这不适用于某些设备。在三星设备中，他们不允许使用下载管理器下载文件。我已经在清单中定义了权限并获得了运行时权限。
DownloadManager downloadManager = (DownloadManager) getActivity().getSystemService(Context.DOWNLOAD_SERVICE);
DownloadManager.Request request = new DownloadManager.Request(uri);
request.setAllowedNetworkTypes(DownloadManager.Request.NETWORK_WIFI | DownloadManager.Request.NETWORK_MOBILE);
request.setTitle("");
request.setDescription("");
request.allowScanningByMediaScanner();
request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);
request.setDestinationUri(Uri.fromFile(imageFile));
request.setMimeType("*/*");
downloadManager.enqueue(request);