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Improvements to the dart:io libraries

Posted by Seth Ladd

Mads Ager, engineer on the Dart server and IO libraries, has just posted some breaking changes to the dart:io libraries.

One-shot methods now take their callback as an argument.

Old 'n busted:

var f = new File('myfile.txt'); 
f.existsHandler = (result) { 
  // do stuff. 

New hotness:

var f = new File('myfile.txt'); 
f.exists((result) { 
  // do stuff. 

For active objects such as streams that emit events, the event handlers have changed names from eventHandler to onEvent.

Old 'n busted:

stdin.dataHandler = () { 
  // do stuff. 

New hotness:

stdin.onData = () { 
  // do stuff. 

Why not follow the lead of the dart:html libraries and use the convention of object.on.event.add(handler)? Mads explains:
We did consider going with object.on.event.add(handler). However,
having a collection of handlers for dart:io events usually doesn't
make sense. Then you are down to object.on.event = stuff. At that
point I think it is better to just have object.onEvent = stuff.
As always, continue the discussion on the mailing list and file issues in our public issue tracker. Thanks!


  1. Wouldn't it be good to have a static method for that?

    File.exists("myfile.txt", (result) {
    // do something

  2. Also.. giving a user a choice to use sync/async invocation would be good. And preferably without delegates! :)

    if (File.exists("myfile.txt")) {
    // do stuff

    if (await File.exists("myfile.txt")) {
    // do stuff in async manner


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