galler3d的源码分析——数据来源

我们这里主要讲本地sd卡的数据，pisaca看情况后续再作分析。



数据操作设计的类包括：CacheService，MediaFeed，LocalDataSource，DiskCache，MediaItem， MediaSet，MediaClustering。



数据操作包括几个方面：寻找媒体源（图片、视频），cache媒体源，将cache数据导出，将cache的媒体渲染到屏幕。

首先是找图片或者视频：

1. 如果当前cache文件不存在，或者当前cache文件记录的语言、国家和变量代码与系统的不一致的场景时，首先将所有cache文件(index和chunk文件）删除，接着将语言、国家和变量代码写入local-album-cachechunk0的文件中。



2. 启动一个线程，寻找图片或者视频，并将相册信息cache。



3. 再启动一个线程，将图片缩略图化，并cache。



4. 如果上一次还有一部分数据cache，执行步骤2和3。



5. 如果这一次有新的媒体文件需要cache，启动一个线程，将新的媒体文件cache。再执行步骤3。

关键的代码如下：

@Override
    protected void onHandleIntent(final Intent intent) {
        Log.i(TAG, "CacheService, onHandleIntent, Starting CacheService");
        if (Environment.getExternalStorageState() == Environment.MEDIA_BAD_REMOVAL) {
            sAlbumCache.deleteAll();
            putLocaleForAlbumCache(Locale.getDefault());
        }
        Locale locale = getLocaleForAlbumCache();
        if (locale != null && locale.equals(Locale.getDefault())) {
            // The cache is in the same locale as the system locale.
            if (!isCacheReady(false)) {
                // The albums and their items have not yet been cached, we need
                // to run the service.
            	Log.i(TAG, "CacheService, onHandleIntent, start new cache thread");
                startNewCacheThread();
            } else {
            	Log.i(TAG, "CacheService, onHandleIntent, start new cache thread for dirty sets");
                startNewCacheThreadForDirtySets();
            }
        } else {
            // The locale has changed, we need to regenerate the strings.
        	Log.i(TAG, "CacheService, onHandleIntent, delete all");
            sAlbumCache.deleteAll();
            putLocaleForAlbumCache(Locale.getDefault());
            startNewCacheThread();
        }
        if (intent.getBooleanExtra("checkthumbnails", false)) {
        	Log.i(TAG, "CacheService, onHandleIntent, start new thumbnail thread");
            startNewThumbnailThread(this);
        } else {
            final Thread existingThread = THUMBNAIL_THREAD.getAndSet(null);
            if (existingThread != null) {
                existingThread.interrupt();
            }
        }
    }

先看这个函数：

private static final Locale getLocaleForAlbumCache() {
    	Log.i(TAG, "CacheService, getLocaleForAlbumCache");
        final byte[] data = sAlbumCache.get(ALBUM_CACHE_LOCALE_INDEX, 0);
        if (data != null && data.length > 0) {
            ByteArrayInputStream bis = new ByteArrayInputStream(data);
            DataInputStream dis = new DataInputStream(bis);
            try {
                String country = Utils.readUTF(dis);
                if (country == null)
                    country = "";
                String language = Utils.readUTF(dis);
                if (language == null)
                    language = "";
                String variant = Utils.readUTF(dis);
                if (variant == null)
                    variant = "";
                final Locale locale = new Locale(language, country, variant);
                dis.close();
                bis.close();
                return locale;
            } catch (IOException e) {
                // Could not read locale in cache.
                Log.i(TAG, "Error reading locale from cache.");
                return null;
            }
        }
        return null;
    }

我们看到 sAlbumCache这个变量，这个变量的定义是这样的：

public static final DiskCache sAlbumCache = new DiskCache("local-album-cache");

我们来看看DiskCache的构造函数做了什么？

public DiskCache(String cacheDirectoryName) {
        String cacheDirectoryPath = CacheService.getCachePath(cacheDirectoryName);

        // Create the cache directory if needed.
        File cacheDirectory = new File(cacheDirectoryPath);
        if (!cacheDirectory.isDirectory() && !cacheDirectory.mkdirs()) {
            Log.e(TAG, "Unable to create cache directory " + cacheDirectoryPath);
        }
        mCacheDirectoryPath = cacheDirectoryPath;
        loadIndex();
    }

public static final String getCachePath(final String subFolderName) {
        return Environment.getExternalStorageDirectory() + "/Android/data/com.luowei.media/cache/" + subFolderName;
    }

原来如此，在sd卡建立了一个目录/Android/data/com.luowei.media/cache/local-album-cache。



建立目录后呢？

private void loadIndex() {
        final String indexFilePath = getIndexFilePath();
        try {
            // Open the input stream.
            final FileInputStream fileInput = new FileInputStream(indexFilePath);
            final BufferedInputStream bufferedInput = new BufferedInputStream(fileInput, 1024);
            final DataInputStream dataInput = new DataInputStream(bufferedInput);

