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集 — Node.js SDK

Atlas Device SDK 已弃用。有关详细信息，请参阅弃用页面。

10.5.0 版本中的新增功能。

Overview

Realm集是一种特殊对象，允许您存储唯一值的集合。 Realm集基于JavaScript 集，但只能包含单一类型的值，并且只能在写事务（write transaction）中修改。集合允许您执行数学运算，例如查找两个集合之间的并集、交集或差值。要学习；了解有关执行这些操作的更多信息，请参阅 MDN 文档中的“实施基本设立操作”。

Realm 对象模型

要将属性类型定义为Realm 集，请在该集中指定所需的数据类型，然后指定 <> 。实例，对于由整数值组成的集合，请指定"int<>" 。

const characterSchema = {
  name: "Character",
  primaryKey: "_id",
  properties: {
    _id: "objectId",
    name: "string",
    levelsCompleted: "int<>",
    inventory: "string<>",
  },
};

使用集合创建对象

要创建具有Realm Set属性的对象，您必须在写事务（write transaction）中创建该对象。定义 Realm 对象时，通过传递空数组或具有初始值的数组来初始化Realm 集。

let playerOne, playerTwo;
realm.write(() => {
  playerOne = realm.create("Character", {
    _id: new BSON.ObjectId(),
    name: "PlayerOne",
    inventory: ["elixir", "compass", "glowing shield"],
    levelsCompleted: [4, 9],
  });
  playerTwo = realm.create("Character", {
    _id: new BSON.ObjectId(),
    name: "PlayerTwo",
    inventory: ["estus flask", "gloves", "rune"],
    levelsCompleted: [1, 2, 5, 24],
  });
});

将项目添加到 Set 中

要将项目添加到集合中，请将新值传递给写事务（write transaction）中的<Realm.Set>.add()方法。

realm.write(() => {
  playerOne.inventory.add("hammer");
  playerOne.levelsCompleted.add(32);
});

检查集是否包含特定项目

要确定集合是否包含特定值，请将该值传递给<Realm.Set>.has()方法。如果集合包含指定的值， set.has()方法将返回 true。

// check if playerTwo has completed level 3 by calling the `has()` method
// on the Realm Set object
const playerTwoHasCompletedLevelThree = playerTwo.levelsCompleted.has(3);
console.log(
  `Is level three completed by playerTwo: ${playerTwoHasCompletedLevelThree}`
);

检查集合的大小

要了解集合中有多少项，可以检查集合的size属性。

// check how many items playerTwo has in his inventory through the `size`
// property of the Realm Set object
const playerTwoInventorySize = playerTwo.inventory.size;
console.log(`playerTwo has ${playerTwoInventorySize} inventory items`);

从集合中删除项目

要从集中删除特定值，请在写事务（write transaction）中将该值传递给<Realm.Set>.delete()方法。

realm.write(() => {
  // remove the compass from playerOne's inventory by calling the
  // `delete()` method of the Realm Set object within a write transaction
  playerOne.inventory.delete("compass");
});

从集中删除所有项目

要清除该集合，请在写事务（write transaction）中运行<Realm.Set>.clear()方法。

realm.write(() => {
  // clear all data from the inventory slot of playerTwo by calling
  // the `clear()` method of the Realm Set object in a write transaction
  playerTwo.inventory.clear();
});

遍历集

要遍历设立，请使用 <Realm.Set>.forEach() 方法或替代迭代方法。

playerOne.inventory.forEach((item) => {
  console.log(item);
});

例子

按顺序遍历集合

Realm 集的顺序可能与添加项目的顺序不同。

您可以通过在添加新值时更新数组来追踪设置顺序。例如：

function updateSetAndOrderedSetArray(set, orderedArray, value) {
  const oldSize = set.size;
  set.add(value);
  if (set.size > oldSize) {
    orderedArray.push(value);
  }
}
let playerOne;
let levelsCompletedInOrder = [];
const realm = await Realm.open({
  path: "realm-files/data-type-realm",
  schema: [characterSchema],
});
realm.write(() => {
  playerOne = realm.create("Character", {
    _id: new BSON.ObjectId(),
    name: "PlayerOne",
    inventory: ["potion", "wand", "spell book"],
    levelsCompleted: [],
  });
});
realm.write(() => {
  updateSetAndOrderedSetArray(
    playerOne.levelsCompleted,
    levelsCompletedInOrder,
    5
  );
});
realm.write(() => {
  updateSetAndOrderedSetArray(
    playerOne.levelsCompleted,
    levelsCompletedInOrder,
    12
  );
});
realm.write(() => {
  updateSetAndOrderedSetArray(
    playerOne.levelsCompleted,
    levelsCompletedInOrder,
    2
  );
});
realm.write(() => {
  updateSetAndOrderedSetArray(
    playerOne.levelsCompleted,
    levelsCompletedInOrder,
    7
  );
});
console.log("set ordered", Array.from(playerOne.levelsCompleted)); // not necessarily [5, 12, 2, 7]
console.log("insert ordered", levelsCompletedInOrder); // [5, 12, 2, 7]
// close the realm
realm.close();

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