MMS源碼中異步處理簡析

1，信息數據的查詢，刪除使用AsycnQueryHandler處理

AsycnQueryHandler繼承了Handler

public abstract class AsyncQueryHandler extends Handler

 內部使用HandleThread來實現異步線程處理數據，成員變量也有一個Handler並擁有HandleThread的looper。

private Handler mWorkerThreadHandler;

public AsyncQueryHandler(ContentResolver cr) {
        super();
        mResolver = new WeakReference<ContentResolver>(cr);
        synchronized (AsyncQueryHandler.class) {
            if (sLooper == null) {
                HandlerThread thread = new HandlerThread("AsyncQueryWorker");
                thread.start();

                sLooper = thread.getLooper();
            }
        }
        mWorkerThreadHandler = createHandler(sLooper);
    }

主線程調用AsycnQueryHandler的startQuery方法，成員變量mWorkerThreadHandler傳遞Message給HandleThread處理，查詢參數使用對象WorkerArgs封裝。

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;
        args.uri = uri;
        args.projection = projection;
        args.selection = selection;
        args.selectionArgs = selectionArgs;
        args.orderBy = orderBy;
        args.cookie = cookie;
        msg.obj = args;

        mWorkerThreadHandler.sendMessage(msg);
    }

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);
                        // Calling getCount() causes the cursor window to be filled,
                        // which will make the first access on the main thread a lot faster.
                        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;

HandleThread將處理結果result封裝到WorkerArgs中，再經過Message傳遞給AsycnQueryHandler, AsycnQueryHandler調用handleMessage就是主線程中處理了。

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;

2，對於信息數據加載後在UI中展示處理。

　　ConversationList中ConversationListItem佈局時在XML中寫好的，只需根據Conversation數據來顯示或隱藏對應的view便可。

　　ComposeMessageActivity中的MessageListItem須要處理彩信附件，彩信附件有多種格式，且UI佈局是從數據庫中讀取解析，因此沒法直接在XML中寫好，須要在代碼中動態加載。也就是說對於彩信，須要同時處理UI加載(MessageListItem)和數據加載(MessageItem)。

　　MessageItem中使用了異步處理加載MMS數據。

private ItemLoadedFuture mItemLoadedFuture;

mItemLoadedFuture = MmsApp.getApplication().getPduLoaderManager()
                    .getPdu(mMessageUri, loadSlideshow,
                    new PduLoadedMessageItemCallback());

PduLoadManager是MMS源碼中定義的異步處理類，使用了ThreadPoolExecutor來實現異步處理。getPdu方法中將處理結果封裝爲PduLoaded類經過PduLoadedMessageItemCallback接口回調將結果在主線程處理。

 ItemLoadedFuture也是使用了接口回調的設計，回調方法setIsDone是在 PduLoadedMessageItemCallback中觸發，但實現是在PduLoadManager中，因此主線程能夠經過ItemLoadedFuture取消PduLoadManager中的Executor運行。 

public class PduLoaderManager extends BackgroundLoaderManager

BackgroundLoaderManager(Context context) {
        mPendingTaskUris = new HashSet<Uri>();
        mCallbacks = new HashMap<Uri, Set<ItemLoadedCallback>>();
        final LinkedBlockingQueue<Runnable> queue = new LinkedBlockingQueue<Runnable>();
        final int poolSize = MAX_THREADS;
        mExecutor = new ThreadPoolExecutor(
                poolSize, poolSize, 5, TimeUnit.SECONDS, queue,
                new BackgroundLoaderThreadFactory(getTag()));
        mCallbackHandler = new Handler();
    }

public ItemLoadedFuture getPdu(Uri uri, boolean requestSlideshow,
            final ItemLoadedCallback<PduLoaded> callback) {
        if (uri == null) {
            throw new NullPointerException();
        }
       //……此處省略掉一些源碼
       //……爲了顯示不太長，便於查看
        if (pduExists && slideshowExists) {
            if (callbackRequired) {
                PduLoaded pduLoaded = new PduLoaded(cacheEntry.getPdu(), slideshow);
                callback.onItemLoaded(pduLoaded, null);
            }//這幾從緩存中返回數據
            return new NullItemLoadedFuture();
        }

        if (callbackRequired) {
            addCallback(uri, callback);
        }

        if (newTaskRequired) {
            mPendingTaskUris.add(uri);
            Runnable task = new PduTask(uri, requestSlideshow);
            mExecutor.execute(task);//後臺線程執行加載數據
        }
        return new ItemLoadedFuture() {//返回接口實現
            private boolean mIsDone;

            public void cancel(Uri uri) {
                cancelCallback(callback);
                removePdu(uri);     // the pdu and/or slideshow might be half loaded. Make sure
                                    // we load fresh the next time this uri is requested.
            }

            public void setIsDone(boolean done) {
                cancelCallback(callback);
                mIsDone = done;
            }

            public boolean isDone() {
                return mIsDone;
            }
        };
    }

