Picasso源码梳理

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添加依赖

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implementation "com.squareup.picasso:picasso:2.4.0"

简单使用

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Picasso.with(this)
  .load("http://****.jpg")
  .into(mImageView);

初始化梳理

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public static Picasso with(Context context) {
  if (singleton == null) {
    synchronized (Picasso.class) {
      if (singleton == null) {
        singleton = new Builder(context).build();
      }
    }
  }
  return singleton;
}

  public Picasso build() {
    Context context = this.context;

    //初始化图片下载器
    if (downloader == null) {
      downloader = Utils.createDefaultDownloader(context);
    }
    //初始化Lru缓存 使用LinkedHashMap实现
    if (cache == null) {
      cache = new LruCache(context);
    }
    //初始化线程池
    if (service == null) {
      service = new PicassoExecutorService();
    }
    if (transformer == null) {
      transformer = RequestTransformer.IDENTITY;
    }

    Stats stats = new Stats(cache);

    //初始化一个分发器
    Dispatcher dispatcher = new Dispatcher(context, service, HANDLER, downloader, cache, stats);

    return new Picasso(context, dispatcher, cache, listener, transformer,
        requestHandlers, stats, indicatorsEnabled, loggingEnabled);
  }
}

Load流程梳理

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public RequestCreator load(String path) {
  if (path == null) {
    return new RequestCreator(this, null, 0);
  }
  if (path.trim().length() == 0) {
    throw new IllegalArgumentException("Path must not be empty.");
  }
  //将String转化为Uri对象
  return load(Uri.parse(path));
}

//返回一个请求创造对象
public RequestCreator load(Uri uri) {
  return new RequestCreator(this, uri, 0);
}

init流程梳理

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public void into(ImageView target, Callback callback) {
  long started = System.nanoTime();
  //在主线程调用直接拋出异常
  checkMain();
  
  //判断View是否存在
  if (target == null) {
    throw new IllegalArgumentException("Target must not be null.");
  }

  //判断请求对象
  if (!data.hasImage()) {
    picasso.cancelRequest(target);
    if (setPlaceholder) {
      setPlaceholder(target, getPlaceholderDrawable());
    }
    return;
  }

  if (deferred) {
    if (data.hasSize()) {
      throw new IllegalStateException("Fit cannot be used with resize.");
    }
    int width = target.getWidth();
    int height = target.getHeight();
    if (width == 0 || height == 0) {
      if (setPlaceholder) {
        setPlaceholder(target, getPlaceholderDrawable());
      }
      picasso.defer(target, new DeferredRequestCreator(this, target, callback));
      return;
    }
    data.resize(width, height);
  }

  //生成请求对象与Key作为唯一标识
  Request request = createRequest(started);
  String requestKey = createKey(request);

  //从内存Cache中找一遍
  if (!skipMemoryCache) {
    Bitmap bitmap = picasso.quickMemoryCacheCheck(requestKey);
    if (bitmap != null) {
      picasso.cancelRequest(target);
      setBitmap(target, picasso.context, bitmap, MEMORY, noFade, picasso.indicatorsEnabled);
      if (picasso.loggingEnabled) {
        log(OWNER_MAIN, VERB_COMPLETED, request.plainId(), "from " + MEMORY);
      }
      if (callback != null) {
        callback.onSuccess();
      }
      return;
    }
  }

  //是否设置了占位图片
  if (setPlaceholder) {
    setPlaceholder(target, getPlaceholderDrawable());
  }

  //新建一个请求Action 
  Action action =
      new ImageViewAction(picasso, target, request, skipMemoryCache, noFade, errorResId,
          errorDrawable, requestKey, tag, callback);

  picasso.enqueueAndSubmit(action);
}

void enqueueAndSubmit(Action action) {
  Object target = action.getTarget();
  if (target != null && targetToAction.get(target) != action) {
    // This will also check we are on the main thread.
    cancelExistingRequest(target);
    targetToAction.put(target, action);
  }
  submit(action);
}

多类型资源处理流程

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//有下面这些资源的处理能力 他们回依次执行
 List<RequestHandler> allRequestHandlers =
      new ArrayList<RequestHandler>(builtInHandlers + extraCount);

  // ResourceRequestHandler needs to be the first in the list to avoid
  // forcing other RequestHandlers to perform null checks on request.uri
  // to cover the (request.resourceId != 0) case.
  allRequestHandlers.add(new ResourceRequestHandler(context));
  if (extraRequestHandlers != null) {
    //客户端自己实现的RequestHandlers 默认为空
    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);
  
