Avoiding Ten Big Mistakes iOS Developers Make with Core Data | | InformIT
To read or fetch records from a persistent store, we make use of the NSFetchRequest class. . Have you noticed that the items relationship is also a fault. Most developers new to Core Data expect an array or an ordered set. I want to batch unfault by groups of 20, in an ordered relationship. 20 == 0), I use a new NSFetchRequest for the next 20 and unfault them by calling a common. Here are some of the things we have not delved into: • NSFetchRequest is a cons Technique Pros Cons Archiving Allows ordered relationships (arrays, not.
The data is available, but Core Data hasn't fetched it from the persistent store to save memory and improve performance. You don't believe me? Remove the print statement of the if clause and replace it with the following to better understand how faulting works.
- Reading and Updating Managed Objects With Core Data
This means that Core Data needs to fire or resolve the fault. Core Data fetches the missing data from the persistent store to fill in the gaps. Note that we only see a fault if the list record is fetched from the persistent store.
A newly created managed object doesn't have any faults because it hasn't been fetched from a persistent store. Take a closer look at the last print statement.
Have you noticed that the items relationship is also a fault. The values of the name and createdAt attributes are resolved, but the items relationship isn't.
relationshipKeyPathsForPrefetching - NSFetchRequest | Apple Developer Documentation
This is another Core Data optimization. Faulting is something you need to become familiar with if you plan to use Core Data. Most of the time, it's something you don't need to worry about. Updating Records How to Update an Attribute of a Managed Object The above output shows us that the list record we fetched from the persistent store doesn't have a name or creation date.
We can fix that by updating the list record and saving the changes. Run the application to update the list record.
This is what you see in the console if the record doesn't have a name or creation date. It should look something like this: The documentation tells us that the return type of value forKey: To increase your visibility of the number of objects in a context, log the results of [[context registeredObjects] count] to the debug console regularly. If large photos are within managed objects cut from the same entity that drives a table-view, performance will suffer. To get around this issue, attributes that will hold large objects should be split off into a related entity.
This way the large objects can remain in the persistent store and can be represented by a fault instead, until they really are needed. If you need to display photos in a table view, you should use auto-generated thumbnail images instead. Not Preloading Data When you ship a new model version with an updated application, you need to be careful not to accidentally ship a default data store based on an old model.
If you do, the update will probably crash on launch for some if not all users.
This threat can deter developers from shipping a default data store at all! If default data is included with an application, it becomes easier to learn to use.
Loading Core Data objects using NSFetchRequest and NSSortDescriptor
The easier a program is to use, the more likely it is that people will continue to use it. The longer people use an application, the more chance that word about the application will spread, thus increasing sales potential.
To avoid crashing updates while still providing default data, you need a good testing strategy. You should deploy an untouched App Store version of the application to your device, add some data, and then test the upgrade process thoroughly. Not Using Multiple Contexts At a minimum, a Core Data implementation should have one context operating on the main thread. The user interface also operates on the main thread, so any slowdowns to the main thread will impact application responsiveness.
While it is easier to use only one context, performance issues can creep in unless your data set is extremely small.
Avoiding Ten Big Mistakes iOS Developers Make with Core Data
For example, if you wanted to generate photo thumbnails or import some data, the application would appear to lock up for the duration of these processes. Since iOS 5, managing multiple contexts has become a lot easier.
You can now configure a context hierarchy and run some contexts in the foreground and others in the background. By positioning a background context as a parent of a foreground context, you can implement background save. By positioning a background context as a child of a foreground context, you can implement an import context that automatically updates the user interface as objects are imported. A key limitation of iCloud, however, is that its data is constrained to one iCloud account.
Because iCloud accounts are deeply intertwined with many aspects of user devices, it is not practical or recommended to share iCloud accounts. Add this to the ViewController class now: This returns 1 by default, and our new method will also return 1, but this will become useful later on. Add these three methods now: But before we jump into the code, you need to learn about one of the most important classes in Core Data: This is the class that performs a query on your data, and returns a list of objects that match.
We're going to use NSFetchRequest in a really basic form for now, then add more functionality later. In this first version, we're going to ask it to give us an array of all Commit objects that we have created, sorted by date descending so that the newest commits come first.
The way fetch requests work is very simple: