Android入门教程 | OkHttp + Retrofit 下载文件与进度监听 android

下载文件是一个比较常见的需求。给定一个url，我们可以使用URLConnection下载文件。使用OkHttp也可以通过流来下载文件。给OkHttp中添加拦截器，即可实现下载进度的监听功能。
使用流来实现下载文件获取并使用字节流，需要注意两个要点，一个是服务接口方法的 @Streaming 注解，另一个是获取到 ResponseBody。
获取流（Stream）。先定义一个服务ApiService。给方法添加上@Streaming的注解。

private interface ApiService { @Streaming @GET Observable download(@Url String url); }

初始化OkHttp。记得填入你的baseUrl。

OkHttpClient okHttpClient = new OkHttpClient.Builder() .connectTimeout(8, TimeUnit.SECONDS) .build(); retrofit = new Retrofit.Builder() .client(okHttpClient) .addCallAdapterFactory(RxJava2CallAdapterFactory.create()) .baseUrl("https://yourbaseurl.com") .build();

发起网络请求。获取到 ResponseBody。

String downUrl = "xxx.com/aaa.apk"; retrofit.create(ApiService.class) .download(downUrl) .subscribeOn(Schedulers.io()) .observeOn(Schedulers.io()) .doOnNext(new Consumer() { @Override public void accept(ResponseBody responseBody) throws Exception { // 处理 ResponseBody 中的流 } }) .doOnError(new Consumer() { @Override public void accept(Throwable throwable) throws Exception { Log.e(TAG, "accept on error: " + downUrl, throwable); } }) .observeOn(AndroidSchedulers.mainThread()) .subscribe(new Observer() { @Override public void onSubscribe(Disposable d) {}@Override public void onNext(ResponseBody responseBody) {}@Override public void onError(Throwable e) { Log.e(TAG, "Download center retrofit onError: ", e); }@Override public void onComplete() {} });

通过 ResponseBody 拿到字节流 body.byteStream()。这里会先创建一个临时文件tmpFile，把数据写到临时文件里。下载完成后再重命名成目标文件targetFile。

public void saveFile(ResponseBody body) { state = DownloadTaskState.DOWNLOADING; byte[] buf = new byte[2048]; int len; FileOutputStream fos = null; try { Log.d(TAG, "saveFile: body content length: " + body.contentLength()); srcInputStream = body.byteStream(); File dir = tmpFile.getParentFile(); if (dir == null) { throw new FileNotFoundException("target file has no dir."); } if (!dir.exists()) { boolean m = dir.mkdirs(); onInfo("Create dir " + m + ", " + dir); } File file = tmpFile; if (!file.exists()) { boolean c = file.createNewFile(); onInfo("Create new file " + c); } fos = new FileOutputStream(file); long time = System.currentTimeMillis(); while ((len = srcInputStream.read(buf)) != -1 && !isCancel) { fos.write(buf, 0, len); int duration = (int) (System.currentTimeMillis() - time); int overBytes = len - downloadBytePerMs() * duration; if (overBytes > 0) { try { Thread.sleep(overBytes / downloadBytePerMs()); } catch (Exception e) { e.printStackTrace(); } } time = System.currentTimeMillis(); if (isCancel) { state = DownloadTaskState.CLOSING; srcInputStream.close(); break; } } if (!isCancel) { fos.flush(); boolean rename = tmpFile.renameTo(targetFile); if (rename) { setState(DownloadTaskState.DONE); onSuccess(url); } else { setState(DownloadTaskState.ERROR); onError(url, new Exception("Rename file fail. " + tmpFile)); } } } catch (FileNotFoundException e) { Log.e(TAG, "saveFile: FileNotFoundException ", e); setState(DownloadTaskState.ERROR); onError(url, e); } catch (Exception e) { Log.e(TAG, "saveFile: IOException ", e); setState(DownloadTaskState.ERROR); onError(url, e); } finally { try { if (srcInputStream != null) { srcInputStream.close(); } if (fos != null) { fos.close(); } } catch (IOException e) { Log.e(TAG, "saveFile", e); } if (isCancel) { onCancel(url); } } }

每次读数据的循环，计算读了多少数据和用了多少时间。超过限速后主动sleep一下，达到控制下载速度的效果。要注意不能sleep太久，以免socket关闭。这里控制的是网络数据流与本地文件的读写速度。
下载进度监听 OkHttp实现下载进度监听，可以从字节流的读写那里入手。也可以使用拦截器，参考官方的例子。这里用拦截器的方式实现网络下载进度监听功能。
定义回调与网络拦截器先定义回调。

public interface ProgressListener { void update(String url, long bytesRead, long contentLength, boolean done); }

自定义 ProgressResponseBody。

public class ProgressResponseBody extends ResponseBody {private final ResponseBody responseBody; private final ProgressListener progressListener; private BufferedSource bufferedSource; private final String url; ProgressResponseBody(String url, ResponseBody responseBody, ProgressListener progressListener) { this.responseBody = responseBody; this.progressListener = progressListener; this.url = url; }@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(final 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; progressListener.update(url, totalBytesRead, responseBody.contentLength(), bytesRead == -1); return bytesRead; } }; } }

【Android入门教程 | OkHttp + Retrofit 下载文件与进度监听】定义拦截器。从 Response 中获取信息。

public class ProgressInterceptor implements Interceptor {private ProgressListener progressListener; public ProgressInterceptor(ProgressListener progressListener) { this.progressListener = progressListener; }@NotNull @Override public Response intercept(@NotNull Chain chain) throws IOException { Response originalResponse = chain.proceed(chain.request()); return originalResponse.newBuilder() .body(new ProgressResponseBody(chain.request().url().url().toString(), originalResponse.body(), progressListener)) .build(); } }

添加拦截器在创建 OkHttpClient 时添加 ProgressInterceptor。

OkHttpClient okHttpClient = new OkHttpClient.Builder() .connectTimeout(8, TimeUnit.SECONDS) .addInterceptor(new ProgressInterceptor(new ProgressListener() { @Override public void update(String url, long bytesRead, long contentLength, boolean done) { // tellProgress(url, bytesRead, contentLength, done); } })) .build();

值得注意的是这里的进度更新非常频繁。并不一定每次回调都要去更新UI。
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