ItemLoadedFuture接口在MessageItem中聲明，但在處理後臺線程的類PduLoadManager中實現，能夠實現主線程管理後臺線程（如取消後臺任務）

PduLoadedMessageItemCallback接口在MessageItem中實現，做爲參數傳遞給PduLoadManager，將後臺線程處理的結果result經過方法onItemLoaded(Object result, Throwable exception)返回到主線程處理。

 

MessageListItem中加載彩信附件

private void bindCommonMessage(final boolean sameItem) {
               ……
               ……
       
                if (mMessageItem.mSlideshow == null) {//若是附件數據還未加載完成 final int mCurrentAttachmentType = mMessageItem.mAttachmentType;
                    mMessageItem.setOnPduLoaded(new MessageItem.PduLoadedCallback() {//這裏又是接口回調實現 public void onPduLoaded(MessageItem messageItem) {
                            if (DEBUG) {
                                Log.v(TAG, "PduLoadedCallback in MessageListItem for item: " + mPosition +
                                        " " + (mMessageItem == null ? "NULL" : mMessageItem.toString()) +
                                        " passed in item: " +
                                        (messageItem == null ? "NULL" : messageItem.toString()));
                            }
                            if (messageItem != null && mMessageItem != null &&
                                    messageItem.getMessageId() == mMessageItem.getMessageId()) {
                                mMessageItem.setCachedFormattedMessage(null);
                                bindCommonMessage(//繼續調用此方法，直到mMessageItem.mSlideshow加載完成
                                        mCurrentAttachmentType == messageItem.mAttachmentType);
                            }
                        }
                    });
                } else {
                    if (mPresenter == null) {
                        mPresenter = PresenterFactory.getPresenter(
                                "MmsThumbnailPresenter", mContext,
                                this, mMessageItem.mSlideshow);
                    } else {
                        mPresenter.setModel(mMessageItem.mSlideshow);
                        mPresenter.setView(this);
                    }
                    if (mImageLoadedCallback == null) {
                        mImageLoadedCallback = new ImageLoadedCallback(this);
                    } else {
                        mImageLoadedCallback.reset(this);
                    }
                    mPresenter.present(mImageLoadedCallback);

                    
                }
            ……
            ……
        requestLayout();
    }
                        

  

3, 幻燈片編輯列表界面(SlideshowEditActivity)，幻燈片中視頻縮略圖加載處理。

SlideListAdapter的getView方法加載幻燈片縮略圖

private View createViewFromResource(int position, View convertView, int resource) {
            SlideListItemView slideListItemView;
            slideListItemView = (SlideListItemView) mInflater.inflate(
                    resource, null);

            // Show slide number.
            TextView text;
            text = (TextView) slideListItemView.findViewById(R.id.slide_number_text);
            text.setText(mContext.getString(R.string.slide_number, position + 1));

            SlideModel slide = getItem(position);
            int dur = slide.getDuration() / 1000;
            text = (TextView) slideListItemView.findViewById(R.id.duration_text);
            text.setText(mContext.getResources().
                         getQuantityString(R.plurals.slide_duration, dur, dur));

            if (mPresenter == null) {
                mPresenter = PresenterFactory.getPresenter(
                        "SlideshowPresenter", mContext, slideListItemView, mSlideshow);
            } else {
                mPresenter.setModel(mSlideshow);
                mPresenter.setView(slideListItemView);
            }
            ((SlideshowPresenter) mPresenter).setLocation(position);            
            mPresenter.present(null);

            return slideListItemView;
        }

SlideshowPresenter中調用present(..)方法，針對視頻類型使用了異步線程處理，避免耗時阻塞UI。也是用接口回調設計，

mItemLoadedFuture = video.loadThumbnailBitmap(mItemLoadedCallback, mLocation, mAdapterHandler);

mItemLoadedFuture的實如今ThumbnailManager中實現，ThumbnailManager也繼承了BackgroundLoaderManager，仍是使用Executor實現後臺線程。

mItemLoadedCallback在SlideshowPresenter中實現，並傳遞給ThumbnailManager，用來將後臺線程處理的結果返回到主線程處理。mItemLoadedCallback的回調方法處理返回的縮略圖時，須要根據幻燈片的postion肯定顯示位置。但源碼中，直接在SlideListAdapter的getView方法中設置positon,這樣等視頻縮率圖返回時，position的值已經被修改了（由於getView在主線程中處理position, 而視頻縮略圖在異步線程中處理，當異步線程比主線程慢時，postion已經給後續處理改變了。）對於這個問題，能夠將positon直接傳給異步線程處理，縮率圖顯示時直接讀取傳遞下去的postion，就不會被主線程的position影響。