思路：
1Retrofit下载地址连接读取成功后，
2开启子线程保存资源文件到本地
3接口监听下载的进度，通知UI主线程更新下载的状态

效果:

依赖：

implementation 'com.squareup.retrofit2:retrofit:2.4.0'
implementation 'com.squareup.retrofit2:adapter-rxjava2:2.4.0'
implementation 'com.jakewharton.retrofit:retrofit2-rxjava2-adapter:1.0.0'
implementation 'io.reactivex.rxjava2:rxandroid:2.0.2'
implementation 'io.reactivex.rxjava2:rxjava:2.1.14'

下载工具类：

package com.example.mvpfour.utils;

import android.content.Context;
import android.os.Environment;
import android.util.Log;

import com.example.mvpfour.retrofit.ApiStores;
import com.example.mvpfour.retrofit.DownloadListener;
import com.example.mvpfour.retrofit.RetrofitClient;
import com.jakewharton.retrofit2.adapter.rxjava2.RxJava2CallAdapterFactory;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.concurrent.TimeUnit;

import io.reactivex.Observable;
import io.reactivex.Observer;
import io.reactivex.android.schedulers.AndroidSchedulers;
import io.reactivex.disposables.Disposable;
import io.reactivex.schedulers.Schedulers;
import okhttp3.OkHttpClient;
import okhttp3.ResponseBody;
import retrofit2.Retrofit;

/**
 * 思路：
 * 1Retrofit下载地址连接读取成功后，
 * 2开启子线程保存资源文件到本地
 * 3接口监听下载的进度，通知UI主线程更新下载的状态
 * Created by agen on 2018/8/24.
 */

public class DownLoadUtils {
    private static final String TAG = "GsonUtils";
    private static final String PLAY_VIDEO_URL = "http://7xstkb.com1.z0.glb.clouddn.com/agen_apple.mp4";
    private static final int TIMEOUT = 10;
    String baseUrl = "https://www.baidu.com/";//baseUrl地址不影响下载地址！！！
    private static final String PATH_CHALLENGE_VIDEO = Environment.getExternalStorageDirectory() + "/DownloadFileTwo";
    private String mVideoPath; //下载到本地的视频路径
    private File mFile;
    private Thread mThread;//子线程进行io读写操作
    private Context context;
    private OkHttpClient mHttpClient;
    private Retrofit mRetrofit;

    //volatile 关键字具有屏蔽指令重排的功能，即对 instance 加上了一把锁，
    // 在完成写操作之前不会允许其他线程进行读操作，因此，在初始化完成前，无法对其进行读操作
    private static volatile DownLoadUtils instance;

    //私有化构造函数
    private DownLoadUtils() {
        OkHttpClient.Builder builder = new OkHttpClient.Builder()
                .connectTimeout(TIMEOUT, TimeUnit.SECONDS)
                .readTimeout(TIMEOUT, TimeUnit.SECONDS)
                .writeTimeout(TIMEOUT, TimeUnit.SECONDS);
        mHttpClient = builder.build();
        mRetrofit = new Retrofit.Builder()
                .baseUrl(baseUrl)
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .client(mHttpClient)
                .build();
    }

    //提供对外的获取单例对象的方法
    public static DownLoadUtils getInstance() {
        //只有当 instance 为 null 的时候才执行同步代码块，二次判空保证了不会产生多个实例
        if (null == instance) {
            synchronized (RetrofitClient.class) {
                if (null == instance) {
                    instance = new DownLoadUtils();
                }
            }
        }
        return instance;
    }

    //清理单例对象
    public static void clearInstance() {
        //只有当 instance 为 null 的时候才执行同步代码块，二次判空保证了不会产生多个实例
        if (null != instance) {
            synchronized (RetrofitClient.class) {
                if (null != instance) {
                    instance = null;
                }
            }
        }
    }

    public void downloadFile(DownloadListener downloadListener){
        downloadFile(PLAY_VIDEO_URL,mRetrofit,downloadListener);
    }
    public void downloadFile(String url,DownloadListener downloadListener){
        downloadFile(url,mRetrofit,downloadListener);
    }

    public void downloadFile(String url, Retrofit retrofit, final DownloadListener downloadListener) {
        Log.d(TAG,"downloadFile");
        //建立一个文件夹
        mFile = new File(PATH_CHALLENGE_VIDEO);
        if (!mFile.exists() || !mFile.isDirectory()) {
            mFile.mkdirs();
        }
        //通过Url得到保存到本地的文件名
        String name = url;
        int index = name.lastIndexOf('/');//一定是找最后一个'/'出现的位置
        if (index != -1) {
            name = name.substring(index);
            mVideoPath = PATH_CHALLENGE_VIDEO + name;
            Log.d(TAG,"mVideoPath=" + mVideoPath);
        }
        mFile = new File(mVideoPath);
        ApiStores apiStores = retrofit.create(ApiStores.class);
        Observable<ResponseBody> observable = apiStores.downloadFile(url);
        observable.subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Observer<ResponseBody>() {
                    @Override
                    public void onSubscribe(Disposable d) {

