綜述

Glide支持Gif加載，且不需要使用自定義的ImageView，直接使用系統(tǒng)的ImageView即可，接入成本很低。在做Gif這個功能的時候，涉及到以下幾個功能點：
- 帶下載進度回調(diào)的圖片下載
- 判斷圖片是否來自緩存
- Gif加載卡頓的問題
- GifWifif自動播放邏輯，以及控制單頁面只有一個Gif播放的邏輯

帶下載進度回調(diào)的圖片下載

Glide底層網(wǎng)絡(luò)庫可選擇使用okhttp，基于Intercepor撰寫ProgressIntercepor：

public class ProgressInterceptor implements Interceptor {
  private DownloadProgressListener progressListener;

  public ProgressInterceptor(DownloadProgressListener progressListener){
    this.progressListener = progressListener;
  }

  @Override
  public Response intercept(Chain chain) throws IOException {
    Response originalResponse = chain.proceed(chain.request());
    return originalResponse.newBuilder()
        .body(new DownloadProgressResponseBody(originalResponse.body(), progressListener))
        .build();
  }

  private static class DownloadProgressResponseBody extends ResponseBody {

    private final ResponseBody responseBody;
    private final DownloadProgressListener progressListener;
    private BufferedSource bufferedSource;

    public DownloadProgressResponseBody(ResponseBody responseBody,
                                        DownloadProgressListener progressListener) {
      this.responseBody = responseBody;
      this.progressListener = progressListener;
    }

    @Override public MediaType contentType() {
      return responseBody.contentType();
    }

    @Override public long contentLength(){
      return responseBody.contentLength();
    }

    @Override public BufferedSource source(){
      if (bufferedSource == null) {
        bufferedSource = Okio.buffer(source(responseBody.source()));
      }
      return bufferedSource;
    }

    private Source source(Source source) {
      return new ForwardingSource(source) {
        long totalBytesRead = 0L;

        @Override public long read(Buffer sink, long byteCount) throws IOException {
          long bytesRead = super.read(sink, byteCount);
          // read() returns the number of bytes read, or -1 if this source is exhausted.
          totalBytesRead += bytesRead != -1 ? bytesRead : 0;

          if (null != progressListener) {
            progressListener.update(totalBytesRead, responseBody.contentLength(), bytesRead == -1);
          }
          return bytesRead;
        }
      };
    }
  }

  public interface DownloadProgressListener {
    void update(long bytesRead, long contentLength, boolean done);
  }
}

實現(xiàn)Glide的DataFetcher接口，ProgressDataFetcher實現(xiàn)網(wǎng)絡(luò)數(shù)據(jù)拉取的具體實現(xiàn)：

public class ProgressDataFetcher implements DataFetcher<InputStream> {
  private String url;
  private ProgressInterceptor.DownloadProgressListener listener;
  private Call progressCall;
  private InputStream stream;
  private boolean isCancelled;


  public ProgressDataFetcher(String url, ProgressInterceptor.DownloadProgressListener listener){
    this.url = url;
    this.listener = listener;
  }

  @Override
  public InputStream loadData(Priority priority) throws Exception {
    Request request = new Request.Builder().url(url).build();
    OkHttpClient client = new OkHttpClient.Builder()
        .addInterceptor(new ProgressInterceptor(listener))
        .build();

    try {
      progressCall = client.newCall(request);
      Response response = progressCall.execute();
      if (isCancelled) {
        return null;
      }
      if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);
      stream = response.body().byteStream();
    } catch (IOException e) {
      e.printStackTrace();
      return null;
    }
    return stream;
  }

  @Override
  public void cleanup() {
    if (stream != null) {
      try {
        stream.close();
        stream = null;
      } catch (IOException e) {
        stream = null;
      }
    }
    if (progressCall != null) {
      progressCall.cancel();
    }
  }

  @Override
  public String getId() {
    return url;
  }

  @Override
  public void cancel() {
    isCancelled = true;
  }
}

將ProgressDataFetcher封裝為ProgressModelLoader：

public class ProgressModelLoader implements StreamModelLoader<String> {
  private ProgressInterceptor.DownloadProgressListener listener;

  public ProgressModelLoader(ProgressInterceptor.DownloadProgressListener listener){
    this.listener = listener;
  }

  @Override
  public DataFetcher<InputStream> getResourceFetcher(String model, int width, int height) {
    return new ProgressDataFetcher(model, listener);
  }
}

