Android开发之——数据库之Realm

简介

数据库Realm，是用来替代sqlite的一种解决方案，它有一套自己的数据库存储引擎，比sqlite更轻量级，拥有更快的速度，并且具有很多现代数据库的特性，比如支持JSON，流式api，数据变更通知，自动数据同步，简单身份验证，访问控制，事件处理，最重要的是跨平台，目前已有Java、Objective C、Swift、React-Native、Xamarin这五种实现。

环境配置

搜索Realm

打开Github后搜索Realm，有realm-java，并没有找到realm-android，但是我们知道，Android Studio中的代码仓库托管在jcenter中，首先打开https://bintray.com/，在搜索框中输入realm后可以看到realm-android

查看Realm

点击realm-android，可以看到当前realm的最新版本和引用方式

配置Realm

compile 'io.realm:realm-android:0.87.5'，
javaCompileOptions { annotationProcessorOptions { includeCompileClasspath = true } }

初始化Realm

在Application的oncreate()方法中对Realm进行相关配置

Realm中并没有init()方法，我们使用默认配置方式

RealmConfiguration config = new RealmConfiguration.Builder(this).build();
Realm.setDefaultConfiguration(config);

在AndroidManifest.xml配置自定义的Application

<application
    android:name=".MyApplication"
    ...
/>

创建实体

新建一个类继承RealmObject

public class Dog extends RealmObject 
{
    private String name;
    private int age;
    @PrimaryKey
    private String id;
    //get/set方法
}

多对多的关系：

public class Contact extends RealmObject 
{
    public String name;
    public RealmList<Email> emails;
}

public class Email extends RealmObject 
{
    public String address;
    public boolean active;
}

其他相关说明

支持的数据类型

boolean,byte,short,int,long,float,double,String,Date,byte[]
在Realm中byte,short,int,long最终都被映射成long类型

注解说明

@PrimaryKey

• 字段必须是String、integer、byte、short、int、long以及它们的封装类Byte、Short、Integer、Long
• 使用了该注解之后可以使用copyToRealmOrUpdate()方法，通过主键查询它的对象，如果查询到了，则更新它，否则新建一个对象来替代。
• 使用了该注解将默认设置@index注解
• 使用了该注解之后，创建和更新数据库将会慢一点，查询数据会快一点

@Required
数据不能为null

@Ignore
忽略，即该字段不被存储到本地

@Index
为这个字段添加一个搜索引擎，这将使插入数据变慢、数据增大，但是查询会变快。建议在需要优化读取性能的情况下使用。

增

实现方式一：实务操作

类型一：新建一个对象，并进行存储

Realm realm=Realm.getDefaultInstance();

realm.beginTransaction();
User user = realm.createObject(User.class); // Create a new object
user.setName("John");
user.setEmail("john@corporation.com");
realm.commitTransaction();  

类型二：复制一个对象到Realm数据库

Realm realm=Realm.getDefaultInstance();
User user = new User("John");
user.setEmail("john@corporation.com");
//Copy the object to Realm. Any further changes must happen on realmUser
realm.beginTransaction();
realm.copyToRealm(user);
realm.commitTransaction();  

实现方法二：使用事务块

Realm  mRealm=Realm.getDefaultInstance();
final User user = new User("John");
user.setEmail("john@corporation.com");
mRealm.executeTransaction(new Realm.Transaction() 
{
        @Override
        public void execute(Realm realm) 
        {
            realm.copyToRealm(user);
        }
 });

删

Realm  mRealm=Realm.getDefaultInstance();
final RealmResults<Dog> dogs=  mRealm.where(Dog.class).findAll();
    mRealm.executeTransaction(new Realm.Transaction() {
        @Override
        public void execute(Realm realm) 
        {
            Dog dog=dogs.get(5);
            dog.removeFromRealm();
            //删除第一个数据
            dogs.removeFirstFromRealm();
            //删除最后一个数据
            dogs.removeLastFromRealm();
            //删除位置为1的数据
            dogs.removeFromRealm(1);
            //删除所有数据
            dogs.removeAllFromRealm();
        }
    });

同样也可以使用同上的beginTransation和commitTransation方法进行删除

改

Realm  mRealm=Realm.getDefaultInstance();
Dog dog = mRealm.where(Dog.class).equalTo("id", id).findFirst();
mRealm.beginTransaction();
dog.setName(newName);
mRealm.commitTransaction();  

同样也可以用事物快来更新数据

查

查询全部

查询结果为RealmResults，可以使用mRealm.copyFromRealm(dogs)方法将它转为List

public List<Dog> queryAllDog() 
{
    Realm  mRealm=Realm.getDefaultInstance();
    RealmResults<Dog> dogs = mRealm.where(Dog.class).findAll();
    return mRealm.copyFromRealm(dogs);
}

