Android JetPack现在已经在android开发中大行其道，以下是个人使用LiveData与数据库查询的一点总结。
LiveData + Room
官方已经出了rxRoom，DAO中已经可以直接使用RxJava。这里有一个使用LiveData+Room的例子 PDFReader
@Dao
public interface PdfDAO {
@Query("SELECT * FROM Pdf WHERE read_time != 0 ORDER BY read_time DESC")
Flowable<List<Pdf>> getRecentPdf();
@Query("SELECT * FROM Pdf WHERE read_time != 0 ORDER BY read_time DESC")
Flowable<List<Pdf>> getAllPdf();
@Query("SELECT * FROM Pdf WHERE file_path = :path LIMIT 1")
Single<Pdf> getPdf(String path);
@Insert(onConflict = OnConflictStrategy.REPLACE)
Completable insertPdf(Pdf pdf);
@Delete
int deletePdf(Pdf pdf);
}
返回结果可以是：Maybe，Flowable，Single，其中Completeable目前只有androidx支持
其中Flowable会一直监听数据库变化，具体如何实现的以后再分享
于是就可以使用Flowable来实时更新界面：
public class DocumentLiveData extends LiveData<List<Pdf>> {
private boolean mRecentLivaData;
private PdfDAO mPdfDao;
private Disposable mDisposable;
public DocumentLiveData(Context context, boolean recent) {
mRecentLivaData = recent;
mPdfDao = AppDatabase.getInstance(context).getPdfDAO();
}
@Override
protected void onActive() {
super.onActive();
Flowable<List<Pdf>> flowable;
if (mRecentLivaData) {
flowable = mPdfDao.getRecentPdf();
} else {
flowable = mPdfDao.getAllPdf();
}
mDisposable = flowable.subscribeOn(Schedulers.io())
.subscribe(this::postValue,
t -> Log.e("PdfRepository", "Query pdf items failed", t));
}
@Override
protected void onInactive() {
if (mDisposable != null) {
mDisposable.dispose();
mDisposable = null;
}
super.onInactive();
}
}
LiveData + RxJava + ContentProvider
上面的方法适用与本地自建数据库，有时需要监听第三方数据库（多通过ContentProvider提供）
以下是一个例子，查询系统CalendarProvider中有事件的天：
public class EventDayLiveData extends MutableLiveData<List<Integer>> {
private static final String[] PROJECTION = new String[]{
Instances.START_DAY,
Instances.END_DAY
};
private static final int INDEX_START_DAY = 0;
private static final int INDEX_END_DAY = 1;
public static final String SELECTION = Calendars.VISIBLE + "=1";
private ContentResolver mContentResolver;
private Handler mMainHandler;
private ContentObserver mContentObserver;
private PublishProcessor<Object> triggers;
private Disposable mEventBus;
private CompositeDisposable mQueries;
// 事件加载的起始天, julianDay
private int mStartDay;
// 事件加载的结束天, julianDay
private int mEndDay;
private Runnable mTimeChangeUpdater = new Runnable() {
@Override
public void run() {
// reload event day
if (triggers != null) {
triggers.onNext("TimeChange");
}
}
};
public EventDayLiveData(Context content) {
mMainHandler = new Handler(Looper.getMainLooper());
mContentResolver = content.getApplicationContext().getContentResolver();
mQueries = new CompositeDisposable();
triggers = PublishProcessor.create();
mContentObserver = new ContentObserver(mMainHandler) {
@Override
public void onChange(boolean selfChange) {
// reload event day
if (triggers != null) {
triggers.onNext("EventChange");
}
}
};
TimeCalendar time = new TimeCalendar(1970, 0, 1);
mStartDay = time.getJulianDay();
time.set(2037, 11,31);
mEndDay = time.getJulianDay();
}
@Override
protected void onActive() {
Timber.d(" onActive");
// 过滤 200 毫秒内的多次加载请求
mEventBus = triggers.throttleFirst(200, TimeUnit.MILLISECONDS)
.doOnNext(new Consumer<Object>() {
@Override
public void accept(Object o) throws Exception {
Timber.d(" receive action : %s", o.toString());
}
})
.subscribe(new Consumer<Object>() {
@Override
public void accept(Object o) throws Exception {
loadEventDays();
}
});
//Utils.setMidnightUpdater(mMainHandler, mTimeChangeUpdater, TimeZone.getDefault().getID());
mContentResolver.registerContentObserver(CalendarContract.CONTENT_URI, true, mContentObserver);
triggers.onNext("InitValue");
}
@Override
protected void onInactive() {
Timber.d(" onInactive");
mContentResolver.unregisterContentObserver(mContentObserver);
//Utils.resetMidnightUpdater(mMainHandler, mTimeChangeUpdater);
if (mQueries != null) {
mQueries.dispose();
}
if (mEventBus != null) {
mEventBus.dispose();
}
}
private void loadEventDays() {
final Disposable disposable = Observable.create(
new ObservableOnSubscribe<List<Integer>>() {
@Override
public void subscribe(ObservableEmitter<List<Integer>> emitter) {
if (emitter.isDisposed()) {
return;
}
try {
emitter.onNext(queryEventDay());
emitter.onComplete();
} catch (Exception e) {
emitter.onError(e);
}
}
})
.filter(new Predicate<List<Integer>>() {
@Override
public boolean test(List<Integer> list) {
// 判断数据是否变化
return eventDaysChanged(getValue(), list);
}
})
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new Consumer<List<Integer>>() {
@Override
public void accept(List<Integer> list) {
setValue(list);
}
}, new Consumer<Throwable>() {
@Override
public void accept(Throwable throwable) {
Timber.i(throwable,"Query event day failed");
}
});
mQueries.add(disposable);
}
private List<Integer> queryEventDay() {
Uri.Builder builder = EventDays.CONTENT_URI.buildUpon();
ContentUris.appendId(builder, mStartDay);
ContentUris.appendId(builder, mEndDay);
Uri uri = builder.build();
Cursor c = mContentResolver.query(uri, PROJECTION, SELECTION, null, null);
List<Integer> dayList = new ArrayList<>();
if (c != null && c.moveToFirst()) {
while (!c.isAfterLast()) {
int startDay = c.getInt(INDEX_START_DAY);
int endDay = c.getInt(INDEX_END_DAY);
if (endDay != startDay) {
for (int i = startDay; i <= endDay; i++) {
if (!dayList.contains(i)) {
dayList.add(i);
}
}
} else {
if (!dayList.contains(startDay)) {
dayList.add(startDay);
}
}
c.moveToNext();
}
}
if (c != null) {
c.close();
}
return dayList;
}
private boolean eventDaysChanged(List<Integer> oldList, List<Integer> newList) {
if (oldList == null) {
return true;
}
if (oldList.size() != newList.size()) {
return true;
}
return !oldList.containsAll(newList);
}
}
主要思路就是通过注册数据库变化来持续获取事件流丢给PublishProcessor下发到下游订阅者处理
