java如何使我的片段利用我的活动数据？

Question

我有一个活动承载3个片段，我的活动包含一些我希望我的第一个片段使用的数据。 我尝试调用以下方法，以便活动可以与片段通信：

FirstFragment fragmentFirst = (FirstFragment)
                getSupportFragmentManager().findFragmentById(R.id.firstFragment);
        assert fragmentFirst != null;
        fragmentFirst.doSomething("some param");

对于活动类和：

public void doSomething(String param) {
        // do something in fragment
    }

对于fragment类，但它产生了一个：Attempt to invoke virtual method 'void com.tex.lightweatherforecast.FirstFragment.doSomething(java.lang.String)' on a null object reference 运行时出错

我的活动代码是：

public class HomeActivity extends AppCompatActivity {
    public static String BaseUrl = "http://api.openweathermap.org/";
    public static String AppId = "9c547bfc852923c3b30d0d62a5ae35e8";
    public static String lat = "9.0574";
    public static String lon = "7.4898";
    // User Timezone name, current time, current temperature, current condition, sunrise, sunset, temperature, pressure, humidity, wind_speed, visibility, UV Index
    TextView time_zone, time_field, current_temp, current_output, rise_time, set_time, temp_out, Press_out, Humid_out, Ws_out, Visi_out, UV_out;
    ConstraintLayout constraintLayout;
    public static int count=0;
    int[] drawable =new int[]{R.drawable.dubai,R.drawable.central_bank_of_nigeria,R.drawable.eiffel_tower,R.drawable.hong_kong,R.drawable.statue_of_liberty};
    Timer _t;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home);
        FirstFragment fragmentFirst = (FirstFragment)
                getSupportFragmentManager().findFragmentById(R.id.firstFragment);
        assert fragmentFirst != null;
        fragmentFirst.doSomething("some param");

        time_zone = findViewById(R.id.textView9);
        time_field = findViewById(R.id.textView4);
        current_temp = findViewById(R.id.textView10);
        current_output = findViewById(R.id.textView11);
        rise_time = findViewById(R.id.textView25);
        set_time = findViewById(R.id.textView26);
        temp_out = findViewById(R.id.textView28);
        Press_out = findViewById(R.id.textView29);
        Humid_out = findViewById(R.id.textView30);
        Ws_out = findViewById(R.id.textView33);
        Visi_out = findViewById(R.id.textView34);
        UV_out = findViewById(R.id.textView35);

        BottomNavigationView bottomNavigationView = findViewById(R.id.bottomNavigationView);
        NavController navController = Navigation.findNavController(this, R.id.fragment);
        NavigationUI.setupWithNavController(bottomNavigationView, navController);

        findViewById(R.id.button2).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                getCurrentData();
                constraintLayout = findViewById(R.id.layout);
        constraintLayout.setBackgroundResource(R.drawable.dubai);
        _t = new Timer();
        _t.scheduleAtFixedRate(new TimerTask() {
            @Override
            public void run() {
                runOnUiThread(new Runnable() { // run on ui thread
                    @Override
                    public void run() {
                        if (count < drawable.length) {

                            constraintLayout.setBackgroundResource(drawable[count]);
                            count = (count + 1) % drawable.length;
                        }
                    }
                });
            }
        }, 5000, 5000);
    }

            void getCurrentData() {
    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl(BaseUrl)
            .addConverterFactory(GsonConverterFactory.create())
            .build();
    WeatherService service = retrofit.create(WeatherService.class);
    Call<WeatherResponse> call = service.getCurrentWeatherData(lat, lon, AppId);
        call.enqueue(new Callback<WeatherResponse>() {
            @Override
            public void onResponse(@NonNull Call<WeatherResponse> call, @NonNull Response<WeatherResponse> response) {
                if (response.code() == 200) {
                    WeatherResponse weatherResponse = response.body();
                    assert weatherResponse != null;

                    assert response.body() != null;
                    time_zone.setText(response.body().getTimezone());
                    time_field.setText(response.body().getCurrent().getDt());
                    current_temp.setText(response.body().getCurrent().getTemp() + " ℃");
                    current_output.setText(response.body().getCurrent().getWeather().get(0).getDescription());
                    rise_time.setText(response.body().getCurrent().getSunrise() + " AM");
                    set_time.setText(response.body().getCurrent().getSunset() + " PM");
                    temp_out.setText(response.body().getCurrent().getTemp() + " ℃");
                    Press_out.setText(response.body().getCurrent().getPressure() + " hpa");
                    Humid_out.setText(response.body().getCurrent().getHumidity() + " %");
                    Ws_out.setText(response.body().getCurrent().getWindSpeed() + " Km/h");
                    Visi_out.setText(response.body().getCurrent().getVisibility() + " m");

                }
            }

            @Override
            public void onFailure(@NonNull Call<WeatherResponse> call, @NonNull Throwable t) {
            }
        });
            }
        });
    }
}

任何帮助都将不胜感激

Answer 1

我认为您得到这个错误是因为片段不活动。如果它是活动的，并且这种方法有效，那么这是非常糟糕的方法，当我们使用新的LiveData和ViewModel arch样式时，还有其他方法

如果您认为子级（片段）将需要数据源（通常是LiveData），则将此数据源归父级（活动）所有，并在片段内监视该数据源（即，在不同片段内观察此LiveData）

如果需要从片段回调到活动，请使用契约（接口）并在活动内部实现该契约（该契约可能包含doSomething（）之类的函数），然后从片段内部调用该契约，如下所示：

(((ContractName)activity).doSomething(passData))

Answer 2

若要在第一个目标片段中传递活动数据，请避免在活动布局文件中设置图形

从活动布局中删除图形实现。并在活动的onCreate方法中实现它，如下所示

    NavController navController = Navigation.findNavController(this, R.id.nav_controller);
    Bundle bundle = new Bundle();
    bundle.putString("key", "value");
    navController.setGraph(navController.getGraph(), bundle);

通过参考以下代码更新图形文件

<navigation xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        android:id="@+id/navigation_graph"
        app:startDestination="@id/homeFragment">

    <fragment android:id="@+id/homeFragment"
          android:name="com.java.HomeFragment"
          android:label="home_fragment"
          tools:layout="@layout/fragment_home">

            <argument 
                android:name="key"
                app:argType="string"/>

    </fragment>

    ...
    ...

</navigation>

在主片段中，获取参数值

 String data = getArguments().getString("key");