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Angular use root scope vs services to share data

I have been reading wrox angular book. Inside the book the author describes that a method of sharing data between controllers is to
Have a property on the root scope
Update that property on the root scope
Broadcast the fact that the property was updated
All children scopes that need to know , will listen for the broadcast.
as opposed to expose an object on a Service and letting angular's two way databinding do all the heavy lifting. Why would anyone go with the 'root scope publish/subscribe' methodology, instead of exposing an object on the service?

That's interesting question.
First we should consider differences on various levels:
Scope
in case of $rootScope we define variable in global scope
in case of shared services we can inject this service to controllers that really use this value
Extensibility
$rootScope - we have limited options to add additional logic to work on this value (we can define another global function)
shared services - we are free to define any kind of logic
Encapsulation
$rootScope - all object defined in $rootScope would be visible in all modules
shared services - we can decide what is visible and what is not
Modularity
$rootScope - global variables is not places in module spaces
shared services - service is a separate module for application
Maintaining
$rootScope - it's very hard to find which components use our $rootScope variable.
shared services - we can see which services we use and we can find in which component we use this service
Binding
$rootScope - it is easy to setup two-way binding in several controllers on one variable in $rootScope
shared services - could be tricky to enable two-way binding
In my opinion this is only useful for make really global variables.

Say you have two controllers A and B, and a service S, storing the common data.
When A changes data in S, B cannot directly change its scope value by understanding that data in S has changed. Someone has to say to it that data in S has changed and update its scope according to this change. This may be done two ways.
One is rootScope broadcast: service S broadcast changes and B listens this broadcast.
The other $scope.$watch: In controller B, scope must watch the changes in service data.

It depends which kind of data you are managing, if you are for example relying on a DB where you perform CRUD actions, you'd like a service to just interact with the DB.
That's called a stateless service, some people vouch for it and some are against and prefer to have state also on the service, exposing the object as you mentioned.
I'll leave you a couple resources with more information on the topic so you can decide which solution suits you best
http://www.johnpapa.net/sharing-data-in-an-angular-controller-or-an-angular-service/
http://www.webdeveasy.com/angularjs-data-model/

Angular Project Architecture

I am building an app in angular, which consumes different APIs and Gives options for the user to select it will be recorded and sent back to the server.
I have designed it as follows.
All the common logic in Main Controller and all other options in different controllers as the child of main controller.
Main Controller retrieve all the data that are required to run the app.
which is consumed by all other child controllers.
To make sure data is loaded I am using promise attached to scope. So all the child controller will know data loaded.
I have moved data updation part of all child controllers to main controller
because all the updates happen in one object.
Child Controller emit/broadcast to communicate between child and main. So when update happens child will emit an event with data which will be captured by Main and it will do the update.
MainController {
$scope.loaded = DataService.get();
$scope.userOptions = {};
$scope.$on('update',function(){
updateUserOptions();
})
}
ChildController {
$scope.loaded.then(function(){
//all logic of child controller
}
$scope.onselect = function(){
$scope.$emit('update',data);
}
}
Questions
Is it a good practice to use events between controllers ?
is it good to use promise attached to scope for child controllers ?
Will it improve my code if I start using services ?

I will try to answer your question based on my own experience. Recently I've built a single page application and I've refactored its architecture.
Here are my answers:
Is it a good practice to use events between controllers? IMHO, it is the most flexible way to share information between all controllers even if they have isolated scope (using $broadcast or $emit for example). This is called the Observer design pattern. However, you can use services instead of events to share data between them. If you are going to use $rootScope, be careful as all the $scopes inherit from $rootScope.
is it good to use promise attached to scope for child controllers ? Firstly, you have to learn about how scope inheritance works. You have to take care to avoid property shadow in JS. Secondly, I would move out all the logic from scope.loaded in ChildController to a service such as ChildService. Keeping the business logic (such as request, etc) in Services instead of Controllers, will ensure it can be re-used.
Segregation of business logic is good design principle.
Will it improve my code if I start using services ? I answered this question above.
In addition, in order to build a good architecture I've read this angular style guide written by John Papa.
I recommend the following changes:
To make sure data is loaded I am using promise attached to scope. So all the child controller will know data loaded.. Instead I would emit a custom 'loaded' event in the MainController using $scope.$emit('loaded'). After that, in the ChildController I would use $scope.$on('loaded', function(){}) to handle the event.
I would move the updateUserOptions function to a service and inject the it into just the controllers that need it.
I hope that helps!

