Important Although Core Data supports SQLite as a store type, the store format—like those of the other native Core Data stores—is private. You cannot create a SQLite database using the native SQLite API and use it directly with Core Data, nor should you manipulate an existing Core Data SQLite store using native SQLite API. If you have an existing SQLite database, you need to import it into a Core Data store.
Given the abstraction that Core Data offers, there is typically no need to use the same store throughout the development process. It is common, for example, to use the XML store early in a project life cycle, because it is fairly human-readable and you can inspect a file to determine whether or not it contains the data you expect. In a deployed application that uses a large data set, you typically use an SQLite store because it offers high performance and does not require that the entire object graph reside in memory. You might use the binary store if you want store writes to be atomic. Limitations of Persistent Store Security Core Data makes no guarantees regarding the security of persistent stores from untrusted sources (as opposed to stores generated internally) and cannot detect whether files have been maliciously modified. The SQLite store offers slightly better security than the XML and binary stores, but it should not be considered inherently secure.
Note also that it is possible for data archived in the metadata to be tampered with independently of the store data. To ensure data security, use a technology such as an encrypted disk image. Fetch Predicates and Sort Descriptors Fetching differs somewhat according to the type of store. In the XML, binary, and in-memory stores, evaluation of the predicate and sort descriptors is performed in Objective-C with access to all Cocoa functionality, including the comparison methods on.
The SQLite store, on the other hand, compiles the predicate and sort descriptors to SQL and evaluates the result in the database itself. This is done primarily for performance, but it means that evaluation happens in a non-Cocoa environment, and so sort descriptors (or predicates) that rely on Cocoa cannot work. The supported sort selectors for SQLite are and,. The latter is Finder-like sorting and what most people should use most of the time. In addition, you cannot sort on transient properties using the SQLite store. There are additional constraints on the predicates you can use with the SQLite store:.
You cannot necessarily translate arbitrary SQL queries into predicates. You can have only one to-many element in a key path in a predicate. For example, no toOne.toMany.toMany or toMany.toOne.toMany type constructions (they evaluate to sets of sets) are allowed. As a consequence, in any predicate sent to the SQL store, there may be only one operator (and one instance of that operator) from ALL, ANY, and IN. CoreData supports a noindex: that can be used to drop indices in queries passed to SQLite.
This is done primarily for performance reasons: SQLite uses a limited number of indices per query, and noindex: allows the user to preferentially specify which indexes should not be used. See documentation regarding function expressions. SQLite-Supported File Systems The SQLite store supports reading data from a file that resides on any type of file system. However, the SQLite store does not generally support writing directly to file systems that do not implement byte-range locking. For DOS file systems and for some NFS file system implementations that do not support byte-range locking correctly, SQLite uses.lock locking, and for SMB file systems it uses flock-style (file level) locking. In summary, byte-range locking file systems have the best concurrent read/write support; these include HFS+, AFP, and NFS.
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File systems with simple file locking are also supported but do not allow for as much concurrent read/write access by multiple processes. Simple file locking systems include SMB and DOS.
The SQLite store does not support writing to WebDAV file systems. SQLite File Size and Record Deletion Simply deleting a record from an SQLite store does not necessarily result in a reduction in the size of the file. If enough items are removed to free up a page in the database file, SQLite’s automatic database vacuuming reduces the size of the file as it rearranges the data to remove that page.
Similarly, the file size is reduced if you remove an item that itself occupies multiple pages (such as a thumbnail image). An SQLite file is organized as a collection of pages. The data within those pages is managed through B-trees, not as simple fixed-length records. This format is more efficient for searching and for overall storage, because it allows SQLite to optimize how it stores both data and indexes in a single file.
This format is also the foundation of SQLite’s data integrity (transaction and journaling) mechanism. However, the cost of this design is that some delete operations may leave holes in the file and impact read and write performance. If you delete some data and add other data, the holes left by the deleted data may be filled by the added data, or the file may be vacuumed to compact its data, whichever SQLite considers most appropriate based on the operations you’re performing.
Configuring Save Behavior for an SQLite Store When Core Data saves an SQLite store, SQLite updates just part of the store file. Loss of that partial update would be catastrophic, so ensure that the file is written correctly before your application continues. Unfortunately, doing partial file updates means that in some situations saving even a small set of changes to an SQLite store can take considerably longer than saving to, say, an XML store. For example, where saving to an XML file might take less than a hundredth of a second, saving to a SQLite store may take almost half a second. This data loss risk is not an issue for XML or binary stores. Because writes to these stores are typically atomic, it is less likely that data loss involves corruption of the file, and the old file is not deleted until the new one has been successfully written.
Important In macOS the fsync command does not guarantee that bytes are written, so SQLite sends a FFULLFSYNC request to the kernel to ensure that the bytes are actually written through to the drive platter. This request causes the kernel to flush all buffers to the drives and causes the drives to flush their track caches. Without this, there is a significantly large window of time within which data will reside in volatile memory. If system failure occurs, you risk data corruption.
Changing a Store’s Type and Location You can migrate a store from one type or location to another (for example, for a Save As operation) using the method. After invocation of this method, the original store is removed from the coordinator; thus the persistent store is no longer a useful reference. The method is illustrated in the following code fragment, which shows how you can migrate a store from one location to another. If the old store type is XML, the example also converts the store to SQLite.
Objective-C. NSPersistentStoreCoordinator. psc = self managedObjectContext persistentStoreCoordinator ;. NSURL. oldURL =;. NSURL. newURL =;.
NSError. error = nil;. NSPersistentStore. xmlStore = psc persistentStoreForURL: oldURL ;. NSPersistentStore.
sqLiteStore = psc migratePersistentStore: xmlStore. toURL: newURL. options: nil.
withType: NSSQLiteStoreType. error: & error.