条件查询

public Dog queryDogById(String id) 
{
    Realm  mRealm=Realm.getDefaultInstance();
    Dog dog = mRealm.where(Dog.class).equalTo("id", id).findFirst();
    return dog;
}

常见的条件如下(详细资料请查官方文档)：

• between(),greaterThan(),lessThan(),greaterThanOrEqualTo()&lessThanOrEqualTo()
• equalTo()&noEqualTo()
• contains(),beginsWith()&endsWith()
• isNull()&isNotNull()
• isEmpty()&isNotEmpty()

对查询结果进行排序

public List<Dog> queryAllDog() 
{
    RealmResults<Dog> dogs = mRealm.where(Dog.class).findAll();
     //增序排列
     dogs=dogs.sort("id");
     //降序排列
     dogs=dogs.sort("id", Sort.DESCENDING);
     return mRealm.copyFromRealm(dogs);
 }

其他查询

sum,min,max,average只支持整形数据字段

//查询平均年龄
private void getAverageAge() 
{
     double avgAge=  mRealm.where(Dog.class).findAll().average("age");
}
//查询总年龄
private void getSumAge() 
{
  Number sum=  mRealm.where(Dog.class).findAll().sum("age");
    int sumAge=sum.intValue();
}

//查询最大年龄
private void getMaxId()
{
  Number max=  mRealm.where(Dog.class).findAll().max("age");
    int maxAge=max.intValue();
}

异步操作

大多数情况下，Realm的增删改查插值足够快，可以在UI线程中执行操作。但是如果遇到较复杂的增删改查或增删改查操作的数据较多时，就可以在子线程进行操作。

异步增

private void addCat(final Cat cat) 
{
  RealmAsyncTask  addTask=  mRealm.executeTransactionAsync(new Realm.Transaction() {
        @Override
        public void execute(Realm realm) {
            realm.copyToRealm(cat);
        }
    }, new Realm.Transaction.OnSuccess() {
        @Override
        public void onSuccess() {
            ToastUtil.showShortToast(mContext,"收藏成功");
        }
    }, new Realm.Transaction.OnError() {
        @Override
        public void onError(Throwable error) {
            ToastUtil.showShortToast(mContext,"收藏失败");
        }
    });

}

最后在销毁Activity或Fragment时，要取消掉异步任务

@Override
protected void onDestroy() 
{
    super.onDestroy();
   if (addTask!=null&&!addTask.isCancelled())
    {
        addTask.cancel();
    }
}

异步删

private void deleteCat(final String id, final ImageView imageView)
{
  RealmAsyncTask  deleteTask=   mRealm.executeTransactionAsync(new Realm.Transaction() {
        @Override
        public void execute(Realm realm) {
            Cat cat=realm.where(Cat.class).equalTo("id",id).findFirst();
            cat.deleteFromRealm();

        }
    }, new Realm.Transaction.OnSuccess() {
        @Override
        public void onSuccess() {
            ToastUtil.showShortToast(mContext,"取消收藏成功");
        }
    }, new Realm.Transaction.OnError() {
        @Override
        public void onError(Throwable error) {
            ToastUtil.showShortToast(mContext,"取消收藏失败");
        }
    });

}

最后在销毁Activity或Fragment时，要取消掉异步任务

@Override
protected void onDestroy() 
{
    super.onDestroy();
   if (deleteTask!=null&&!addTask.isCancelled())
    {
        deleteTask.cancel();
    }
}

异步改

RealmAsyncTask  updateTask=   mRealm.executeTransactionAsync(new Realm.Transaction() {
        @Override
        public void execute(Realm realm) {
            Cat cat=realm.where(Cat.class).equalTo("id",mId).findFirst();
            cat.setName(name);
        }
    }, new Realm.Transaction.OnSuccess() {
        @Override
        public void onSuccess() {
            ToastUtil.showShortToast(UpdateCatActivity.this,"更新成功");
         
        }
    }, new Realm.Transaction.OnError() {
        @Override
        public void onError(Throwable error) {
            ToastUtil.showShortToast(UpdateCatActivity.this,"失败成功");
        }
    });

最后在销毁Activity或Fragment时，要取消掉异步任务

@Override
protected void onDestroy() 
{
    super.onDestroy();
   if (updateTask!=null&&!addTask.isCancelled())
    {
        updateTask.cancel();
    }
}