業(yè)務層調(diào)用方法：

/**
 * Gilde 帶進度回調(diào)的圖片加載
 * Note: Gif 圖片加載，disk cache必須是SOURCE/NONE；否則Gif有卡頓
 * 
 * @param imageView
 * @param url
 * @param drawable
 * @param roundDp 可設(shè)置圓角大小
 * @param listener 可為null
 */
public static Target<GlideDrawable> bindGifWithProgress(ImageView imageView, String url,
    Drawable drawable, int roundDp,
    ProgressInterceptor.DownloadProgressListener listener) {
  RequestManager requestManager = Glide.with(imageView.getContext().getApplicationContext());
  DrawableTypeRequest request = null;
  if (listener != null) {
    request = requestManager.using(new ProgressModelLoader(listener)).load(url);
  } else {
    request = requestManager.load(url);
  }
  DrawableRequestBuilder builder = request.dontAnimate();

  if (drawable != null)
    builder.placeholder(drawable);

  if (roundDp != 0)
    builder.transform(
        new GlideRoundTransform(imageView.getContext().getApplicationContext(), roundDp));

  builder.diskCacheStrategy(DiskCacheStrategy.SOURCE);

  return builder.into(imageView);
}

判斷圖片是否來自緩存

業(yè)務需要判斷Gif圖片是否已經(jīng)在本地緩存，如果未緩存，加載靜態(tài)占位圖，從網(wǎng)絡(luò)加載顯示Gif字樣的圓形進度條；如果已緩存，加載Gif，判斷是否播放。

因為Glide的緩存邏輯是會緩存原圖，以及根據(jù)ImageView大小裁剪之后的圖片；所以沒有辦法僅根據(jù)一個圖片URL判斷緩存是否存在。這里想了一個折中的方法：先調(diào)用一次禁用網(wǎng)絡(luò)加載的Glide圖片加載，從onResoureReady和onException判斷圖片是否被緩存命中。

這里還是對Glide的單次加載實現(xiàn)一個NetworkDisablingLoader，來實現(xiàn)這個僅從本地加載的功能：

public class NetworkDisablingLoader implements StreamModelLoader<String> {
  @Override public DataFetcher<InputStream> getResourceFetcher(final String model, int width, int height) {
    return new DataFetcher<InputStream>() {
      @Override public InputStream loadData(Priority priority) throws Exception {
        throw new IOException("Forced Glide network failure");
      }
      @Override public void cleanup() { }
      @Override public String getId() { return model; }
      @Override public void cancel() { }
    };
  }
}

業(yè)務層調(diào)用方法：

/**
 * Glide 僅從本地加載圖片
 * 
 * @param imageView
 * @param url
 * @param defaultImage 不需要，填 -1
 * @param roundDp 可設(shè)置圓角大小
 * @param listener 可為null
 */
public static Target<GlideDrawable> bindWithoutNet(ImageView imageView, String url,
    int defaultImage, int roundDp,
    RequestListener listener) {
  RequestManager requestManager = Glide.with(imageView.getContext().getApplicationContext());
  DrawableTypeRequest request = requestManager.using(new NetworkDisablingLoader()).load(url);
  DrawableRequestBuilder builder = request.dontAnimate();
  if (roundDp != 0) {
    builder.transform(
        new GlideRoundTransform(imageView.getContext().getApplicationContext(), roundDp));
  }
  if (defaultImage != -1) {
    builder.placeholder(defaultImage);
  }
  if (listener != null) {
    builder.listener(listener);
  }
  builder.diskCacheStrategy(DiskCacheStrategy.SOURCE);

  return builder.into(imageView);

}

Gif加載卡頓的問題

Gilde在緩存Gif資源的時候，可以有兩種模式：SOURCE和RESULT?？D的原因是RESULT類型的緩存造成的。（Glide為了節(jié)省空間，會對原始緩存做壓縮生成RESULT；對于Gif類型而言，這種壓縮算法顯示效率有問題，從RESULT到原始Gif的轉(zhuǎn)化會很慢）。這里的解決方法是將緩存的類型設(shè)置為ALL/SOURCE。（在ALL模式下，應該優(yōu)先那SOURCE緩存使用，所有也不會有問題）。

Gif播放控制邏輯

為什么做Gif的播放控制邏輯？Gif的播放非常消耗資源，我們應該控制單個頁面正在播放的gif個數(shù)，這里限定為一個。此外，還可以在此基礎(chǔ)上實現(xiàn)Gif Wifi下自動播放的邏輯。（當頁面滾動時自動播放下一個）。
這個功能主要是根據(jù)RecycleView的LayoutManager實現(xiàn)的，主要涉及到如下方法：
- linearLayoutManager.findFirstVisibleItemPosition()
- linearLayoutManager.findLastVisibleItemPosition()
- linearLayoutManager.findViewByPosition
- recyclerView.getChildViewHolder(itemView)
- linearLayoutManager.findFirstCompletelyVisibleItemPosition()
- linearLayoutManager.findLastCompletelyVisibleItemPosition()

具體的業(yè)務流程：
- 通過LinearLayoutManager獲取可見范圍內(nèi)item的范圍
- 在可見的item范圍內(nèi)，找到正在播放Gif的item是否滿足播放條件，如果滿足，直接跳出流程
- 當正在播放Gif的item不滿足播放條件，先對所有可見item執(zhí)行暫停Gif播放的功能。
- 然后在所有已經(jīng)暫停的Gif item中找到第一個滿足播放條件的Gif item

具體代碼如下：

public class GifFeedHelper {
  public static final String GIF_TAB_ID = "10283";

  private LinearLayoutManager linearLayoutManager;
  private RecyclerView recyclerView;

  public static WeakReference<GifFeedHelper> gifFeedHelper;


  public GifFeedHelper(LinearLayoutManager linearLayoutManager, RecyclerView recyclerView){
    this.linearLayoutManager = linearLayoutManager;
    this.recyclerView = recyclerView;
    gifFeedHelper = new WeakReference<GifFeedHelper>(this);
  }

