I have a main Container View controller which has a table view controller and another view controller. In table view I am displaying certain items which can be selected and grouped. This group details are shown on the view controller(like a summary). once grouped these items will no longer be in table view. If needed I can even ungroup them and add it back to table view.
So these two view controllers need to have a communication channel between them. What would be the best approach here ? Protocols or blocks ? Notifications are a strict no.
The business logic of your application should be handled separately from the code that glues things into views — you're conflating model and controller.
So the items and the groupings would be maintained by a third class. Both view controllers would talk to it. They would nit talk to one another beyond transient UI information like, say, one saying "this was the specific item selected".
A good way to think about it is to start with the goal of no direct communication. Instead, both VCs have access to the application's model (usually a form of a singleton), and configure their state based on the state of the model. In other words, the view controllers are stateful with respect to the app's model, stateless with respect to each other.
But sometimes vcs in the same container must communicate, like in a navigation controller when a selection is made. That's typically done in prepare for segue. The selection is conveyed to the vc being pushed, and then it carries on. To the extent that vc needs to communicate back down the stack, I try to go by the first principal: can it just change the model state and get popped, counting on its parent to notice the changes in viewDidAppear?
If I need further communication still, I begin to worry about my design, but the next place I go is KVO. There again, view controllers are passive with respect to each other, watching the app's model for changes they care about. I classify NSNotification in this category. Even though it sounds like you want to rule it out, it's my third favorite idea after nothing.
While I'm editing here, I notice #Tommy has provided concise advice that I agree with (and consistent with my opinion here). I'll leave the answer since it adds a little nuance.
Related
This is a concept question.
Like many others, I have a parent scene and a child or detail scene. When I click on a specific button in my parent scene, prepare to segue gets called and I pass over a couple of properties. In my detail scene, I gather more information and need to save it for use by the parent. I have seen various methods involving delegates, using singletons, and passing directly back to the parent properties. Here is my question, would it be more correct to store the data in a database in the detail controller, or pass it back to the parent controller to store it? It seems to me that since it was collected in the child, it should be stored there.
Would that be the more correct way of handling it?
For simplicity, say the larger model is an array of custom objects and the detail view presents and edits one of those objects. If you pass a custom object to the detail vc on before the segue, there's no need to "pass it back" later. The parent vc passed that object in the first place, so we know it already has a pointer.
Instead, the parent vc should notice that the work has been done on the object it passed and react accordingly (usually update it's view) This can be done by one of a handful techniques:
just assume something changed and update the view on viewWillAppear
notice the custom object changed via KVO (observing one of its properties)
be notified that the object changed because the detail vc posts an NSNotification
learn that the object changed by arranging to be the delegate of the detail vc
pass the detail vc a block to execute when it updates the custom object
It's hard to say what's better without knowing more about your situation. I can say that simple is usually better, and that favors (1). I can also say that delegates are fine, but often overused.
If you're dealing with more than a few pieces of information, it would likely be much easier in the long run to use Core Data. If, down the road, you decided to add more functionality to the app, or wanted to store different kinds of information, it would take no more than a few extra lines of code to do this.
But from what it sounds like, you have a really simple app, so you should probably be fine just making an instance of your "parent scene" in your "detail scene" and storing whatever values you may have in the "detail scene" into properties that already exist in the "parent scene".
Does that answer your question well enough?
I found this in SO; it doesn't exactly answer my question, which is: is there a way to clone a UITableView from one controller to another while using Storyboards and maintain synchronization?
You can clone them in the sense that their initial property values remain the same, like position, layout etc. For this, just copy the UITableView from storyboard, go to destination view controller and paste it there.
If you share same UITableView object between two view controllers, it is still possible, but you must estimate how much work you would have to do yourself. When a view controller solely handles a table view, much of the work is done under the hood and table is handed over to you. In case of your UITableView shared between two view controllers, you would have to play with it quite carefully. You need to take care of entire life cycle of the view - see the usual methods like viewDidLoad, viewDidAppear and so on. How would you take care of them when your view exists in two scenes? Maybe you would be inventing a unique design pattern - if at all you find the most optimistic and programmatically correct way to accomplish it.
