FlutterFutureBuilder 异步编程 ( FutureBuilder 构造方法 | AsyncSnapshot 异步计算 )

Posted 2021-09-11 韩曙亮

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文章目录

一、FutureBuilder 简介
二、FutureBuilder 构造方法
三、AsyncSnapshot 异步计算
四、相关资源

一、FutureBuilder 简介

FutureBuilder 将异步操作与异步 UI 更新结合在一起 ; 它可以将异步操作的结果 , 异步的更新到 UI 界面中 ;

异步操作结果 : 网络请求 , 数据库读取 , 等耗时操作得到的结果 ;

二、FutureBuilder 构造方法

FutureBuilder 构造方法如下 :

/// Creates a widget that builds itself based on the latest snapshot of
/// interaction with a [Future].
///
/// The [builder] must not be null.
const FutureBuilder({
  Key? key,
  this.future,
  this.initialData,
  required this.builder,
}) : assert(builder != null),
     super(key: key);

FutureBuilder({Key key, Future<T> future, T initialData, @required AsyncWidgetBuilder<T> builder })

FutureBuilder 构造方法参数解析 :

Future<T> future : 与异步操作相关的异步计算 ;

  /// The asynchronous computation to which this builder is currently connected,
  /// possibly null.
  ///
  /// If no future has yet completed, including in the case where [future] is
  /// null, the data provided to the [builder] will be set to [initialData].
  final Future<T>? future;

T initialData : 异步计算完成前的初始化数据 ;

  /// The data that will be used to create the snapshots provided until a
  /// non-null [future] has completed.
  ///
  /// If the future completes with an error, the data in the [AsyncSnapshot]
  /// provided to the [builder] will become null, regardless of [initialData].
  /// (The error itself will be available in [AsyncSnapshot.error], and
  /// [AsyncSnapshot.hasError] will be true.)
  final T? initialData;

@required AsyncWidgetBuilder<T> builder : AsyncWidgetBuilder 类型的回调函数 , 这是基于异步交互构建 Widget 的函数 ;

/// Signature for strategies that build widgets based on asynchronous
/// interaction.
///
/// See also:
///
///  * [StreamBuilder], which delegates to an [AsyncWidgetBuilder] to build
///    itself based on a snapshot from interacting with a [Stream].
///  * [FutureBuilder], which delegates to an [AsyncWidgetBuilder] to build
///    itself based on a snapshot from interacting with a [Future].
typedef AsyncWidgetBuilder<T> = Widget Function(BuildContext context, AsyncSnapshot<T> snapshot);

三、AsyncSnapshot 异步计算

AsyncWidgetBuilder<T> 回调函数的实际类型是 Widget Function(BuildContext context, AsyncSnapshot<T> snapshot) , 接收两个参数 BuildContext context 和 AsyncSnapshot<T> snapshot , 返回值是 Widget 组件 ;

AsyncSnapshot<T> snapshot 参数中包含有异步计算的信息 ;

class AsyncSnapshot<T> {
  /// Creates an [AsyncSnapshot] with the specified [connectionState],
  /// and optionally either [data] or [error] with an optional [stackTrace]
  /// (but not both data and error).
  const AsyncSnapshot._(this.connectionState, this.data, this.error, this.stackTrace)
    : assert(connectionState != null),
      assert(!(data != null && error != null)),
      assert(stackTrace == null || error != null);

  /// Current state of connection to the asynchronous computation.
  final ConnectionState connectionState;

  /// The latest data received by the asynchronous computation.
  ///
  /// If this is non-null, [hasData] will be true.
  ///
  /// If [error] is not null, this will be null. See [hasError].
  ///
  /// If the asynchronous computation has never returned a value, this may be
  /// set to an initial data value specified by the relevant widget. See
  /// [FutureBuilder.initialData] and [StreamBuilder.initialData].
  final T? data;