  /**
   * 首頁Gif Feed Tab
   * Gif 調(diào)度播放邏輯
   */
  public void dispatchGifPlay() {

    int start_index = linearLayoutManager.findFirstVisibleItemPosition();
    int end_index = linearLayoutManager.findLastVisibleItemPosition();

    if (findCurrentPlayGifCardSatisfy(start_index, end_index))
      return;

    stopAllGifCardOnScreen(start_index, end_index);

    actionFirstGifCardOnScreen(start_index, end_index);
  }

  /**
   * 外部可強制停止頁面內(nèi)Gif Card 的播放
   * Note：HPGifNormalCardPresenter中的點擊事件，需要暫停其他的Gif Card
   * */
  public void forceStopAllGifs() {
    int start_index = linearLayoutManager.findFirstVisibleItemPosition();
    int end_index = linearLayoutManager.findLastVisibleItemPosition();

    stopAllGifCardOnScreen(start_index, end_index);
  }

  private boolean findCurrentPlayGifCardSatisfy(int start_index,int end_index) {

    for(int i= start_index; i<=end_index; i++){
      View itemView = linearLayoutManager.findViewByPosition(i);
      if (null == itemView)
        continue;
      RippleViewHolder holder = (RippleViewHolder) recyclerView.getChildViewHolder(itemView);
      CardPresenter presenter = holder.presenter;
      if(presenter == null)
        continue;

      BasePresenter basePresenter = presenter.get(0); //默認是0
      if (basePresenter != null && basePresenter instanceof HPGifNormalCardPresenter) {
        HPGifNormalCardPresenter gifNormalCardPresenter = (HPGifNormalCardPresenter) basePresenter;
        int fullHeight = itemView.getHeight();
        if(fullHeight <=0)
          continue;
        Rect rect = new Rect();
        boolean visible = itemView.getGlobalVisibleRect(rect);
        int visibleHeight = rect.height();
        if(gifNormalCardPresenter.isGifActive() &&
            visible && ((1.0f * visibleHeight / fullHeight) >= 0.3f)) {
          //Fix: 找到滿足條件的第一個Gif，之后的Gif強制停止播放
          if (i<end_index)
            stopAllGifCardOnScreen(i+1,end_index);
          return true;
        }
      }
    }

    return false;
  }

  /**
   * 暫停屏幕中所有可見Gif Card的播放
   * */
  private void stopAllGifCardOnScreen(int start_index, int end_index) {


    for (int i = start_index; i <= end_index; i++) {
      View itemView = linearLayoutManager.findViewByPosition(i);
      if (null == itemView)
        continue;
      RippleViewHolder holder = (RippleViewHolder) recyclerView.getChildViewHolder(itemView);
      CardPresenter presenter = holder.presenter;
      if (presenter == null)
        return;
      BasePresenter basePresenter = presenter.get(0); //默認是0
      if (basePresenter != null && basePresenter instanceof HPGifNormalCardPresenter) {
        HPGifNormalCardPresenter gifNormalCardPresenter = (HPGifNormalCardPresenter) basePresenter;
        gifNormalCardPresenter.onStop();
      }
    }
  }

  /**
   * 在所有已經(jīng)暫停的Gif Card中找到第一個滿足播放條件的Gif Card
   * */
  private void actionFirstGifCardOnScreen(int start_index, int end_index) {
    View activeGifView = null;
    for (int i = start_index; i <= end_index; i++) {
      View view = linearLayoutManager.findViewByPosition(i);
      if (null == view)
        continue;

      int fullHeight = view.getHeight();
      if (fullHeight <= 0) {
        continue;
      }

      Rect rect = new Rect();
      boolean visible = view.getGlobalVisibleRect(rect);
      int visibleHeight = rect.height();

      RippleViewHolder holder = (RippleViewHolder) recyclerView.getChildViewHolder(view);
      CardPresenter presenter = holder.presenter;
      if (presenter == null)
        continue;
      BasePresenter basePresenter = presenter.get(0); //默認是0
      if (basePresenter == null)
        continue;
      if (visible && ((1.0f * visibleHeight / fullHeight) >= 0.3f)
          && (basePresenter instanceof HPGifNormalCardPresenter)) {
        activeGifView = view;
        break;
      }
    }

    if (activeGifView != null) {
      RippleViewHolder holder = (RippleViewHolder) recyclerView.getChildViewHolder(activeGifView);
      CardPresenter presenter = holder.presenter;
      BasePresenter basePresenter = presenter.get(0); //默認是0

      if (basePresenter instanceof HPGifNormalCardPresenter) {
        HPGifNormalCardPresenter gifNormalCardPresenter = (HPGifNormalCardPresenter) basePresenter;
        if (gifNormalCardPresenter.canAutoPlay()) {
          gifNormalCardPresenter.performAutoPlay();
        } else {
          gifNormalCardPresenter.onStart();
        }
      }
    }
  }
}