Future

Future#

Key Ideas#

  • Future - this is a callback function.

Concept

Future - Flutter is a single threaded program. This means that each function must complete before moving on to the next one. Problem - what if a function needs more time to wait for something? This would stop the program leaving the user waiting for it to complete. A call backback function will all the program to continue. In Flutter, future is the command to tell the program to continue and that function will finish sometime in the future.

Key Terms#

Key terms:

  • async: You can use the async keyword before a function’s body to mark it as asynchronous.

  • async function: An async function is a function labeled with the async keyword.

  • await: You can use the await keyword to get the completed result of an asynchronous expression. The await keyword only works within an async function.

    https://dart.dev/codelabs/async-await#why-asynchronous-code-matters

Lecture Code#

import 'dart:async';

import 'dart:io';

Duration mywaittime = Duration(seconds: 5);

Future<void> function1() async {

  // simulates a wait state
  Future.delayed(mywaittime, () async {
    var thedata = await myDelayedData();
    print(thedata);
  });

  stdout.writeln("Who wants a hotdog?");

}

Future <String> myDelayedData() =>
    Future.delayed(mywaittime,()=>"** This is the delayed data **");

void function1a()  {

  // simulates a wait state
  Future.delayed(mywaittime, () {
    print(myDelayedData());
  });

  stdout.writeln("Function 1a");
}


void function2() {
  print("This is function 2");
}

void function3() {
  print("This is function 3");
}

main(List<String> arguments) {

  // toggle to comments to see the right and wrong ways
  
  stdout.writeln("\nThe Wrong Way");
  function1a();
  function2();
  function3();

/*
  stdout.writeln("\nThe Right Way");
  function1();
  function2();
  function3();
*/

}

Example 2#

import 'dart:async';
 
Duration myseconds = Duration(seconds: 10);
 
Future<void> waitOnInfo() async{
 
    await waitOnSomething();
 
}
 
Future<void> waitOnSomething() async{
	//code that takes a while goes here
	Future.delayed(myseconds,(){
        
	//this is programming a pause
	//for this example 
	//get some data wait a while;
	print("Now its Done");
	});
	//return "Item 1a - thedata";	
}
 
void function2()
{
	print("Item 2");
}
void function3()
{
	print("Item 3");
}
main(List<String> arguments) {
 
    waitOnInfo();
    function2();
    function3();

    waitOnSomething();
 
}
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