Is it a good practice to use events between controllers ? Not as the main form of data sharing, but you can use it to notify about system events, such as data ready.
Is it good to use promise attached to scope for child controllers ? Don't use scope inheritance, it causes lots of annoying problems.
Will it improve my code if I start using services ? Yep.
This is what I would do in your place:
dataService - this service is responsible for all data coming in / going out. Whenever a request for data is made (no matter which controller asked for the data), the service caches the data (save the promise is good enough). All further requests get the cached data unless they specify they want a fresh data. Whenever data is updated (1st time or refresh), the service broadcasts a 'dataReady' event via $rootScope to which the main controller and other subscribers can listen.
The service is also responsible for data updates, and when the data is updated you can also broadcast an event via the $rootScope.
When the event is activated, all subscribers query the service, and get the data they need.
Controllers - avoid controllers, use directives with isolated scope, and pass the data between them using attributes. In this way you can be sure that each directive gets what it needs, and not everything. The directives can communicate using attributes, services, broadcast / emit or require their parents / siblings if they work closely together.

Is it a good practice to use events between controllers ?
No it's not, it will be deprecated by Angular JS 2.0. It also often leads to unmanagable tangle of events which are hard to understand and debug. Use services to share data between controllers. (Inject same service into multiple controllers, service then holds data, controllers bind to that data and are automatically synchronized) I wrote a blog post explaining this use case.
Is it good to use promise attached to scope for child controllers ?
No it's not. Use promises and resolve data in services. Don't use $scope at all but use controllerAs syntax instead. $scope was deprecated also in Angular JS 1.X because it's usage leads to many different problems with scope inheritance.
Will it improve my code if I start using services ?
YES! Use services for all logic and data manipulation. Use controllers only for UI interaction and delegate everything to services. Also use ui-router for managing state of your application.

I'm not going to answer your questions directly as I have some other comments as well. I think the approach you mentioned is not the best way to build angular applications.
All the common logic in Main Controller and all other options in different controllers as the child of main controller.
It's against all angular style guides to place common logic in controllers. Controllers should only be used for the logic related to the view (data binding, validation, ...). Because the code inside a controller is not reusable, the less code you have in a controller the better. The more logic you have in services, the more scalable your application becomes.
Fix: I suggest you create a service that retrieves data from the server, and inject this service in controllers as you need. Notice also this way offers better dependency management as you can keep track of which controllers need which services exactly.
Nested controllers should be avoided when possible, because angular keeps track of all the active scopes and re-evaluates them in every $apply() loop.
Fix: same as #1, use services instead of the main controller.
To make sure data is loaded I am using promise attached to scope. So all the child controller will know data loaded.
Using a promise for data retrieval is a good practice. But, again, keeping it in a service is much cleaner than main controller.
I have moved data updation part of all child controllers to main controller because all the updates happen in one object.
Child Controller emit/broadcast to communicate between child and main. So when update happens child will emit an event with data which will be captured by Main and it will do the update.
Fix: use a service with an update function instead of events. Events are harder to debug and track. And you need to unregister event handlers on destroying a controller. If you can use a function/promise instead of events, then it's usually a better idea.

Is it a good practice to use events between controllers ?
A problem with your current set-up is that you're implicitly relying on the hierarchy of your controllers (the fact that one is the child of the other) - because you emit the event, only scopes higher up on the hierarchy can catch it. Besides being an implicit connection (that a developer has to remember), this also limits he extendability of this feature.
On the other hand, if you injected a shared service into all the controllers that need it, the connection between the controllers would become explicit and documented, and their scopes' position in the hierarchy independent. This will make your architecture easier to maintain, with the added benefit of also being easier to test, for one.
You can still implement an observer pattern with a service.
is it good to use promise attached to scope for child controllers ?
The issue of polluting scopes pointed out in other answers is valid. This is one of the reasons why it's better to limit the number of objects you attach to your scope, and to use objects as bundles of variables on your scope instead of attaching all the variables to the scope directly.
(For an explanation of these reasons, see discussions about "always having a . in your bindings".)
(Of course, don't do this blindly just to reduce the number of variables, try to find semantic connections between variables that might be bundled together sensefully.)
Will it improve my code if I start using services ?
I think the above answers already outline the answer for this: yes. There are other benefits too, but this format is not best for too long answers, so I won't list anything else now.
All in all, these above pointers are not big issues with your code currently, but if you're looking for the best architecture, I think you can do better.

