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

问题

I'm exploring Futures in Dart, and I'm confused about these two methods that Future offers, .then() and .whenCompleted(). What's 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 the Future is completed.

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

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

The difference I see here is that .then() has access to data that was returned! What is .whenCompleted() used for? When should we choose one over the other?

回答1:

.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.

回答2:

.whenComplete = The function inside .whenComplete is called when this future completes, whether it does so with a value or with an error.

.then = Returns a new Future which is completed with the result of the call to onValue (if this future completes with a value) or to onError (if this future completes with an error)

Read detail on API DOC

whenComplete then

回答3:

Even if there is an error, then can still be invoked, provided you have specified catchError.

someFuture.catchError(
  (onError) {
    print("called when there is an error catches error");
  },
).then((value) {
  print("called with value = null");
}).whenComplete(() {
  print("called when future completes");
});

So, if there is an error, all the above callbacks gets called.

来源：https://stackoverflow.com/questions/55381236/difference-between-then-and-whencompleted-methods-when-working-with-future

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