Android如何從資料庫中載入海量資料

摘要：   在Android3.0之前，很多應用程式響應效能方面有缺陷，其中比較典型的錯誤行為是在UI執行緒中執行了查詢資料操作，尤其是一次性從database查出大量資料並載入到ListView裡，用這種方式載入資料是最差的選擇，硬體偏弱的手機會假死會兒。 其實體驗最好的還屬手機自帶通訊錄...

  在Android3.0之前，很多應用程式響應效能方面有缺陷，其中比較典型的錯誤行為是在UI執行緒中執行了查詢資料操作，尤其是一次性從database查出大量資料並載入到ListView裡，用這種方式載入資料是最差的選擇，硬體偏弱的手機會假死會兒。 其實體驗最好的還屬手機自帶通訊錄App這類應用，滑動絲般順滑。

  在Android 3.0版本之前一般的做法是用Activity提供的startManagingCursor()和stopManagingCursor(),已經deprecated的API我們就不談了，3.0之後取而代之的是Loader，想必Loader的使用大家都有所知道：

public class CursorLoaderListFragment extends ListFragment {
SimpleCursorAdapter mAdapter;
SearchView mSearchView;
String mCurFilter;


static final String[] CONTACTS_SUMMARY_PROJECTION = new String[] {
ContactsContract.Contacts._ID,
ContactsContract.Contacts.DISPLAY_NAME,
ContactsContract.Contacts.CONTACT_STATUS,
};

private LoaderManager.LoaderCallbacks<Cursor> mLoaderCallback = new LoaderManager.LoaderCallbacks<Cursor>() {

@Override
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
// This is called when a new Loader needs to be created.
String select = "((" + ContactsContract.Contacts.DISPLAY_NAME + " NOTNULL) AND ("
+ ContactsContract.Contacts.HAS_PHONE_NUMBER + "=1) AND ("
+ ContactsContract.Contacts.DISPLAY_NAME + " != '' ))";
return new CursorLoader(getActivity(),
ContactsContract.Contacts.CONTENT_URI,
CONTACTS_SUMMARY_PROJECTION, select, null,
ContactsContract.Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");
}

@Override
public void onLoadFinished(@NonNull Loader<Cursor> loader, Cursor data) {
// Swap the new cursor in.(The framework will take care of closing the
// old cursor once we return.)
mAdapter.swapCursor(data);

// The list should now be shown.
if (isResumed()) {
setListShown(true);
} else {
setListShownNoAnimation(true);
}
}

@Override
public void onLoaderReset(@NonNull Loader<Cursor> loader) {
// This is called when the last Cursor provided to onLoadFinished()
// above is about to be closed.We need to make sure we are no
// longer using it.
mAdapter.swapCursor(null);
}
};

@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
setEmptyText("No phone numbers");

setHasOptionsMenu(true);
mAdapter = new SimpleCursorAdapter(getActivity(),
android.R.layout.simple_list_item_2, null,
new String[] { ContactsContract.Contacts.DISPLAY_NAME, ContactsContract.Contacts.CONTACT_STATUS },
new int[] { android.R.id.text1, android.R.id.text2 }, 0);
setListAdapter(mAdapter);

LoaderManager.getInstance(this).initLoader(0, null, mLoaderCallback);
}

public boolean onQueryTextChange(String newText) {
String newFilter = !TextUtils.isEmpty(newText) ? newText : null;
if (mCurFilter == null && newFilter == null) {
return true;
}
if (mCurFilter != null && mCurFilter.equals(newFilter)) {
return true;
}
mCurFilter = newFilter;
LoaderManager.getInstance(this).restartLoader(0, null, mLoaderCallback);
return true;
}
}

不難看出只要實現三個回撥函式就能創建出一個LoaderCallbacks ，並將此丟給LoaderManager 去initLoader或者restartLoader，initLoader是第一次查詢使用的，restartLoader是二次查詢使用的。簡單是簡單不過有個東西不知有沒有發現：在此demo 中onCreateLoader()方法返回值是CursorLoader 物件，它的建構函式必須是Uri，意味著必須是基於Content Provider 實現的資料庫才可以使用，可是現實專案需要ContentProvider 的不多吧，很多是純sqlite的，為了此場景硬生生將sqlite的實現改成ContentProvider 得不償失。