Introduction to Core Data: Your First Step to Persistent Data. Editor’s note: After we published the, some readers asked about Core Data and how we can use it to save persistent information. This week, we work with, an independent iOS developer, to give you an introduction of Core Data and work with you to build a sample app using Core Data. This tutorial talks about persistence on iPhone (or other iOS devices). What I mean by persistence is to make data that’s in your apps stay around between application launches. Persistence lets users store persistent data and also retrieve it, so that users don’t have to reenter all their data each time they use their applications.
There are multiple ways to store data in iOS devices but most of them aren’t good enough to store a complicated data. They are usually used to save settings or to preload some data such as “Property List” and “Archiving Objects”. So that’s why we’ll go through Core Data to see how you can utilize it to manage data in database. The focus of the tutorial is to provide a practical introduction of Core Data framework. I expect you’ve already gone through our tutorials about. I will not give in-depth explanation about how to create view controller in Storyboard but you can always refer to the to gain better understanding.
Core Data is not a Database When we talk about persistent data, people probably think of database. If you are familiar with Oracle or MySQL, you know that relational database stores data in the form of table, row and column, and it usually facilitates access through what-so-called SQL query. However, don’t mix up Core Data with database. Though database is the default persistent store for Core Data on iPhone, Core Data is not a relational database. It is actually a framework that lets developers store (or retrieve) data in database in an object-oriented way.
With Core Data, you can easily map the objects in your apps to the table records in the database without even knowing any SQL. To illustrate the concept, let’s begin and create your first app using Core Data. This app is called My Store.
It is a very simple app that stores all devices you have by collecting the name, version, company. Set up Xcode Project Options – Remember to select Use Core Data Core Data Stack Before we start working on the project, you first have to understand the Core Data Stack: Managed Object Model – It describes the schema that you use in the app. If you have a database background, think of this as the database schema. However, the schema is represented by a collection of objects (also known as entities).
In Xcode, the Managed Object Model is defined in a file with the extension. You can use the visual editor to define the entities and their attributes, as well as, relationships.
Persistent Store Coordinator – SQLite is the default persistent store in iOS. However, Core Data allows developers to setup multiple stores containing different entities. The Persistent Store Coordinator is the party responsible to manage different persistent object stores and save the objects to the stores. Forget about it you don’t understand what it is. You’ll not interact with Persistent Store Coordinator directly when using Core Data. Managed Object Context – Think of it as a “scratch pad” containing objects that interacts with data in persistent store. Its job is to manage objects created and returned using Core Data.
Among the components in the Core Data Stack, the Managed Object Context is the one you’ll work with for most of the time. In general, whenever you need to fetch and save objects in persistent store, the context is the first component you’ll talk to. The below illustration can probably give you a better idea about the Core Data Stack. Note: While we encourage you to build the user interface, you can also skip the procedures.
The template already comes with the Storyboard and set up all the view controller classes for you. This gives you a good starting point to work on Core Data. If you use the template, you can skip this section and go directly to the “Diving Core Data” section. The next thing we need to do is to create the Storyboard that defines the views of our app. Navigate to File New New File and choose Storyboard in the User Interface template.
Click next and select the iPhone device family, click create. Creating the Table View Controller Drag a View Controller to the Storyboard and add a Navigation Bar to the top of the screen. Next, drag two buttons into the navigation bar. Name one as “Cancel” and the other one as “Save”.
In the content view, add three text fields and name the placeholder attributes as “Name”, “Version” and “Company”. This detail view will be shown when user taps the “+” button in the table view controller.
So finally, press and hold the Control key, click the “+” button and drag towards the detail view controller. Select “Modal” as the Segue action to connect the table view controller and detail view controller. ( IBAction ) save: ( id ) sender; Diving into Core Data With the user interface, it’s time to go into the details of Core Data.
Core Data Tutorial For Ios
Apparently, there are a couple of areas we have to implement:. Save device information in the Detail View Controller. Fetch device information from persistent store (i.e.
SQLite database) and populate the data into Table View Controller We’ll look into the implementation one by one. Saving Device Information First, we need to implement the DeviceDetailViewController to let user add the devices to the database. Open up the DeviceDetailViewController.m file and add the following code after @implementation DeviceDetailViewController. When user taps the “Cancel” button, we expect the app to close the detail view controller. Line 2 of the above code invokes the dismissViewControllerAnimated method to dismiss the current view controller with animation. For the “save” method, we first grab the managed object context. Every object that Core Data stores is inherited from NSManagedObject.
Nspersistentdocument Core Data Tutorial For Mac
So we first create a new instance of NSManagedObject for the “Device” entity that we’ve defined in the object model. NSEntityDescription class provides a method named “insertNewObjectForEntityForName” for developer to create a managed object. Once you created the managed object (i.e. NewDevice), you can set the attributes (name, version, company) using the user input.
Lastly, we call up the “save:” method of the context to save the object into database. You can now hit the Run button to try out your app. Tap the “+” button to bring up the Detail View and save a new device. However, the new device is not yet displayed in the table. Let’s move on to see how you can fetch the device information from database.
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Fetching Device Information Open DeviceViewController.m, add a “devices” property to it so we can save all the devices received. MyStore App using Core Data What’s Coming Next It’s a lengthy tutorial but we try to elaborate the implementation as detail as possible so everyone can follow.
As you can see, with Core Data, you don’t have to care about SQL to save and retrieve data from database. Everything is done behind the scene. This tutorial kicks off the first part of Core Data series. Later we’ll talk more about object relationship. Lastly, let me end the tutorial with an exercise. Try to complete the app by adding the functions that let user update and delete an existing device by selecting a row in the table view.
Hope you enjoy the tutorial and feel to leave us comment. Update: Check out!
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