Answers:
No, it will be deprecated soon.
$scope is deprecated already.
Services is a great choice. Services allow us to share data and behaviour across other objects like controllers.

Managing globally needed services in web applications

I struggle to find a statisfying solution on how to expose service instances whose methods need to be accessed through multiple parts of my applications.
The situation
First things first, by a 'service', I mean an instance of a function that holds properties & methods which are exposed through an API.
Consider a REST service whose purpose it is to provide convenient methods to access REST points on a server. I would make the following assumptions on that service:
It must be available throughout the application. It is likely that as the app grows, there will be new components that need access.
There is no need of multiple instances of this service. We can consider it a singleton.
My solutions
I can think of 2 possible solutions:
Concatenating scripts & utilizing the global object
I could combine all my script files (e.g rest.service.js, app.js) into a single file and create an object property on the global object (like App).
Then, I could attach service instances to this object. This allows me to do something like this from everywhere within the app:
App.restService.get()
However, even if I wrap each service in an IIFE, i still have to add some variables on window in order to retrieve instances.
Using commonJS / AMD modules
I could require() my service instances from everywhere by using require.js / browserify
The issues
Now I got a headache because on the one hand, people are telling me that polluting the global object is bad practice. Singletons are bad practice also.
On the other hand, we make a lot of effort to 'uglify' scripts, each byte saved considered an enhancement. Using browserify would lead to the same script injected in multiple files, though (I'm using web-components, therefore I've got a lot of isolated scripts). Not mentioning the fact that I have no idea on how to provide a state-safe service using browserify.
So how should I approach this problem?
How should I expose standard services that may or may not be instantiated multiple times? How should I implement state-safe ones?

Just a starting point (but too long to be a comment) I really enjoy the strategy used by AngularJs, where you always instantiate services within a container - and every time you instantiate something you also specify which modules should be injected into it:
angular.module('myApp.services', []); // the second argument are the dependencies (an empty array
At any point, you can retrieve your modules and add functionalities:
var services = angular.module('myApp.services');
services.factory('yourServiceName', //
['other', 'service', 'dependencies'],
function(other, service, dependencies){
other.doStuff();
service.doStuff();
dependencies.doStuff();
[..]
});
You can then inject your module in other modules
var myApp = angular.module('na', ['yourServiceName'])
In angular, the app is instantiated by the framework itself - but I guess you can develop a entry point for your app, so that you can use your services.
..unfortunately, I do not know exactly how this pattern is implemented - probably all the modules are stored within an instance of the application, so the global namespace is not polluted.

This problem also confuses me a lot, I think there are two points I can figure out:
1) There must be an entry point for each service in global, otherwise it is impossible to get the one you need everywhere. It's not good to add many things in global, but I think service reference is the one deserved.
2) Config the service object other than initialization, for example, they can be only one ajax service object with different configuration to do different things. There are objects, so they can be merged and extended.
This is an interesting topic, I would like to see more opinions about, not just management of services, also other resources like templates, objects, files, etc.

