摘要：從網(wǎng)絡(luò)加載圖片加載從加載從網(wǎng)絡(luò)加載從加載具體的方法實(shí)現(xiàn)接口的類以后再做分析，而從網(wǎng)絡(luò)加載兩步從網(wǎng)絡(luò)獲取數(shù)據(jù)處理數(shù)據(jù)。

4.從網(wǎng)絡(luò)加載

        EngineJob current = jobs.get(key);
        if (current != null) {
            current.addCallback(cb);
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                logWithTimeAndKey("Added to existing load", startTime, key);
            }
            return new LoadStatus(cb, current);
        }

        EngineJob engineJob = engineJobFactory.build(key, isMemoryCacheable);
        DecodeJob decodeJob = new DecodeJob(key, width, height, fetcher, loadProvider, transformation,
                transcoder, diskCacheProvider, diskCacheStrategy, priority);
        EngineRunnable runnable = new EngineRunnable(engineJob, decodeJob, priority);
        jobs.put(key, engineJob);
        engineJob.addCallback(cb);
        engineJob.start(runnable);

EngineRunnable.java:

    @Override
    public void run() {
        if (isCancelled) {
            return;
        }

        Exception exception = null;
        Resource resource = null;
        try {
            resource = decode();
        } catch (Exception e) {
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                Log.v(TAG, "Exception decoding", e);
            }
            exception = e;
        }

        if (isCancelled) {
            if (resource != null) {
                resource.recycle();
            }
            return;
        }

        if (resource == null) {
            onLoadFailed(exception);
        } else {
            onLoadComplete(resource);
        }
    }
    private Resource decode() throws Exception {
        if (isDecodingFromCache()) {
            return decodeFromCache();
        } else {
            return decodeFromSource();
        }
    }

圖片加載：
1.從disk加載
2.從網(wǎng)絡(luò)加載
從disk加載
DecodeJob.java:

   private Resource loadFromCache(Key key) throws IOException {
        File cacheFile = diskCacheProvider.getDiskCache().get(key);
        if (cacheFile == null) {
            return null;
        }

        Resource result = null;
        try {
            result = loadProvider.getCacheDecoder().decode(cacheFile, width, height);
        } finally {
            if (result == null) {
                diskCacheProvider.getDiskCache().delete(key);
            }
        }
        return result;
    }

具體decode的方法 實(shí)現(xiàn)ResourceDecoder接口的類

以后再做分析，而從網(wǎng)絡(luò)加載

Decoder.java:

    private Resource decodeSource() throws Exception {
        Resource decoded = null;
        try {
            long startTime = LogTime.getLogTime();
            final A data = fetcher.loadData(priority);
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                logWithTimeAndKey("Fetched data", startTime);
            }
            if (isCancelled) {
                return null;
            }
            decoded = decodeFromSourceData(data);
        } finally {
            fetcher.cleanup();
        }
        return decoded;
    }

兩步：
1.從網(wǎng)絡(luò)獲取數(shù)據(jù)；
2.處理數(shù)據(jù)。
HttpUrlFetcher.java:

    private InputStream loadDataWithRedirects(URL url, int redirects, URL lastUrl, Map headers)
            throws IOException {
        if (redirects >= MAXIMUM_REDIRECTS) {
            throw new IOException("Too many (> " + MAXIMUM_REDIRECTS + ") redirects!");
        } else {
            // Comparing the URLs using .equals performs additional network I/O and is generally broken.
            // See http://michaelscharf.blogspot.com/2006/11/javaneturlequals-and-hashcode-make.html.
            try {
                if (lastUrl != null && url.toURI().equals(lastUrl.toURI())) {
                    throw new IOException("In re-direct loop");
                }
            } catch (URISyntaxException e) {
                // Do nothing, this is best effort.
            }
        }
        urlConnection = connectionFactory.build(url);
        for (Map.Entry headerEntry : headers.entrySet()) {
          urlConnection.addRequestProperty(headerEntry.getKey(), headerEntry.getValue());
        }
        urlConnection.setConnectTimeout(2500);
        urlConnection.setReadTimeout(2500);
        urlConnection.setUseCaches(false);
        urlConnection.setDoInput(true);

        // Connect explicitly to avoid errors in decoders if connection fails.
        urlConnection.connect();
        if (isCancelled) {
            return null;
        }
        final int statusCode = urlConnection.getResponseCode();
        if (statusCode / 100 == 2) {
            return getStreamForSuccessfulRequest(urlConnection);
        } else if (statusCode / 100 == 3) {
            String redirectUrlString = urlConnection.getHeaderField("Location");
            if (TextUtils.isEmpty(redirectUrlString)) {
                throw new IOException("Received empty or null redirect url");
            }
            URL redirectUrl = new URL(url, redirectUrlString);
            return loadDataWithRedirects(redirectUrl, redirects + 1, url, headers);
        } else {
            if (statusCode == -1) {
                throw new IOException("Unable to retrieve response code from HttpUrlConnection.");
            }
            throw new IOException("Request failed " + statusCode + ": " + urlConnection.getResponseMessage());
        }
    }

3XX重定向

   URL redirectUrl = new URL(url, redirectUrlString);
   return loadDataWithRedirects(redirectUrl, redirects + 1, url, headers);

處理數(shù)據(jù)：

 private Resource decodeFromSourceData(A data) throws IOException {
        final Resource decoded;
        if (diskCacheStrategy.cacheSource()) {
            decoded = cacheAndDecodeSourceData(data);
        } else {
            long startTime = LogTime.getLogTime();
            decoded = loadProvider.getSourceDecoder().decode(data, width, height);
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                logWithTimeAndKey("Decoded from source", startTime);
            }
        }
        return decoded;
    }

具體decode的方法 實(shí)現(xiàn)ResourceDecoder接口的類

EngineJob.java:

    private final ExecutorService diskCacheService;
    
    public void start(EngineRunnable engineRunnable) {
        this.engineRunnable = engineRunnable;
        future = diskCacheService.submit(engineRunnable);
    }