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从 Java SDK 迁移到 Kotlin SDK

注意

什么是 Kotlin SDK?

Kotlin SDK 是完全使用 Kotlin 编程语言构建的全新 Realm 客户端 SDK。Kotlin SDK 使用与 Java SDK 完全不同的代码库,专为利用 Kotlin 语言功能(如协程和挂起函数)而设计。Java SDK 还支持其中一些功能以及用 Kotlin 编写的 Android 应用程序。但 Kotlin SDK 比 Java SDK 更符合 Kotlin 习惯。

Java SDK 和 Kotlin SDK 在很多方面有所不同。在本页面中,您将发现大多数 SDK 不同之处的高级比较。

Java SDK 提供了活动对象、查询和域,这些对象、查询和领域会在底层数据发生变化时自动更新。Kotlin SDK 仍然在写事务中提供此活动接口,但在其他事务中会依赖新的冻结架构,该架构降低了 Realm 对象的处理难度。以下是 Java SDK 架构与 Kotlin SDK 架构的一些主要区别:

  • 默认已冻结:现在所有对象都已冻结。与活动对象不同,冻结对象在数据库写入后不会自动更新。您仍然可以在写事务中访问活动对象,但将活动对象传递出写入事务会冻结该对象。

  • 线程安全性:所有 Realm 实例、对象、查询结果和集合现在都可以跨线程传输。

  • Singleton :现在每个域只需要一个实例。 无需在单个线程上打开和关闭 Realm。

提示

Java SDK 会自动检测应用程序中定义的 Realm 对象模型,并在打开的 Realm 模式中使用所有这些对象模型,除非您另外指定。Kotlin SDK 要求您手动指定要在 Realm 模式中使用的 Realm 对象模型。 此外:

  • Kotlin SDK 不提供在应用程序中设置和访问默认 Realm 的功能。 由于您现在可以跨线程共享 Realm、对象和结果,因此您可以改为依赖全局单例。

  • Java SDK 使用RealmConfiguration.Builder().build()生成RealmConfiguration的实例。 对于 Kotlin SDK,请使用RealmConfiguration.create() 伴随方法RealmConfiguration

  • Java SDK 使用静态Realm.getInstance()方法打开具有给定配置的 Realm。 使用 Kotlin SDK,请改用静态 Realm.open() 方法。

val config = RealmConfiguration.Builder()
.build()
var realm: Realm
realm = Realm.getInstance(config)
Log.v(
"EXAMPLE",
"Successfully opened a realm: "
+ realm.path
)
RealmConfiguration config =
new RealmConfiguration.Builder()
.build();
Realm realm;
realm = Realm.getInstance(config);
Log.v("EXAMPLE",
"Successfully opened a realm: "
+ realm.getPath());
val config = RealmConfiguration.create(
setOf(Frog::class, Sample::class))
val realm = Realm.open(config)
Log.v("Successfully opened realm:" +
"${realm.configuration.name}")

或者,使用 RealmConfiguration.Builder 进一步自定义配置:

Kotlin SDK
val config = RealmConfiguration.Builder(
setOf(Frog::class, Sample::class))
.name(REALM_NAME)
.deleteRealmIfMigrationNeeded()
.directory(PATH)
.encryptionKey(KEY)
.build()
val realm = Realm.open(config)
Log.v("Successfully opened realm:" +
realm.configuration.name
)

在 Java SDK 中,您可以通过以下两种方式之一来声明 Realm 对象模型:

  • 扩展 RealmObject

  • 实施 RealmModel

Kotlin SDK 改用 RealmObject 接口中的默认方法。对于 Kotlin SDK,从 RealmObject 继承以声明 Realm 对象模型。对于具有特殊属性的字段,如忽略的字段、主键和索引,注释的工作方式与 Java 中的相同。

