Android Realm資料庫的查詢及自動更新
阿新 • • 發佈：2019-02-14
上一篇文章Android Realm資料庫的使用寫了Relam資料庫的基本配置和基本用法，這次主要看看Realm資料庫的查詢以及自動更新。
1.查詢
查詢分為同步和非同步，同步和非同步的使用請看上一篇文章，這次主要是學習查詢條件的使用：
//查詢年齡在10~50歲的Student,並且結果按降序排列
RealmResults<Student> list = mRealm.where(Student.class)
.between("age", 10, 50)
.findAllSorted("age", Sort.DESCENDING);
//查詢年齡大於等於70或者名字叫"斯巴達1"的所有Student,結果按升序排列 基本語法和GreenDao差不多，用過類似ORM的應該都很容易上手。
2.自動更新
如果Realm的例項存在於帶有Looper，也就是UI執行緒，那麼這個Realm就擁有自動更新的功能。也就是說如果Realm資料庫發生了變化，那麼資料庫就會在下一個事件迴圈中自動更新，這樣也就實現了資料與UI的同步。
如果你在子執行緒中新增資料到Realm資料庫，你可以新增一個onChangeListener來監聽資料的變化，這樣在資料發生變化的時候就會執行相應的操作。
public class MyActivity extends Activity {
private Realm realm;
private RealmChangeListener realmListener;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
realm = Realm.getDefaultInstance();
realmListener = new RealmChangeListener() {
@Override
public void onChange(Realm realm) {
// ... do something with the updates (UI, etc.) ...
}};
realm.addChangeListener(realmListener);
}
@Override
protected void onDestroy() {
super.onDestroy();
// Remove the listener.
realm.removeChangeListener(realmListener);
//or use realm.removeAllChangeListeners() if needed.
// Close the Realm instance.
realm.close();
}
}這些變化的監聽也可以新增在RealmObject或者RealmResults的例項中，你可以通過這樣的方式來監視物件和查詢結果的改變。另外，當監聽回撥函式被呼叫時，相應的資料已經被更新，不需要去做重新整理操作。
public class MyActivity extends Activity {
private Realm realm;
private RealmChangeListener puppiesListener;
private RealmChangeListener dogListener;
private RealmResults<Dog> puppies;
private Dog dog;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
realm = Realm.getDefaultInstance();
puppiesListener = new RealmChangeListener() {
@Override
public void onChange(RealmResults<Dog> puppies) {
// ... do something with the updated puppies instance
}};
// Find all the puppies
puppies = realm.where(Dog.class).lessThanOrEqualTo("age", 2).findAll();
puppies.addChangeListener(puppiesListener);
dogListener = new RealmChangeListener() {
@Override
public void onChange(Dog dog) {
// ... do something with the updated Dog instance
}};
dog = realm.where(Dog.class).equalTo("name", "Fido").findFirst();
dog.addChangeListener(dogListener);
}
@Override
protected void onDestroy() {
super.onDestroy();
// Remove the listeners
puppies.removeChangeListener(puppiesListener);
dog.removeChangeListener(dogListener);
// Close the Realm instance.
realm.close();
}
}如果Realm例項存在與非UI執行緒，那麼資料不會發生自動重新整理，知道手動呼叫waitForChange()方法。如果您想確定當前 Realm 例項是否有自動更新功能，可以通過呼叫 isAutoRefresh() 方法查詢。官方是這樣說的：
Change listeners only work on Looper threads. For non-looper threads, you manually have to use Realm.waitForChange() instead.
也就是說對Realm資料變化的監聽只會在主執行緒執行，要想在非UI執行緒使用，需要使用waitForChange()代替。另外，官方還重寫了ListView和RecyclerView的Adapter,可以實現資料的實時重新整理。用法大同小異，需要在module一級的build.gradle中新增依賴：
compile 'io.realm:android-adapters:1.4.0'具體用法以RecyclerView為例：
public class DetailActivity extends AppCompatActivity {
public static final String STUDENT = "STUDENT";
private Realm mRealm;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_detail);
getSupportActionBar().setTitle("RealmAdapter");
mRealm = Realm.getDefaultInstance();
RecyclerView mRecyclerView = (RecyclerView) findViewById(R.id.mRecyclerView);
mRecyclerView.setHasFixedSize(true);
mRecyclerView.setLayoutManager(new LinearLayoutManager(this));
//Realm的查詢方法
//查詢年齡在10~50歲的Student,並且結果按降序排列
RealmResults<Student> list = mRealm.where(Student.class)
.between("age", 10, 50)
.findAllSorted("age", Sort.DESCENDING);
mRecyclerView.setAdapter(new MyAdapter(this, list, true));//這裡就設定了自動更新
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
if (item.getItemId() == R.id.insert) {
mRealm.beginTransaction();
List<Student> list = new ArrayList<>();
for (int i = 0; i < 100; i++) {
Student s = new Student(UUID.randomUUID().toString(), "斯巴達" + i, i, (i % 2) + 1);
list.add(s);
}
mRealm.copyToRealm(list);
mRealm.commitTransaction();
}
return super.onOptionsItemSelected(item);
}
class MyAdapter extends RealmRecyclerViewAdapter<Student, MyAdapter.ViewHolder> {
public MyAdapter(@NonNull Context context, @Nullable OrderedRealmCollection<Student> data, boolean autoUpdate) {
super(context, data, autoUpdate);
}
class ViewHolder extends RecyclerView.ViewHolder {
private TextView name;
private TextView clazz;
public ViewHolder(View itemView) {
super(itemView);
name = (TextView) itemView.findViewById(R.id.name);
clazz = (TextView) itemView.findViewById(R.id.clazz);
}
}
@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View itemView = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_layout, parent, false);
return new ViewHolder(itemView);
}
@Override
public void onBindViewHolder(ViewHolder holder, int position) {
Student student = getData().get(position);
holder.name.setText(student.getName());
holder.clazz.setText(String.format(Locale.CHINA, "%d班", student.getClazz()));
}
}
@Override
protected void onDestroy() {
super.onDestroy();
if (mRealm != null && !mRealm.isClosed()) {
mRealm.close();
mRealm = null;
}
}
}想了解更詳細的內容可以看官方介紹。