同样地，开始之前先思考1个问题：
- Glide是怎么实现那么多资源Model的加载的？比如可以从Url、Asset、FileDescriptor、Uri、File等来源加载数据
1.ModelLoader
Glide的所有数据加载都实现一个接口ModelLoader,其中Model类型就是来源的类型，Data是加载得到的数据类型，看下接口的具体源码,包含一个内部类LoadData，和两个接口方法，一个buildLoadData方法用来构造返回一个LoadData,另外一个方法handles用来返回这个ModelLoader能否处理这个Model。LoadData这个类里面有三个字段，一个sourceKey用来表示这次下载，一个DataFetcher用来获取不在缓存中的数据：
public interface ModelLoader {
/**
* @param The type of data that well be loaded.
*/
class LoadData {
public final Key sourceKey;
public final List alternateKeys;
public final DataFetcher fetcher;
public LoadData(@NonNull Key sourceKey, @NonNull DataFetcher fetcher) {
this(sourceKey, Collections.emptyList(), fetcher);
}
public LoadData(@NonNull Key sourceKey, @NonNull List alternateKeys,
@NonNull DataFetcher fetcher) {
this.sourceKey = Preconditions.checkNotNull(sourceKey);
this.alternateKeys = Preconditions.checkNotNull(alternateKeys);
this.fetcher = Preconditions.checkNotNull(fetcher);
}
}
@Nullable
LoadData buildLoadData(@NonNull Model model, int width, int height,
@NonNull Options options);
boolean handles(@NonNull Model model);
}
看下Glide里面所有的实现类，这些种类就代表了Glide支持从哪些数据类型下载数据：
比如我们看一个最常用的从http/https uris下载数据的UrlUriLoader, 先看它的接口方法handles，如果scheme类型是http/https类型的就返回true;另外一个接口方法buildLoadData通过字段urlloader返回一个LoadData,还有一个内部工厂类StramFactory, UrlUriLoader不建议外面通过构造函数实例化，而是通过这个工厂类的build方法进行实例化。基本上ModelLoader的实现类都是这种设计模式。
// UrlUriLoader.java
public class UrlUriLoader implements ModelLoader {
private static final Set SCHEMES = Collections.unmodifiableSet(
new HashSet<>(
Arrays.asList(
"http",
"https"
)
)
);
private final ModelLoader urlLoader;
// Public API.
@SuppressWarnings("WeakerAccess")
public UrlUriLoader(ModelLoader urlLoader) {
this.urlLoader = urlLoader;
}
@Override
public LoadData buildLoadData(@NonNull Uri uri, int width, int height,
@NonNull Options options) {
GlideUrl glideUrl = new GlideUrl(uri.toString());
return urlLoader.buildLoadData(glideUrl, width, height, options);
}
@Override
public boolean handles(@NonNull Uri uri) {
return SCHEMES.contains(uri.getScheme());
}
/**
* Loads {@link java.io.InputStream InputStreams} from {@link android.net.Uri Uris} with http
* or https schemes.
*/
public static class StreamFactory implements ModelLoaderFactory {
@NonNull
@Override
public ModelLoader build(MultiModelLoaderFactory multiFactory) {
return new UrlUriLoader<>(multiFactory.build(GlideUrl.class, InputStream.class));
}
@Override
public void teardown() {
// Do nothing.
}
}
}
StreamFactory也是实现一个工厂接口ModelLoaderFactory,具体看下这个接口代码：
// ModelLoaderFactory.java
/**
* An interface for creating a {@link ModelLoader} for a given model type.
*
* @param The type of the model the {@link com.bumptech.glide.load.model.ModelLoader}s built by
* this factory can handle
* @param The type of data the {@link com.bumptech.glide.load.model.ModelLoader}s built by this
* factory can load.
*/
public interface ModelLoaderFactory {
/**
* Build a concrete ModelLoader for this model type.
*
* @param multiFactory A map of classes to factories that can be used to construct additional
* {@link ModelLoader}s that this factory's {@link ModelLoader} may depend on
* @return A new {@link ModelLoader}
*/
@NonNull
ModelLoader build(@NonNull MultiModelLoaderFactory multiFactory);
/**
* A lifecycle method that will be called when this factory is about to replaced.
