Coredata data persistence

CoreData Introduce

CoreData Is a powerful data persistence technology , be located SQLite Above the database , It avoids the SQL Complexity , It allows us to interact with the database in a more natural way .CoreData Provide data –OC Object mapping relationship to achieve data and object management , This does not require any SQL Statements can manipulate them .
CoreData The data persistence framework is Cocoa API Part of ,⾸ Next in iOS5 Version of the system , It allows for Entity - attribute - Value model Organization data , And XML、⼆ Base file perhaps SQLite data ⽂ Pieces of The format of persistent data

CoreData And SQLite Contrast

SQLite

1、 be based on C Interface , Need to use SQL sentence , Code cumbersome 
2、 When dealing with large amounts of data , Table relationships are more intuitive 
3、 stay OC Is not visualization , Not easy to understand 

CoreData

1、 visualization , And undo/redo Ability 
2、 Can achieve a variety of file formats ：
    * NSSQLiteStoreType
    * NSBinaryStoreType
    * NSInMemoryStoreType
    * NSXMLStoreTyp
3、 Apple official API Support , And iOS It's closer together 

CoreData Core classes and structures

NSManagedObjectContext( Data context )

• Object management context , Responsible for the actual operation of data ( important )
• effect ： insert data , Query data , Delete data , Update data

NSPersistentStoreCoordinator( Persistent storage assistant )

• Equivalent to a database connector
• effect ： Set the name of the data store , Location , storage , And storage time

NSManagedObjectModel( Data model )

• All tables or data structures in the database , Contains the definition information of each entity
• effect ： Add the properties of the entity , Building relationships between attributes
• Operation method ： View editor , Or code

NSManagedObject( Managed data records )

• Table records in the database

NSEntityDescription( Physical structure )

• Equivalent to table structure

NSFetchRequest( Data request )

• Equivalent to a query statement

The suffix is .xcdatamodeld My bag

• Inside is .xcdatamodel file , Edit with the data model editor
• Compiled as .momd or .mom file

Diagram of the relationship between various types

CoreData database Manually create

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The steps are as follows

1. Create model file  ［ It's like a database ］
2. Add entities  ［ A watch ］
3. Create entity class  [ Equivalent model -- Table structure ]
4. Generate context   Generate database by associating model files 

0、 Create a project

1、 Manually create CoreData Data time , We create a project as usual , There is no need to check Use Core Data:

1、 Create model file

1、 Go to create new file ,command+N Or as shown in the figure below

2、 Select the model file type , Here's the picture ：

3、 Set file name , Here's the picture ：

4、 Model file created successfully , Will appear later

2、 Create entities

1、 Create entities in a visual way , The functions of entities are similar to our Model class , The specific operations are as follows: ：

3、 Create entity class

Create entities with visualization , But we want to get the corresponding data and name , You must associate classes , So create an entity class , The steps are as follows ：

1、 Go to create new file

2、 Select file type , Entity class file type selection :NSManagedObject subclass

3、 Select the model file

4、 Select entities

5、 Create success

After the entity file is created successfully, the system automatically generates the corresponding Classes and properties , Class name Corresponding The entity name , attribute Corresponds to The attribute name ; If our attribute is a basic data type , Then the default will help us convert to NSNumber Properties of type .

*  The old version ： Only one pair of files are generated , That is, classes and attributes are all together 
*  The new version ： Generate a pair of class files , Regenerate into a pair of categories , Generate attributes in the class 

4、 Generate context Associated database

Before creating the context and the associated database, let's take a look at the relational dependency graph ：

From the picture above we can see , To generate context , Need to have Database assistant and Model Support for ; It's like SQLite In the same , Want to operate the database , You have to have database , And create surface ; Let's look at the implementation code ：

<code class="hljs objectivec has-numbering">- (<span class="hljs-keyword">void</span>)viewDidLoad {
    [<span class="hljs-keyword">super</span> viewDidLoad];


<span class="hljs-comment">//1、 Create model objects </span>
    <span class="hljs-comment">// Get model path </span>
    <span class="hljs-built_in">NSURL</span> *modelURL = [[<span class="hljs-built_in">NSBundle</span> mainBundle] URLForResource:@<span class="hljs-string">"School"</span> withExtension:@<span class="hljs-string">"momd"</span>];
    <span class="hljs-comment">// Create model objects from model files </span>
    NSManagedObjectModel *model = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];


<span class="hljs-comment">//2、 Create a persistence assistant </span>
    <span class="hljs-comment">// Using model objects to create helper objects </span>
    NSPersistentStoreCoordinator *store = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:model];

    <span class="hljs-comment">// The name and path of the database </span>
    <span class="hljs-built_in">NSString</span> *docStr = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, <span class="hljs-literal">YES</span>) lastObject];
    <span class="hljs-built_in">NSString</span> *sqlPath = [docStr stringByAppendingPathComponent:@<span class="hljs-string">"mySqlite.sqlite"</span>];
    <span class="hljs-built_in">NSLog</span>(@<span class="hljs-string">"path = %@"</span>, sqlPath);
    <span class="hljs-built_in">NSURL</span> *sqlUrl = [<span class="hljs-built_in">NSURL</span> fileURLWithPath:sqlPath];

    <span class="hljs-comment">// Set up database related information </span>
    [store addPersistentStoreWithType:NSSQLiteStoreType configuration:<span class="hljs-literal">nil</span> URL:sqlUrl options:<span class="hljs-literal">nil</span> error:<span class="hljs-literal">nil</span>];


<span class="hljs-comment">//3、 Create context </span>
    NSManagedObjectContext *context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];

    <span class="hljs-comment">// Association persistence assistant </span>
    [context setPersistentStoreCoordinator:store];

    _context = context;

}
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• After this , Our whole CoreData Even if the database is created , The whole process is manual , So for principle Can better understand ;
• After running, the database has been created , Enter the corresponding path , We can see the created database file ;
• Use the tool to open , It is found that the corresponding table has also been created for us ;
• After the above steps are completed , In the follow-up work, we do not need to do any work related to the database , All the work dealing with the database is left to CoreData To achieve .

CoreData database System creation

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Using the built-in method to create a database is the same as the manual method , Only the system will create the model file 、 Generate context 、 The work of associated database helps us do , We don't have to do any more of this ;
Saying so much , Let's take a look at how to use the system self creation CoreData database

For those created by the system itself , There are only two steps ：

• Create a project
• The created entity is already associated with the entity class

Create a project

1、 Manually create CoreData Database time , We create a project as usual , Particular attention ： Be sure to check Use Core Data:

2、 After the project is created , The system will automatically help us create a project with the same name Model file ; And help us write Generate context and Associated database Code for ：

After the above is done , We just need to create entities , And associated entity classes

Create entities Associated entity class

Create entities and associated entity classes and Create the database manually The way is the same , Refer to manual creation 2、3 that will do

Jane books links ：http://www.jianshu.com/p/880dd63c5f5e