I come from a .NET web application background and have just started iOS development. The initial design of my app focuses around the NSNotificationCenter. I was reasonably happy with this until I saw various posts explaining how reaching for the NSNotificationCentre was a common newbie mistake.
Here is a simplified version of the problem I am trying to address:
My application is trying to show a list of messages that are populated using web service calls, think Facebook messaging.
When the app is first loaded it pulls a collection of messages from the server and displays them in a table to the user. The user can add new messages (which get sent back over the API) and the app can receive Push Notifications about new messages which are added to the feed.
The messages are never persisted to disk so I'm just using POCOs for the model to keep things simple.
I have a MessageFeedController which is responsible for populating the message feed view (in a storyboard). I also have a message feed model, which stores the currently retrieved values and has various methods:
(void) loadFromServer;
(void) createMessage: (DCMMessage*) message;
(void) addMessage: (DCMMessage*) message;
(NSArray*) messages;
(int) unreadMessages;
The current implementation I have is this:
Use case 1 : Initial Load
When the view first appears the "loadFromServer" method is called. This populates the messages collection and raises an NSNotificationCenter event.
The controller observes this event, and when received it populates the tableview
Use Case 2: New Message
When a user clicks the "add" button a new view appears, they enter their message, hit send and then the view is dismissed.
This calls the createMessage method on the model, which calls the API
Once we have a response the model raises the NSNotificationCenter event
Once again the MessageFeedController listens for this event and re-populates the table
Use Case 3: Push Message
A push notification is received while the app is open, with the new message details
The AppDelegate (or some other class) will call the addMessage method on the model, to add it to the collection
Once again, assuming the MessageFeed view is open, it re-populates
In all three cases the MessageFeed view is updated. In addition to this a BadgeManager also listens to these events which has the responsibility of setting the app icon badge and the tabbar badge, in both cases the badge number relates to the number of unread messages.
It's also possible that another view is open and is listening to these events, this view holds a summary of messages so needs to know when the collection changes.
Right, thanks for sticking with me, my question is: Does this seem like a valid use of NSNotificationCentre, or have I misused it?
One concern I have is that I'm not 100% sure what will happen if the messages collection changes half-way through re-populating the message table. The only time I could see this happening is if a push notification was received about a new message. In this case would the population of the table have to finish before acting upon the NSNotification anyway?
Thanks for your help
Dan.
In other words, you're posting a notification whenever the message list is updated. That's a perfectly valid use of NSNotificationCenter.
Another option is to use Key-Value Observing.
Your controller (and anyone else) can register as an observer to the "messages" property, and will be notified whenever that property changes. On the model side, you get KVO for free; simply calling a method named setMessages: will trigger the KVO change notification. You can also trigger the notification manually, and, if so desired, the KVO notification can include which indexes of the array have been added, removed, or changed.
KVO is a standardized way to do these kinds of change notifications. It's particularly important when implementing an OS X app using Cocoa Data Binding.
NSNotificationCenter is more flexible in that you can bundle any additional info with each notification.
It's important to ensure that your messages list is only updated on the main thread, and that the messages list is never modified without also posting a corresponding change notification. Your controller should also take care to ignore these notifications whenever it is not the top-most view controller or not on screen. It's not uncommon to register for change notifications in viewWillAppear: and unregister in viewWillDisappear:.
In my opinion using a delegate protocol pattern would be a much better fit for this scenario. Consider the scenario where your "api layer" needs used across many view controllers in an application. If another developer were to be introduced to your code, they would have to hunt around for notificationcenter subscriptions instead of just following a clean 'interface' like protocol.
That being said, your code will work just fine and this is a valid use of notification center. It is just my personal preference for 'cleaner' code to use a protocol based approach. Take a look around in the iOS SDK itself and you will see scenarios where Apple themselves use protocols and use notifications. I feel it is much more easy to comprehend and use the protocols than having to dig around and determine what I must listen to for a notification.
NSNotifications run the receivers code synchronously as soon as they are posted, so a new message during repopulation would join the back of that execution queue. On the whole it seems valid to me, and it keeps a reasonable degree of separation between The view controllers and the model.
