What you are going to learn: What is Future? How to create Future? How to use Future in Dart code? How to use Future in Flutter UI?
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What is Future?
A future represents the result of an asynchronous operation, and can have two states: 1- uncompleted, 2- completed (Completing with a value,Completing with an error).
1- Uncompleted
When asynchronous function called, it returns an uncompleted future. After that future keep waiting for the function’s asynchronous operation to completed(with value or error).
2- Completed
i- Completing with a value: If asynchronous operation successfully completed, It will return value.
ii- Completing with an error: In case it fails, It throw error.
How to create Future?
Number of way to create Future in Dart, Here are some of them:
i- Create Future by Future class constructor:
//Create Future by Future class constructor
Future<int> createFutureMethodOne() {
Future<int> future = Future(workDoInFuture);
return future;
}int workDoInFuture(){
//Do any work here it can be Async/Isolate
print('workDoInFuture');
Random rng = Random();
if(rng.nextBool()){
return rng.nextInt(100);
}else{
throw("There is some error");
}
}
ii- Create Future by Completer
//Create future by Completer
Future<int> createFutureMethodTwo(){
Completer completer = Completer<int>();
//Do any work here it can be Async/Isolate
Random rng = Random();
if(rng.nextBool()){
completer.complete(rng.nextInt(100));
}else{
throw("There is some error");
}
return completer.future;
}iii- Create Future by marking method “async”
//Create future by marking method "async"
Future<int> createFutureMethodThree() async {
//Do any work here it can be Async/Isolate
Random rng = Random();
if(rng.nextBool()){
return rng.nextInt(100);
}else{
throw("There is some error");
}
}iv- Create Future by Future class factory constructors(Its mostly helpful in unit tests)
//Create future class factory constructors(Its mostly helpful in unit tests)
Future<int> createFutureMethodFour(){
Random rng = Random();
if(rng.nextBool()){
int value= rng.nextInt(100);
//First e.g.
// return Future.value(value);
//Second e.g.
// return Future.error('There is some error');
//Third e.g.
return Future.delayed( Duration(seconds: 3),(){
return value;
});
}else{
throw("There is some error");
}
}Here is complete example:
How to use Future in Dart code?
In Dart code two ways to use Futures: 1- Using Future class methods then/catchError/whenComplete, 2- Using async/await with try/catch/finally
1- Using Future class methods then/catchError/whenComplete
//Use Future by then/catchError/whenComplete
void useFutureMethodOne(){
Future<String> future =getFuture();
future.then((String onValue){
print('On success : $onValue');
})
.catchError((Object onError){
print('On error : $onError');
})
.whenComplete((){
print('On success/error its commpeted');
});
}2- Using async/await with try/catch/finally
//Use Future by async/await/try,catch,finally
void useFutureMethodTwo()async{
Future<String> future =getFuture();
try{
String onValue=await future;
print('On success : $onValue');
}on Exception catch( onError){
print('On error : $onError');
}finally{
print('On success/error its commpeted');
}
}Here is complete code:
How to use Future in Flutter UI?
FutureBuilder Widget use to render based on current state of Future(Uncompleted, complete with value , complete with error)
FutureBuilder takes two parameters :
1- future: put method name which is providing future
2- builder: need to implement method which returning Widget to show based on state of Future that can be Uncompleted, complete with value , complete with error
