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RealmDictionary — Java SDK

Atlas Device SDK 已弃用。有关详细信息，请参阅弃用页面。

版本 10.6.0 中的新增内容。

您可以使用 RealmDictionary数据类型来管理由唯一的 String键值对组成的集合。 RealmDictionary实现了 Java 的Map接口，因此它的工作方式与内置HashMap类相同，不同之处在于托管RealmDictionary实例将其内容持久保存到域中。包含Realm对象的RealmDictionary实例会存储对这些对象的引用。当您从域中删除Realm 对象时， RealmDictionary中对该对象的任何引用都会变为null值。

使用

要创建RealmDictionary类型的字段，请定义RealmDictionary<T>类型的对象属性，其中T定义要存储在RealmDictionary中的值。目前， RealmDictionary实例只能使用String类型的键。

下表显示了可以使用哪些方法与RealmDictionary一起完成常见collection任务：

任务	方法
将对象添加到 `RealmDictionary`	put() （或 Kotlin 中的`[]`操作符）
将多个对象添加到 `RealmDictionary`	putAll()
检查字典是否包含特定键	containsKey()
检查字典是否包含特定值	containsValue()

import io.realm.RealmDictionary;
import io.realm.RealmObject;
public class Frog extends RealmObject {
    String name;
    RealmDictionary<Frog> nicknamesToFriends;
    // realm-required empty constructor
    public Frog() {}
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public RealmDictionary<Frog> getNicknamesToFriends() { return nicknamesToFriends; }
    public void setNicknamesToFriends(RealmDictionary<Frog> nicknamesToFriends) { this.nicknamesToFriends = nicknamesToFriends; }
}

Frog frog = realm.createObject(Frog.class);
frog.setName("George Washington");
// get the RealmDictionary field from the object we just created
RealmDictionary<Frog> dictionary = frog.getNicknamesToFriends();
// add key/value to the dictionary
Frog wirt = realm.createObject(Frog.class);
wirt.setName("Wirt");
dictionary.put("tall frog", wirt);
// add multiple keys/values to the dictionary
Frog greg = realm.createObject(Frog.class);
greg.setName("Greg");
Frog beatrice = realm.createObject(Frog.class);
beatrice.setName("Beatrice");
dictionary.putAll(Map.of("small frog", greg, "feathered frog", beatrice));
// check for the presence of a key
Assert.assertTrue(dictionary.containsKey("small frog"));
// check for the presence of a value
Assert.assertTrue(dictionary.containsValue(greg));
// remove a key
dictionary.remove("feathered frog");
Assert.assertFalse(dictionary.containsKey("feathered frog"));
// deleting a Realm object does NOT remove it from the dictionary
int sizeOfDictionaryBeforeDelete = dictionary.size();
greg.deleteFromRealm();
// deleting greg object did not reduce the size of the dictionary
Assert.assertEquals(sizeOfDictionaryBeforeDelete, dictionary.size());
// but greg object IS now null:
Assert.assertEquals(dictionary.get("small frog"), null);

import io.realm.RealmDictionary
import io.realm.RealmObject
open class Frog
    : RealmObject() {
    var name: String? = null
    var nicknamesToFriends: RealmDictionary<Frog> = RealmDictionary<Frog>()
}

val frog =
    realm.createObject(Frog::class.java)
frog.name = "George Washington"
// get the RealmDictionary field from the object we just created
val dictionary = frog.nicknamesToFriends
// add key/value to the dictionary
val wirt =
    realm.createObject(Frog::class.java)
wirt.name = "Wirt"
dictionary["tall frog"] = wirt
// add multiple keys/values to the dictionary
val greg =
    realm.createObject(Frog::class.java)
greg.name = "Greg"
val beatrice =
    realm.createObject(Frog::class.java)
beatrice.name = "Beatrice"
dictionary.putAll(mapOf<String, Frog>(
    Pair("small frog", greg),
    Pair("feathered frog", beatrice)))
// check for the presence of a key
Assert.assertTrue(dictionary.containsKey("small frog"))
// check for the presence of a value
Assert.assertTrue(dictionary.containsValue(greg))
// remove a key
dictionary.remove("feathered frog")
Assert.assertFalse(dictionary.containsKey("feathered frog"))
// deleting a Realm object does NOT remove it from the dictionary
val sizeOfDictionaryBeforeDelete = dictionary.size
greg.deleteFromRealm()
// deleting greg object did not reduce the size of the dictionary
Assert.assertEquals(
    sizeOfDictionaryBeforeDelete.toLong(),
    dictionary.size.toLong()
)
// but greg object IS now null:
Assert.assertEquals(dictionary["small frog"], null)

通知

要订阅对RealmDictionary的更改，请将 MapChangeListener实施传递给RealmSet.addChangeListener 方法。您的MapChangeListener实施必须定义一个onChange()方法，该方法接受对已更改RealmDictionary的引用和一设立更改作为参数。您可以通过MapChangeSet参数访问权限添加到字典中的键以及从字典中删除的键。

AtomicReference<Frog> frog = new AtomicReference<Frog>();
realm.executeTransaction(r -> {
    frog.set(realm.createObject(Frog.class));
    frog.get().setName("Jonathan Livingston Applesauce");
});
MapChangeListener<String, Frog> mapChangeListener =
    new MapChangeListener<String, Frog>() {
        @Override
        public void onChange(RealmMap<String, Frog> map,
                             MapChangeSet<String> changes) {
            for (String insertion : changes.getInsertions()) {
                Log.v("EXAMPLE",
                        "Inserted key:  " + insertion +
                                ", Inserted value: " + map.get(insertion).getName());
            }
        }
    };
frog.get().getNicknamesToFriends().addChangeListener(mapChangeListener);
realm.executeTransaction(r -> {
    // get the RealmDictionary field from the object we just created
    RealmDictionary<Frog> dictionary = frog.get().getNicknamesToFriends();
    // add key/value to the dictionary
    Frog wirt = realm.createObject(Frog.class);
    wirt.setName("Wirt");
    dictionary.put("tall frog", wirt);
    // add multiple keys/values to the dictionary
    Frog greg = realm.createObject(Frog.class);
    greg.setName("Greg");
    Frog beatrice = realm.createObject(Frog.class);
    beatrice.setName("Beatrice");
    dictionary.putAll(Map.of("small frog", greg, "feathered frog", beatrice));
});

var frog: Frog? = null
realm.executeTransaction { r: Realm? ->
    frog = realm.createObject(Frog::class.java)
    frog?.name = "Jonathan Livingston Applesauce"
}
val mapChangeListener: MapChangeListener<String, Frog>
        = MapChangeListener<String, Frog> { map, changes ->
    for (insertion in changes.insertions) {
        Log.v("EXAMPLE",
                "Inserted key:  $insertion, Inserted value: ${map[insertion]!!.name}")
    }
}
frog?.nicknamesToFriends?.addChangeListener(mapChangeListener)
realm.executeTransaction { r: Realm? ->
    // get the RealmDictionary field from the object we just created
    val dictionary = frog!!.nicknamesToFriends
    // add key/value to the dictionary
    val wirt = realm.createObject(Frog::class.java)
    wirt.name = "Wirt"
    dictionary["tall frog"] = wirt
    // add multiple keys/values to the dictionary
    val greg = realm.createObject(Frog::class.java)
    greg.name = "Greg"
    val beatrice = realm.createObject(Frog::class.java)
    beatrice.name = "Beatrice"
    dictionary.putAll(mapOf<String, Frog>(
            Pair("small frog", greg),
            Pair("feathered frog", beatrice)))
}

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