open class Sample : RealmObject() {
@PrimaryKey
var stringField = "Realm"
var byteField: Byte = 0xA
// no support for chars: no charField
var shortField: Short = 17
var intField = 42
@Index
var longField = 256L
var booleanField = true
var floatField = 3.14f
var doubleField = 1.19840122
var timestampField = Date()
}
public class Sample extends RealmObject {
@PrimaryKey
public String stringField = "Realm";
public Byte byteField = 0xA;
// no support for chars: no charField
public Short shortField = 17;
public Integer intField = 42;
@Index
public Long longField = 256L;
public Boolean booleanField = true;
public Float floatField = 3.14f;
public Double doubleField =
1.19840122;
public Date timestampField =
new Date();
}
class Sample : RealmObject {
@PrimaryKey
var stringField: String = "Realm"
var byteField: Byte = 0xA
var charField: Char = 'a'
var shortField: Short = 17
var intField: Int = 42
@Index
var longField: Long = 256L
var booleanField: Boolean = true
var floatField: Float = 3.14f
var doubleField: Double = 1.19840122
var timestampField: RealmInstant =
RealmInstant.from(
100,
1000)
var objectIdField: ObjectId = ObjectId()
}

Java 和 Kotlin SDK 都通过 Realm 对象字段声明关系:

open class Child : RealmObject() {
var frog: Frog? = null
}
public class Child
extends RealmObject {
public Frog frog = null;
}
class Child : RealmObject {
var frog: Frog? = null
}

使用 Java SDK,您可以定义与RealmList类型字段的一对多关系。 Kotlin SDK 仍然使用RealmList类型的字段,但您应使用realmListOf()伴随方法实例化RealmList实例。

open class Kid : RealmObject() {
var frogs = RealmList<Frog>()
}
public class Kid
extends RealmObject {
public RealmList<Frog> frogs =
new RealmList<Frog>();
}
class Kid : RealmObject {
var frogs: RealmList<Frog> =
realmListOf()
}

对于 Java SDK,您需要使用 @Required 注释使基元列表在 Realm 对象模型中不可为空。Kotlin SDK 默认使基元列表不可为空。使用 ? 操作符,使基元列表可为空。

open class CollegeStudent : RealmObject() {
@Required
var notes = RealmList<String>()
var nullableNotes = RealmList<String>()
}
public class CollegeStudent
extends RealmObject {
@Required
public RealmList<String> notes =
new RealmList<String>();
public RealmList<String> nullableNotes =
new RealmList<String>();
}
class Student : RealmObject {
var notes: RealmList<String> =
realmListOf()
var nullableNotes: RealmList<String?> =
realmListOf()
}

Kotlin SDK 为写入 Realm 的方法引入了新名称。

使用 Java SDK,您可以异步写入具有 realm.executeTransactionAsync()的域。 Kotlin SDK 改用挂起函数 realm.write()

realm.executeTransactionAsync {
transactionRealm: Realm ->
val sample: Sample =
Sample()
sample.stringField = "Sven"
transactionRealm.copyToRealm(
sample
)
}
realm.executeTransactionAsync(
transactionRealm -> {
Sample sample = new Sample();
sample.stringField = "Sven";
transactionRealm.copyToRealm(sample);
});
realm.write {
// this: MutableRealm
val sample = Sample()
sample.stringField = "Sven"
this.copyToRealm(sample)
}

使用 Java SDK,您可以同步写入具有realm.executeTransaction()的 Realm。 Kotlin SDK 使用realm.writeBlocking():

realm.executeTransaction {
transactionRealm: Realm ->
val sample: Sample =
Sample()
sample.stringField = "Sven"
transactionRealm.copyToRealm(
sample
)
}
realm.executeTransaction(
transactionRealm -> {
Sample sample = new Sample();
sample.stringField = "Sven";
transactionRealm.copyToRealm(sample);
});
realm.writeBlocking {
// this: MutableRealm
val sample = Sample()
sample.stringField = "Sven"
this.copyToRealm(sample)
}

