android DownloadManager实现下载显示进度条

要实现在Android应用程序中下载文件并同时显示进度条，可以使用Android DownloadManager类。 DownloadManager是Android系统提供的一个下载管理器，它可以轻松地执行文件下载，并且可以方便地显示进度条。

以下是实现此功能的步骤：

1. 添加权限

在AndroidManifest.xml文件中添加以下权限：

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.DOWNLOAD_WITHOUT_NOTIFICATION" />

2. 创建DownloadManager请求

创建一个DownloadManager.Request对象，设置下载请求的URL、下载文件的名称和类型。

DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));
request.setTitle("下载文件");
request.setDescription("正在下载");
request.setMimeType("application/pdf");
request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "file.pdf");

3. 获取DownloadManager实例

获取DownloadManager实例，并将DownloadManager.Request对象传递给enqueue()方法。

DownloadManager manager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);
long downloadId = manager.enqueue(request);

4. 显示进度条

使用一个ProgressDialog对象来显示下载进度条，通过查询DownloadManager的状态来更新进度条的值。

ProgressDialog progressDialog = new ProgressDialog(MainActivity.this);
progressDialog.setMessage("正在下载");
progressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
progressDialog.setCancelable(true);
progressDialog.show();

new Thread(new Runnable() {
    @Override
    public void run() {
        boolean downloading = true;
        while (downloading) {
            DownloadManager.Query q = new DownloadManager.Query();
            q.setFilterById(downloadId);
            Cursor cursor = manager.query(q);
            cursor.moveToFirst();
            int bytesDownloaded = cursor.getInt(cursor.getColumnIndex(DownloadManager.COLUMN_BYTES_DOWNLOADED_SO_FAR));
            int bytesTotal = cursor.getInt(cursor.getColumnIndex(DownloadManager.COLUMN_TOTAL_SIZE_BYTES));
            if (cursor.getInt(cursor.getColumnIndex(DownloadManager.COLUMN_STATUS)) == DownloadManager.STATUS_SUCCESSFUL) {
                downloading = false;
            }
            final int dl_progress = (int) ((bytesDownloaded * 100l) / bytesTotal);
            runOnUiThread(new Runnable() {
                @Override
                public void run() {
                    progressDialog.setProgress(dl_progress);
                }
            });
            cursor.close();
        }
    }
}).start();

在以上代码中，我们启动一个新线程来查询下载进度，并使用runOnUiThread()方法更新ProgressDialog对象的进度条。

完整代码示例：

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        String url = "http://www.example.com/file.pdf";

        DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));
        request.setTitle("下载文件");
        request.setDescription("正在下载");
        request.setMimeType("application/pdf");
        request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "file.pdf");

        DownloadManager manager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);
        long downloadId = manager.enqueue(request);

        ProgressDialog progressDialog = new ProgressDialog(MainActivity.this);
        progressDialog.setMessage("正在下载");
        progressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
        progressDialog.setCancelable(true);
        progressDialog.show();

        new Thread(new Runnable() {
            @Override
            public void run() {
                boolean downloading = true;
                while (downloading) {
                    DownloadManager.Query q = new DownloadManager.Query();
                    q.setFilterById(downloadId);
                    Cursor cursor = manager.query(q);
                    cursor.moveToFirst();
                    int bytesDownloaded = cursor.getInt(cursor.getColumnIndex(DownloadManager.COLUMN_BYTES_DOWNLOADED_SO_FAR));
                    int bytesTotal = cursor.getInt(cursor.getColumnIndex(DownloadManager.COLUMN_TOTAL_SIZE_BYTES));
                    if (cursor.getInt(cursor.getColumnIndex(DownloadManager.COLUMN_STATUS)) == DownloadManager.STATUS_SUCCESSFUL) {
                        downloading = false;
                    }
                    final int dl_progress = (int) ((bytesDownloaded * 100l) / bytesTotal);
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            progressDialog.setProgress(dl_progress);
                        }
                    });
                    cursor.close();
                }
            }
        }).start();
    }
}