*/
void teardown();
}
接口比较简单，比如特殊的是还有一个工厂MultiModelLoaderFactory，其实也可以看成一种代理模式，真正去buildLoadData的是通过工厂类StreamFactory构造返回的urlLoader，它能处理的Model类型是GlideUrl,返回的类型是InputStream，而urlloader本身是通过MultiModelLoaderFactory构造：
public ModelLoader build(MultiModelLoaderFactory multiFactory) {
return new UrlUriLoader<>(multiFactory.build(GlideUrl.class, InputStream.class));
}
所以我们总结下下ModelLoader,UrlUriLoader,StreamFactory,ModelLoaderFactory,MultiModelLoaderFactory的关系, 具体的ModelLoader里面有一个代理的ModelLoader,这个代理的ModelLoder由MultiModelLoaderFactory实例化，不管是外层的ModelLoader或者是代理ModelLoader都是由Factory这种方式生成，外层的ModelLoader的Factory在自己的类名下面，而代理ModelLoader统一都由MultiModelLoaderFactory这个工厂生成，类似门面模式。
StreamFactory是在Glide初始化的时候注册到ModelLoaderRegistry中：
//Glide.java
registry.append(URL.class, InputStream.class, new UrlLoader.StreamFactory())
而ModelLoaderRegistry其实只是MultiModelLoaderFactory的外壳，它会把Glide初始化注册的所有ModelFactory保存到MultiModelLoaderFactory:
// ModelLoaderRegistry.java
private final MultiModelLoaderFactory multiModelLoaderFactory;
private final ModelLoaderCache cache = new ModelLoaderCache();
public synchronized void append(
@NonNull Class modelClass,
@NonNull Class dataClass,
@NonNull ModelLoaderFactory factory) {
multiModelLoaderFactory.append(modelClass, dataClass, factory);
cache.clear();
}
按照上面的逻辑，再看一个具体的ModelLoader实现类FileLoader implements ModelLoader, 实现从File加载Data,这里Data有两个类型java.io.InputStream和java.io.FileDescriptor,看下具体实现。实际工作会通过内部实例变量FileOpener完成，在FileFetcher loadData中通过FileOpener打开文件，返回需要的数据类型，所以关键就在这个FileOpener，在这里是对文件打开关闭的一个封装接口，根据具体的返回类型构造不同的FileOpener.
// FileLoader.java
public class FileLoader implements ModelLoader {
private static final String TAG = "FileLoader";
private final FileOpener fileOpener;
// Public API.
@SuppressWarnings("WeakerAccess")
public FileLoader(FileOpener fileOpener) {
this.fileOpener = fileOpener;
}
@Override
public LoadData buildLoadData(@NonNull File model, int width, int height,
@NonNull Options options) {
return new LoadData<>(new ObjectKey(model), new FileFetcher<>(model, fileOpener));
}
@Override
public boolean handles(@NonNull File model) {
return true;
}
/**
* Allows opening a specific type of data from a {@link java.io.File}.
* @param The type of data that can be opened.
*/
public interface FileOpener {
Data open(File file) throws FileNotFoundException;
void close(Data data) throws IOException;
Class getDataClass();
}
private static final class FileFetcher implements DataFetcher {
private final File file;
private final FileOpener opener;
private Data data;
FileFetcher(File file, FileOpener opener) {
this.file = file;
this.opener = opener;
}
@Override
public void loadData(@NonNull Priority priority, @NonNull DataCallback callback) {
try {
data = opener.open(file);
} catch (FileNotFoundException e) {
if (Log.isLoggable(TAG, Log.DEBUG)) {
Log.d(TAG, "Failed to open file", e);
}
callback.onLoadFailed(e);
return;
}
callback.onDataReady(data);
}
@Override
public void cleanup() {
if (data != null) {
try {
opener.close(data);
} catch (IOException e) {
// Ignored.
}
}
}
@Override
public void cancel() {
// Do nothing.
}
@NonNull
@Override
public Class getDataClass() {
return opener.getDataClass();
}
@NonNull
@Override
public DataSource getDataSource() {
return DataSource.LOCAL;
}
}
/**
* Base factory for loading data from {@link java.io.File files}.
* @param The type of data that will be loaded for a given {@link java.io.File}.
*/
public static class Factory implements ModelLoaderFactory {
private final FileOpener opener;
public Factory(FileOpener opener) {
this.opener = opener;
}
@NonNull
@Override
public final ModelLoader build(@NonNull MultiModelLoaderFactory multiFactory) {
return new FileLoader<>(opener);
}
@Override
public final void teardown() {
// Do nothing.
}
}
在FileLoader内部有两个静态内部类StreamFactory和FileDescriptorFactory分别返回两个数据类型java.io.InputStream和java.io.FileDescriptor。分别构造两种类型的FileOpener,一个返回InputStream类型，另外一个返回ParcelFileDescriptor.
/**
* Factory for loading {@link InputStream}s from {@link File}s.
*/
public static class StreamFactory extends Factory {
public StreamFactory() {
super(new FileOpener() {
@Override
public InputStream open(File file) throws FileNotFoundException {
return new FileInputStream(file);
}
@Override
public void close(InputStream inputStream) throws IOException {
inputStream.close();
}
@Override
public Class getDataClass() {
return InputStream.class;
}
});
}
}
/**
* Factory for loading {@link ParcelFileDescriptor}s from {@link File}s.
