【安卓筆記】AsyncQueryHandler源碼剖析，asyncqueryhandler

【安卓筆記】AsyncQueryHandler源碼剖析，asyncqueryhandler
AsyncQueryHandler類用於提供非同步資料庫操作（增刪改查）功能。本文視圖從源碼角度分析這個類。
1.AsyncQueryHandler使用方式：

AsyncQueryHandler handler = new AsyncQueryHandler(this.getContentResolver()){@Overrideprotected void onQueryComplete(int token, Object cookie,Cursor cursor){}@Overrideprotected void onUpdateComplete(int token, Object cookie, int result){}@Overrideprotected void onInsertComplete(int token, Object cookie, Uri uri){}@Overrideprotected void onDeleteComplete(int token, Object cookie, int result){}};handler.startQuery(token, cookie, uri, projection, selection, selectionArgs, orderBy);handler.startDelete(token, cookie, uri, selection, selectionArgs);handler.startInsert(token, cookie, uri, initialValues);handler.startUpdate(token, cookie, uri, values, selection, selectionArgs);

2.源碼分析

分析源碼前，從整體上把握這個類：每個AsyncQueryHandler對象都會開啟一個後台線程，線上程中執行與ContentProvider組件的資料互動，進行增刪改查。調用時，可以通過AsyncQueryHandler.startXXX系列方法將請求打包發送到後台線程，當相關處理完成後，會將結果非同步回傳給主線程並調用AsyncQueryHandler.onXXXComplete方法通知調用者。調用者每次請求時，需要傳入一個整型值token作為這次請求的標識，當該請求完成後進行回調時，會將token傳回，協助調用者確定這是哪一次請求。

AsyncQueryHandler是個抽象類別，不能直接被執行個體化，這點需要注意：

public abstract class AsyncQueryHandler extends Handler

其內部封裝了Looper和Handler，這個訊息迴圈工作在子線程。另提供4個常量標識當前操作類型。ContentResolver由構造器注入。

 private static final int EVENT_ARG_QUERY = 1;    private static final int EVENT_ARG_INSERT = 2;    private static final int EVENT_ARG_UPDATE = 3;    private static final int EVENT_ARG_DELETE = 4;    /* package */ final WeakReference<ContentResolver> mResolver;    private static Looper sLooper = null;    private Handler mWorkerThreadHandler;

背景工作執行緒的handler，即mWorkThreadHandler的handleMessage方法負責操作資料庫：

 protected class WorkerHandler extends Handler {        public WorkerHandler(Looper looper) {            super(looper);        }        @Override        public void handleMessage(Message msg) {            final ContentResolver resolver = mResolver.get();            if (resolver == null) return;            WorkerArgs args = (WorkerArgs) msg.obj;            int token = msg.what;            int event = msg.arg1;            switch (event) {                case EVENT_ARG_QUERY:                    Cursor cursor;                    try {                        cursor = resolver.query(args.uri, args.projection,                                args.selection, args.selectionArgs,                                args.orderBy);                        if (cursor != null) {                            cursor.getCount();                        }                    } catch (Exception e) {                        Log.w(TAG, "Exception thrown during handling EVENT_ARG_QUERY", e);                        cursor = null;                    }                    args.result = cursor;                    break;                case EVENT_ARG_INSERT:                    args.result = resolver.insert(args.uri, args.values);                    break;                case EVENT_ARG_UPDATE:                    args.result = resolver.update(args.uri, args.values, args.selection,                            args.selectionArgs);                    break;                case EVENT_ARG_DELETE:                    args.result = resolver.delete(args.uri, args.selection, args.selectionArgs);                    break;            }            //將結果返回給AsyncQueryHandler的handleMessage處理            Message reply = args.handler.obtainMessage(token);            reply.obj = args;            reply.arg1 = msg.arg1;            if (localLOGV) {                Log.d(TAG, "WorkerHandler.handleMsg: msg.arg1=" + msg.arg1                        + ", reply.what=" + reply.what);            }            reply.sendToTarget();        }    }

AsyncQueryHandler在構造的時候，會建立一個HandlerThread，start之，然後擷取其Looper，並利用其Looper構建子線程訊息迴圈，並與mWorkThreadHandler綁定：

  public AsyncQueryHandler(ContentResolver cr) {        super();        mResolver = new WeakReference<ContentResolver>(cr);        synchronized (AsyncQueryHandler.class) {            if (sLooper == null) {                HandlerThread thread = new HandlerThread("AsyncQueryWorker");                thread.start();//啟動子線程,啟動子線程訊息迴圈                sLooper = thread.getLooper();            }        }        mWorkerThreadHandler = createHandler(sLooper);    }    protected Handler createHandler(Looper looper) {        return new WorkerHandler(looper);    }

這個類提供了WorkerArgs封裝資料庫操作的請求參數：

 protected static final class WorkerArgs {        public Uri uri;        public Handler handler;        public String[] projection;        public String selection;        public String[] selectionArgs;        public String orderBy;        public Object result;        public Object cookie;        public ContentValues values;    }

當外界調用startXXX方法時，其實會構造WorkerArgs對象，然後將這個對象以訊息的形式發送給mWorkThreadHandler  ，處理完成後將結果發送給AsyncQueryHandler本身：startXXX:

public void startQuery(int token, Object cookie, Uri uri,            String[] projection, String selection, String[] selectionArgs,            String orderBy) {        // Use the token as what so cancelOperations works properly        Message msg = mWorkerThreadHandler.obtainMessage(token);        msg.arg1 = EVENT_ARG_QUERY;        WorkerArgs args = new WorkerArgs();        args.handler = this;//注意這行制定了Handler為本身，即AsyncQueryHandler        args.uri = uri;        args.projection = projection;        args.selection = selection;        args.selectionArgs = selectionArgs;        args.orderBy = orderBy;        args.cookie = cookie;        msg.obj = args;        mWorkerThreadHandler.sendMessage(msg);//交給子線程處理    }

WorkerHandler:

 Message reply = args.handler.obtainMessage(token);            reply.obj = args;            reply.arg1 = msg.arg1;   ...          reply.sendToTarget();//在startXXX方法中已經制定了target為AsyncQuertHandler本身

最終，操作結果交由AsyncQueryHandler類的handleMessage處理：

  @Override    public void handleMessage(Message msg) {        WorkerArgs args = (WorkerArgs) msg.obj;        if (localLOGV) {            Log.d(TAG, "AsyncQueryHandler.handleMessage: msg.what=" + msg.what                    + ", msg.arg1=" + msg.arg1);        }        int token = msg.what;        int event = msg.arg1;        // pass token back to caller on each callback.        switch (event) {            case EVENT_ARG_QUERY:                onQueryComplete(token, args.cookie, (Cursor) args.result);                break;            case EVENT_ARG_INSERT:                onInsertComplete(token, args.cookie, (Uri) args.result);                break;            case EVENT_ARG_UPDATE:                onUpdateComplete(token, args.cookie, (Integer) args.result);                break;            case EVENT_ARG_DELETE:                onDeleteComplete(token, args.cookie, (Integer) args.result);                break;        }    }

而AsyncQueryHandler類的handleMessage方法回調了onXXXcomplete系列方法。我們重寫onXXXcomplete方法即可拿到資料庫操作結果。

這個設計太贊了！！！！