I cannot find documents about the way, in which Interface Builder determines the Sent Message outlets for the graphical connections between components triggering events and messages of other components.
I want to generate components encapsulating Finite State Automata. The input part is simple, just define IBAction messages and you can connect them in in Interface Builder. The tricky part is obviously the other end of such connections.
I want to provide for each event triggered by the FSM a distinct outlet, like the 'selector' outlet of a NSButton (listed under 'Sent Messages' on the 'Connections' tab of the inspector).
How do I specify such interfaces programmatically and can I specify more than one of these?
Or is this approach not suitable; would Notifications be a better way? (I am used graphical connections from Visual Age and Parts, so I would prefer them, but in Interface Builder, the support for such connections seems somehow limited).
Thanks in advance
The first part of my question has been ansered in the question 'Send An Action Cocoa - IBAction'. I am still looking for a possibility to define more than one 'Sent Message'.
When you implement your method using IBActions, the object that generated the message (the sender) is passed to the message. So if I have a button on my interface that says "Logout" and an action on some controller object named logout: and I have wired these up, the method receives the instance of the button that triggered it. For example:
- (void)logout:(id)sender
{
// sender is the instance of whichever wired button triggered
// this action. We just NSLog() it for now.
NSLog(#"-[%# logout:%#]", self, sender);
}
Other objects may call this action as well, and may pass themselves as the sender or may pass nil. The details of this would be left up to you as the designer.
Related
I have a UIBarButtonItem. When it receives a message it cannot handle, I want it to forward that message to a particular view controller.
I thought I might be able to accomplish this using the bar button item's forwardingTargetForSelector: method, but apparently no such property is found on objects of type UIBarButtonItem. (Point of terminology: Does that mean forwardingTargetForSelector: is a private property? edit: Wait, I think I'm confused... methods with a colon at the end aren't properties... so can you ever make public a method (like a getter/setter) to which parameters are passed?)
And does that mean that in order to set the value of forwardingTargetForSelector: I must do it from within the .m file of the object for which I want to set it? Which would mean that I would have to subclass my UIBarButtonItem?
And if so, why is this not a public property of NSObjects?
And moreover, what's the best way to achieve my forwarding goal, preferably avoiding subclassing?
additional information:
It all stems from my inclination to reuse a single action in response to various instances of an identical button being pressed. The action is currently contained in my delegate (see How should I implement [almost] identical actions used throughout various VCs? (Answer: use a category)) and varies only in that it should send a presentViewController message to the view controller that instantiated the button that sent the action. Thus, in the action, I can send a presentViewController message to sender, which is an instance of the button, and I want to be able to forward that message to the view controller that created that instance of the button, which I can do if I set each button's forwarding property immediately after it is instantiated in its respective view controller.
I hoped to avoid the "why" just to make the question shorter, but there ya go.
forwardingTargetForSelector: is not really a property; it's more like a question the runtime asks an instance when the instance doesn't respond to a message.
It can't be a property in the #property/declared-property sense, because each selector could have a different target; there would need to be a mapping behind it. That's just not how declared properties work.
UIBarButtonItem descends from NSObject, and it inherits this method along with all the others, but you can't "set" the forwarding target for a selector from outside an instance (without creating some other machinery to allow you to do so, anyways -- possible, but not available by default).
In order to utilize this method, yes, you have to implement it in the class that is doing the forwarding. This does indeed mean subclassing. It also means that the forwarding instance needs to have a reference to the object to which it is forwarding; this requires careful design.
forwardingTargetForSelector: is all but certainly not the correct way to achieve whatever your goal is. In general, in fact, it's a bit esoteric.
I'm not sure exactly what problem you're trying to solve ("making a button forward messages it doesn't respond to" is still rather general -- in particular, why is that necessary?), so it's hard to be more precise.
OK, here is my situation and I'm really not sure on which design approach to use. So, I'd be glad to read some input on my particular case...
The scenario :
I've got a tab-based application
Initially we create an instance of NSWindowController (e.g. MyDocumentManager) which takes care of the tab creation/manipulation
Each tab contains (= is attached to) an instance of MyDocument
Now, I'm about to implement the menu actions.
For each menu there is a separate Menu Controller (actually a subclass of NSObject grouping all relevant functions), e.g. MyFileMenuController
File Menu's actions are linked to actions in a MyFileMenuController object, via Interface Builder
The question :
How is it possible that MyFileMenuController "knows" about MyDocumentManager (created in my AppDelegate.m), so that we can access current document details and perform all relevant actions? Any ideas? Which approach is preferable?
My ideas :
Passing object from class to class (not sounding that great)
Singletons (although I've honestly never used them, and do not know whether/how it could be my particular case)
Notifications & Notification Listeners
(Looking at it from the opposite side, though not sure) Delegate methods
OK (not sure if that's the best way to do it), but this is what I decided to do :
[[[NSApplication sharedApplication] delegate] MyDocumentManager]
So, as long as an object is part of my AppDelegate, this way I can access it from anywhere.