  /// The latest error object received by the asynchronous computation.
  ///
  /// If this is non-null, [hasError] will be true.
  ///
  /// If [data] is not null, this will be null.
  final Object? error;

  /// Returns whether this snapshot contains a non-null [data] value.
  ///
  /// This can be false even when the asynchronous computation has completed
  /// successfully, if the computation did not return a non-null value. For
  /// example, a [Future<void>] will complete with the null value even if it
  /// completes successfully.
  bool get hasData => data != null;

  /// Returns whether this snapshot contains a non-null [error] value.
  ///
  /// This is always true if the asynchronous computation's last result was
  /// failure.
  bool get hasError => error != null;
}

AsyncSnapshot<T> snapshot 中的 ConnectionState connectionState 是连接状态 , 是个枚举值 , 有四种取值 :

none
waiting
active
done

/// The state of connection to an asynchronous computation.
///
/// The usual flow of state is as follows:
///
/// 1. [none], maybe with some initial data.
/// 2. [waiting], indicating that the asynchronous operation has begun,
///    typically with the data being null.
/// 3. [active], with data being non-null, and possible changing over time.
/// 4. [done], with data being non-null.
///
/// See also:
///
///  * [AsyncSnapshot], which augments a connection state with information
///    received from the asynchronous computation.
enum ConnectionState {
  /// Not currently connected to any asynchronous computation.
  ///
  /// For example, a [FutureBuilder] whose [FutureBuilder.future] is null.
  none,

  /// Connected to an asynchronous computation and awaiting interaction.
  waiting,

  /// Connected to an active asynchronous computation.
  ///
  /// For example, a [Stream] that has returned at least one value, but is not
  /// yet done.
  active,

  /// Connected to a terminated asynchronous computation.
  done,
}

final T? data 是异步计算接收的最新数据 ;

Object? error 是异步计算接收的错误对象 ;

AsyncSnapshot<T> snapshot 中还有 hasData 和 hasError 两个属性 , hasData 用于检查该对象是否包含非空数据值 , hasError 用于检查是否包含错误值 ;

  /// Returns whether this snapshot contains a non-null [data] value.
  ///
  /// This can be false even when the asynchronous computation has completed
  /// successfully, if the computation did not return a non-null value. For
  /// example, a [Future<void>] will complete with the null value even if it
  /// completes successfully.
  bool get hasData => data != null;

  /// Returns whether this snapshot contains a non-null [error] value.
  ///
  /// This is always true if the asynchronous computation's last result was
  /// failure.
  bool get hasError => error != null;

四、相关资源

参考资料 :

Flutter 官网 : https://flutter.dev/
Flutter 插件下载地址 : https://pub.dev/packages
Flutter 开发文档 : https://flutter.cn/docs ( 强烈推荐 )
官方 GitHub 地址 : https://github.com/flutter
Flutter 中文社区 : https://flutter.cn/
Flutter 实用教程 : https://flutter.cn/docs/cookbook
Flutter CodeLab : https://codelabs.flutter-io.cn/
Dart 中文文档 : https://dart.cn/
Dart 开发者官网 : https://api.dart.dev/
Flutter 中文网 : https://flutterchina.club/ , http://flutter.axuer.com/docs/
Flutter 相关问题 : https://flutterchina.club/faq/ ( 入门阶段推荐看一遍 )
GitHub 上的 Flutter 开源示例 : https://download.csdn.net/download/han1202012/15989510
Flutter 实战电子书 : https://book.flutterchina.club/chapter1/
Dart 语言练习网站 : https://dartpad.dartlang.org/

重要的专题 :

Flutter 动画参考文档 : https://flutterchina.club/animations/

博客源码下载 :

GitHub 地址 : https://github.com/han1202012/flutter_http( 随博客进度一直更新 , 有可能没有本博客的源码 )
博客源码快照 : https://download.csdn.net/download/han1202012/21528472 ( 本篇博客的源码快照 , 可以找到本博客的源码 )

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