With storyboards, you cannot achieve cloning up to the level wherein data changes will reflect seamlessly between the two. Storyboard is, as the name suggest, just a board, where you can draw things to know how would they look initially, before your code takes over.
In order to achieve what you want, you have to create a shared model that updates two table views through proper delegate methods. Most likely such a model (NSArray, or any such collections as per your requirement) can reside inside a shared class (app delegate isn't a wrong choice), from where both your view controllers can refer to it. That is neat practice, it not only is right from programming perspective but also extensible and helpful to anyone who deals with the code any time.
For details about how to update UI from your model, there is nothing better than starting from scratch, going through the books I mean.
I am not aware of such possibilities.
I would pass the tableview source object amongst different controllers and let the view controller handle their own table view.
I think the best approach would be to use a framework such as the freely available Sensible TableView, then use the same model for both table views. This should be really straight forward.
I have an app that was extremely simple until today. It had a tab bar view controller with 3 tabs. The middle tab was a camera, and the other 2 were table views. The tab bar view controller was the central hub for all the data in the app. So from there, I would set a table's data array as:
(PLEListViewController*)[self.viewControllers objectAtIndex:0] setList:newList];
Obviously, PLEListViewController is my UITableView subclass.
So now, I want to wrap the table views in a UINavigationController, which is fairly simple. But now, that line of code turns into:
[(PLEListViewController*)((UINavigationController*)[self.viewControllers objectAtIndex:0]).topViewController setList:newList];
There are 15 lines in the code that do this, which is not pleasant.
So my question: what is a more elegant way of doing this that I'm missing?
It's good that you're asking this and seeing the issue now. Your problem is can be found in your question. The answer to "the correct way to communicate between several different view controllers in Objective C" is "don't." Specifically, your mistake is here:
The tab bar view controller was the central hub for all the data in the app.
A view controller should never hold any of the data in the app. Your data should live in your model classes. All the view controllers should talk to the model classes. They should very seldom talk to each other. That's the heart of MVC.
So, you move your "list" (whatever that is, doesn't matter) into some model object that all the view controllers know about. That model object can be a singleton, or often better, it can be passed to the view controllers when they are created. When things change, you change the model. And in viewWillAppear: you update your view controller to match the current state of the model.
Never assume that a view controller exists when it is not currently on screen. If your design requires that a non-active view controller exist, then your design needs fixing.
You need to work with your architecture. Make the appropriate datasource and delegation protocols to ensure your classes can communicate anonymously. What you currently have is very inflexible and it will get worse as your app grows/changes.
You want to make things more loosely-coupled, instead of coding explicit traversal of links between your objects in your code.
Assuming you have one data model that is displayed in various places in your application, I think there are 2 approaches that could help...
One is to use your view controller hierarchy.. For example, use [ self enclosingTabBarController ] to find your closest parent tab bar controller and get it's data model property. Substitute -enclosingTabBarController with what works better for your application.
The other approach would be a "data model as a singleton" approach. For this you can either
move the data to your application delegate and access it via ((MyApplicationDelegateClass*)[ UIApplication sharedApplication ].delegate).dataModel
or
have a singleton data model object for your app, and access it via [ MyDataModelClass sharedModel ]
In any case you are moving to a looser coupling, which requires less explicit traversal of links between objects in your app. Less is more!
At the moment I'm using a Singleton class to do some work but I'm wondering if there is something better.
I have an app that has a completely dynamic work flow. It uses a navigation controller but the order of the view controllers depends entirely on some data that is downloaded from our server.
The entire workflow is downloaded and saved in an array.
The "main menu" screen of the app has several choices (settings, recent, etc...) these are fixed but one of them is the dynamic one. It always starts with the same type of view controller but from then on it depends on what you choose.
Description
There are 4 different types of these dynamic controllers.
Table View Controller with single selection and detail indicators.
Table View Controller with multiple selection and checkmarks.
View Controller with a text field and keyboard.
View Controller (with other related VCs) used for searching for accounts on the server.
When you press the option "New Event" on the main menu the menu then goes off to the singleton (EventManager) and tells it to start a new event.
The singleton then pushes a single selection dynamic view on the nav controller and gives it the initial options.