            // Read the header.
            final int magic = dataInput.readInt();//4
            final int version = dataInput.readInt();//4
            boolean valid = true;
            if (magic != INDEX_HEADER_MAGIC) {
                Log.e(TAG, "Index file appears to be corrupt (" + magic + " != " + INDEX_HEADER_MAGIC + "), " + indexFilePath);
                valid = false;
            }
            if (valid && version != INDEX_HEADER_VERSION) {
                // Future versions can implement upgrade in this case.
                Log.e(TAG, "Index file version " + version + " not supported");
                valid = false;
            }
            if (valid) {
                mTailChunk = dataInput.readShort();//2
            }

            // Read the entries.
            if (valid) {
                // Parse the index file body into the in-memory map.
                final int numEntries = dataInput.readInt();//4
                mIndexMap = new LongSparseArray<Record>(numEntries);
                synchronized (mIndexMap) {
                    for (int i = 0; i < numEntries; ++i) {
                        final long key = dataInput.readLong();//8
                        final int chunk = dataInput.readShort();//2
                        final int offset = dataInput.readInt();//4
                        final int size = dataInput.readInt();//4
                        final int sizeOnDisk = dataInput.readInt();//4
                        final long timestamp = dataInput.readLong();//8
                        mIndexMap.append(key, new Record(chunk, offset, size, sizeOnDisk, timestamp));
                    }
                }
            }

            dataInput.close();
            if (!valid) {
                deleteAll();
            }

        } catch (FileNotFoundException e) {
            // If the file does not exist the cache is empty, so just continue.
        } catch (IOException e) {
            Log.e(TAG, "Unable to read the index file " + indexFilePath);
        } finally {
            if (mIndexMap == null) {
                mIndexMap = new LongSparseArray<Record>();
            }
        }
    }

首先找索引文件，索引文件路径是什么？

private String getIndexFilePath() {
        return mCacheDirectoryPath + INDEX_FILE_NAME;
    }

具体路径是 sd卡下/Android/data/com.luowei.media/cache/local-album-cacheindex。如果文件不存在，就只做一件事，将mIndexMap初始化；如果文件存在，读取文件头，如果文件头是00 00 CA FE 00 00 00 02这个格式，说明是标准的索引文件，继续分析此文件，将数据写入mIndexMap映射表。



接着继续分析getLocaleForAlbumCache函数，sAlbumCache.get做了什么？

public byte[] get(long key, long timestamp) {
        // Look up the record for the given key.
        Record record = null;
        synchronized (mIndexMap) {
            record = mIndexMap.get(key);
        }
        if (record != null) {
            // Read the chunk from the file.
            if (record.timestamp != timestamp) {
                return null;
            }
            try {
                RandomAccessFile chunkFile = getChunkFile(record.chunk);
                if (chunkFile != null) {
                    byte[] data = new byte[record.size];
                    chunkFile.seek(record.offset);
                    chunkFile.readFully(data);
                    return data;
                }
            } catch (Exception e) {
                Log.e(TAG, "Unable to read from chunk file");
            }
        }
        return null;
    }

如果mIndexMap没有ALBUM_CACHE_LOCALE_INDEX这个key的数据，返回null；如果有，接着比较。如果记录的时间戳也不一致，返回null。那getChunkFile又有什么呢？

private RandomAccessFile getChunkFile(int chunk) {
        RandomAccessFile chunkFile = null;
        synchronized (mChunkFiles) {
            chunkFile = mChunkFiles.get(chunk);
        }
        if (chunkFile == null) {
            final String chunkFilePath = mCacheDirectoryPath + CHUNK_FILE_PREFIX + chunk;
            
            Log.i(TAG, "DiskCache, chunkFilePath:"+chunkFilePath);
            try {
                chunkFile = new RandomAccessFile(chunkFilePath, "rw");
            } catch (FileNotFoundException e) {
                Log.e(TAG, "Unable to create or open the chunk file " + chunkFilePath);
            }
            synchronized (mChunkFiles) {
                mChunkFiles.put(chunk, chunkFile);
            }
        }
        return chunkFile;
    }

mChunkFiles也是一个chunk映射表，local-album-cache对应的chunk文件路径对应为/Android/data/com.luowei.media/cache/local-album-cachechunk0。如果chunk文件不存在，需要建立这个文件，同时将这个文件加入mChunkFiles映射表。



回到getLocaleForAlbumCache这个函数，发现这个chunk文件的头记录语言、地区和变量代码。再返回onHandleIntent函数，如果getLocaleForAlbumCache取回的代码是空的或者跟本机的代码不一致，就需要做三件事情：

1. 将/Android/data/com.luowei.media/cache/local-album-cache目录下所有文件，其实这里面有个bug，请读者自行发现；

2. 创建chunk和index文件，并将语言、地区和变量代码写入chunk文件，这就是putLocaleForAlbumCache函数所作的事情；

3. 开启新的cache线程：startNewCacheThread。