FileDownload使用，知乎正在使用的download库，功能强大。

1.引入依赖:

 implementation 'com.liulishuo.filedownloader:library:1.7.6'

2.在自定义的Application子类的onCreate中初始化

    public void onCreate() {
        super.onCreate();
        app = this;
//        FileDownload初始化
        FileDownloader.setupOnApplicationOnCreate(this)
                .connectionCreator(new FileDownloadUrlConnection
                        .Creator(new FileDownloadUrlConnection.Configuration()
                        .connectTimeout(15_000) // set connection timeout.
                        .readTimeout(15_000) // set read timeout.
                ))
                .commit();
    }

3.创建一个下载任务

   String directory= Environment.getExternalStorageDirectory()+ File.separator+CAMERA+File.separator;
        String path = directory+ System.currentTimeMillis()+".png";
        BaseDownloadTask downloadTask =  FileDownloader.getImpl().create(uri.toString())
                .setPath(path, false)
                .setListener(new FileDownloadListener() {
                    @Override
                    protected void pending(BaseDownloadTask task, int soFarBytes, int totalBytes) {

                    }

                    @Override
                    protected void progress(BaseDownloadTask task, int soFarBytes, int totalBytes) {

                    }

                    @Override
                    protected void completed(BaseDownloadTask task) {
                        Toast.makeText(UIUtils.getContext(),"图片已保存到:"+directory+" 文件夹",Toast.LENGTH_SHORT).show();
                    }

                    @Override
                    protected void paused(BaseDownloadTask task, int soFarBytes, int totalBytes) {

                    }

                    @Override
                    protected void error(BaseDownloadTask task, Throwable e) {

                    }

                    @Override
                    protected void warn(BaseDownloadTask task) {

                    }
                });
            downloadTask.start();  //开始下载    
                

over.结束