android下载文件功能怎么实现

在Android中实现下载文件功能通常需要使用DownloadManager类。以下是一个简单的示例代码，演示如何使用DownloadManager来下载文件：

public class MainActivity extends AppCompatActivity {

    private DownloadManager downloadManager;
    private long downloadID;

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

        downloadManager = (DownloadManager) getSystemService(DOWNLOAD_SERVICE);

        Uri uri = Uri.parse("http://example.com/file.txt");

        DownloadManager.Request request = new DownloadManager.Request(uri);
        request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);
        request.setDestinationInExternalFilesDir(this, Environment.DIRECTORY_DOWNLOADS, "file.txt");

        downloadID = downloadManager.enqueue(request);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        unregisterReceiver(onDownloadComplete);
    }

    private BroadcastReceiver onDownloadComplete = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            long id = intent.getLongExtra(DownloadManager.EXTRA_DOWNLOAD_ID, -1);
            if (id == downloadID) {
                // 下载完成后的处理逻辑
                Toast.makeText(MainActivity.this, "Download Completed", Toast.LENGTH_SHORT).show();
            }
        }
    };

    @Override
    protected void onResume() {
        super.onResume();
        registerReceiver(onDownloadComplete, new IntentFilter(DownloadManager.ACTION_DOWNLOAD_COMPLETE));
    }
}

在上面的示例代码中，我们首先通过DownloadManager类创建一个用于下载文件的Request对象，然后调用enqueue方法开始下载。在下载完成后，我们使用BroadcastReceiver监听DownloadManager.ACTION_DOWNLOAD_COMPLETE广播，并在接收到该广播时处理下载完成后的逻辑。

请注意，为了使下载功能正常工作，您需要在AndroidManifest.xml文件中声明权限android.permission.INTERNET和android.permission.WRITE_EXTERNAL_STORAGE。