Depending on the number of classes that are likely to want to listen for the same information arriving, you may want to use a delegate pattern, maybe keeping an dictionary of delegate objects, but I personally don't feel as though this scales so well, and you also have to take care of nil-ing out delegates if a page is dealloced to avoid crashes. To sum up, your approach seems good to me.
Related
Lately I have been playing around with the bluetooth framework and grew a strong enough knowledge to start building an application. The only problem is that all the examples I found and all the practice I have made consist in putting the core bluetooth core code inside the same file as the UIView with which the user is interacting.
I would like my future application to have multiple views in which the BLE scan occurs on the background. I initially thought about creating an object with a name similar to bleDeviceFinder and pass this object through each view. However, after thinking about it I realised that if I want something to happen in the current view I need the function didDiscoverPeripheral to have direct access to the UIView objects which it is supposed to affect.
I know it is probably a stupid question, what would be the best way to do so? I was thinking maybe to set and alert and subscribe every view to that alert; is this a good solution?
A quasi singleton BTLEManager that you pass around in the app. It sends NSNotifications for events like discovery, and your ViewControllers observe these notifications. The truth (i.e. list of discovered devices) stays in BTLEManager. Once a viewController has received such a notification it asks the BTLEManager for the list of current devices and then the viewController changes your views accordingly. The Views should never talk to the BTLEManager directly.
That's how I would do it.
In many of my UIViewControllers, I update certain controls based on the state of my data. For example, I might have an edit button on a UITableViewController that should only be enabled when there is one or more items. Or perhaps I want to limit the number of items that can be added, and disable the 'add' button otherwise.
Every time I add or delete an item (or take any other action that can add/remove items), I have to remember to update any controls that might need enabling/disabling. This is trivial for the most part, but doesn't feel comfortable - there is a lot of repetition, and I have to remember to add the calls to updateControlEnabled (or whatever) whenever I add new functionality that might affect the data.
And then I noticed NSManagedObjectContextObjectsDidChangeNotification. Reading the docs, it looks like I can receive a notification whenever something changes in my managed object context. This seems ideal, but I have a few questions:
Is this an appropriate use of
NSManagedObjectContextObjectsDidChangeNotification?
Should I anticipate any performance impact if a controller
subscribes to these and parses each one to see if it needs to update
the UI? I will be checking the userInfo for every change, instead of
only those that I know I will care about.
Where should I subscribe to the notifications? My UIViewController has a
reference to the context, which helps, but I don't know where to
subscribe (loadView? viewDidLoad? init?) such that the view
controller will always have one and only one subscription.
The view controller will continue to receive and process notifications
when it's offscreen - enabling and disabling controls as the
data model is affected from elsewhere. Is this ok?
I guess I'm mostly just wondering if anyone else uses this approach and if so, what their experience is.
Q) Is this an appropriate use of NSManagedObjectContextObjectsDidChangeNotification?
A) Yes - I used it on OSX for a similar purpose.
Q) Should I anticipate any performance impact if a controller subscribes to these and parses each one to see if it needs to update the UI? I will be checking the userInfo for every change, instead of only those that I know I will care about.
A) NO - it will normally be a very small set of objects - ones that were directly changed.
Q) Where should I subscribe to the notifications? My UIViewController has a reference to the context, which helps, but I don't know where to subscribe (loadView? viewDidLoad? init?) such that the view controller will always have one and only one subscription.
A) Well, you cannot affect the UI til the view shows - so probably viewDidLoad or viewWillAppear. The problem with the later is you may get it a few times depending on push/pops, so maybe I'd do it in viewDidLoad.
Q) The view controller will continue to receive and process notifications when it's offscreen - enabling and disabling controls as the data model is affected from elsewhere. Is this ok?
A) Sure - when the view reappears all the elements will be setup correctly.
What you want to do is a classical use of that notification. Just check the thread it comes in on - if its not the mainThread then you want to make all your changes in a block posted to the mainThread.
I have subclassed RMMessageComposeViewController : MFMessageComposeViewController. The extra functionality that I'm aiming for is for the MFMessageComposeViewController to be able by itself to present a new message compose controller (over itself).
So I should from one RMMessageComposeViewController instance present a new one. The message result from the new instance should be sent to the parent (or "old" one). So I suppose I need to set the parent message compose controller as the delegate when I'm creating the child ("new" one).
