flutter pub add after_layout根據套件公開資料產生的選型評估，僅供參考。
after_layout 套件讓開發者能在小部件首次佈局後立即執行程式碼，確保在小部件渲染到螢幕上後能存取準確的尺寸與位置。它特別適用於需要依賴最終佈局測量值的操作，例如動畫、定位調整或與其他 UI 元件整合。此套件可在 Flutter 支援的所有主要平台（包含行動裝置、網頁及桌面）上運作。它透過一個混合（mixin）提供簡單的 API，可加入任何 StatefulWidget 中。
以下為關聯原始碼儲存庫的 README 原文快照，最新內容請造訪原始碼儲存庫。
Brings functionality to execute code after the first layout of a widget has been performed, i.e. after the first frame has been displayed.
Add with AfterLayoutMixin<MyWidget> mixin to your State<MyWidget> class and then implement the FutureOr<void> afterFirstLayout(BuildContext context) abstract method. Code in this method will be called the first time this widget is laid out on the screen.
If you want to display a widget that depends on the layout, such as a Dialog or BottomSheet, you can not use that widget in initState.
You might have tried this.
BAD CODE
import 'package:flutter/material.dart';
void main() => runApp(const MyApp());
@immutable
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
@override
Widget build(BuildContext context) {
return const MaterialApp(
title: 'After Layout - Good Example',
home: HomeScreen(),
);
}
}
@immutable
class HomeScreen extends StatefulWidget {
const HomeScreen({Key? key}) : super(key: key);
@override
HomeScreenState createState() => HomeScreenState();
}
class HomeScreenState extends State<HomeScreen> {
@override
void initState() {
super.initState();
// NOTE: Calling this function here would crash the app.
showHelloWorld();
}
@override
Widget build(BuildContext context) {
return Scaffold(
body: Container(color: Colors.red),
);
}
void showHelloWorld() {
showDialog(
context: context,
builder: (BuildContext context) {
return AlertDialog(
content: const Text('Hello World'),
actions: <Widget>[
TextButton(
onPressed: () => Navigator.of(context).pop(),
child: const Text('DISMISS'),
)
],
);
},
);
}
}
This demo showcases how this package resolves the shortcomings shown above:
GOOD CODE
import 'package:flutter/material.dart';
import 'package:after_layout/after_layout.dart';
void main() => runApp(const MyApp());
@immutable
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
@override
Widget build(BuildContext context) {
return const MaterialApp(
title: 'After Layout - Good Example',
home: HomeScreen(),
);
}
}
@immutable
class HomeScreen extends StatefulWidget {
const HomeScreen({Key? key}) : super(key: key);
@override
HomeScreenState createState() => HomeScreenState();
}
class HomeScreenState extends State<HomeScreen> with AfterLayoutMixin<HomeScreen> {
@override
Widget build(BuildContext context) {
return Scaffold(
body: Container(color: Colors.red),
);
}
@override
void afterFirstLayout(BuildContext context) {
// Calling the same function "after layout" to resolve the issue.
showHelloWorld();
}
void showHelloWorld() {
showDialog(
context: context,
builder: (BuildContext context) {
return AlertDialog(
content: const Text('Hello World'),
actions: <Widget>[
TextButton(
onPressed: () => Navigator.of(context).pop(),
child: const Text('DISMISS'),
)
],
);
},
);
}
}