异步查

RealmResults<Cat>   cats=mRealm.where(Cat.class).findAllAsync();
    cats.addChangeListener(new RealmChangeListener<RealmResults<Cat>>() 
{
        @Override
        public void onChange(RealmResults<Cat> element) {
           element= element.sort("id");
            List<Cat> datas=mRealm.copyFromRealm(element);
          
        }
    });

最后在销魂Activity或Fragment时，要取消掉异步任务

@Override
protected void onDestroy() 
{
    super.onDestroy();
    cats.removeChangeListeners();
}

数据迁移(版本升级)

方法一：删除旧版本的数据

RealmConfiguration config = new RealmConfiguration.Builder()
.deleteRealmIfMigrationNeeded()
.build()  

方法二：设置schema版本和migration，对改变的数据进行处理

RealmConfiguration config = new RealmConfiguration.Builder()
.schemaVersion(2) // Must be bumped when the schema changes
.migration(new Migration()) // Migration to run instead of throwing an exception
.build()  

处理版本数据变化Migration

public class Migration implements RealmMigration 
{
    @Override
    public void migrate(final DynamicRealm realm, long oldVersion, long newVersion) {
    // During a migration, a DynamicRealm is exposed. A DynamicRealm is an untyped variant of a normal Realm, but
    // with the same object creation and query capabilities.
    // A DynamicRealm uses Strings instead of Class references because the Classes might not even exist or have been
    // renamed.

    // Access the Realm schema in order to create, modify or delete classes and their fields.
    RealmSchema schema = realm.getSchema();

    /************************************************
        // Version 0
        class Person
            @Required
            String firstName;
            @Required
            String lastName;
            int    age;
        // Version 1
        class Person
            @Required
            String fullName;            // combine firstName and lastName into single field.
            int age;
    ************************************************/
    // Migrate from version 0 to version 1
    if (oldVersion == 0) {
        RealmObjectSchema personSchema = schema.get("Person");

        // Combine 'firstName' and 'lastName' in a new field called 'fullName'
        personSchema
                .addField("fullName", String.class, FieldAttribute.REQUIRED)
                .transform(new RealmObjectSchema.Function() {
                    @Override
                    public void apply(DynamicRealmObject obj) {
                        obj.set("fullName", obj.getString("firstName") + " " + obj.getString("lastName"));
                    }
                })
                .removeField("firstName")
                .removeField("lastName");
        oldVersion++;
    }

    /************************************************
        // Version 2
            class Pet                   // add a new model class
                @Required
                String name;
                @Required
                String type;
            class Person
                @Required
                String fullName;
                int age;
                RealmList<Pet> pets;    // add an array property
     ************************************************/
    // Migrate from version 1 to version 2
    if (oldVersion == 1) {

        // Create a new class
        RealmObjectSchema petSchema = schema.create("Pet")
                .addField("name", String.class, FieldAttribute.REQUIRED)
                .addField("type", String.class, FieldAttribute.REQUIRED);

        // Add a new field to an old class and populate it with initial data
        schema.get("Person")
            .addRealmListField("pets", petSchema)
            .transform(new RealmObjectSchema.Function() {
                @Override
                public void apply(DynamicRealmObject obj) {
                    if (obj.getString("fullName").equals("JP McDonald")) {
                        DynamicRealmObject pet = realm.createObject("Pet");
                        pet.setString("name", "Jimbo");
                        pet.setString("type", "dog");
                        obj.getList("pets").add(pet);
                    }
                }
            });
        oldVersion++;
    }

    /************************************************
        // Version 3
            class Pet
                @Required
                String name;
                int type;               // type becomes int
            class Person
                String fullName;        // fullName is nullable now
                RealmList<Pet> pets;    // age and pets re-ordered (no action needed)
                int age;
     ************************************************/
    // Migrate from version 2 to version 3
    if (oldVersion == 2) {
        RealmObjectSchema personSchema = schema.get("Person");
        personSchema.setNullable("fullName", true); // fullName is nullable now.

        // Change type from String to int
        schema.get("Pet")
            .addField("type_tmp", int.class)
            .transform(new RealmObjectSchema.Function() {
                @Override
                public void apply(DynamicRealmObject obj) {
                    String oldType = obj.getString("type");
                    if (oldType.equals("dog")) {
                        obj.setLong("type_tmp", 1);
                    } else if (oldType.equals("cat")) {
                        obj.setInt("type_tmp", 2);
                    } else if (oldType.equals("hamster")) {
                        obj.setInt("type_tmp", 3);
                    }
                }
            })
            .removeField("type")
            .renameField("type_tmp", "type");
        oldVersion++;
    }
}
}

参考：
Realm for Android详细教程
RealmSample