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Angular.JS,

I Promise Uri Shaked, Jan 2014

Jan, 2014 Copyright 2013, 2014, Uri Shaked

SECTION 1

WHY PROMISES?


Angular.JS, I Promise!

Synchronous Approach

Consider the following Pseudo Code:

fileName = prompt('Please enter a file name');

if (fileName) {

data = loadUrl('http://server/pictureService');

fs = requestFileSystem(…);

file = fs.getFile(fileName);

file.write(data);

alert('Your changes were saved!');

}



Asynchronous Version

This is what it would look like in the async world of JS:

showPrompt("Please enter a file name", function(fileName) {

loadData("http://www.server/pictureService",

function(data) {

requestFileSystem(…, function(fs) {

fs.getFile(fileName, function(file) {

file.write(data, function() {

showAlert("You changes were saved!");

});

});

});

});

});



• Nesting makes the Async version hard to read –

Pyramid Code

• We could split into many functions, but the control flow

would be hard to follow

• The Async version doesn’t even have any error handling

code and yet it is so messy

• Would you enjoy reading and maintaining code like this?


SECTION 2

PROMISES TO RESCUE



Definition

A promise represents a future value, usually a future result

of an asynchronous operation, and allows us to define what

will happen once this value becomes available, or when an

error occurs.



Basic Usage Pattern

var promise = asyncFunction(parameters);

promise.then(

function (result) {

// Do Something with the result

},

function (error) {

// Handle error (exception, etc).

});

Success Handler

Error Handler



AngularJS 1.2 Extensions


promise.then(

function (result) {

// Do Something with the result

},

function (error) {


},

function (update) {

// Receive progress updates (e.g.

// amount of data downloaded so far)

});

Success Handler

Error Handler


Progress Handler


Catch and Finally


promise.catch(function (error) {

// Syntax sugar for defining an error handler

});

promise.finally(function() {

// Will be executed regardless of success/failure.

});



The Power of Promises

• You can chain them to create code flows

• Error propagates, so you can catch it on the end of the

chain

• Actually, they are much like an asynchronous equivalent

for the well-known try-catch-finally clause.



Chaining Promises

var finalResult = getCurrentUserId()

.then(function (userId) {

if (!userId) { throw "Invalid User"; }

return loadUserData(userId);

}).then(function(userData) {

if (!userData) { return "default.png"; };

return prepareUserThumbnail(userData.profilePic);

}).then(function(thumbnail) {

console.log("User thumbnail: " + thumbnail);

return thumbnail;

}, function(error) {


});


SECTION 3

USING ANGULAR PROMISES



$HTTP Returns a Promise

function promiseCtrl($scope, $http) {

$http.get("/README.txt").then(function(result) {

$scope.readme = result.data;

});

}

<div ng-controller="promiseCtrl">

<h2>HTTP Request Demo</h2>

<pre>{{readme || "Loading…"}}</pre>

<b ng-show="readme">Request finished.</b>

</div>



Promise Chaining

function promiseCtrl($scope, $http) {

$http.get("/userInfo.json").then(function(result) {

var user = $result.data;

$scope.user = user;

return $http.get("/users/" + user.id + "/score”);

}).then(function(result) {

$scope.score = result.data.total;

});

}


<p>User: {{user.name}}</p>

<p>Score: {{score}}</p>

</div>



$timeout and $interval Jan, 2014 Copyright 2013, 2014, Uri Shaked

• $timeout(fn, delay) -> Promise

$timeout returns a promise that will be resolved once the

timeout is reached.

• $interval(fn, delay, count) -> Promise

$interval returns a promise which will be notified on each

iteration and will be resolved after count iterations if

count > 0.


$timeout Example

function promiseCtrl($scope, $timeout, $http) {

$timeout(function(){

return $http.get("/status");

}, 1000).then(function(result) {

$scope.result = result.data

});

}


{{result || "Preparing…"}

</div>


SECTION 4

$Q, A FACTORY OF PROMISES



Create Your Own

function requestFS(type, size) {

var deferred = $q.defer();

webkitRequestFileSystem(type, size,

function(fs) {

$rootScope.$apply(function() {

deferred.resolve(fs);

});

}, function (fsError) {


deferred.reject(fsError);

});

});

return deferred.promise;

}



Wrapping as a Promise

You can wrap any value with a promise, using the utility

method $q.when().

$q.when(promise) → promise

$q.when(nonPromise) → a new promise, that will

asynchronously resolve to the given value nonPromise.



Get & Return a Promise

function getFile(fs, name, options) {

var deferred = $q.defer();

$q.when(fs).then(function(resolveFS) {

resolveFS.getFile(name, options, function (file) {


deferred.resolve(file);

});

}, function (error) {


deferred.reject(error);

});

});

});


}



Usage Example

// Assume we also implemented readFileAsText() in a similar

// fashion.

function fileExampleCtrl($scope) {

var fs = requestFS(window.PERSISTENT, 1024*1024);

var file = getFile(fs, "log.txt");

readFileAsText(file).then(function(content) {

$scope.content = content;

});

}

<div ng-controller="fileExampleCtrl">

{{ content || 'Loading…' }}

</div>



Promise Many

• Sometimes you want to wait for multiple promises

• Angular provide a utility method:

$q.all([promise, …]) → newPromise

• newPromise will resolve once all the given promises have

been resolved

• If any of the given promises is rejected, newPromise will

also be rejected



Notify (Angular 1.2)

function downloadUserProfiles(users) {

var deferred = $q.defer(), promises = [];

angular.forEach(users, function(user) {

promises.push(

$http.get("/profile/" + user)

.then(function(result) {

deferred.notify(user);

return result.data;

}));

});

$q.all(promises).then(function(results) {

deferred.resolve(results);

});


}



Gotcha’s

• You always want to call deferred.reject() and

deferred.resolve() within a scope context (e.g.

$rootScope.$apply()).

• Remember, you can resolve/reject a promise only once!

• Don’t forget to return deferred.promise



About the Speaker

Uri Shaked, [email protected]

• Co-Organizer of Google Developer Group Tel-Aviv

• Salsa Instructor&Dancer, created the Salsa Beat Machine

• Musician, created the Zampoña mobile pan-flute app

• Loves to hack Electronics and Hardware


mailto:[email protected]

http://www.meetup.com/GDG-Tel-Aviv/



http://www.salsabeatmachine.org/

http://www.zampona.org/

http://www.youtube.com/watch?v=dw9U0WxtK9c

http://www.youtube.com/watch?v=dw9U0WxtK9c


Hope You Enjoyed !


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