How to process async code in order - dart

Question

I have read a lot of postings and articles on this (async and Futures), and I am unsure what is the current situation. I have attempted a number of different variations.
I am attempting the process the following code in sequence :
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;
}
The intention of the program is to loop three times and :
a) Select the Table
b) Insert a row into the table.
c) Select the Table
The output from the program clearly shows all three Inserts simultaneously at the end.
The output from the program is as follows :
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
A Select in psql on termination of the program shows :
testdb=# select * from test01;
id | name
-----+--------
157 | Vern
158 | Peter
159 | Robert
(3 rows)
Is there a way to achieve what I want IE. For the Selects after the Inserts show the new values in the table?
Any relevant comments are welcome.

Answer 1

To start, here's a great article about how Futures work in Dart.
What you really need to know is that the code inside Futures run after all synchronous code has completed. Further, Futures by definition complete asynchronously, so it is not reasonable to expect one Future to complete before another unless they are chained.
That is to say:
main() {
print('first');
someFutureCall().then((_) => print('second?'));
someOtherFutureCall().then((_) => print('third?'));
print('last?');
}
This will print: 'first' and then 'last?' and then the two Futures can complete in any order. You cannot tell which will complete first. All you know is that the synchronous code happens first.
To have the correct order, do:
main() {
print('first');
someFutureCall()
.then((_) => print('second?'))
.then((_) => someOtherFutureCall())
.then((_) => print('third?'))
.then((_) => print('last?'));
}
So your sample uses Future in a way that the order is not preserved. What you want is this:
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));
});
});
}
See also the API documentation for Futures.