react-native之Realm数据库的使用

realm数据库是一款小型数据库系统，可以支持多个平台，如android、ios、javascript等。当然了realm也是可以支持react-native的，官网有具体介绍，官网文档

安装realm

npm install --save realm

然后link

react-native link realm

或者

rnpm link realm

如果link不完全，可以手动检查添加

1.Add the following lines to android/settings.gradle:

include ':realm'
project(':realm').projectDir =new File(rootProject.projectDir,'../node_modules/realm/android')

2.Add the compile line to the dependencies in android/app/build.gradle:

dependencies {
    compile project(':realm')
}

3.Add the import and link the package in MainApplication.java:

import io.realm.react.RealmReactPackage; // add this import

public class MainApplication extends Application implements ReactApplication {
    @Override
    protected List<ReactPackage> getPackages() {
        return Arrays.<ReactPackage>asList(
            new MainReactPackage(),new RealmReactPackage() // add this line
        );
    }
}

React-native中用到的realm是一款对象数据库，因此使用起来相关简单方便；
然后关于其操作：
1. 打开数据库的方式，按照其介绍有两种方式
其一、通过 open方法，eg：

// Define your models and their properties
const CarSchema = {
  name: 'Car',properties: {
    make:  'string',model: 'string',miles: {type: 'int',default: 0},}
};
const PersonSchema = {
  name: 'Person',properties: {
    name:     'string',birthday: 'date',cars:     'Car[]',picture:  'data?' // optional property
  }
};

Realm.open({schema: [CarSchema,PersonSchema]})
  .then(realm => {
    // Create Realm objects and write to local storage
    realm.write(() => { const myCar = realm.create('Car',{ make: 'Honda',model: 'Civic',miles: 1000,}); myCar.miles += 20; // Update a property value }); // Query Realm for all cars with a high mileage const cars = realm.objects('Car').filtered('miles > 1000'); // Will return a Results object with our 1 car cars.length // => 1

    // Add another car
    realm.write(() => { const myCar = realm.create('Car',{ make: 'Ford',model: 'Focus',miles: 2000,}); }); // Query results are updated in realtime cars.length // => 2
  });

其二、通过new产生一个对象

const PersonSchema = {
  name: 'Person',properties: {
    name: 'string',testScores: 'double?[]'
  }
};

let realm = new Realm({schema: [PersonSchema,CarSchema]});

realm.write(() => { let charlie = realm.create('Person',{ name: 'Charlie',testScores: [100.0] }); // Charlie had an excused absense for the second test and was allowed to skip it charlie.testScores.push(null); // And then he didn't do so well on the third test charlie.testScores.push(70.0); });

1. 插入数据

let realm = new Realm({schema: [PersonSchema,CarSchema]});
realm.write(() => { let charlie = realm.create('Person',testScores: [100.0] }); });

1. 查询数据

假设有表 Dog，下面获取Dog表所有数据。

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

有时候我们需要做数据筛选，可以这样写

query(name){
    let dogs = realm.objects('Dog');
    let tanDogs = dogs.filtered('color = "tan" AND name "'+name+'"');
}

具体的筛选条件可以用到下面这些

At the moment only a subset of the NSPredicate syntax is supported in the query language. Basic comparison operators ==,!=,>,>=,<,and <= are supported for numeric properties. ==,BEGINSWITH,ENDSWITH,and CONTAINS are supported for string properties. String comparisons can be made case insensitive by appending [c] to the operator: ==[c],BEGINSWITH[c] etc. Filtering by properties on linked or child objects can by done by specifying a keypath in the query eg car.color == 'blue'

有时候我们还需要排序

let hondas = realm.objects('Car').filtered('make = "Honda"');

// Sort Hondas by mileage
let sortedHondas = hondas.sorted('miles');

// Sort in descending order instead
sortedHondas = hondas.sorted('miles',true);

// Sort by price in descending order and then miles in ascending
sortedHondas = hondas.sorted(['price',true],['miles']);

results也可以通过存储对象的链接对象进行排序，如：

let people = realm.objects('Person');

// Sort people by the milage of their cars
let sortedPeople = people.sorted('car.miles');

未完待续..

（编辑：李大同）

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