From here on the singleton picks up all the selections and works out what type of view is required next.
I hope this is making sense
Anyway, I don't like the singleton pattern here as I don't think it should be a singleton.
What I would like is a class that I can create from a ViewController and this class will then control the pushing and popping and flow of data between a load of different view controllers. Then when you go back to the main menu this class can go away so I create a new class each time.
Is there a pattern that I can look at that will do this? Or should I stick with a singleton like I am now?
I hope this makes sense.
EDIT
Could I use a UIPageViewController for this? Then the datasource/delegate object of the UIPageViewController will take the place of the Singleton I am currently using... or something?
ADDING PHOTO FROM TWITTER REQUEST
Each VC along the flow has no idea what cam before it or what comes next. All they do is call back to the singleton to say "This value was selected" or "This text was entered" etc...
The singleton then stores that info and works out what comes next and pushes the next VC onto the stack.
It needs to be able to move back along the stack so the user can go back to change something etc...
It's all working as it is I just don't like the use of the singleton.
Lots of comments here in order of importance.
Everything you've described here sounds really good, even down to the naming. "EventManager" sort of sounds like it manages all "events" in the system (so I'd expect there to be a class called Event, but that's a tiny quibble and the name is likely still very good). There are other good designs, but I wouldn't have any problem with yours.
I agree that this does look like a good fit for UIPageViewController. You should certainly investigate that to see if it's the right fit. It's always nice to use a built-in controller if you can.
There's no reason to strongly avoid singletons. They are a natural part of iOS development and fairly common in good Cocoa design. They should be "shared" singletons generally (never create "strict" singletons that override +allocWithZone:). This just creates an easy-to-access instance rather than a true "singleton." This is the way things like NSNotificationCenter work and is often a very good pattern.
Singletons are best when many random pieces of the system need to access them directly and passing them around to everyone would be a lot of overhead (especially if many of the pieces you'd have to pass the object to don't need it themselves). Again, think NSNotificationCenter. If the users of it are mostly contiguous (i.e. most objects you would pass it to actually need it themselves), then just create one at the start of the program and pass it around. That sounds like your situation, so your intuition about it seems good. Just de-singleton it and pass it. Easy change.
But I'd definitely dig into UIPageViewController. It could match your problem very well.
As an exercise I am working on a simple drawing app for the iPad.
I am using UISplitView, with the drawing view as the detail view. In the master view controller I present (in a table view) a list of the shapes drawn so far.
The user can edit or delete any shape from the master view controller, and also select and edit a shape by touching it in the detail view controller.
To notify each of the view controllers of the changes made by the other, I thought of using delegates, but I am not sure if this is the right pattern to use.
First, as I understand it, delegates are supposed to be used when a certain object encounters an event which they don't know how to handle. in that case they pass all the information to the delegate and let it handle the event. This is not the case here since both view controllers need to do something with the information. Using delegates here can cause code repetition.
Another reason I am thinking not to use delegates is that in the future I might want other view controllers to get the information of changes in the drawing. I can use multiple delegates (is it good practice in general?) but I'm not sure this is a good solution either.
Are there other solutions I should consider?
On the contrary, I think delegates might be the right pattern to use here - you don't necessarily delegate only when you can't handle an event (though that might be one situation where you would delegate).
Instead, consider delegates a way of getting information to or from another object, when you simply don't know what that object might be. For example, Apple uses the delegate pattern when working with UITableViews; in that delegate protocol, the table view knows perfectly well what to do in each situation, but still notifies your code when certain actions are about to happen. I think that's an excellent parallel for your situation. (Note that a parallel to your question's assumptions also exists, in the UITableView "data source," where the table view does require some bits of information.)
Another technology you could consider using, if you're really dead-set against delegates, would be to use notifications. You can have each controller subscribe to particular notifications, then have your shape (or detail view controller) post instances of NSNotification when observable changes happen. That way, you still get to handle events happening in a different controller, but without needing to maintain a delegate list.
As for code duplication, just consider refactoring as you start encountering situations where you would duplicate; maybe you should design a single delegate or notification subscriber object for the common code, then only do class-specific things in each of the other controllers?