I looked through a core data tutorial which puts the core data code in the AppDelegate class that is created upon creating a Window project. I was able to successfully save and retrieve data from the premade AppDelegate class.
My question is, if i have a ViewController that saves data, how do i go about saving data from that specific class.
Do i need to redefine the persistent store and managedObjectModel in that class?
If that is the case, what is the programmatic way to do it (since previously it was generated for me) i.e what methods/instancevariables/properties do i need to declare.
Standing by for any clarifications there may be
No need to go through the app delegate or redefine persistent store etc. All you need is a reference to the managedObjectContext. Normally, this is a property (say, of a view controller) that you can set from the outside after creation and easily access from inside the view controller class (much easier than going through the app delegate!). You can pass this on easily to other view controllers, and read from and write to core data with the standard methods.
If you want to save your own objects, yes - you have to create your own managed object model (by modifying the one the template generated for you).
Why not just have a reference to your application delegate in the view controller you wish to save data in? You can then access the managed object context and call its save method as usual.
In the .h file of your view controller have a property of same type as you application delegate. Then in the viewDidLoad method in the .m file set the reference like this:
self.appDelegate = (MyApplicationDelegate*)[[UIApplication sharedApplication] delegate];
Then when you want to save you can access the application delegates managed object context like so:
[self.appDelegate.managedObjectContext save:&error];
Hope that helps, I wrote that off the top of my head so there might be some syntax errors but the compiler will let you know.
Related
I am trying to use Core Data without storyboarding in my app. I want to manage many relationships and I have a kind of singleton class to manage the access to the managedObjectContext, model, and persistentStoreCordinator I got it from this link Singleton datalayer without appdelegate
My problem is that I dont want to use StoryBoards but all the examples that I found use Storyboards. I have problems with UITableView because appDelegate doesnot found it.
I want to create a Single Application and then with a singleton to manage the context and everything, show the objects that I get in a JSON format with nsfetchedresultcontroller or with fetchrequests if I only need one item.
I am using this class. also I have another class called CoreDataTableViewController.h/m it is a subclass of UITableViewController and it has the methods of NSFetchedResultControllers. I got this of the Stanford IOS course. Then I have my MainViewController its a subclass of CoreDataTableViewController and there I get the data from the Json and put it in my ManagedObject and also I overwrite the method cellForRowAtIndexPath. But when I start the simulator I have an error. loaded the "ViewController" nib but didn't get a UITableView.
Core Data doesn't care whether you're using Storyboards or not. Just copy the Core Data related code that you need (you can create a singleton containing the relevant managedObjectModel/context, persistentStoreCoordinator objects and methods) and add it to your own project.
There are no relation between the two. If you need an example of core data that doesnot use storyboard, try Apple's. Good luck!
I'm new to programming, iphone application programming in specific. After reading a bunch about MVC I decided to give it a try in a small application. As to my understanding, MVC works like this:
Model: data, manipulating data, retrieving data.
ViewController: formats data from the model (NSDate to specific style), etc.
View: the actual gui.
If this is indeed a correct formulation of basic MVC theory, my confusion lies in how data is passed between the model, VC, and view. Example: if I make calls to twitter and get the data in the model, how do I (correctly) pass this information to the VC for further work. I know that between the VC and View one mostly uses IBOutlets. The Model is my real problem.
In my last app I made an NSString variable in the app delegate so I could access that data from any class. However, I read that this is not the best approach when the app becomes complex because the delegate is in charge of starting, ending the app, not holding data.
I've read of delegation methods, singleton's, NSNotification (which I've used to call methods in other classes). The problem is that I don't really understand how to use these techniques to pass data from the model to other views.
Please let me know if my question is unclear.
If you think about reusability, the main components that can be reused again are your model objects and view objects. They can be moved to different apps and still used properly. Your view controller is what is really specific to your app and where most of the app logic lies.
So in your example, you could have a twitter object that stores information and tweets from a user perhaps. You would create that class with all its functions separately within its own .h and .m file. Then in your view controller, instantiate the twitter class with data that is retrieved and begin using it from within the view controller.
Your view controller is actually retrieving the data but your model object is the one maintaining the data. In this way, you can pass on the model data with your twitter object to other view controllers.
Control over the application resides in the controller, so it is the object that will retrieve or save persisted data, update views with that data, and handle various events. Consider it the glue between the model and the view!
For example, if you were to click on a button to open a new modal view, you'd handle that event in your view controller. In the method that responds to the clicked button, you will create or access the new view controller and present it using presentModalViewController:animated:. If that new view and controller needs data that your current controller has access to, you could set a property in the new controller to refer to the object.
In my iPhone app, the user will be making multiple NSStrings. Once these are made, I need to pass them to another view completely in the app. How can i do this? All I know at the moment os I can't access objects or variables declared in one view, in another. Thanks.
One way would be to follow the MVC (model view controller) design pattern. Whichever controllers are responsible for your respective views can then store and retrieve the NSStrings from/to a common data model object.
As to how you can make the strings stored in an object visible to the outside, the easiest way is to use Objective-C properties to save you from writing the accessor methods yourself.