  當翻看onCreateLoader()方法定義時候，發現返回值不是CursorLoader 而是Loader ，CursorLoader 只是Loader 的一個子類而已，因此轉機來了：定義一個類繼承Loader 並內部用Cursor實現，最終返回自定義類的物件給onCreateLoader()：

public abstract class SQLiteCursorLoader extends AsyncTaskLoader<Cursor> {
private Cursor lastCursor;
private Cursor queryCursor;

public SQLiteCursorLoader(Context context, Cursor cursor) {
super(context);
queryCursor = cursor;
}

/**
* Runs on a worker thread, loading in our data. Delegates the real work to concrete subclass'
* buildCursor() method.
*/
@Override
public Cursor loadInBackground() {
final Cursor cursor = queryCursor;

if (cursor != null) {
// Ensure the cursor window is filled
cursor.getCount();
}

return cursor;
}

/**
* Runs on the UI thread, routing the results from the background thread to whatever is using
* the Cursor (e.g., a CursorAdapter).
*/
@Override
public void deliverResult(final Cursor cursor) {
if (isReset()) {
// An async query came in while the loader is stopped
if (cursor != null) {
cursor.close();
}

return;
}

final Cursor oldCursor = lastCursor;
lastCursor = cursor;

if (isStarted()) {
super.deliverResult(cursor);
}

if (oldCursor != null && oldCursor != cursor && !oldCursor.isClosed()) {
oldCursor.close();
}
}

/**
* Starts an asynchronous load of the list data. When the result is ready the callbacks will be
* called on the UI thread. If a previous load has been completed and is still valid the result
* may be passed to the callbacks immediately. Must be called from the UI thread.
*/
@Override
protected void onStartLoading() {
if (lastCursor != null) {
deliverResult(lastCursor);
}

if (takeContentChanged() || lastCursor == null) {
forceLoad();
}
}

/**
* Must be called from the UI thread, triggered by a call to stopLoading().
*/
@Override
protected void onStopLoading() {
// Attempt to cancel the current load task if possible.
cancelLoad();
}

/**
* Must be called from the UI thread, triggered by a call to cancel(). Here, we make sure our
* Cursor is closed, if it still exists and is not already closed.
*/
@Override
public void onCanceled(final Cursor cursor) {
if (cursor != null && !cursor.isClosed()) {
cursor.close();
}
}

/**
* Must be called from the UI thread, triggered by a call to reset(). Here, we make sure our
* Cursor is closed, if it still exists and is not already closed.
*/
@Override
protected void onReset() {
super.onReset();

// Ensure the loader is stopped
onStopLoading();

if (lastCursor != null && !lastCursor.isClosed()) {
lastCursor.close();
}

lastCursor = null;
}
}

事實上並沒有直接繼承Loader， 而是繼承的AsyncTaskLoader ，從名字看就知道它是類似AsyncTask的原理實現的，SDK的CursorLoader 也是基於AsyncTaskLoader 實現的，當有了SQLiteCursorLoader 我們就可以用它建立LoaderManager.LoaderCallbacks<Cursor> 了：

private LoaderManager.LoaderCallbacks<Cursor> mLoaderCallback = new LoaderManager.LoaderCallbacks<Cursor>() {

@Override
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
// This is called when a new Loader needs to be created.
String sql = "SELECT * FROM TABLE_XX";
return new SQLiteCursorLoader(getActivity(), sql);
}

@Override
public void onLoadFinished(@NonNull Loader<Cursor> loader, Cursor data) {
// ...
}

@Override
public void onLoaderReset(@NonNull Loader<Cursor> loader) {
// ...
}
};

關於Android資料庫方面的開發用純SQL的確有點累，可以考慮ORM的思路，以前我也寫了一個輕量級的，也一直使用中，SQLiteCursorLoader其實我提供了2個構造方法，一個如上傳Cursor，另外一個是傳BuilderSupport , 它有2個實現，分別為ConditionBuilder 和MultiTableConditionBuilder , 通過他們可以以面向物件方式來查詢資料庫，一個是用於單表查詢，一個用於多表查詢，具體可以參考下：Light-DAO