                    }

                    @Override
                    public void onNext(final ResponseBody responseBody) {
                        mThread = new Thread() {
                            @Override
                            public void run() {
                                super.run();
                                writeFileSDcard(responseBody, mFile, downloadListener);
                            }
                        };
                        mThread.start();
                    }

                    @Override
                    public void onError(Throwable e) {
                        Log.d(TAG,"onError=" + e.getMessage());
                    }

                    @Override
                    public void onComplete() {
                        Log.d(TAG,"onComplete");
                    }
                });
    }

    public void writeFileSDcard(ResponseBody responseBody, File mFile, DownloadListener downloadListener) {
        downloadListener.onStart();
        Log.d(TAG,"writeFileSDcard");
        long currentLength = 0;
        OutputStream os = null;
        InputStream is = responseBody.byteStream();
        long totalLength = responseBody.contentLength();
        Log.d(TAG,"totalLength=" + totalLength);
        try {
            os = new FileOutputStream(mFile);
            int len;
            byte[] buff = new byte[1024];
            while ((len = is.read(buff)) != -1) {
                os.write(buff, 0, len);
                currentLength += len;
                Log.d(TAG,"当前长度: " + currentLength);
                int progress = (int) (100 * currentLength / totalLength);
                Log.d(TAG,"当前进度: " + progress);
                downloadListener.onProgress(progress);
                if (progress == 100) {
                    downloadListener.onFinish(mVideoPath);
                }
            }
        } catch (FileNotFoundException e) {
            Log.d(TAG,"Exception=" + e.getMessage());
            downloadListener.onFailure("未找到文件！");
            e.printStackTrace();
        } catch (IOException e) {
            Log.d(TAG,"Exception=" + e.getMessage());
            downloadListener.onFailure("IO错误！");
            e.printStackTrace();
        } finally {
            if (os != null) {
                try {
                    os.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (is != null) {
                try {
                    is.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

使用：

package com.example.mvpfour.activity;

import android.os.Bundle;
import android.os.Environment;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;

import com.example.mvpfour.R;
import com.example.mvpfour.retrofit.DownloadListener;
import com.example.mvpfour.utils.DownLoadUtils;

import java.io.File;

/**
 * Created by agen on 2018/8/23.
 */

public class FileDownTest extends AppCompatActivity implements View.OnClickListener {
    private static final String TAG = "GsonUtils";
    private Button down_file;
    private static final String PLAY_VIDEO_URL = "http://7xstkb.com1.z0.glb.clouddn.com/agen_apple.mp4";
    String baseUrl = "https://sapi.daishumovie.com/";//baseUrl不影响操作
    private static final String PATH_CHALLENGE_VIDEO = Environment.getExternalStorageDirectory() + "/DownloadFileTwo";
    private String mVideoPath; //下载到本地的视频路径
    private File mFile;
    private ProgressBar progress_bar;
    private Thread mThread;//子线程进行io读写操作
    private TextView tv_progress;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.file_downtest);
        initView();
    }

    private void initView() {
        down_file = (Button) findViewById(R.id.down_file);
        down_file.setOnClickListener(this);
        progress_bar = (ProgressBar) findViewById(R.id.progress_bar);
        progress_bar.setOnClickListener(this);
        tv_progress = (TextView) findViewById(R.id.tv_progress);

    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.down_file:
                downloadFileUtils();
                break;
        }
    }

    private void downloadFileUtils() {
        DownLoadUtils instance = DownLoadUtils.getInstance();
        instance.downloadFile(new DownloadListener() {
            @Override
            public void onStart() {
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        Toast.makeText(FileDownTest.this, "开始下载", Toast.LENGTH_SHORT).show();
                    }
                });
            }

            @Override
            public void onProgress(final int currentLength) {
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        progress_bar.setProgress(currentLength);
                        tv_progress.setText(currentLength);
                    }
                });
            }

            @Override
            public void onFinish(String localPath) {
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        Toast.makeText(FileDownTest.this, "下载成功", Toast.LENGTH_SHORT).show();
                    }
                });
            }

            @Override
            public void onFailure(String erroInfo) {
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        Toast.makeText(FileDownTest.this, "下载失败", Toast.LENGTH_SHORT).show();
                    }
                });
            }
        });
    }
}

。。。

布局

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:orientation="vertical">

    <Button
        android:id="@+id/down_file"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:text="下载文件"/>

    <ProgressBar
        android:id="@+id/progress_bar"
        style="@android:style/Widget.ProgressBar.Horizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:max="100"/>
</LinearLayout>

参考：https://blog.csdn.net/k_bb_666/article/details/79500623