使用的方式相当的简单，相信大家都使用过。这也是部分人喜欢Glide的原因之一。说实话如果其他好资源都这样，那真是共产主义了。
这里对于使用方式就不多介绍了，相信看一遍就一目了然。常规的图片预加载、图片下载、图片缓存、图片加载回调、图片占位图以及一些图片的设置等等Glide都支持，功能可以说是很完善的。感兴趣的网上查查吧。 ### Glide解析 #### 一
那么先从 >Glide.with(this).load(url).into(imageView);

public static RequestManager with(Activity activity) {
    RequestManagerRetriever retriever = RequestManagerRetriever.get();
    return retriever.get(activity);
}

public static RequestManager with(FragmentActivity activity) {
    RequestManagerRetriever retriever = RequestManagerRetriever.get();
    return retriever.get(activity);
}

@TargetApi(Build.VERSION_CODES.HONEYCOMB)
public static RequestManager with(android.app.Fragment fragment) {
    RequestManagerRetriever retriever = RequestManagerRetriever.get();
    return retriever.get(fragment);
}

public static RequestManager with(Fragment fragment) {
    RequestManagerRetriever retriever = RequestManagerRetriever.get();
    return retriever.get(fragment);
}

>Glide.with(this).load(url).into(imageView); with方法结束之后得到RequestManager对象RequestManager.load(）,那么这个load方法肯定也在RequestManager中。接下来就研究下load方法。 ``` /**
 * Returns a request builder to load the given {@link String}.
 * signature.
 *
 * @see #fromString()
 * @see #load(Object)
 *
 * @param string A file path, or a uri or url handled by {@link com.bumptech.glide.load.model.UriLoader}.
 */
public DrawableTypeRequest<String> load(String string) {
    return (DrawableTypeRequest<String>) fromString().load(string);
}

/**
 * Returns a request builder that loads data from {@link String}s using an empty signature.
 *
 * <p>
 *     Note - this method caches data using only the given String as the cache key. If the data is a Uri outside of
 *     your control, or you otherwise expect the data represented by the given String to change without the String
 *     identifier changing, Consider using
 *     {@link GenericRequestBuilder#signature(Key)} to mixin a signature
 *     you create that identifies the data currently at the given String that will invalidate the cache if that data
 *     changes. Alternatively, using {@link DiskCacheStrategy#NONE} and/or
 *     {@link DrawableRequestBuilder#skipMemoryCache(boolean)} may be appropriate.
 * </p>
 *
 * @see #from(Class)
 * @see #load(String)
 */
public DrawableTypeRequest<String> fromString() {
    return loadGeneric(String.class);
}

private <T> DrawableTypeRequest<T> loadGeneric(Class<T> modelClass) {
    ModelLoader<T, InputStream> streamModelLoader = Glide.buildStreamModelLoader(modelClass, context);
    ModelLoader<T, ParcelFileDescriptor> fileDescriptorModelLoader =
            Glide.buildFileDescriptorModelLoader(modelClass, context);
    if (modelClass != null && streamModelLoader == null && fileDescriptorModelLoader == null) {
        throw new IllegalArgumentException("Unknown type " + modelClass + ". You must provide a Model of a type for"
                + " which there is a registered ModelLoader, if you are using a custom model, you must first call"
                + " Glide#register with a ModelLoaderFactory for your custom model class");
    }
    return optionsApplier.apply(
            new DrawableTypeRequest<T>(modelClass, streamModelLoader, fileDescriptorModelLoader, context,
                    glide, requestTracker, lifecycle, optionsApplier));
} 