I would make an initial UIViewController linked to appDelegate.rootViewController.
In this new view controller (just call it "mainViewController") I would put my tabBar and the menuController.
I understand your interface is similar to facebook with a leftBarbutton which makes horizontal scroll and discover the menu. So in the selector for this leftBarButton I would call a method like:
- (void)discoverMenuForDocument:(MyDocument*)document {
// Set menu configuration for specific document
// Make animation to discover menu
}
Where document could be something like:
self.selectedViewController (<-- You cand make this in several ways depending on your code...)
Realize that (MyDocument*) is not an object but just a reference, so in my opinion there isn't any problem doing this.
Good luck!
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.
I just created a custom UIViewController with some user actions like touch. I would like to handle the user interaction in the parentObject. In other words the one that created the ViewController.
From other languages I am used to use Events that are pushed up. So my parent object would have some kind of listener on the reference of the ViewController object it can react to.
How would that type of interaction handled by Objective C?
This can be done by 1) responder chain, 2) notifications and 3) delegates.
All UI objects form the responder chain, starting from the currently focused element, then it's parent view and so on, usually until the application object. By sending action to the special First Responder object in your nib, you'll throw it down the responder chain until someone handles it. You can use this mechanism for firing events without knowing who and when will handle them. This is similar to HTML event model.
Notifications send by NSNotificationCenter can be received by any number of listeners. This is the closest analog to e.g. C# events.
Delegates is the simplest mechanism of sending event to one single object. The class declares weak property named delegate that can be assigned with any object, and a protocol that this object is supposed to implement. Many classes use this approach; the main problem is that you can't have more than one listener this way.
you should look into delegations/delegate for interactions between two viewControllers. You need to understand how it works first.
https://developer.apple.com/library/mac/#documentation/General/Conceptual/DevPedia-CocoaCore/Delegation.html
It sounds like you need to implement a delegate protocol, which will allow your 'child' view controller to communicate back to it's 'parent'
See http://developer.apple.com/library/ios/#documentation/General/Conceptual/DevPedia-CocoaCore/Delegation.html
Hey guys, I currently have a root table view which has a toolbar at the bottom and has labels and a refresh button within it, much like the Mail app's toolbar. This root table view controller obtains data from a server by allocating and initializing a DataUpdater class. Within this class are the NSURLConnection delegate methods that are called while communicating with the server.
As you can probably guess, I need to know when certain (delegate) functions are called within the DataUpdater class and the values of the parameters passed to these delegate functions so that I can update the labels on the toolbar accordingly (i.e. Connecting..., Updated, etc).
The problem I am having is determining how to notify the root table view controller of what is going on in these delegate methods. Would I use protocols, if so how? I have been skimming the documentation and don't quite see how I would get this effect. Or would you suggest I implement my program another way?
Thanks in advance!
A protocol is a kind of contract that says: I promise to provide the non-optional methods defined in the protocol, and maybe even the optional ones. It's purpose is like Java interfaces: to work around missing multiple-inheritence.
The delegate pattern in Objective-C normally works like this: you define a protocol, and then in your class, you define a variable like id<MyProtocol> myDelegate; and define a setter and maybe getter (either via normal methods, e.g. - (void)setDelegate:(id<MyProtocol>)aDelegate; or via properties.
Note that the delegate is not retained ! So if you work with a property, you need the assign option, not retain.
Now back in your class, you check whether myDelegate is nil and if not, you can directly call its non-optional methods. If you want to call an optional method, you first need to verify its presence via respondsToSelector:.
So if you decide to use the delegate pattern, you need to define a protocol, add that protocol to your root table view controller, implement the necessary methods there, and make sure to call [foo setDelegate:self]; or something similar to inform your other class that the root table view controller is the delegate. And of course implement the delegate calls in your class.
Edit:
An alternative might be to use NSNotifications, BTW. The advantage of notifications is that you can have multiple objects listen and react to them. The disadvantage is that you cannot (directly) pass values back. For example, you can define a delegate method that asks the delegate whether to do something or not. That's not possible with notifications, it's more like shouting into a room instead of having a one-to-one conversation.
DarkDust's answer about protocols is fine but I would like to add some things to it.
One underlying thing that is often forgotten when it comes to delegation is object ownership. When a program is running it creates a tree of objects. Its root object is the application delegate and for example it owns a navigation controller, which owns the individual view controllers, which own the view and the view owns its subviews and so on.
Often the question comes up: "Why is the delegate not retained, just assigned?" The problem is that if you send a message to a deallocated object the program crashes. So how do you make sure the delegate stays around? The answer is object ownership.
I give you an example: a UITableView and its data source which is the TableViewController which is nothing but a delegate. The TableViewController holds a reference with its view property to the UITableView, so it owns the TableView. That means when the tableView is alive there must also be its parent object present, which is the UITableView's delegate. So there is no danger that the delegate goes away somehow.
In the end it is again all about memory management.
Take home message is: think upfront about object ownership will make your program mode modular, easier to maintain and will lead to a looser coupling between individual objects.