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 extends Target<TranscodeType>> 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<A, T> modelLoader = loadProvider.getModelLoader();
final DataFetcher<T> dataFetcher = modelLoader.getResourceFetcher(model, width, height);
if (dataFetcher == null) {
onException(new Exception("Failed to load model: \'" + model + "\'"));
return;
}
ResourceTranscoder<Z, R> 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<T> dataFetcher = modelLoader.getResourceFetcher(model, width, height);看到子类重写为：
// HttpUrlGlideUrlLoader.java
@Override
public DataFetcher<InputStream> 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<T, Z, R> decodeJob = new DecodeJob<T, Z, R>(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<T> decodeSource() throws Exception {
Resource<T> 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<T> 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<String, String> 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<String, String> 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。