@SuppressWarnings("unchecked")
public <Z> Target<Z> buildTarget(ImageView view, Class<Z> clazz) {
    if (GlideDrawable.class.isAssignableFrom(clazz)) {
        return (Target<Z>) new GlideDrawableImageViewTarget(view);
    } else if (Bitmap.class.equals(clazz)) {
        return (Target<Z>) new BitmapImageViewTarget(view);
    } else if (Drawable.class.isAssignableFrom(clazz)) {
        return (Target<Z>) new DrawableImageViewTarget(view);
    } else {
        throw new IllegalArgumentException("Unhandled class: " + clazz
                + ", try .as*(Class).transcode(ResourceTranscoder)");
    }
} } ``` 构建不同的Target，例如BitmapImageViewTarget、DrawableImageViewTarget等。又回到最后一行代码既into(target),接下来看看into调用的是什么方法。 ```  ....省略 Request request = buildRequest(target);
target.setRequest(request);
lifecycle.addListener(target);
requestTracker.runRequest(request);
return target; ``` 最终into里面构造了request并执行这个request（即执行request请求）。构造request时建立了带了很多参数，这些参数之前也提到过。runQuest方法中有一个队列，如果已经在执行了，则进入等待队列待执行完成在执行此任务。当开始执行request时，有一个begin方法。 ```@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));
} } ``` 这个begin方法里面就是一些基本设置，例如model为空就会执行onException方法，这个方法就会执行图片加载错误的占位图，还有设置图片大小即resize加载图片宽高。可以理解begin方法里面就执行了这些设置。设置完之后就执行request请求了。下面看如何执行的。 ```public <T, Z, R> LoadStatus load(Key signature, int width, int height, DataFetcher<T> fetcher,
        DataLoadProvider<T, Z> loadProvider, Transformation<Z> transformation, ResourceTranscoder<Z, R> transcoder,
        Priority priority, boolean isMemoryCacheable, DiskCacheStrategy diskCacheStrategy, ResourceCallback cb) {
    Util.assertMainThread();
    long startTime = LogTime.getLogTime();

    final String id = fetcher.getId();
    EngineKey key = keyFactory.buildKey(id, signature, width, height, loadProvider.getCacheDecoder(),
            loadProvider.getSourceDecoder(), transformation, loadProvider.getEncoder(),
            transcoder, loadProvider.getSourceEncoder());

    EngineResource<?> cached = loadFromCache(key, isMemoryCacheable);
    if (cached != null) {
        cb.onResourceReady(cached);
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logWithTimeAndKey("Loaded resource from cache", startTime, key);
        }
        return null;
    }

    EngineResource<?> active = loadFromActiveResources(key, isMemoryCacheable);
    if (active != null) {
        cb.onResourceReady(active);
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logWithTimeAndKey("Loaded resource from active resources", startTime, key);
        }
        return null;
    }

    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<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);
    jobs.put(key, engineJob);
    engineJob.addCallback(cb);
    engineJob.start(runnable);

    if (Log.isLoggable(TAG, Log.VERBOSE)) {
        logWithTimeAndKey("Started new load", startTime, key);
    }
    return new LoadStatus(cb, engineJob);
}

private EngineResource<?> getEngineResourceFromCache(Key key) {
    Resource<?> cached = cache.remove(key);
    final EngineResource result;
    if (cached == null) {
        result = null;
    } else if (cached instanceof EngineResource) {
        result = (EngineResource) cached;
    } else {
        result = new EngineResource(cached, true /*isCacheable*/);
    }
    return result;
}

private EngineResource<?> loadFromActiveResources(Key key, boolean isMemoryCacheable) {
    if (!isMemoryCacheable) {
        return null;
    }
    EngineResource<?> active = null;
    WeakReference<EngineResource<?>> activeRef = activeResources.get(key);
    if (activeRef != null) {
        active = activeRef.get();
        if (active != null) {
            active.acquire();
        } else {
            activeResources.remove(key);
        }
    }
    return active;
}

private String mUrl;

public MyGlideUrl(String url) {
    super(url);
    mUrl = url;
}

@Override
public String getCacheKey() {
    return mUrl.replace(findTokenParam(), "");
}

private String findTokenParam() {
    String tokenParam = "";
    int tokenKeyIndex = mUrl.indexOf("?token=") >= 0 ? mUrl.indexOf("?token=") : mUrl.indexOf("&token=");
    if (tokenKeyIndex != -1) {
        int nextAndIndex = mUrl.indexOf("&", tokenKeyIndex + 1);
        if (nextAndIndex != -1) {
            tokenParam = mUrl.substring(tokenKeyIndex + 1, nextAndIndex + 1);
        } else {
            tokenParam = mUrl.substring(tokenKeyIndex);
        }
    }
    return tokenParam;
} }

Glide.with(this)
     .load(new MyGlideUrl(url))
     .into(imageView);