*/
public static class FileDescriptorFactory extends Factory {
public FileDescriptorFactory() {
super(new FileOpener() {
@Override
public ParcelFileDescriptor open(File file) throws FileNotFoundException {
return ParcelFileDescriptor.open(file, ParcelFileDescriptor.MODE_READ_ONLY);
}
@Override
public void close(ParcelFileDescriptor parcelFileDescriptor) throws IOException {
parcelFileDescriptor.close();
}
@Override
public Class getDataClass() {
return ParcelFileDescriptor.class;
}
});
}
}
上面两个Factory也是Glide初始化的时候注册到ModelLoaderRegistry中：
.append(File.class, InputStream.class, new FileLoader.StreamFactory())
.append(File.class, ParcelFileDescriptor.class, new FileLoader.FileDescriptorFactory())
2. DataFetcher
从前面ModelLoader中有提到过DataFetcher这个类，这是真正做数据拉取的功能接口，基本上对应不同的ModelLoader会有对应的DataFetcher去实现拉取数据的工作，看下DataFetcher这个类的接口定义。里面有个内部接口DataCallback，在数据拉取成功后会通过回调接口返回。loadData就是真正发起数据请求的地方，getDataSource返回数据来源，这是个枚举。
// DataSource.java
public interface DataFetcher {
/**
* Callback that must be called when data has been loaded and is available, or when the load
* fails.
*
* @param The type of data that will be loaded.
*/
interface DataCallback {
/**
* Called with the loaded data if the load succeeded, or with {@code null} if the load failed.
*/
void onDataReady(@Nullable T data);
/**
* Called when the load fails.
*
* @param e a non-null {@link Exception} indicating why the load failed.
*/
void onLoadFailed(@NonNull Exception e);
}
void loadData(@NonNull Priority priority, @NonNull DataCallback callback);
void cleanup();
void cancel();
/**
* Returns the class of the data this fetcher will attempt to obtain.
*/
@NonNull
Class getDataClass();
/**
* Returns the {@link com.bumptech.glide.load.DataSource} this fetcher will return data from.
*/
@NonNull
DataSource getDataSource();
}
看下在Glide中DataFetcher的继承结构，实现类还是很多，可以从AssetPath/Url/File/Uri等地方拉取数据。
看下比较常用的HttpUrlFetcher，实现从Url加载返回一个InputStream:
/**
* A DataFetcher that retrieves an {@link java.io.InputStream} for a Url.
*/
public class HttpUrlFetcher implements DataFetcher
在loadData方法中,通过loadDataWithRedirects加载url返回result,如果成功就通过onDataReady返回数据：
// HttpUrlFetcher.java
InputStream result = loadDataWithRedirects(glideUrl.toURL(), 0, null, glideUrl.getHeaders());
callback.onDataReady(result);
在loadDataWithRedirects中通过HttpURLConnection发起网络连接：
// HttpUrlFetcher.java
urlConnection = connectionFactory.build(url);
for (Map.Entry headerEntry : headers.entrySet()) {
urlConnection.addRequestProperty(headerEntry.getKey(),headerEntry.getValue());
}
urlConnection.setConnectTimeout(timeout);
urlConnection.setReadTimeout(timeout);
urlConnection.setUseCaches(false);
urlConnection.setDoInput(true);
urlConnection.setInstanceFollowRedirects(false);
// Connect explicitly to avoid errors in decoders if connection fails.
urlConnection.connect();
// Set the stream so that it's closed in cleanup to avoid resource leaks. See #2352.
stream = urlConnection.getInputStream();
看下另外几个接口方法的实现,这里DataSource明显就是REMOTE，返回的数据类型是InputStream.class,在cleanup中就是把sream和connection关闭:
@Override
public void cleanup() {
if (stream != null) {
try {
stream.close();
} catch (IOException e) {
// Ignore
}
}
if (urlConnection != null) {
urlConnection.disconnect();
}
urlConnection = null;
}
@Override
public void cancel() {
isCancelled = true;
}
@NonNull
@Override
public Class getDataClass() {
return InputStream.class;
}
@NonNull
@Override
public DataSource getDataSource() {
return DataSource.REMOTE;
}
3.总结
再回头看前面提的问题，Glide提供很多ModelLoader接口的实现类来实现不同资源的数据加载，每个ModelLoader不是直接通过构造函数实例化，而是每个Loader内部提供的工厂类进行实例化，在Glide初始化的时候，会把这些工厂类都注册到ModelLoaderRegistry中，构造的时候从里面取出对应的工厂实例化ModelLoader。ModelLoader下载数据会通过DataFetcher，DataFetcher的实现类基本和ModelLoader对应。