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

Android平台Gallery2应用分析(七)—PhotoPage图片解码分类： Android多媒体 2013-12-23 16:15 1347人阅读评论(1) 收藏举报 AndroidGallery2应用相册多媒体 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： [java] view plaincopy在CODE上查看代码片派生到我的代码片 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： [java] view plaincopy在CODE上查看代码片派生到我的代码片 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： [java] view plaincopy在CODE上查看代码片派生到我的代码片 public Future submit(Job job, FutureListener listener) {
Worker w = new Worker(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： [java] view plaincopy在CODE上查看代码片派生到我的代码片 @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()。 [java] view plaincopy在CODE上查看代码片派生到我的代码片 private class FullImageJob implements Job {
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)。 [java] view plaincopy在CODE上查看代码片派生到我的代码片 private void updateFullImage(Path path, Future 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。 [java] view plaincopy在CODE上查看代码片派生到我的代码片 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。 [java] view plaincopy在CODE上查看代码片派生到我的代码片 @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。欢迎转载和技术交流，转载请帮忙注明出处，http://blog.csdn.net/discovery_by_joseph%EF%BC%8C%E8%B0%A2%E8%B0%A2%EF%BC%81