Glide原理及源码简述

Glide是如何发起网络请求获取图片的呢？

流程图：

Glide调用：

  Glide.with(OpApplication.getInstance().getApplicationContext())
                .load(drawableId)
                .asGif()
                .error(R.mipmap.default_error)
                .crossFade()
                .into(iv);

with

通过匹配context对应的Activity返回一个RequestManager，在RequestManager中完成对Glide对象的初始化Glide.get(context);

——> GlideBuilder builder = new GlideBuilder(applicationContext);

load

指定静态图片返回BitmapTypeRequest，动态图片返回GifTypeRequest，不指定默认返回DrawbleTypeRequest，总之，load方法返回一个DrawbleTypeRequest对象

into

  public > Y into(Y target) {
        Util.assertMainThread();
        if (target == null) {
            throw new IllegalArgumentException("You must pass in a non null Target");
        }
        if (!isModelSet) {
            throw new IllegalArgumentException("You must first set a model (try #load())");
        }

        Request previous = target.getRequest();

        if (previous != null) {
            previous.clear();
            requestTracker.removeRequest(previous);
            previous.recycle();
        }

        Request request = buildRequest(target);
        target.setRequest(request);
        lifecycle.addListener(target);
        requestTracker.runRequest(request);

        return target;
    }

通过传入target判断是否要更新当前request，构建一个Request，在调用requestTracker.runRequest(request);方法：

 public void runRequest(Request request) {
        requests.add(request);
        if (!isPaused) {
            request.begin();
        } else {
            pendingRequests.add(request);
        }
    }

request.begin()跳转到子类实现方法

// GenericRequest.java
   
 @Override
    public void begin() {
        startTime = LogTime.getLogTime();
        if (model == null) {
            onException(null);
            return;
        }

        status = Status.WAITING_FOR_SIZE;
        if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
            onSizeReady(overrideWidth, overrideHeight);
        } else {
            target.getSize(this);
        }

        if (!isComplete() && !isFailed() && canNotifyStatusChanged()) {
            target.onLoadStarted(getPlaceholderDrawable());
        }
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("finished run method in " + LogTime.getElapsedMillis(startTime));
        }
    }

进入重点方法onSizeReady(overrideWidth, overrideHeight);之中：

 @Override
    public void onSizeReady(int width, int height) {
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("Got onSizeReady in " + LogTime.getElapsedMillis(startTime));
        }
        if (status != Status.WAITING_FOR_SIZE) {
            return;
        }
        status = Status.RUNNING;

        width = Math.round(sizeMultiplier * width);
        height = Math.round(sizeMultiplier * height);

        ModelLoader modelLoader = loadProvider.getModelLoader();
        final DataFetcher dataFetcher = modelLoader.getResourceFetcher(model, width, height);

        if (dataFetcher == null) {
            onException(new Exception("Failed to load model: \'" + model + "\'"));
            return;
        }
        ResourceTranscoder transcoder = loadProvider.getTranscoder();
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("finished setup for calling load in " + LogTime.getElapsedMillis(startTime));
        }
        loadedFromMemoryCache = true;
        loadStatus = engine.load(signature, width, height, dataFetcher, loadProvider, transformation, transcoder,
                priority, isMemoryCacheable, diskCacheStrategy, this);
        loadedFromMemoryCache = resource != null;
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("finished onSizeReady in " + LogTime.getElapsedMillis(startTime));
        }
    }

 在onSizeReady()方法中完成图片压缩。

通过final DataFetcher dataFetcher = modelLoader.getResourceFetcher(model, width, height);看到子类重写为：

//  HttpUrlGlideUrlLoader.java
 
@Override
    public DataFetcher getResourceFetcher(GlideUrl model, int width, int height) {
        // GlideUrls memoize parsed URLs so caching them saves a few object instantiations and time spent parsing urls.
        GlideUrl url = model;
        if (modelCache != null) {
            url = modelCache.get(model, 0, 0);
            if (url == null) {
                modelCache.put(model, 0, 0, model);
                url = model;
            }
        }
        return new HttpUrlFetcher(url);
    }

可以看到返回一个HttpUrlFetcher对象，到这先返回到上面：

 engine.load(signature, width, height, dataFetcher, loadProvider, transformation, transcoder,
        priority, isMemoryCacheable, diskCacheStrategy, this);

跟进去看，发现有下面一段代码：

DecodeJob decodeJob = new DecodeJob(key, width, height, fetcher, loadProvider, transformation,
        transcoder, diskCacheProvider, diskCacheStrategy, priority);
EngineRunnable runnable = new EngineRunnable(engineJob, decodeJob, priority);

注意DecodeJob中传入的一个fetcher对象，在DecodeJob对象中看到一段代码：

// DecodeJob.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;
    }

接着看EngineRunnable类中的run()方法，

decode();——>decodeFromSource()——>decodeJob.decodeFromSource()——>Resource decoded = decodeSource();

发现这时候调用的是上面提到的DecodeJob对象的decodeSource()方法，至此调用HttpUrlFetcher类的loadData()方法：

fetcher.loadData(priority);

下面我们继续看HttpUrlFetcher类中的loadDataWithRedirects方法的内容：

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());
        }
    }

终于，我们看到了Glide的网络请求了，在statusCode为2xx时，将网络数据通过InputStream返回。