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

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;
    }
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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