(another) request for clarification about the meaning of "singleton" in Angular

I'm learning angular, experimenting with different ways of using services/factories, and am trying to wrap my head around the unique sense in which they are "singletons".
I have an API service that exposes domain models and wraps functionality for retrieving them from my REST server. This service can be easily comprehended as a singleton in the classic sense: I want a single instance to be shared across my application, with a state that can be observed by many different controllers, enabling those controllers to "synchronize" with each other through the conduit of the service: The controllers stay in sync not by being aware of (and thus coupled with) each other directly, but by being aware of this common service. (side question: is this a correct characterization of the role of a service?)
This is a use-case where a singleton service is clearly and unambiguously appropriate. But:
One of the domain objects that gets returned by the API service is called thread, which is essentially a wrapper around a linked-list of points. In addition to the list of points, a thread has a currentPoint variable and a next() function, which pops the next value from the list and makes it the currentPoint.
My UI visualizes a thread simultaneously in two different ways - in two separate elements with their own controllers and directives. Each of these elements contains a button that, when clicked, calls nextPoint() and thereby changes the state of the thread. When this button is pushed in one element, the state of the thread needs to be updated in both elements - so, here again we have a situation where a service seems ideal.
But, at any given time, there can be an arbitrary number of threads being displayed, each of which should be independent and unaware of each other - which conflicts with (what I understand to be) the "classic" sense of "singleton".
Is there a word for this sort of thing in angular?
I've experimented/looked into this enough to know that it's certainly possible to create these "non-singleton singletons" using factorys, but surely there must be a better term for them than "non-singleton singletons" - I cringed both times I just typed it.
Second, what is the best way of implementing one? Can the strategy illustrated below (which I found in an angular github issue here) be regarded as a best practice?
myApp.factory('myServiceProvider',function(){
var serviceProvider=function(){};
serviceProvider.prototype.foo=function(){};
var service = {
getInstance:function(){ return new serviceProvider(); }
}
return service;
});
//Singleton Services
myApp.factory('myService',['myServiceProvider',function(myServiceProvider){
return myServiceProvider.getInstance();
}]);
//Where non-singletons are required
function myController(myServiceProvider){
var instance_a=myServiceProvider.getInstance();
var instance_b=myServiceProvider.getInstance();
};
//Where singleton service is required
function myOtherController(myService){
myService.foo();
};

An angular singleton is not exactly the classic. A singleton in angular is basically one instance of an object that holds a single state. That state can be data bindable via a services allowing for value sharing. This means that you can have a service property that is bound across all of its uses an app; one change will be reflected everywhere. The singleton persists through the life of the app. If the app is refreshed the singleton will loose value until it is set again by the app via a user interaction or a storage retrieval.
I like the pattern you are using. I also tend to use it. Here is an example of a web storage service I recently made similar to your service, https://github.com/breck421/WebStorage/blob/master/src/js/WebStorage.js.
I hope this helps and feel free to continue this dialog :)
Thanks,
Jordan

SPA design with AngularJS - sharing state across some, but not all, pages

How would you implement something like this in Angular:
I have a multi-page user interaction that shares state across pages/controllers. Page A launches a multi-page process across pages B, C, and D. I need to share state across pages B-C-D but as soon as the user goes back to page A (either at completion, or abandoning halfway through) the shared B-C-D state should go away.
I could put the shared state in a service and that would take care of sharing across pages. But then it's a global singleton for the whole application. Is there a way to ensure the service is disposed if the transaction is abandoned or completed?
In the server-side, Java EE world this was called "conversation scope" - I'm wondering what the equivalent might be in Knockout or Angular.
Or there a better way to approach the design? Should I use nested controllers?

This is a pretty broad question and is going to certainly elicit some opinion based responses. That said, one of the things you can rely on is that services (and factories) are singletons. What that means is if you need to share data between controllers, or across the life of a SPA you can create a service to hold and share that data. That service and its data will exist as long as the page isn't re-loaded.
The basic pattern for a shared data factory is like this is:
// define the shared data service
app.factory("SharedData", function () {
return {
// just one data object for the purposes of the example
sharedDataObject: {}
};
});
Once you have defined the service you can define a controller that uses the service something like:
app.controller("FirstController", function ($scope, SharedData) {
$scope.localData = SharedData.sharedDataObject;
});
Because we have defined the shared data as an object and not as a value - binding to "localData" will allow us to set values that can then be retrieved for as long as the page isn't changed. I put together an example that shows sharing data between two controllers here. This pattern works just as well if the controllers were on separate views.
Again - I'm not sure if this is the recommended pattern but it is one I have seen around and it works pretty well. Best of luck!
NOTE - This answer is referenced in your question - so it probably isn't exactly what you are looking for. That said, I am posting for others that may need a simple method of saving state on an SPA.

If you put the shared state into a service, just add a reset() function to it that gets called when they go to the "A" step or when they complete the process.
And I think you could do it with a nested controller too. You'd probably still need to add a reset() function for handling all the cases.