// retrofit, 基于Okhttp，考慮到項目中經常會用到retrofit，就導入這個了。  compile 'com.squareup.retrofit2:retrofit:2.1.0'// ButterKnife  compile 'com.jakewharton:butterknife:7.0.1'// rxjava 本例中線程切換要用到，代替handler  compile 'io.reactivex:rxjava:1.1.6'  compile 'io.reactivex:rxandroid:1.2.1'

// 參考okhttp的官方demo，此類當中我們主要把注意力放在ProgressListener和read方法中。在這里獲取文件總長我寫在了構造方法里，這樣免得在source的read方法中重復調用或判斷。讀者也可以根據個人需要定制自己的監聽器。public class ProgressResponseBody extends ResponseBody {  public interface ProgressListener {    void onPreExecute(long contentLength);    void update(long totalBytes, boolean done);  }  private final ResponseBody responseBody;  private final ProgressListener progressListener;  private BufferedSource bufferedSource;  public ProgressResponseBody(ResponseBody responseBody,                ProgressListener progressListener) {    this.responseBody = responseBody;    this.progressListener = progressListener;    if(progressListener!=null){      progressListener.onPreExecute(contentLength());    }  }  @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 totalBytes = 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.        totalBytes += bytesRead != -1 ? bytesRead : 0;        if (null != progressListener) {          progressListener.update(totalBytes, bytesRead == -1);        }        return bytesRead;      }    };  }}

//帶進度監聽功能的輔助類public class ProgressDownloader {  public static final String TAG = "ProgressDownloader";  private ProgressListener progressListener;  private String url;  private OkHttpClient client;  private File destination;  private Call call;  public ProgressDownloader(String url, File destination, ProgressListener progressListener) {    this.url = url;    this.destination = destination;    this.progressListener = progressListener;    //在下載、暫停后的繼續下載中可復用同一個client對象    client = getProgressClient();  }  //每次下載需要新建新的Call對象  private Call newCall(long startPoints) {    Request request = new Request.Builder()        .url(url)        .header("RANGE", "bytes=" + startPoints + "-")//斷點續傳要用到的，指示下載的區間        .build();    return client.newCall(request);  }  public OkHttpClient getProgressClient() {  // 攔截器，用上ProgressResponseBody    Interceptor interceptor = new Interceptor() {      @Override      public Response intercept(Chain chain) throws IOException {        Response originalResponse = chain.proceed(chain.request());        return originalResponse.newBuilder()            .body(new ProgressResponseBody(originalResponse.body(), progressListener))            .build();      }    };    return new OkHttpClient.Builder()        .addNetworkInterceptor(interceptor)        .build();  }// startsPoint指定開始下載的點  public void download(final long startsPoint) {    call = newCall(startsPoint);    call.enqueue(new Callback() {          @Override          public void onFailure(Call call, IOException e) {          }          @Override          public void onResponse(Call call, Response response) throws IOException {            save(response, startsPoint);          }        });  }  public void pause() {    if(call!=null){      call.cancel();    }  }  private void save(Response response, long startsPoint) {    ResponseBody body = response.body();    InputStream in = body.byteStream();    FileChannel channelOut = null;    // 隨機訪問文件，可以指定斷點續傳的起始位置    RandomAccessFile randomAccessFile = null;    try {      randomAccessFile = new RandomAccessFile(destination, "rwd");      //Chanel NIO中的用法，由于RandomAccessFile沒有使用緩存策略，直接使用會使得下載速度變慢，親測緩存下載3.3秒的文件，用普通的RandomAccessFile需要20多秒。      channelOut = randomAccessFile.getChannel();      // 內存映射，直接使用RandomAccessFile，是用其seek方法指定下載的起始位置，使用緩存下載，在這里指定下載位置。      MappedByteBuffer mappedBuffer = channelOut.map(FileChannel.MapMode.READ_WRITE, startsPoint, body.contentLength());      byte[] buffer = new byte[1024];      int len;      while ((len = in.read(buffer)) != -1) {        mappedBuffer.put(buffer, 0, len);      }    } catch (IOException e) {      e.printStackTrace();    }finally {      try {        in.close();        if (channelOut != null) {          channelOut.close();        }        if (randomAccessFile != null) {          randomAccessFile.close();        }      } catch (IOException e) {        e.printStackTrace();      }    }  }}

<uses-permission android:name="android.permission.INTERNET"/><uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

public class MainActivity extends AppCompatActivity implements ProgressResponseBody.ProgressListener {  public static final String TAG = "MainActivity";  public static final String PACKAGE_URL = "http://gdown.baidu.com/data/wisegame/df65a597122796a4/weixin_821.apk";  @Bind(R.id.progressBar)  ProgressBar progressBar;  private long breakPoints;  private ProgressDownloader downloader;  private File file;  private long totalBytes;  private long contentLength;  @Override  protected void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    setContentView(R.layout.activity_main);    ButterKnife.bind(this);  }  @OnClick({R.id.downloadButton, R.id.cancel_button, R.id.continue_button})  public void onClick(View view) {    switch (view.getId()) {      case R.id.downloadButton:      // 新下載前清空斷點信息        breakPoints = 0L;        file = new File(Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS), "sample.apk");        downloader = new ProgressDownloader(PACKAGE_URL, file, this);        downloader.download(0L);        break;      case R.id.pause_button:        downloader.pause();        Toast.makeText(this, "下載暫停", Toast.LENGTH_SHORT).show();        // 存儲此時的totalBytes，即斷點位置。        breakPoints = totalBytes;        break;      case R.id.continue_button:        downloader.download(breakPoints);        break;    }  }  @Override  public void onPreExecute(long contentLength) {    // 文件總長只需記錄一次，要注意斷點續傳后的contentLength只是剩余部分的長度    if (this.contentLength == 0L) {      this.contentLength = contentLength;      progressBar.setMax((int) (contentLength / 1024));    }  }  @Override  public void update(long totalBytes, boolean done) {    // 注意加上斷點的長度    this.totalBytes = totalBytes + breakPoints;    progressBar.setProgress((int) (totalBytes + breakPoints) / 1024);    if (done) {    // 切換到主線程      Observable          .empty()          .observeOn(AndroidSchedulers.mainThread())          .doOnCompleted(new Action0() {            @Override            public void call() {              Toast.makeText(MainActivity.this, "下載完成", Toast.LENGTH_SHORT).show();            }          })          .subscribe();    }  }}