Question

我在swift中有这个功能

func *<T1:Sequence, T2:Sequence>(lhs: T1,rhs : T2) ->
              [(T1.Iterator.Element,T2.Iterator.Element)] 
{
    let product = lhs.flatMap({ x in rhs.lazy.map{y in (x,y)}})
    return product
}

我想让它评价懒惰。我知道我可以使用lhs.lazy.flatmap但是返回类型应该是什么？或者有更好的或其他方式来做这样的事情吗？

Answer 1

您可以创建一个type-erased sequence，将其操作转发给基础具有相同Element类型的序列，隐藏了该特征基础序列：

func *<T1:Sequence, T2:Sequence>(lhs: T1,rhs : T2) -> AnySequence<(T1.Iterator.Element,T2.Iterator.Element)> 
{
    return AnySequence (
        lhs.lazy.flatMap { x in rhs.lazy.map { y in (x,y) }}
    )
}

然后，您的代码独立于lazy.flatMap的实际实现及其确切的返回类型（甚至可能随之改变更新的Swift版本。）

Answer 2

感谢亚历山大，我想出了

 public class MainActivity extends AppCompatActivity {

        private CountryService countryService = new CountryServiceImpl();
        private static final String INSERT_DATA_ITEM_MESSAGE = "Insert data item selected";
        private static final String EXIT_ITEM_MESSAGE = "Exit. Bye, bye";
        private static final String ERROR_DURING_CONVERTING_JSON_TO_MAP_MESSAGE = "Error during converting JSON to Map. ";
        private static final String JSON_FILE_NAME = "countries";
        private static final String ASSET_FOLDER = "raw";
        private static final String DATA_ARE_ALREADY_UPLOADED = "Data are already uploaded";
        private static final String DATA_UPLOADED_SUCCESSFUL = "Data uploaded successful";
        private static final String KEYWORD = "search";
        private SQLiteDatabase sqLiteDatabase;
        private SQLiteConnector sqLiteConnector;
        private Spinner countryListSpinner;
        private ListView listView;
        private List<CountryWithCitiesDto> listCountryiesAndCities;
        private ProgressBar progressBar;

        private Map<String, String[]> getParsedMapFromJSONFile() {
            InputStream inputStream = getResources().openRawResource(
                    getResources().getIdentifier(JSON_FILE_NAME,
                            ASSET_FOLDER, getPackageName()));
            byte[] byteArrayFromJSONFile = JSONReader.getByteArrayFromJsonFile(inputStream);
            if (byteArrayFromJSONFile != null) {
                return buildMapFromByteArrayJSONFile(byteArrayFromJSONFile);
            }
            return Collections.emptyMap();
        }


        private Map<String, String[]> buildMapFromByteArrayJSONFile(byte[] jsonByteArray) {
            try {
                ObjectMapper mapper = new ObjectMapper();
                return mapper.readValue(jsonByteArray, new TypeReference<Map<String, String[]>>() {
                });
            } catch (IOException e) {
                Log.e(ClassNameTags.MAIN_ACTIVITY, ERROR_DURING_CONVERTING_JSON_TO_MAP_MESSAGE + e.getMessage());
                return Collections.emptyMap();
            }
        }

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            initDatabase();
            countryListSpinner = (Spinner) findViewById(R.id.countryListSpinner);
            progressBar = (ProgressBar) findViewById(R.id.progressBar);
            listView = (ListView) findViewById(R.id.cityListView);
            progressBar.setVisibility(View.VISIBLE);
            setAdapterToSpinner(setCountryListSpinner());
            progressBar.setVisibility(View.INVISIBLE);
            countryListSpinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
                @Override
                public void onItemSelected(AdapterView<?> arg0, View arg1,
                                           int position, long arg3) {
                    List<String> cityList = new ArrayList<>();
                    for (int i = 0; i < listCountryiesAndCities.get(position).getCityList().size(); i++) {
                        cityList.add(listCountryiesAndCities.get(position).getCityList().get(i).getNameCity());
                    }
                    setAdapterToCityList(cityList);
                }


                @Override
                public void onNothingSelected(AdapterView<?> arg0) {


                }
            });


            listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
                @Override
                public void onItemClick(AdapterView<?> parent, View view, int position,
                                        long id) {
                    Intent intent = new Intent(MainActivity.this, WikiActivity.class);
                    intent.putExtra(KEYWORD, listCountryiesAndCities.get((int) countryListSpinner.getSelectedItemId()).getCityList().get(position).getNameCity());
                    startActivity(intent);
                }
            });
        }


        @Override
        public boolean onCreateOptionsMenu(Menu menu) {
            getMenuInflater().inflate(R.menu.main_menu, menu);
            return super.onCreateOptionsMenu(menu);
        }

        @Override
        public boolean onOptionsItemSelected(MenuItem item) {
            switch (item.getItemId()) {
                case R.id.insert_data_menu:
                    if (listCountryiesAndCities.isEmpty()) {
                        progressBar.setVisibility(View.VISIBLE);
                        uploadFromJsonToDatabase();
                        setAdapterToSpinner(setCountryListSpinner());
                        progressBar.setVisibility(View.INVISIBLE);
                        Toast.makeText(this, DATA_UPLOADED_SUCCESSFUL, Toast.LENGTH_SHORT).show();
                    } else {
                        Toast.makeText(this, DATA_ARE_ALREADY_UPLOADED, Toast.LENGTH_SHORT).show();
                    }
                    Log.i(ClassNameTags.MAIN_ACTIVITY, INSERT_DATA_ITEM_MESSAGE);
                    break;
                case R.id.exit_menu:
                    Log.i(ClassNameTags.MAIN_ACTIVITY, EXIT_ITEM_MESSAGE);
                    break;
            }
            return super.onOptionsItemSelected(item);
        }


        private void initDatabase() {
            sqLiteConnector = new SQLiteConnector(this);
            sqLiteDatabase = sqLiteConnector.getWritableDatabase();
        }

        private void uploadFromJsonToDatabase() {
            countryService.insertCountryWithCities(getParsedMapFromJSONFile(), sqLiteDatabase);
        }

        private String[] setCountryListSpinner() {
            listCountryiesAndCities = countryService.getListCountriesWithCities(sqLiteDatabase);
            String[] outArray = new String[listCountryiesAndCities.size()];
            for (int i = 0; i < listCountryiesAndCities.size(); i++) {
                System.out.println(listCountryiesAndCities.get(i).getCountry().getNameCountry());
                outArray[i] = listCountryiesAndCities.get(i).getCountry().getNameCountry();
            }
            return outArray;
        }

        private void setAdapterToSpinner(String[] spinnerCountryArray) {
            ArrayAdapter<String> adapter = new ArrayAdapter<>(this,
                    android.R.layout.simple_spinner_item, spinnerCountryArray);
            countryListSpinner.setAdapter(adapter);
        }


        private void setAdapterToCityList(List<String> cityNames) {
            ArrayAdapter<String> adapterToCityList
                    = new ArrayAdapter<>(this,
                    android.R.layout.simple_list_item_1,
                    cityNames);
            listView.setAdapter(adapterToCityList);
            adapterToCityList.notifyDataSetChanged();

        }


    }

这样可行，但不知何故，返回类型对我来说似乎有点太多了。

Swift懒惰的笛卡儿产品

2 个答案: