[fluent] futurebuilder asynchronous programming (futurebuilder construction method | asyncsnapshot asynchronous calculation)

One 、FutureBuilder brief introduction

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FutureBuilder take Asynchronous operations And asynchronous UI to update Bind together ; It can be Asynchronous operations Result , Asynchronous Update to UI In the interface ;

Asynchronous operation results : Network request , Database read , Wait for time-consuming operations The result ;

Two 、FutureBuilder Construction method

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FutureBuilder The construction method is as follows :

/// 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 Parameter analysis of construction method :

• Future<T> future : Asynchronous computation related to asynchronous operations ;

  /// 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 : Initialization data before asynchronous calculation ;

  /// 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 Type of callback function , This is based on asynchronous interaction Widget Function of ;

/// 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);

3、 ... and 、AsyncSnapshot Asynchronous computation

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AsyncWidgetBuilder<T> The actual type of callback function is Widget Function(BuildContext context, AsyncSnapshot<T> snapshot) , Receive two parameters BuildContext context and AsyncSnapshot<T> snapshot , The return value is Widget Components ;

AsyncSnapshot<T> snapshot The parameter contains the information of asynchronous calculation ;

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 Medium ConnectionState connectionState It's the connection state , Is an enumeration value , There are four values :

• 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 It is the latest data received by asynchronous calculation ;

Object? error Is the error object received by asynchronous calculation ;

AsyncSnapshot<T> snapshot There is also hasData and hasError Two attributes , hasData Used to check whether the object contains non empty data values , hasError Used to check whether the error value is included ;

  /// 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;

Four 、 Related resources

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Reference material :

• Flutter Official website : https://flutter.dev/
• Flutter Plug in download address : https://pub.dev/packages
• Flutter Developing documents : https://flutter.cn/docs ( Strongly recommend )
• official GitHub Address : https://github.com/flutter
• Flutter The Chinese community : https://flutter.cn/
• Flutter Practical tutorial : https://flutter.cn/docs/cookbook
• Flutter CodeLab : https://codelabs.flutter-io.cn/
• Dart Chinese document : https://dart.cn/
• Dart Developer website : https://api.dart.dev/
• Flutter Chinese net : https://flutterchina.club/ , http://flutter.axuer.com/docs/
• Flutter Related issues : https://flutterchina.club/faq/ ( It is recommended to watch it at the introductory stage )
• GitHub Upper Flutter Open source examples : https://download.csdn.net/download/han1202012/15989510
• Flutter Practical e-books : https://book.flutterchina.club/chapter1/
• Dart Language practice website : https://dartpad.dartlang.org/

Important topics :

• Flutter Animation reference documentation : https://flutterchina.club/animations/

Blog source download :

• GitHub Address : https://github.com/han1202012/flutter_http( Keep updating with the progress of the blog , There may not be the source code of this blog )

• Blog source snapshot : https://download.csdn.net/download/han1202012/21528472 ( The source code snapshot of this blog , You can find the source code of this blog )