Could someone please help me think this out, what instance variables I need to add (parents, children?) How to setup the child message compose controller?
From the docs:
The message composition interface itself is not customizable and must not be modified by your application. In addition, after presenting the interface, your application is unable to make further changes to the SMS content.
What you're trying to do there is explicitly not supported because of security concerns: It would make it easy for an application to forge messages. While you can probably push a view on top of it, I suspect your app would get rejected from the App Store for doing it.
I wouldn't be surprised if MFMessageComposeViewController prevents an application from creating more than one instance at a time, though I haven't confirmed this.
I'm using Apple's view controllers quite alot in my app (like MFMessageComposeViewController and ABPeoplePickerNavigationController). So when I receive a notification (Local or Remote notification) how would I deal with it the most elegant way since I can not interact (send messages) to Apple's view controllers.
My assumption is that if the user is actively using the app and in i.e MFMessageComposeViewController he does not want to get disturbed/interrupted with what he is doing. But if it was me I would get a bit confused if I was doing something and I would hear boing sound (from the notification) and then nothing more happened.
So would a reasonable way to handle this do let the user finish doing his task in whatever Apple view controller he is inside and then display the notification for the user?
Or dismiss the Apple view controller and handle the notifaction and then put the user back in the Apple view controller?
I'm not quite sure I understood you exactly correct, but to me it sound like you want to do something like this:
If the user is composing a message in your app and there is an incoming notification, lets say a push notification from the facebook app, you want to handle this event somehow in your app by letting your MFMessageComposerViewController do something.
As answer to this request I would like to state the following:
You should be able to send messages to your MFMessageComposerViewController if you extend this class (i.e. create a class called MyMessageComposerController and let it extend the standard controller, and do whatever you need there)
However, you can't really do anything about the push notification, it comes from another app and this functionality is built into iOS, you can not make the push arrive later when the user is finished typing, it will always arrive and the user will always be the one to decide if he/she should keep typing or look at the notification. The only thing you can do is to make sure that your app saves everything that the user has typed so that he/she can continue typing when returning to your app.
Assuming you don't just want to display an AlertView or something similarly modal over the message composer, I would do something like the following:
assign a MFMessageComposeViewControllerDelegate to the composer.
show the composer as usual.
queue/remember the notifications received while the composer is in play (through the normal application:didReceive{Local|Remote}Notification: messages)
when the composer is closed, the delegate receives its messageComposeViewController:didFinishWithResult: message.
Do... whatever... with the stored/queued notifications.
Whether you delay responding to the notification until the user is finished with the composition or interrupt them is a trickier situation. If it's a relatively unimportant notification or notifications that are frequent, I'd tend towards the queue/delay approach. But for "important" notifications, I'd have no problems in interrupting the user. Of course, that just moves the question to what determines importance. And that is very much dependent on the app.
I wonder what the optimal way is to respond to notifications sent out by the notification centers.
Here is an example:
I have a model which receives updates from the server.
Whenever new information is received a notification is generated and posted though the NSNotificationCenter.
There is a view controller with lots of (partially nested) subviews; depending on the type of information received I have to update a particular subview.
For me, there are currently two solutions:
The view controller becomes observer and tells a particular view to update based on the notification name. [subviewx pleaseUpdate];
Each view registers an a observer and depending on the notification name.
The downside of 1 is that the vc has to deal with all notifications although he is not really affected.
Is there any proposed way to do this? Should the responsible view controller deal with all notifications or is it ok for a UILabel, for instance, to become an observer and be somewhat independent.
Thanks for your opinion!
An interesting question - technically, both approaches produce the same result.
However, personally I'd lean towards keeping your notification handling in the view controller, because that's closer to the model-view-controller (MVC) pattern in iOS.
The other advantage of having your notification in the view controller is that you may want to reuse your views elsewhere in your app, and you don't want adverse side effects happening when views start responding to notifications they weren't intended to receive. Collating all your notifications in the view controller will also make handling them much easier - don't forget you need to remove your notification observers when you're done with the view, and having all your removeObserver statements in one place is arguably far better than spread across multiple classes.