Picasso源码阅读笔记六

RequestHandler是Picasso获取图片逻辑的实现类。

RequestHandler.Result

Result类是图片请求Request的处理结果。

public static final class Result {
    private final Picasso.LoadedFrom loadedFrom;//图片的来源：内存、磁盘、网络
    private final Bitmap bitmap;//图片数据，和InputStream只能二者选一
    private final InputStream stream;//图片数据，和Bitmap只能二者选一
  ...
}

自定义RequestHandler

Picasso框架提供了7个RequestHandler和一个BitmapHunter.ERRORING_HANDLER，已经能够很好地满足我们的需求。同时，Picasso也允许我们根据实际需求自定义RequestHandler。 自定义RequestHandler只需要继承RequestHandler并实现两个抽象方法，然后通过Picasso.Builder的addRequestHandler方法添加到Picasso中。

public abstract class RequestHandler {
    ...
    public abstract boolean canHandleRequest(Request data);//判断是否能够处理图片请求
    @Nullable public abstract Result load(Request request, int networkPolicy) throws IOException;//根据图片请求和网络策略获取图片，并返回处理结果Result
    ...
}

RequestHandler还提供了静态方法createBitmapOptions、requiresInSampleSize、calculateInSampleSize方便对图片进行压缩。

RequestHandler责任链

Picasso把默认和自定义的RequestHandler添加到属性requestHandlers中。添加顺序如下：

//extraRequestHandlers是自定义RequestHandler列表。
int extraCount = (extraRequestHandlers != null ? extraRequestHandlers.size() : 0);
List<RequestHandler> allRequestHandlers =
        new ArrayList<RequestHandler>(builtInHandlers + extraCount);
allRequestHandlers.add(new ResourceRequestHandler(context));
if (extraRequestHandlers != null) {
    allRequestHandlers.addAll(extraRequestHandlers);
}
allRequestHandlers.add(new ContactsPhotoRequestHandler(context));
allRequestHandlers.add(new MediaStoreRequestHandler(context));
allRequestHandlers.add(new ContentStreamRequestHandler(context));
allRequestHandlers.add(new AssetRequestHandler(context));
allRequestHandlers.add(new FileRequestHandler(context));
allRequestHandlers.add(new NetworkRequestHandler(dispatcher.downloader, stats));
requestHandlers = Collections.unmodifiableList(allRequestHandlers);

这些RequestHandler是以责任链模式被调用的，请看BitmapHunter的forRequest方法。

static BitmapHunter forRequest(Picasso picasso, Dispatcher dispatcher, Cache cache, Stats stats,
    Action action) {
    Request request = action.getRequest();
    List<RequestHandler> requestHandlers = picasso.getRequestHandlers();

    for (int i = 0, count = requestHandlers.size(); i < count; i++) {
      RequestHandler requestHandler = requestHandlers.get(i);
      if (requestHandler.canHandleRequest(request)) {//从前向后迭代直到找到能处理该Request的RequestHandler。
        return new BitmapHunter(picasso, dispatcher, cache, stats, action, requestHandler);
      }
    }
    //如果没能找到合适的RequestHandler，则交给ERRORING_HANDLER，该Handler的处理方式是直接抛出异常。
    return new BitmapHunter(picasso, dispatcher, cache, stats, action, ERRORING_HANDLER);
}

NetworkRequestHandler

NetworkRequestHandler处理网络图片请求，图片下载逻辑是由Downloader实现的。

class NetworkRequestHandler extends RequestHandler {
  static final int RETRY_COUNT = 2;
 //处理http或者https请求
 @Override public boolean canHandleRequest(Request data) {
    String scheme = data.uri.getScheme();
    return (SCHEME_HTTP.equals(scheme) || SCHEME_HTTPS.equals(scheme));
  }
  //下载失败会重试2次
  @Override int getRetryCount() {
    return RETRY_COUNT;
  }
  //支持失败重试
  @Override boolean supportsReplay() {
    return true;
  }
}