dojo/promise at London Ajax User Group

Details on the new dojo/promise API in Dojo 1.8.

Transcript

1. What’s new in 1.8 dojo/promise A talk given at London Ajax User Group on July 10th, 2012.
2. Brand new deferred/ Promise code Better tools for async programming Foundation for 2.0 Deprecated deferreds still around
3. What’s old is new again This is for those of you who used Deferreds before.
4. Deprecated, since 1.5… callback() & errback() addCallback/s(), addErrback(), addBoth() .fired, .results
5. Deprecated, since 1.5… Mutation!
6. But still available dojo.Deferred dojo/_base/Deferred Returned by older APIs
7. Don’t use it!
8. But note… dojo/_base/Deferred has then() and .promise, which return the new promise object
9. Promises Are objects that are returned instead of the actual result Promise a result in the future Can be passed to other functions Of course promises are in jQuery as well, though they behave slightly differently.
10. function doAsync(cb){ … cb(null, {}); } doAsync(function(error, result){ … }); A typical example with Node-style callbacks.
11. function doAsync(){ … return promise; } doAsync().then(function(result){ … }); Here doAsync returns a promise, and we hook up the callback later.
12. var promise = doAsync(); useAsyncResult(promise); But because the promise encapsulates the async operation, we can pass it to another function as well. You can’t do this with callbacks.
13. dojo/Deferred Provides the Deferred base class Let's explain through the new API.
14. require([ "dojo/Deferred" ], function(Deferred){ var deferred = new Deferred(); … }); We can require the constructor into our code.
15. function doAsync(){ var deferred = new Deferred(); … return deferred.promise; } So let's say we have a function that is asynchronous and should return a promise. Construct the deferred at the top of the function, and return the promise at the end.
16. // Resolve a deferred if the async operation // completed successfully. deferred.resolve(result);
17. // Reject a deferred if the async operation // failed. deferred.reject(error);
18. // Add a callback deferred.then(function(result){ … }); deferred.promise.then(function(result){ … }); We can then add callbacks to the returned promise.
19. // Add an errback deferred.then(null, function(error){ … }); deferred.promise.then(null, function(error){ … }); Or errbacks.
20. dojo/promise/Promise Used as the return value Can't be resolved itself, that's the deferred Chaining!
21. // Construct a promise chain promise.then(function(result){ … }).then(function(result){ … }); then() itself also returns a promise, to which we can add another callback.
22. // Return a promise from the callback promise.then(function(){ return doAsync(); });
23. // Add callbacks before deferred is resolved deferred.promise.then(function(){ … }); deferred.resolve(result);
24. // … or after deferred.resolve(result); deferred.promise.then(function(){ … });
25. // Results & errors bubble even if no // callbacks are registered promise.then(function(){ … }).then(null, function(error){ … }); deferred.reject(error);
26. // Thrown errors are handled by the resulting // promise promise.then(function(){ throw new Error(); }, function(error){ // Not called! });
27. Values must be returned, errors (re)thrown
28. Message channels Success: resolve(), callbacks Failure: reject(), errbacks
29. More message channels Progress: progress(), progbacks Cancelation: cancel(), canceler
30. // Report progress on the deferred deferred.progress("50% done!");
31. // Add a progback promise.then(null, null, function(update){ … );
32. Progress updates are not cached by the promise. Progbacks must be registered or updates will be missed
33. // Create a deferred with a canceler var deferred = new Deferred(function(reason){ … });
34. // Cancel a deferred deferred.cancel(); deferred.promise.cancel();
35. // Canceling works with chaining promise.then().then().then().cancel();
36. // Tell the deferred why it's being canceled promise.cancel(new OperationAbortedError());
37. Sugar
38. // Determine state of the deferred deferred.isResolved(); deferred.isRejected(); deferred.isCanceled(); deferred.isFulfilled();
39. // Determine state of the promise promise.isResolved(); promise.isRejected(); promise.isCanceled(); promise.isFulfilled();
40. // Easier callbacks promise.always(function(resultOrError){ … }); promise.otherwise(function(error){ … });
41. Resolving twice is ignored, usually
42. // With a fulfilled deferred, resolving // is normally ignored: deferred.resolve("again!"); // Unless strict mode is desired: deferred.resolve("again!", true); Same for reject(), progress() and cancel(). With the old Deferred this throws an error.
43. dojo/when Newer version of dojo.when Require separately
44. // When you don't know if you have a promise when(promiseOrValue, function(value){ … });
45. // Used without direct callback, converts // into a dojo/promise/Promise when(promiseOrValue).then(function(result){ … });
46. // With direct callback, will call right away // if passed a value, without catching errors // thrown by callback. Does not return a // promise. when(value, function(value){ throw new Error(); }); // Throws!
47. dojo/promise/first dojo/promise/all Replaces dojo.DeferredList
48. // Returns promise fulfilled when the first // promise is resolved or rejected. Resolves // returned promise with result of first. first([promise1, promise2]) first({ p1: promise1, p2: promise2 })
49. // Returns promise fulfilled when all // promises are resolved or one is rejected. // Resolves returned promise with list or // object of results. all([promise1, promise2]) all({ p1: promise1, p2: promise2 })
50. cancel() doesn't cancel the promises passed to first() or all(), only the returned promise
51. Error instrumenting Automatically log unhandled errors during development Pluggable interface dojo/promise/instrumenting
52. Tracing Trace results, errors and progress updates Again automatic during development
53. promise.trace(); deferred.resolve("hello?"); // Logs "resolved", "hello?"
54. I’m @novemberborn or mark@novemberborn.net Cheers! And that’s all folks. Further questions?