context,
+          Theme(
+            data: theme,
+            child: Directionality(
+              textDirection: TextDirection.ltr,
+              child: MediaQuery(
+                data: MediaQuery.of(context),
+                child: Material(
+                  child: Builder(
+                    builder: (context) {
+                      return widget;
+                    },
+                  ),
+                ),
+              ),
+            ),
+          ),
+        ),
+      ).attachToRenderTree(buildOwner);
+  buildOwner.buildScope(rootElement);
+  buildOwner.finalizeTree();
+  pipelineOwner.flushLayout();
+  pipelineOwner.flushCompositingBits();
+  pipelineOwner.flushPaint();
+  final image = await repaintBoundary.toImage(pixelRatio: devicePixelRatio * 2);
+  final byteData = await image.toByteData(format: ImageByteFormat.png);
+  return byteData?.buffer.asUint8List();
+}
+
+Future<Uint8List?> captureWidgetWithKey(
+  BuildContext context,
+  GlobalKey globalKey,
+) async {
+  final boundary =
+      globalKey.currentContext?.findRenderObject() as RenderRepaintBoundary;
+  final pixelRatio = MediaQuery.devicePixelRatioOf(context);
+  final image = await boundary.toImage(pixelRatio: pixelRatio);
+  final data = await image.toByteData(format: ImageByteFormat.png);
+  return data?.buffer.asUint8List();
+}