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Difference between .then() and .whenCompleted() methods when working with Futures?

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2

I'm exploring Futures in Dart, and I'm confused about these two methods that Future offers. Whats the main difference between them?

Lets say I want to read a .txt using .readAsString(), I would do it like this:

void main()
 File file = new File('text.txt');
 Future content = file.readAsString();
 content.then((data) 
 print(content);
 ); 

So .then() is like a callback that fires a function once Future is completed.

But I see there is also .whenComplete() that can also fire an function once Future completes. Something like this :

void main()
 File file = new File('text.txt');
 Future content = file.readAsString();
 content.whenComplete(() 
 print("Completed");
 );

Difference I see here is that .then() has access to data that was returned!
What is .whenCompleted() used for? When to chose one over the other?

And also on these two links, .then() and .whenCompleted() at the end of a page there are implementations:

.then():

Future<R> then<R>(FutureOr<R> onValue(T value), Function onError);

.whenCompleted():

Future<T> whenComplete(FutureOr action());

What does Future<R> means? Or Future<T>? I get that Future is a type, but what are R and T?

Thanks!

dart

share|improve this question

asked Mar 27 at 15:43

MatijaMatija

1,00655 silver badges1212 bronze badges

add a comment | 

2

I'm exploring Futures in Dart, and I'm confused about these two methods that Future offers. Whats the main difference between them?

Lets say I want to read a .txt using .readAsString(), I would do it like this:

void main()
 File file = new File('text.txt');
 Future content = file.readAsString();
 content.then((data) 
 print(content);
 ); 

So .then() is like a callback that fires a function once Future is completed.

But I see there is also .whenComplete() that can also fire an function once Future completes. Something like this :

void main()
 File file = new File('text.txt');
 Future content = file.readAsString();
 content.whenComplete(() 
 print("Completed");
 );

Difference I see here is that .then() has access to data that was returned!
What is .whenCompleted() used for? When to chose one over the other?

And also on these two links, .then() and .whenCompleted() at the end of a page there are implementations:

.then():

Future<R> then<R>(FutureOr<R> onValue(T value), Function onError);

.whenCompleted():

Future<T> whenComplete(FutureOr action());

What does Future<R> means? Or Future<T>? I get that Future is a type, but what are R and T?

Thanks!

dart

share|improve this question

asked Mar 27 at 15:43

MatijaMatija

1,00655 silver badges1212 bronze badges

add a comment | 

I'm exploring Futures in Dart, and I'm confused about these two methods that Future offers. Whats the main difference between them?

Lets say I want to read a .txt using .readAsString(), I would do it like this:

void main()
 File file = new File('text.txt');
 Future content = file.readAsString();
 content.then((data) 
 print(content);
 ); 

So .then() is like a callback that fires a function once Future is completed.

But I see there is also .whenComplete() that can also fire an function once Future completes. Something like this :

void main()
 File file = new File('text.txt');
 Future content = file.readAsString();
 content.whenComplete(() 
 print("Completed");
 );

Difference I see here is that .then() has access to data that was returned!
What is .whenCompleted() used for? When to chose one over the other?

And also on these two links, .then() and .whenCompleted() at the end of a page there are implementations:

.then():

Future<R> then<R>(FutureOr<R> onValue(T value), Function onError);

.whenCompleted():

Future<T> whenComplete(FutureOr action());

What does Future<R> means? Or Future<T>? I get that Future is a type, but what are R and T?

Thanks!

dart

share|improve this question

asked Mar 27 at 15:43

MatijaMatija

1,00655 silver badges1212 bronze badges

void main()
 File file = new File('text.txt');
 Future content = file.readAsString();
 content.then((data) 
 print(content);
 ); 

void main()
 File file = new File('text.txt');
 Future content = file.readAsString();
 content.whenComplete(() 
 print("Completed");
 );

Future<R> then<R>(FutureOr<R> onValue(T value), Function onError);

Future<T> whenComplete(FutureOr action());

share|improve this question

asked Mar 27 at 15:43

MatijaMatija

1,00655 silver badges1212 bronze badges

share|improve this question

asked Mar 27 at 15:43

MatijaMatija

1,00655 silver badges1212 bronze badges

asked Mar 27 at 15:43

MatijaMatija

1,00655 silver badges1212 bronze badges

asked Mar 27 at 15:43

MatijaMatija

1,00655 silver badges1212 bronze badges

1 Answer
1

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oldest

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6

.whenComplete will fire a function either when the Future completes with an error or not, instead .then will fire a function after the Future completes without an error.

Quote from the .whenComplete API DOC

This is the asynchronous equivalent of a "finally" block.

share|improve this answer

answered Mar 27 at 16:40

MattiaMattia

96411 gold badge66 silver badges1717 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks Mattia, and what type is <R> and <T>?
  
  – Matija
  Mar 27 at 17:50
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  They are not actual types, but generics.
  
  – Mattia
  Mar 27 at 18:05
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thank you, this helped me a lot!
  
  – Matija
  Mar 27 at 18:58
  

  
  
  
  

add a comment | 

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6

.whenComplete will fire a function either when the Future completes with an error or not, instead .then will fire a function after the Future completes without an error.

Quote from the .whenComplete API DOC

This is the asynchronous equivalent of a "finally" block.

share|improve this answer

answered Mar 27 at 16:40

MattiaMattia

96411 gold badge66 silver badges1717 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks Mattia, and what type is <R> and <T>?
  
  – Matija
  Mar 27 at 17:50
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  They are not actual types, but generics.
  
  – Mattia
  Mar 27 at 18:05
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thank you, this helped me a lot!
  
  – Matija
  Mar 27 at 18:58
  

  
  
  
  

add a comment | 

6

.whenComplete will fire a function either when the Future completes with an error or not, instead .then will fire a function after the Future completes without an error.

Quote from the .whenComplete API DOC

This is the asynchronous equivalent of a "finally" block.

share|improve this answer

answered Mar 27 at 16:40

MattiaMattia

96411 gold badge66 silver badges1717 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks Mattia, and what type is <R> and <T>?
  
  – Matija
  Mar 27 at 17:50
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  They are not actual types, but generics.
  
  – Mattia
  Mar 27 at 18:05
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thank you, this helped me a lot!
  
  – Matija
  Mar 27 at 18:58
  

  
  
  
  

add a comment | 

.whenComplete will fire a function either when the Future completes with an error or not, instead .then will fire a function after the Future completes without an error.

Quote from the .whenComplete API DOC

This is the asynchronous equivalent of a "finally" block.

share|improve this answer

answered Mar 27 at 16:40

MattiaMattia

96411 gold badge66 silver badges1717 bronze badges

share|improve this answer

answered Mar 27 at 16:40

MattiaMattia

96411 gold badge66 silver badges1717 bronze badges

answered Mar 27 at 16:40

MattiaMattia

96411 gold badge66 silver badges1717 bronze badges

answered Mar 27 at 16:40

MattiaMattia

96411 gold badge66 silver badges1717 bronze badges