Java SDK 中的查询与 Kotlin SDK 中的查询有几处不同:

val samples =
realm.where(
Sample::class.java
).findAll()
val samplesThatBeginWithN =
realm.where(
Sample::class.java
)
.beginsWith(
"stringField",
"N"
).findAll()
RealmResults<Sample> samples =
realm
.where(Sample.class)
.findAll();
RealmResults<Sample> samplesThatBeginWithN =
realm
.where(Sample.class)
.beginsWith("stringField",
"N")
.findAll();
val samples: RealmResults<Sample> =
realm.query<Sample>().find()
val samplesThatBeginWithN:
RealmResults<Sample> =
realm.query<Sample>(
"stringField BEGINSWITH 'N'"
).find()
val aggregates =
realm.where(
Sample::class.java
)
.distinct("stringField")
.sort(
"stringField",
Sort.ASCENDING
)
.limit(2)
.findAll()
RealmResults<Sample> aggregates =
realm.where(Sample.class)
.distinct("stringField")
.sort("stringField",
Sort.ASCENDING)
.limit(2)
.findAll();
val aggregates: RealmResults<Sample> =
realm.query<Sample>()
.distinct(Sample::stringField.name)
.sort(Sample::stringField.name,
Sort.ASCENDING)
.limit(2)
.find()

在这两个 SDK 中,您只能删除活动对象。 Kotlin SDK 提供了mutableRealm.findLatest() 访问任何冻结对象的活动版本。 在写事务中,您可以直接查询和删除活动对象,而无需使用findLatest()

val sample =
realm.where(
Sample::class.java
).findFirst()
// delete one object synchronously
realm.executeTransaction {
transactionRealm: Realm? ->
sample!!.deleteFromRealm()
}
// delete a query result asynchronously
realm.executeTransactionAsync {
backgroundRealm: Realm ->
backgroundRealm.where(
Sample::class.java
).findFirst()!!.deleteFromRealm()
}
Sample sample =
realm.where(Sample.class)
.findFirst();
// delete one object synchronously
realm.executeTransaction(
transactionRealm ->
sample.deleteFromRealm());
// delete a query result asynchronously
realm.executeTransactionAsync(
backgroundRealm ->
backgroundRealm
.where(Sample.class)
.findFirst()
.deleteFromRealm());
val sample: Sample? =
realm.query<Sample>()
.first().find()
// delete one object synchronously
realm.writeBlocking {
if (sample != null) {
findLatest(sample)
?.also { delete(it) }
}
}
// delete a query result asynchronously
GlobalScope.launch {
realm.write {
query<Sample>()
.first()
.find()
?.also { delete(it) }
}
}

在这两个 SDK 中,您都可以订阅结果集合的变化。借助 Java SDK,只要域结果发生变化,您就可以通过以下接口收到通知:

  • realmResults.addChangeListener()

  • RxJava,通过 asFlowable()

  • Kotlin 扩展,带有 toFlow()

Kotlin SDK会用 realmQuery.asFlow() 替换所有这些选项。获得结果流后,您可以调用collection来订阅更改。该流程发出的任何类型为 UpdatedResults 的对象都表示对结果设立的更改。

realm.where(Sample::class.java)
.findAllAsync()
.addChangeListener {
samples: RealmResults<Sample>?,
changeSet: OrderedCollectionChangeSet ->
// log change description
Log.v(
"EXAMPLE",
("Results changed. " +
"change ranges: " +
Arrays.toString(
changeSet
.changeRanges
) +
", insertion ranges: " +
Arrays.toString(
changeSet
.insertionRanges
) +
", deletion ranges: " +
Arrays.toString(
changeSet
.deletionRanges
))
)
}
realm.where(Sample.class).findAllAsync()
.addChangeListener(
(samples, changeSet) -> {
// log change description
Log.v("EXAMPLE",
"Results changed. " +
"change ranges: " +
Arrays.toString(
changeSet
.getChangeRanges()) +
", insertion ranges: " +
Arrays.toString(
changeSet
.getInsertionRanges()) +
", deletion ranges: " +
Arrays.toString(
changeSet
.getDeletionRanges()));
});
// in a coroutine or a suspend function
realm.query<Sample>().asFlow().collect {
results: ResultsChange<Sample> ->
when (results) {
is InitialResults<Sample> -> {
// do nothing with the
// initial set of results
}
is UpdatedResults<Sample> -> {
// log change description
Log.v("Results changed. " +
"change ranges: " +
results.changeRanges +
", insertion ranges: " +
results.insertionRanges +
", deletion ranges: " +
results.deletionRanges
)
}
}
}