I hope this helps with your problem or at least gets you started in the right direction.
Place the strings in a data model object (the M of the MVC pattern), with accessor methods (getter and setters, which can be automagicly created by properties). Then create and place that model object in some central location, a controller common to all views requiring that data, or the appDelegate, a reference for which can be found from any view.
Josh,
I would add to the MVC thing, that still you can do this in several ways.
What I would do for example, is to make your other "View Controller" (MVC), to "observe" when does the user create a new string, and to fetch it accordingly. In that way you would reduce coupling and it will be a cleaner implementation.
Another way would be to create a "delegate" so that the First View controller, "notifies" or calls the delegate method that you created, each time the user creates a new string ( again reducing coupling )
I have following problem:
I have built a tabbar application with 4 tabs. I want to pass a object/variable from the first tab controller to the third one and initialize this controller with the corresponding object.
I've already done some research. The best way, corresponding to a clean model approach, would be to call some initWithObject: method on the called viewcontroller.
How can I achieve this? How can I call the init method of the receivercontroller within the callercontroller? Can you give me some code example?
Edit:
To pass data between several views/classes etc simply create some Kind of data class which holds the data beeing shared between several classes. For more information follow the link:
Singleton
You need a data model object that stores the data for application.
A data model is a customized, standalone object accessible from anywhere in the application. The data model object knows nothing about any views or view controllers. It just stores data and the logical relationships between that data.
When different parts of the app need to write or read data, they write and read to the data model. In your case, view1 would save its data to the data model when it unloads and then view2 would read that data from the data model when it loads (or vice versa.)
In a properly designed app, no two view controllers should have access to the internal data of another controller. (The only reason a view controllers needs to know of the existence of another controller is if it has to trigger the loading of that other controller.)
The quick and dirty way to create a data model is to add attributes to the app delegate and then call the app delegate from the view controllers using:
YourAppDelegateClass *appDelegate = [[UIApplication sharedApplication] delegate];
myLocalProperty = appDelegate.someDataModelProperty;
This will work for small project but as your data grows complex, you should create a dedicated class for your data model.
Edit:
To clarify for your specific case, you would add the call to the data model when the receiver viewController becomes active.
Placing the data in an init method or a viewDidLoad won't work because in a UITabBar the users can switch back and forth without unloading the view or reinitializing the view controller.
The best place to retrieve changing data is in the viewWillAppear controller method. That way the data will be updated every time the user switches to that tab.
You might want to consider NSNotificationCenter (Reference); you register the one viewcontroller with the application notification center, and send a notification when a selection is made. When the notification is received, the other viewcontroller updates itself accordingly.
I don't think this is best practice (also check syntax) however I have got away with:
in the .h
otherclassref *otherclassname
#property (assign) otherclassname otherclassref;
and in the .m
#synthesize otherclassref;
then I just assign the reference from somewhere convenient e.g. the app delegate or wherever you are instantiating your viewcontrollers.
then the view controller can get a reference to the other view controller.
I add #class secondviewcontroller to the .h file for the firstviewcontroller and put put the #imports "secondviewcontroller.h" in the .m file of the first view controller. These are called forward references and prevent compiler errors resulting from having .h files referencing each other.
I am developing an iPhone app for some sweet undergrad research I've been working on. Sadly, my school doesn't offer software engineering / design classes so when it comes to questions of best practices in OO Design, I do a lot of reading.
My Dilemma:
My application loads a view (v1) where, upon user's button click, v1's controller class executes an action method. This action method should fill an array with objects. After that, the user will either execute the action again or click a different tab to load another view. Other views in the application will use the array that v1 populated.
So, where should this shared array be declared? Right now, it's in the AppDelegate class from when I was testing features without a GUI. Should I grab the AppDelegate singleton and add items to it in the v1ViewController? Should it be declared as static?
Thanks for the help!
^Buffalo
EDIT:
Follow-up Question: When interacting with a singleton, which is the better way to talk to it:
[[MyAwesomeSingleton sharedInstance] gimmeSomePizza];
or
MySingleton *s = [MySingleton sharedInstance];
[s gimmeSomePizza];
I guess what I'm wondering is, do you make the sharedInstance method call every time or do you define a pointer to the sharedInstance and then reference the pointer?
Using the app delegate to store data that's shared between views and view controllers is reasonable and appropriate.
In my apps, I view the app delegate as the controller part of MVC, with UIViews and view controllers all being part of the "view". I prefer to use a variant of MVC called Passive View that keeps the model and view parts of my app strictly segregated with only the controller connecting them.
I'm assuming that the array of objects you're storing is your app's model, so storing them on your app delegate makes sense. As Daniel D said, there's no need to make it static.
The app delegate is really the heart of your program. You create and initialize your model and views in your -applicationDidFinishLaunching: method and save your model data and view state in -applicationWillTerminate:. When your view controllers receive events that changes your model, you can call methods on your app delegate to make those changes.
You could store it in an ivar in the app delegate. You don't need to make it static since the app delegate is a singleton anyways (there's never more than 1 instance).
If the app delegate is getting a bit complicated, you can factor out the data storage into a separate model object or perhaps use Core Data.