如何按顺序处理异步代码

Question

我已经阅读了很多关于此的帖子和文章(异步和期货),我不确定目前的情况如何.我尝试了许多不同的变化.

我按顺序尝试以下代码:

import 'dart:async' as async;
import 'dart:io';
import 'dart:math';
import 'package:postgresql/postgresql.dart' as pg;

var uri = 'postgres://admin:admin@localhost:5432/testdb';
List lgNames = ['Peter', 'Robert', 'Mary', 'Marg', 'William',
           'Vern', 'Dermot', 'Monty', 'Peggy', 'Sue', 'William'];

List lgUsed = [];

Random rand1 = new Random();

void main() {
  pg.connect(uri).then((oDb) {
   print ("Connected to database");

    fClearTable(oDb).then((String sResult){
      print(sResult);      

      for (int iPos = 0; iPos < 3; iPos++) {   
        fListTable(oDb, "Select before Insert number ${iPos+1}").then((String sResult) =>
          print(sResult));  

        fInsertData(oDb, "Insert Number ${iPos+1}").then((String sResult) =>
          print(sResult));        

        fListTable(oDb, "Select after Insert number ${iPos+1}").then((String sResult) =>
          print(sResult));
      }
    });
  });
}  

async.Future<String> fClearTable(oDb) {
  async.Completer oCompleter = new async.Completer();
  oDb.execute("DELETE FROM test01").then((oResult){
    oCompleter.complete("Table has been cleared");
  });
  return oCompleter.future;
}

async.Future<List> fListTable(oDb, sMsg) {
  async.Completer oCompleter = new async.Completer();
  oDb.query("SELECT * FROM test01").toList().then((lResult){
    String sResult = "$sMsg = $lResult";
    oCompleter.complete(sResult);
  }); 
  return oCompleter.future;  
}

async.Future<String> fInsertData(oDb, sMsg) {
  async.Completer oCompleter = new async.Completer();  
  oDb.execute("Start Transaction").then((oResult){
    String sName;
    for (bool tFound = true; tFound;) {
      int iPos = rand1.nextInt(10);
      sName = lgNames[iPos];
      tFound = false;    // init
      for (iPos = 0; iPos < lgUsed.length && !tFound; iPos++){
        tFound = (sName == lgUsed[iPos]);
      }
    }
    lgUsed.add(sName);
    String sSql = """INSERT INTO test01 (name)
      VALUES ('$sName')""";
    print("$sSql");
    oDb.execute(sSql).then((oVal){
      oDb.execute("COMMIT").then((oVal){
        oCompleter.complete(sMsg);
      });
    });
  });
  return oCompleter.future;
}

程序的目的是循环三次并且:a)选择表b)在表中插入一行.c)选择表格

程序的输出结果清楚地显示了所有三个插入.

该计划的输出如下:

Connected to database
Table has been cleared
Select before Insert number 1 = []
INSERT INTO test01 (name)
  VALUES ('Vern')
Select after Insert number 1 = []
Select before Insert number 2 = []
INSERT INTO test01 (name)
  VALUES ('Peter')
Select after Insert number 2 = []
Select before Insert number 3 = []
INSERT INTO test01 (name)
  VALUES ('Robert')
Select after Insert number 3 = []
Insert Number 1
Insert Number 2
Insert Number 3

在程序终止时选择psql显示:

testdb=# select * from test01;
 id  |  name
-----+--------
 157 | Vern
 158 | Peter
 159 | Robert
 (3 rows)

有没有办法实现我想要的IE.对于Inserts在表中显示新值后的选择？

欢迎任何相关评论.

Answer 1

首先,这是一篇关于Futures如何在Dart中工作的精彩文章.

你真正需要知道的是,Futures中的代码在所有同步代码完成后运行.此外,根据定义,期货是异步完成的,因此期望一个Future在另一个之前完成是不合理的,除非它们被链接.

也就是说:

main() {
  print('first');
  someFutureCall().then((_) => print('second?'));
  someOtherFutureCall().then((_) => print('third?'));
  print('last?');
}

这将打印:'first'然后'last？' 然后两个期货可以按任何顺序完成.你无法分辨哪些将首先完成.所有你知道的是同步代码首先发生.

要获得正确的订单,请执行:

main() {
  print('first');
  someFutureCall()
    .then((_) => print('second?'))
    .then((_) => someOtherFutureCall())
    .then((_) => print('third?'))
    .then((_) => print('last?'));
}

因此,您的示例使用Future的方式不会保留订单.你想要的是这个:

void main() {
  pg.connect(uri).then((oDb) {
   print ("Connected to database");
   return fClearTable(oDb);
  }).then((String sResult){
    print(sResult);      
    return Future.forEach([0, 1, 2], (i) {
      return fListTable(oDb, "Select before Insert number ${iPos+1}")
       .then((String sResult) => print(sResult))
       .then((_) => fInsertData(oDb, "Insert Number ${iPos+1}"))
       .then((String sResult) => print(sResult))
       .then((_) => fListTable(oDb, "Select after Insert number ${iPos+1}"))
       .then((String sResult) => print(sResult));
    });
  });
}  

另请参阅期货的API文档.