对于 Java SDK,Realm、Realm 对象和结果不能在线程之间传递。Kotlin SDK 默认会冻结这些对象,使其具有线程安全性。与 Java SDK 使用的活动对象不同,Kotlin SDK 中的冻结对象不会在底层数据变更时自动更新。对于 Kotlin SDK,您必须使用通知来订阅更新。

realm = Realm.getInstance(config)
val sample =
realm.where(
Sample::class.java
).findFirst()
// save sample field in a
// separate variable
// for access on another thread
val sampleStringField =
sample!!.stringField
val executorService =
Executors.newFixedThreadPool(4)
executorService.execute {
// cannot pass a realm
// into another thread,
// so get a new instance
// for separate thread
val threadRealm =
Realm.getInstance(config)
// cannot access original
// sample on another
// thread, use
// sampleStringField instead
val threadSample =
threadRealm.where(
Sample::class.java
)
.equalTo(
"stringField",
sampleStringField
).findFirst()
Log.v(
"EXAMPLE",
"Separate thread sample: " +
threadSample
)
}
realm = Realm.getInstance(config);
Sample sample = realm
.where(Sample.class).findFirst();
// save sample field in a variable
// for access on another thread
String sampleStringField =
sample.stringField;
ExecutorService executorService =
Executors.newFixedThreadPool(4);
executorService.execute(() -> {
// cannot pass a realm
// into another thread,
// so get a new instance
// for separate thread
Realm threadRealm =
Realm.getInstance(config);
// cannot access original
// sample on another
// thread, use
// sampleStringField instead
Sample threadSample =
threadRealm
.where(Sample.class)
.equalTo("stringField",
sampleStringField)
.findFirst();
Log.v("EXAMPLE",
"Separate thread sample: "
+ threadSample);
});
val realm = Realm.open(config)
val sample: Sample? =
realm.query<Sample>()
.first()
.find()
launch(Dispatchers.Unconfined) {
// can access the realm opened on
// a different thread
realm.query<Sample>().find()
// can access realm object queried
// on a different thread
Log.v(sample!!.stringField)
}.join()

对于 Java SDK,迁移是手动进程。Kotlin SDK 可自动执行迁移,还允许您访问类似的动态 Realm 接口,对迁移逻辑进行自定义调整。

val config =
RealmConfiguration.Builder()
.migration { realm: DynamicRealm,
oldVersion: Long,
newVersion: Long ->
val schema: RealmSchema =
realm.schema
if (oldVersion == 0L) {
// perform schema migration
schema.get("Sample")
?.addField(
"new_field",
String::class.java
)
}
// migrate data
schema.get("Sample")
?.transform {
obj: DynamicRealmObject ->
obj.set(
"longField",
42L
)
}
}.build()
val realm: Realm =
Realm.getInstance(config)
Log.v(
"EXAMPLE",
"Successfully opened a realm: "
+ realm.path
)
RealmConfiguration config =
new RealmConfiguration.Builder()
.migration((realm,
oldVersion,
newVersion) -> {
RealmSchema schema =
realm.getSchema();
if (oldVersion == 0L) {
// perform schema migration
schema.get("Sample")
.addField("new_field",
String.class);
}
// migrate data
schema.get("Sample")
.transform(obj ->
obj.set("longField",
42L));
}).build();
Realm realm;
realm = Realm.getInstance(config);
Log.v("EXAMPLE",
"Successfully opened a realm: "
+ realm.getPath());
// A Realm migration that performs
// automatic schema migration
// and allows additional custom
// migration of data.
RealmConfiguration.Builder(
schema = setOf(Sample::class))
.migration(AutomaticSchemaMigration {
context:
AutomaticSchemaMigration.MigrationContext ->
val oldRealm:
DynamicRealm =
context.oldRealm
val newRealm:
DynamicMutableRealm =
context.newRealm
// dynamic realm gives access
// to realm data
// through a generic string
// based API
context.enumerate("Sample") {
oldObject:
DynamicRealmObject,
newObject:
DynamicMutableRealmObject? ->
newObject?.set("longField",
42L)
}
})
.build()
val realm = Realm.open(config)

现在您已经了解了 Java SDK 和 Kotlin SDK 的区别,请查看 Kotlin SDK 文档 的其他部分。

后退

SDK 遥测

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