Question

我是初学者。我在android中编写一个天气应用程序。我想在活动中调用片段方法，但它有错误

我的主要活动代码在这里：

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并显示错误

public class MainActivity extends FragmentActivity {
    ViewPager viewpager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        viewpager = (ViewPager)findViewById(R.id.viewPager);
        MyFragmentAdapter adapter = new MyFragmentAdapter(getSupportFragmentManager());
        viewpager.setAdapter(adapter);
    }

    private class MyFragmentAdapter extends FragmentPagerAdapter{

        public MyFragmentAdapter(FragmentManager fm) {
            super(fm);
            // TODO Auto-generated constructor stub
        }

        @Override
        public Fragment getItem(int position) {
            // TODO Auto-generated method stub
            switch (position) {
            case 0:
                return new WeatherCurrentFragment();
            case 1:
                return new WeatherForeCastFragment();
            }
            return null;
        }

        @Override
        public int getCount() {
            // TODO Auto-generated method stub
            return 2;
        }

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.weather, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        if(item.getItemId() == R.id.change_city){
            showInputDialog();
        }
        return super.onOptionsItemSelected(item);
    }

    private void showInputDialog(){
        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setTitle("Change city");
        final EditText input = new EditText(this);
        input.setInputType(InputType.TYPE_CLASS_TEXT);
        builder.setView(input);
        builder.setPositiveButton("Go", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                changeCity(input.getText().toString());
            }
        });
        builder.show();
    }

    public void changeCity(String city){
         WeatherCurrentFragment cf = (WeatherCurrentFragment)getSupportFragmentManager().findFragmentById(R.id.currentweather);
         cf.updateWeatherData(city);
         WeatherForeCastFragment ff = (WeatherForeCastFragmen)getSupportFragmentManager().findFragmentById(R.id.forecast);
         ff.updateWeatherData(city);
         new CityPreference(this).setCity(city);
    }
}

如你所见，当我点击按钮时，我想从CurrentWeatherFragment调用方法一，从ForecastWeatherFragment调用一个方法。

Answer 1

以下代码：

getSupportFragmentManager().findFragmentById(R.id.currentweather);

返回一个空对象引用。

这个问题可能会对您有所帮助，findFragmentById return null

顺便说一下，通常使用回调接口而不是在这样的活动中调用属于fragment的方法：

public interface OnCityChangedListener(){
    public void onCityChanged(String city);
}

在Fragment中实现此接口：

public MyFragment extends Fragment implements OnCityChangedListener

并在Fragment中重写此方法：

@override
public void onCityChanged(String city){
    updateWeatherData(city);
}

在活动中实例化片段，使用ArrayList将其保存在活动中，将此ArrayList传递给PageAdapter的构造函数，

public MyFragmentAdapter(FragmentManager fm, ArrayList<Fragment> fs) {
    super(fm);
    this.fs = fs;
}

点击按钮更改城市时：

public void changeCity(String city){
    fs.onCityChanged(city);
}

Answer 2

问题是FragmentPagerAdapter通过FragmentManager管理传递给FragmentPagerAdapter构造函数的片段。

所以，只要你致电getSupportFragmentManager().findFragmentById()，就不能保证它不会返回null。

更好的设计方法是制作CityPreference单例对象并在Application.onCreate()方法中实例化它：

public class WeatherApplication extends Application {
    private static WeatherApplication instance;
    private CityPreference cityPreference;

    public static WeatherApplication getInstance() {
        return instance;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        instance = this;
        cityPreference = new CityPreference(this);
    }

    public CityPreference getCityPreference() {
        return cityPreference;
    }    
}

然后你应该为＆＃34;城市改变＆＃34;添加听众。事件：

class CityPreference {
    public CityPreference(Context context) {
        // initialize SharedPreferences or any other storage
    }

    private final List<CityChangedListener> listeners = new ArrayList<>();

    public void addListener(CityChangedListener listener) {
        listeners.add(listener);
    }

    public void removeListener(CityChangedListener listener) {
        listeners.remove(listener);
    }

    public void setCity(String city) {
        // save city to shared preferences or some other place

        for (listener: listeners) {
            listener.onCityChanged();
        }
    }

    public interface CityChangedListener {
        void onCityChanged();
    }
}

在每个片段中进行以下更改：

class WeatherCurrentFragment extends Fragment implements CityChangedListener {
    @Override
    public void onResume() {
        super.onResume();
        onCityChanged(); // force fragment update because city could be changed when fragment was hidden or even not yet created
        WeatherApplication.getInstance().getCityPreference().addListener(this);
    }

    @Override
    public void onPause() {
        super.onPause();
        WeatherApplication.getInstance().getCityPreference().removeListener(this);
    }

    public void onCityChanged() {
        // update fragment
    }
}

最后是你的MainActivity.changeCity()方法：

public class MainActivity extends FragmentActivity {
    // other code

    private void changeCity(String city) {
        WeatherApplication.getInstance().getCityPreference().setCity(city);
    }
}

我不能在我的活动中调用我的片段方法，我想在2个不同的片段中调用2方法？

2 个答案: