三、下載速度計算

okhttp可以通過下載速度來評估網絡帶寬并優化下載的效率。以下是代碼示例：

public class SpeedMonitorRequestBody extends RequestBody { 
  private RequestBody requestBody; 
  private ProgressListener listener; 

  public SpeedMonitorRequestBody(RequestBody requestBody, ProgressListener listener) { 
      this.requestBody = requestBody; 
      this.listener = listener; 
  } 

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

  @Override 
  public long contentLength() throws IOException { 
      return requestBody.contentLength(); 
  } 

  @Override 
  public void writeTo(BufferedSink sink) throws IOException { 
      long startTime = System.currentTimeMillis(); 
      long bytesWritten = 0; 
      long writeTimeout = 0; 
      long total = contentLength(); 
      long lastScreenTimestamp = 0;
      int screenSpan = 1000;//1秒

      Buffer buffer = new Buffer(); 
      requestBody.writeTo(buffer); 
      BufferedSource source = buffer.clone().inputStream().source(); 
      long readCount = 0; 
      long lastReadCount = 0; 
      long lastUpdateTime = 0; 

      while ((readCount = source.read(sink.buffer(), 2048)) != -1) { 
          sink.flush(); 
          bytesWritten += readCount; 

          long curTimestamp = System.currentTimeMillis(); 
          long costTime = curTimestamp - startTime; 
          long speed = (bytesWritten - lastReadCount) / (curTimestamp - lastUpdateTime + 1) * 1000; 

          if((curTimestamp - lastScreenTimestamp) > screenSpan) 
          {
              lastScreenTimestamp = curTimestamp;

              String downloadSpeed = getSpeedString(speed);
              //可以把下載速度設置到UI上
          }

          lastReadCount = bytesWritten; 
          lastUpdateTime = curTimestamp; 

          // update progress 
          listener.onProgress(bytesWritten, total, bytesWritten == total); 

          if (writeTimeout != 0) { 
              source.timeout().timeout(writeTimeout, TimeUnit.MILLISECONDS); 
          } 
      } 
  } 

  Public String getSpeedString(long speed) 
  { 
      if(speed > 1024 *1024) 
      { 
          float sizeF = (float)speed / (1024.0f*1024.0f); 
          DecimalFormat df = new DecimalFormat("#.00"); 
          String fileSizeString = df.format(sizeF);
          return fileSizeString + " MB/s"; 
      } 
      else if(speed > 1024) 
      { 
          float sizeF = (float)speed / 1024.0f; 
          DecimalFormat df = new DecimalFormat("#.00"); 
          String fileSizeString = df.format(sizeF);
          return fileSizeString + " KB/s"; 
      } 
      else 
      { 
          return Long.toString(speed) + " B/s"; 
      } 
  } 
}

OkHttpClient client = new OkHttpClient(); 
Request request = new Request.Builder() 
    .url(url) 
    .post(new SpeedMonitorRequestBody(RequestBody.create(MediaType.parse("application/octet-stream"), bytes), 
      new ProgressListener() { 
          @Override 
          public void onProgress(long currentBytes, long totalBytes, boolean done) { 
              Log.d(TAG, currentBytes + "/" + totalBytes); 
          } 
      }) 
  ).build(); 

client.newCall(request).enqueue(new Callback() { 
  //... 
});

OkHttpClient client = new OkHttpClient(); 
Request.Builder builder = new Request.Builder().url(url); 
File file = new File("文件路徑"); 
if (file.exists() && file.length() > 0) { 
    builder.addHeader("RANGE", "bytes=" + file.length() + "-"); 
} 
Request request = builder.build(); 

client.newCall(request).enqueue(new Callback() { 
  //... 

  @Override 
  public void onResponse(Call call, Response response) throws IOException { 
      if (response.code() == 206) { 
          //斷點續傳 
      } else { 
          //重新下載 
      } 
  } 
});