废话不多说，直接上代码：

public class ReadLocalFileActivity extends AppCompatActivity {

    private ListView listView;

    private List<File> files = new ArrayList<>();

    private ArrayAdapter adapter;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_read_loacal_file);
        listView = findViewById(R.id.lv_file);
        adapter = new ArrayAdapter(this, android.R.layout.simple_list_item_1, files);
        listView.setAdapter(adapter);
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                String absolutePath = files.get(position).getAbsolutePath();
                Log.i("lala","=position=>" + position + "   absolutePath==>" + absolutePath);

                Intent intent = new Intent(Intent.ACTION_VIEW);
                intent.addCategory(Intent.CATEGORY_DEFAULT);
                intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                intent.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION | Intent.FLAG_GRANT_WRITE_URI_PERMISSION);

                File file = files.get(position);//这是.txt文件
                Uri contentUri = FileProvider
                        .getUriForFile(ReadLocalFileActivity.this,
                        getPackageName()+".read_txt_provider", file);
                intent.setDataAndType(contentUri,   "text/plain");
                startActivity(intent);

            }
        });

        getLocalFileData();
    }

    @SuppressLint("CheckResult")
    private void getLocalFileData() {
        String basePath = Environment.getExternalStorageDirectory().getPath();
        File rootFile = new File(basePath);
        boolean exists = rootFile.exists();
        if (!exists){
            adapter.notifyDataSetChanged();
            return;
        }

        Observable.just(rootFile)
                .flatMap(new Function<File, Observable<File>>() {
                    @Override
                    public Observable<File> apply(@NonNull File file) throws Exception {
                        return listFiles(file);
                    }
                })
                .filter(new Predicate<File>() {
                    @Override
                    public boolean test(@NonNull File file) throws Exception {
                        return file.getName().endsWith(".txt");
                    }
                })
                .toList()
                .observeOn(AndroidSchedulers.mainThread())
                .subscribeOn(Schedulers.io())
                // 通过订阅发送给观察者
                .subscribe(new Consumer<List<File>>() {
                    @Override
                    public void accept(List<File> s) throws Exception {
                        files.addAll(s);
                        adapter.notifyDataSetChanged();
                    }
                }, new Consumer<Throwable>() {
                    @Override
                    public void accept(Throwable throwable) throws Exception {
                        Log.e("lala","=throwable==>" + throwable.getMessage());
                    }
                });
    }

    private Observable<File> listFiles(File file) {
        if (file.isDirectory()) {
            return Observable.fromArray(file.listFiles())
                    .flatMap(new Function<File, Observable<File>>() {
                        @Override
                        public Observable<File> apply(@NonNull File file) throws Exception {
                            return listFiles(file);
                        }
                    });
        } else {
            return Observable.just(file);
        }
    }
}

layout布局就是一个简单的listview

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