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;
}
};
}
}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。