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packages/when/README.md

Index: packages/when/README.md
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+when [![pub package](http://img.shields.io/pub/v/when.svg)](https://pub.dartlang.org/packages/when) [![Build Status](https://drone.io/github.com/seaneagan/when.dart/status.png)](https://drone.io/github.com/seaneagan/when.dart/latest) [![Coverage Status](https://img.shields.io/coveralls/seaneagan/when.dart.svg)](https://coveralls.io/r/seaneagan/when.dart?branch=master)
+====
+
+It's often useful to provide sync (convenient) and async (concurrent) versions 
+of the same API.  `dart:io` does this with many APIs including [Process.run][] 
+and [Process.runSync][].  Since the sync and async versions do the same thing, 
+much of the logic is the same, with just a few small bits differing in their 
+sync vs. async implementation.
+
+The `when` function allows for registering `onSuccess`, `onError`, and 
+`onComplete` callbacks on another callback which represents that sync/async 
+dependent part of the API.  If the callback is sync (returns a non-`Future` or 
+throws), then the other callbacks are invoked synchronously, otherwise the 
+other callbacks are registered on the returned `Future`.
+
+For example, here's how it can be used to implement sync and async APIs for
+reading a JSON data structure from the file system with file absence handling:
+
+```dart
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:when/when.dart';
+
+/// Reads and decodes JSON from [path] asynchronously.
+///
+/// If [path] does not exist, returns the result of calling [onAbsent].
+Future readJsonFile(String path, {onAbsent()}) => _readJsonFile(
+    path, onAbsent, (file) => file.exists(), (file) => file.readAsString());
+
+/// Reads and decodes JSON from [path] synchronously.
+///
+/// If [path] does not exist, returns the result of calling [onAbsent].
+readJsonFileSync(String path, {onAbsent()}) => _readJsonFile(
+    path, onAbsent, (file) => file.existsSync(),
+    (file) => file.readAsStringSync());
+
+_readJsonFile(String path, onAbsent(), exists(File file), read(File file)) {
+  var file = new File(path);
+  return when(
+      () => exists(file),
+      onSuccess: (doesExist) => doesExist ?
+          when(() => read(file), onSuccess: JSON.decode) :
+          onAbsent());
+}
+
+main() {
+  var syncJson = readJsonFileSync('foo.json', onAbsent: () => {'foo': 'bar'});
+  print('Sync json: $syncJson');
+  readJsonFile('foo.json', onAbsent: () => {'foo': 'bar'}).then((asyncJson) {
+    print('Async json: $asyncJson');
+  });
+}
+```
+
+[Process.run]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart:io.Process#id_run
+[Process.runSync]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart:io.Process#id_runSync