当前位置：网站首页>[fluent special topic] 91 illustration of future (II) yyds dry goods inventory of dart single thread asynchronous processing

[fluent special topic] 91 illustration of future (II) yyds dry goods inventory of dart single thread asynchronous processing

2022-04-22 13:34:00 【A Ze little monk】

Xiaocai just learned a few days ago Future Part of the way to implement asynchronous operations , It mainly includes the construction method and the commonly used static method ; Xiaocai continues to learn today Future Other knowledge and async-await Mode to implement asynchronous operation ;

Future nesting

The side dish was not made in the last blog Future Nesting attempts , There are many scenarios that require multiple asynchronous processing , And each asynchrony requires the result returned by the last asynchrony then() Then you can continue , You can use Future Nesting ; But if there are many hidden methods, it may affect the future code maintenance , That is to say Callback hell;

_futureMoreThen() {
  Future.delayed(Duration(seconds: 3), () {
    return 'Future.delayed 3s!';
  }).then((val) {
    print('Future -> then(1) -> $val');
    return 'then 1';
  }).then((val) {
    print('Future -> then(2) -> $val');
    return 'then 2';
  }).then((val) {
    print('Future -> then(3) -> $val');
    return 'then 3';
  }).whenComplete(() => print('Future whenComplete!'));
}

     
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【Flutter project 】91 The illustration Dart Implementation of asynchronous processing with single thread Future ( Two ) #yyds Dry inventory #_Android Vegetable bird

_futureMoreThen02() {
  Future.delayed(Duration(seconds: 3), () {
    return 'Future.delayed 3s!';
  }).then((val) {
    print('Future -> then(1) -> $val');
    return Future.delayed(Duration(seconds: 2), () {
      return 'Future.delayed 2s!';
    }).then((val) {
      print('Future -> then(2) -> $val');
      return Future.delayed(Duration(seconds: 2), () {
        return 'Future.delayed 2s!';
      }).then((val) {
        print('Future -> then(3) -> $val');
      });
    });
  }).whenComplete(() => print('Future whenComplete!'));
}

     
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【Flutter project 】91 The illustration Dart Implementation of asynchronous processing with single thread Future ( Two ) #yyds Dry inventory #_0 Based on learning Flutter_02

async-await

Future It can also be done through async-await Implement asynchronous operations ; It is usually used in multiple scenarios Future In series , Multi level nesting results in Callback hell, Use async-await Achieve asynchrony ;

async

async The asynchronous method used to decorate eventually encapsulates the return value as Future object ;

await

await Will automatically put the method into the blocking state , Wait until the task is completed and return the corresponding value ;

Case study

I tried the basic async-await usage ;

The side dish is not used async and await key word , here Future.delayed() Back to a Future object , Can't get the returned data synchronously ;

print(_function01());

_function01() {
  var result = Future.delayed(Duration(seconds: 3), () {
    return 'Future.delayed 3s!';
  }).then((val) => print('Function01 -> then() -> $val'))
    .whenComplete(() => print('Function01 -> whenComplete!'));
  return 'Function01 -> $result';
}

     
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【Flutter project 】91 The illustration Dart Implementation of asynchronous processing with single thread Future ( Two ) #yyds Dry inventory #_Android Vegetable bird _03
2. The side dish only adds async key word , The method is modified as Asynchronous methods , Still can't get the returned data directly

print(_function01());

Future<String> _function02() async {
  var result = Future.delayed(Duration(seconds: 3), () {
    return 'Future.delayed 3s!';
  }).then((val) => print('Function02 -> then() -> $val'))
    .whenComplete(() => print('Function02 -> whenComplete!'));
  return 'Function02 -> $result';
}

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【Flutter project 】91 The illustration Dart Implementation of asynchronous processing with single thread Future ( Two ) #yyds Dry inventory #_Android Vegetable bird _04
3. The side dish has been added async and await Two key words , The compiler will eventually convert it into a Promise(Future) The call chain of , You can get the return result after asynchronous completion ; here Future Cannot set then() Callback methods, etc ;

print(await _function03());

_function03() async {
  var result = await Future.delayed(Duration(seconds: 3), () {
    return 'Future.delayed 3s!';
  });
  return 'Function03 -> $result';
}

     
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【Flutter project 】91 The illustration Dart Implementation of asynchronous processing with single thread Future ( Two ) #yyds Dry inventory #_Android Vegetable bird _05

Try to use only await, Prompt at this time The await expression can only used in an async function,await Can only be used in async Methods the internal ;
use async-await When the way , For exception capture , Can pass Future.catchError() To deal with it , You can also use the most commonly used try-catch-finally The way , Simple understanding of the corresponding then()-catchError()-whenComplete();

await _function04();

_function04(index) async {
  switch (index) {
    case 1:
      await Future.error(ArgumentError.notNull('Input')).catchError((val) => print('Function04 -> $val'));
      break;
    case 2:
      try {
        await Future.error(ArgumentError.notNull('Input'));
      } catch (e) {
        print('Function04 -> catch -> $e');
      } finally {
        print('Function04 -> Finally!');
      }
      break;
  }
}

     
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【Flutter project 】91 The illustration Dart Implementation of asynchronous processing with single thread Future ( Two ) #yyds Dry inventory #_Flutter Side dish _06
6. For multiple Future Nesting leads to Callback hell,async-await The way to deal with it should be concise ;

await _functionThen();

_functionThen() async {
  await _itemThen01();
}

_itemThen01() async {
  await Future.delayed(Duration(seconds: 3), () {
    print('Future -> then(1) -> Future.delayed 3s!');
    return _itemThen02();
  });
}

_itemThen02() async {
  await Future.delayed(Duration(seconds: 2), () {
    print('Future -> then(2) -> Future.delayed 2s!');
    return _itemThen03();
  });
}

_itemThen03() async {
  await Future.delayed(Duration(seconds: 2), () {
    print('Future -> then(3) -> Future.delayed 2s!');
    return 'Future.delayed 2s!';
  }).whenComplete(() => print('Future whenComplete!'));
}

     
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【Flutter project 】91 The illustration Dart Implementation of asynchronous processing with single thread Future ( Two ) #yyds Dry inventory #_0 Based on learning Flutter_07

Small expansion

The side dish is trying async-await When I met async*, Where is the side dish bloc State management is always used in async* and Stream, Take a quick look at the differences ;

async*

async* It can also be used asynchronously , Use before method async* Keyword to mark the method as an asynchronous generator , Back to a Stream object , Use yield Statement to pass the value ;

about Stream Use , I have a basic understanding before the course , Usually by skin Add data , adopt listen Data monitoring ;

yield The key word is going to async* The output stream of the declared one-step generator adds a value , It's kind of similar return, It does not terminate the function ;

 _function06() async* {
  for (int i = 1; i <= 10; i++) {
    await Future.delayed(const Duration(seconds: 1));
    yield i;
  }
}

     
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【Flutter project 】91 The illustration Dart Implementation of asynchronous processing with single thread Future ( Two ) #yyds Dry inventory #_Android Vegetable bird _08

【Flutter project 】91 The illustration Dart Implementation of asynchronous processing with single thread Future ( Two ) #yyds Dry inventory #_0 Based on learning Flutter_09

Dart async-await Case study

It's a piece of cake Dart Asynchronous cognition is not complete yet , I'll continue to try isolate as well as EventLoop Execution order, etc ; If there are mistakes and omissions, please give more guidance ！