Realm for iOS

Realm 是除了 CoreData 和 Sqlite 之外的第三个选择，一个近几年移动端数据库的全新方案，一直保持着活跃的更新，而且引起了iOS开发圈广泛的关注。它的使用上也比 CoreData 简单方便，更重要的是快，性能更好。

Using the Realm framework

初始化和配置 realm 

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let realm = try! Realm()

// Get our Realm file's parent directory
let folderPath = realm.configuration.fileURL!.deletingLastPathComponent().path

// Disable file protection for this directory
try! FileManager.default.setAttributes([FileAttributeKey(rawValue: NSFileProtectionKey): NSFileProtectionNone],
                                       ofItemAtPath: folderPath)

可以为每个登录的用户设置独立的数据库

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func setDefaultRealmForUser(username: String) {
    var config = Realm.Configuration()

    // Use the default directory, but replace the filename with the username
    config.fileURL = config.fileURL!.deletingLastPathComponent().appendingPathComponent("\(username).realm")

    // Set this as the configuration used for the default Realm
    Realm.Configuration.defaultConfiguration = config
}

realm 对数据库的操作都是在 model 层，并且自动更新。

Models

基于 Realm 的 model 都必须继承自 Object

Realm 支持的属性字段类型包括: Bool, Int, Int8, Int16, Int32, Int64, Double, Float, String, Date, and Data.

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class Person: Object {
    // Optional string property, defaulting to nil
    @objc dynamic var name: String? = nil

    // Optional int property, defaulting to nil
    // RealmOptional properties should always be declared with `let`,
    // as assigning to them directly will not work as desired
    let age = RealmOptional<Int>()
}

let realm = try! Realm()
try! realm.write() {
    var person = realm.create(Person.self, value: ["Jane", 27])
    // Reading from or modifying a `RealmOptional` is done via the `value` property
    person.age.value = 28
}

添加主键，可以指定某个属性字段为主键，但必须唯一。

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class Person: Object {
    @objc dynamic var id = 0
    @objc dynamic var name = ""

    override static func primaryKey() -> String? {
        return "id"
    }
}

添加索引，可以查询速度

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class Book: Object {
    @objc dynamic var price = 0
    @objc dynamic var title = ""

    override static func indexedProperties() -> [String] {
        return ["title"]
    }
}

Relationship

在 realm 中我们可以指定 Realm objects 之间的关系，这样方便关联查询。

Many-to-one

多对一的关系

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class Dog: Object {
    // ... other property declarations
    @objc dynamic var owner: Person? // to-one relationships must be optional
}

let jim = Person()
let rex = Dog()
rex.owner = jim

Many-to-many

多对多的关系, 通过 List 属性指定

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class Person: Object {
    // ... other property declarations
    let dogs = List<Dog>()
}

let someDogs = realm.objects(Dog.self).filter("name contains 'Fido'")
jim.dogs.append(objectsIn: someDogs)
jim.dogs.append(rex)

Writes

对 Reaml objects 的所有写相关的操作都必须在 transaction 事务中进行

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// Get the default Realm
let realm = try! Realm()
// You only need to do this once (per thread)

// Add to the Realm inside a transaction
try! realm.write {
    realm.add(myDog)
}

// Update an object with a transaction
try! realm.write {
    author.name = "Thomas Pynchon"
}

// Create or Insert an record
try! realm.write {
    realm.create(Book.self, value: ["id": 1, "price": 9000.0], update: true)
}

// Delete an object with a transaction
try! realm.write {
    realm.delete(cheeseBook)
}

Queries

通过 Realm, 我们可以很方便的查询数据库

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// retrieves all Dogs from the default Realm
let dogs = realm.objects(Dog.self)

通过指定查询条件查询

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// Query using a predicate string
var tanDogs = realm.objects(Dog.self).filter("color = 'tan' AND name BEGINSWITH 'B'")

// Query using an NSPredicate
let predicate = NSPredicate(format: "color = %@ AND name BEGINSWITH %@", "tan", "B")
tanDogs = realm.objects(Dog.self).filter(predicate)

还可以通过指定 keyPath 对查询结果排序

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lass Person: Object {
    @objc dynamic var name = ""
    @objc dynamic var dog: Dog?
}
class Dog: Object {
    @objc dynamic var name = ""
    @objc dynamic var age = 0
}

let dogOwners = realm.objects(Person.self)
let ownersByDogAge = dogOwners.sorted(byKeyPath: "dog.age")

Migrations

Realm 数据库迁移也比较方便，在 Realm.Configuration 中配置 migrationBlock 就可以手动或自动迁移

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// Inside your application(application:didFinishLaunchingWithOptions:)

let config = Realm.Configuration(
    // Set the new schema version. This must be greater than the previously used
    // version (if you've never set a schema version before, the version is 0).
    schemaVersion: 1,

    // Set the block which will be called automatically when opening a Realm with
    // a schema version lower than the one set above
    migrationBlock: { migration, oldSchemaVersion in
        // We haven’t migrated anything yet, so oldSchemaVersion == 0
        if (oldSchemaVersion < 1) {
            // The enumerateObjects(ofType:_:) method iterates
            // over every Person object stored in the Realm file
            migration.enumerateObjects(ofType: Person.className()) { oldObject, newObject in
                // combine name fields into a single field
                let firstName = oldObject!["firstName"] as! String
                let lastName = oldObject!["lastName"] as! String
                newObject!["fullName"] = "\(firstName) \(lastName)"
            }
        }
    })

// Tell Realm to use this new configuration object for the default Realm
Realm.Configuration.defaultConfiguration = config

// Now that we've told Realm how to handle the schema change, opening the file
// will automatically perform the migration
let realm = try! Realm()

Realm notifications

Realm notifications 是一个消息通知机制，通过添加 realm.observe，当我们对数据库进行更新操作时，就会收到对应的消息。在这里就很做方便刷新 UI 等操作。 

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// Observe Realm Notifications
let token = realm.observe { notification, realm in
    viewController.updateUI()
}

// later
token.invalidate()