  
  //依次去上边几个RequestHandler中去请求
  for (int i = 0, count = requestHandlers.size(); i < count; i++) {
    RequestHandler requestHandler = requestHandlers.get(i);
    if (requestHandler.canHandleRequest(request)) {
      return new BitmapHunter(picasso, dispatcher, cache, stats, action, requestHandler);
    }
  }
  
  //ResourceRequestHandler 本地资源文件 判断scheme
  @Override 
  public boolean canHandleRequest(Request data) {
    if (data.resourceId != 0) {
      return true;
    }

    return SCHEME_ANDROID_RESOURCE.equals(data.uri.getScheme());
  }
  
  //ContactsPhotoRequestHandler 联系人
  @Override 
  public boolean canHandleRequest(Request data) {
    final Uri uri = data.uri;
    return (SCHEME_CONTENT.equals(uri.getScheme())
        && ContactsContract.Contacts.CONTENT_URI.getHost().equals(uri.getHost())
        && !uri.getPathSegments().contains(ContactsContract.Contacts.Photo.CONTENT_DIRECTORY));
  }
  
  //MediaStoreRequestHandler 媒体库
  @Override 
  public boolean canHandleRequest(Request data) {
    final Uri uri = data.uri;
    return (SCHEME_CONTENT.equals(uri.getScheme())
            && MediaStore.AUTHORITY.equals(uri.getAuthority()));
  }
  
  //ContentStreamRequestHandler Content
  @Override 
  public boolean canHandleRequest(Request data) {
    return SCHEME_CONTENT.equals(data.uri.getScheme());
  }
  
  //AssetRequestHandler assets中资源
  @Override 
  public boolean canHandleRequest(Request data) {
    Uri uri = data.uri;
    return (SCHEME_FILE.equals(uri.getScheme())
        && !uri.getPathSegments().isEmpty() && ANDROID_ASSET.equals(uri.getPathSegments().get(0)));
  }
  
  //FileRequestHandler 文件资源
  @Override 
  public boolean canHandleRequest(Request data) {
    return SCHEME_FILE.equals(data.uri.getScheme());
  }
  
  //NetworkRequestHandler 最后一个网络资源
  @Override 
  public boolean canHandleRequest(Request data) {
    String scheme = data.uri.getScheme();
    return (SCHEME_HTTP.equals(scheme) || SCHEME_HTTPS.equals(scheme));
  }

网络请求流程梳理

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@Override 
public Result load(Request data) throws IOException {
  //加入到下载器中 支持Okhttp和UrlConnection 有OKhttp优先使用Okhttp
  //Picsso没有自己实现硬盘缓存 他的硬盘缓存通过Http的头文件来控制的
  //extraRequestHandlers 可以自己实现
  Response response = downloader.load(data.uri, data.loadFromLocalCacheOnly);
  if (response == null) {
    return null;
  }

  Picasso.LoadedFrom loadedFrom = response.cached ? DISK : NETWORK;

  Bitmap bitmap = response.getBitmap();
  if (bitmap != null) {
    return new Result(bitmap, loadedFrom);
  }

  InputStream is = response.getInputStream();
  if (is == null) {
    return null;
  }
  // Sometimes response content length is zero when requests are being replayed. Haven't found
  // root cause to this but retrying the request seems safe to do so.
  if (response.getContentLength() == 0) {
    Utils.closeQuietly(is);
    throw new IOException("Received response with 0 content-length header.");
  }
  if (loadedFrom == NETWORK && response.getContentLength() > 0) {
    stats.dispatchDownloadFinished(response.getContentLength());
  }
  try {
    return new Result(decodeStream(is, data), loadedFrom);
  } finally {
    Utils.closeQuietly(is);
  }
}

//OkHttp网络请求流程
@Override 
public Response load(Uri uri, boolean localCacheOnly) throws IOException {
  HttpURLConnection connection = openConnection(uri);
  connection.setUseCaches(true);
  //网络层的缓存
  if (localCacheOnly) {
    connection.setRequestProperty("Cache-Control", "only-if-cached,max-age=" + Integer.MAX_VALUE);
  }

  int responseCode = connection.getResponseCode();
  if (responseCode >= 300) {
    connection.disconnect();
    throw new ResponseException(responseCode + " " + connection.getResponseMessage());
  }

  String responseSource = connection.getHeaderField(RESPONSE_SOURCE_OKHTTP);
  if (responseSource == null) {
    responseSource = connection.getHeaderField(RESPONSE_SOURCE_ANDROID);
  }

  long contentLength = connection.getHeaderFieldInt("Content-Length", -1);
  boolean fromCache = parseResponseSourceHeader(responseSource);

  return new Response(connection.getInputStream(), fromCache, contentLength);
}