Android平台Gallery2应用分析(七)---PhotoPage图片解码

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PhotoPage图片解码

从前文可知，PhotoPage的图片解码始于PhotoPage的onResume()调用updateImageRequests()。先看下代码：

    private void updateImageRequests() {        ……        int currentIndex = mCurrentIndex;        MediaItem item = mData[currentIndex % DATA_CACHE_SIZE];        ……        // 1. 遍历sImageFetchSeq，查看当前图片符合哪种类型，调用startTaskIfNeeded        Future<?> task = null;        for (int i = 0; i < sImageFetchSeq.length; i++) {            int offset = sImageFetchSeq[i].indexOffset;            int bit = sImageFetchSeq[i].imageBit;            if (bit == BIT_FULL_IMAGE && !mNeedFullImage) continue;            task = startTaskIfNeeded(currentIndex + offset, bit);            if (task != null) break;        }        // 2. 释放任务和内存        for (ImageEntry entry : mImageCache.values()) {            if (entry.screenNailTask != null && entry.screenNailTask != task) {                entry.screenNailTask.cancel();                entry.screenNailTask = null;                entry.requestedScreenNail = MediaObject.INVALID_DATA_VERSION;            }            if (entry.fullImageTask != null && entry.fullImageTask != task) {                entry.fullImageTask.cancel();                entry.fullImageTask = null;                entry.requestedFullImage = MediaObject.INVALID_DATA_VERSION;            }        }}

接下来，重点分析startTaskIfNeeded()，看它是如何对一张图片做解析的。还是先看下面代码：

    private Future<?> startTaskIfNeeded(int index, int which) {        if (index < mActiveStart || index >= mActiveEnd) return null;        ImageEntry entry = mImageCache.get(getPath(index));        if (entry == null) return null;        // 先得到当前图片，类型为LocalImage        MediaItem item = mData[index % DATA_CACHE_SIZE];        Utils.assertTrue(item != null);        long version = item.getDataVersion();                // 第一次代码执行暂时screenNailTask和fullImageTask为null        if (which == BIT_SCREEN_NAIL && entry.screenNailTask != null                && entry.requestedScreenNail == version) {            return entry.screenNailTask;        } else if (which == BIT_FULL_IMAGE && entry.fullImageTask != null                && entry.requestedFullImage == version) {            return entry.fullImageTask;        }        // 匹配到格式后，先创建ScreenNailJob、ScreenNailListener并返回screenNailTask        if (which == BIT_SCREEN_NAIL && entry.requestedScreenNail != version) {            entry.requestedScreenNail = version;            entry.screenNailTask = mThreadPool.submit(                    new ScreenNailJob(item),                    new ScreenNailListener(item));            // request screen nail            return entry.screenNailTask;        }// 匹配到格式后，先创建FullImageJob、FullImageListener并返回fullImageTask        if (which == BIT_FULL_IMAGE && entry.requestedFullImage != version                && (item.getSupportedOperations()                & MediaItem.SUPPORT_FULL_IMAGE) != 0) {            entry.requestedFullImage = version;            entry.fullImageTask = mThreadPool.submit(                    new FullImageJob(item),                    new FullImageListener(item));            // request full image            return entry.fullImageTask;        }        return null;}

参数which就是静态数组sImageFetchSeq中的图片类型，android原生代码默认两种BIT_SCREEN_NAIL和BIT_FULL_IMAGE。当然我们也可以自己加入一种解析图片的格式，例如BIT_GIF_IMAGE。在详细分析下面的代码后，详细你自己加入一种图片格式应该问题不大。流程大致如下：

1）从传入参数index获取当前图片MediaItem，图片为LocalImage类型。第一次执行时screenNailTask和fullImageTask为null，匹配到格式后，创建ScreenNailJob、ScreenNailListener（或者screenNailTask和fullImageTask为null），并最终返回screenNailTask或者fullImageTask。

2）以FullImage为例。ThreadPool的机制这里再大致讲述一下。看下mThreadPool.submit的代码：

    public <T> Future<T> submit(Job<T> job, FutureListener<T> listener) {        Worker<T> w = new Worker<T>(job, listener);        mExecutor.execute(w);        return w;    }

由前文分析，execute会启动Worker的线程进入run()函数：

       @Override        public void run() {            T result = null;            // A job is in CPU mode by default. setMode returns false            // if the job is cancelled.            if (setMode(MODE_CPU)) {                try {                    // 1、执行job的run()                    result = mJob.run(this);                } catch (Throwable ex) {                    Log.w(TAG, "Exception in running a job", ex);                }            }            synchronized(this) {                setMode(MODE_NONE);                mResult = result;                mIsDone = true;                notifyAll();            }            // 2、执行完毕，调用listener的onFutureDone            if (mListener != null) mListener.onFutureDone(this);        }

这里面分两步：

2.1）执行job的run()。这里会调用传入参数new FullImageJob (item).run()。

    private class FullImageJob implements Job<BitmapRegionDecoder> {        private MediaItem mItem;        public FullImageJob(MediaItem item) {            mItem = item;        }        @Override        public BitmapRegionDecoder run(JobContext jc) {            if (isTemporaryItem(mItem)) {                return null;            }            return mItem.requestLargeImage().run(jc);        }    }

这段代码实际就是在线程池的某个线程中执行LocalImage的requestLargeImage().run(jc)。而该函数会创建一个LocalLargeImageRequest对象，而run(jc)实际就是DecodeUtils.createBitmapRegionDecoder(jc, mLocalFilePath, false)，最终创建一个BitmapRegionDecoder实例。该实例会调用JNI层的BitmapRegionDecoder中的nativeNewInstanceFromStream接口，在doBuildTileIndex(JNIEnv* env, SkStream* stream)里可以看到，图片会根据stream的header判断解码器是SkJPEGImageDecoder，SkPNGImageDecoder，SkBMPImageDecoder还是SkWEBPImageDecoder等，最后得到解码数据。

2.2）调用FullImageListener的onFutureDone。此时会将2.1)中创建好的BitmapRegionDecoder实例返回传给mFuture。而FullImageListener则再发送一个MSG_RUN_OBJECT消息给MainThread，MainThread再执行FullImageListener的run()，即再执行updateFullImage(mPath, mFuture)。

    private void updateFullImage(Path path, Future<BitmapRegionDecoder> future) {        ImageEntry entry = mImageCache.get(path);        ……        entry.fullImageTask = null;        entry.fullImage = future.get();        if (entry.fullImage != null) {            if (path == getPath(mCurrentIndex)) {                updateTileProvider(entry);                mPhotoView.notifyImageChange(0);            }        }        updateImageRequests();}

其中mPhotoView.notifyImageChange(0)取到当前图片后并reload，其中mPictures当前为FullPicture。

    public void notifyImageChange(int index) {        ……        mPictures.get(index).reload();        ……        invalidate();}

那么再看看FullPicture的reload(), 该函数会对mTileView的screenNail做更新。

        @Override        public void reload() {            // mImageWidth and mImageHeight will get updated            mTileView.notifyModelInvalidated();            ……            setScreenNail(mModel.getScreenNail(0));            ……        }

代码段中的mModel就是PhotoDataAdapter。mModel.getScreenNail(0)得到当前图片的ScreenNail以做更新。由此可知，我们看到的全屏的图片，就是TileImageView类型的。

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