下载文件是一个比拟常见的需要。给定一个url，咱们能够应用URLConnection下载文件。 应用OkHttp也能够通过流来下载文件。 给OkHttp中增加拦截器，即可实现下载进度的监听性能。
应用流来实现下载文件
获取并应用字节流，须要留神两个要点，一个是服务接口办法的 @Streaming 注解，另一个是获取到 ResponseBody。
获取流（Stream）。先定义一个服务ApiService。给办法增加上@Streaming的注解。
private interface ApiService { @Streaming @GET Observable<ResponseBody> 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();发动网络申请。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<ResponseBody>() { @Override public void accept(ResponseBody responseBody) throws Exception { // 解决 ResponseBody 中的流 } }) .doOnError(new Consumer<Throwable>() { @Override public void accept(Throwable throwable) throws Exception { Log.e(TAG, "accept on error: " + downUrl, throwable); } }) .observeOn(AndroidSchedulers.mainThread()) .subscribe(new Observer<ResponseBody>() { @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()。这里会先创立一个临时文件tmpFile，把数据写到临时文件里。 下载实现后再重命名成指标文件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工夫。超过限速后被动sleep一下，达到管制下载速度的成果。 要留神不能sleep太久，免得socket敞开。 这里管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，能够从字节流的读写那里动手。也能够应用拦截器，参考官网的例子。 这里用拦截器的形式实现网络下载进度监听性能。
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; } }; }}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(); }}增加拦截器
在创立 OkHttpClient 时增加 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十分频繁。并不一定每次回调都要去更新UI。
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