如何从 Activity 调用 Fragment 的 TextView 和 Button？

Question

首先，我还是 Android 开发的新手，我之前在这里问过类似的问题 How to make my Fragment make use of my Activity Data? and How to send data from Activity to Fragment?(Android) 但是似乎大多数人都不太明白我的意思，所以让我解释一下。假设我有一个 TextView:

<TextView
            android:id="@+id/textView3"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="16dp"
            android:text="@string/current"
            android:textColor="#FF3D19"
            android:textSize="24sp"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.498"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent" />

和一个按钮：

<Button
            android:id="@+id/button2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginStart="14dp"
            android:layout_marginTop="92dp"
            android:shadowColor="#FFFFFF"
            android:text="Click"
            android:textColor="#FF5722"
            app:layout_constraintStart_toStartOf="@+id/imageView3"
            app:layout_constraintTop_toTopOf="@+id/imageView3" />

在我的 FirstFragment layout.xml 中，然后我想从我的 MainActivity 调用它们，即 current_temp = findViewById(R.id.textView10); 从天气 API 和 findViewById(R.id.button2).setOnClickListener(new View.OnClickListener() { 中获取数据，即 current_temp.setText(getString(R.string.blank, response.body().getCurrent().getTemp() + " ℃"));。 Android Studio 不允许您像那样使用它们，所以我收到此错误：

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

每当我运行应用程序时。在运行ning 之前我没有收到任何其他错误，所以原因是我没有实例化任何方法或者回调或接口来告诉 Activity 使用我的片段的 TextViews 和按钮。所以这就是我几天来一直试图解决的问题，但大多数人只是误解了我并开始建议我应该学习 ViewModel 或 LiveData。我看了几个关于 ViewModel 的教程，包括 Codinginflow，他们从来没有谈到将 activity 链接到片段 TextViews，他们只是让一个片段将数据发送到另一个片段，这不是我想要的。我的请求与此类似尝试过但失败了，所以我需要一步一步的程序来应用它。

完整代码： 主页Activity

public class HomeActivity extends AppCompatActivity {
    public static String BaseUrl = "http://api.openweathermap.org/";
    public static String AppId = "";
    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);

        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(getString(R.string.blank, response.body().getCurrent().getTemp() + " ℃"));
                            current_output.setText(response.body().getCurrent().getWeather().get(0).getDescription());
                            rise_time.setText(getString(R.string.blank, response.body().getCurrent().getSunrise() + " AM"));
                            set_time.setText(getString(R.string.blank, response.body().getCurrent().getSunset() + " PM"));
                            temp_out.setText(getString(R.string.blank, response.body().getCurrent().getTemp() + " ℃"));
                            Press_out.setText(getString(R.string.blank, response.body().getCurrent().getPressure() + " hpa"));
                            Humid_out.setText(getString(R.string.blank, response.body().getCurrent().getHumidity() + " %"));
                            Ws_out.setText(getString(R.string.blank, response.body().getCurrent().getWindSpeed() + " Km/h"));
                            Visi_out.setText(getString(R.string.blank, response.body().getCurrent().getVisibility() + " m"));
                        }
                    }

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

第一个片段

public class FirstFragment extends Fragment {
// TODO: Rename parameter arguments, choose names that match
// the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
private static final String ARG_PARAM1 = "param1";
private static final String ARG_PARAM2 = "param2";

// TODO: Rename and change types of parameters
private String mParam1;
private String mParam2;

public FirstFragment() {
    // Required empty public constructor
}

/**
 * Use this factory method to create a new instance of
 * this fragment using the provided parameters.
 *
 * @param param1 Parameter 1.
 * @param param2 Parameter 2.
 * @return A new instance of fragment SecondFragment.
 */
// TODO: Rename and change types and number of parameters
public static FirstFragment newInstance(String param1, String param2) {
    FirstFragment fragment = new FirstFragment();
    Bundle args = new Bundle();
    args.putString(ARG_PARAM1, param1);
    args.putString(ARG_PARAM2, param2);
    fragment.setArguments(args);
    return fragment;
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if (getArguments() != null) {
        mParam1 = getArguments().getString(ARG_PARAM1);
        mParam2 = getArguments().getString(ARG_PARAM2);
    }
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    return inflater.inflate(R.layout.fragment_first, container, false);
}
}

将全部功能都放到 Fragment 上也行不通，因为改造只能从 Activity 调用。但如果您有任何可靠的建议，我将不胜感激。

Answer 1

我建议你有两种方法可以将数据从 Activity 更新到 Fragment 的文本视图。

1.从 Activity 调用方法更新片段的文本视图（简单方法）

在片段 class 中创建方法 updateData() 以将数据更新到文本视图。
你在activity中声明了一个fragment参数class并在添加片段到activity时分配给这个参数。
当您在 activity 中收到来自 api 的数据时，调用 fragment.updateData()

fragment_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/temp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginTop="16dp"
        android:text="TextView"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/hud"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginTop="16dp"
        android:text="TextView"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/temp" />

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

home_activity.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <FrameLayout
        android:id="@+id/container"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

FirstFragment.kt

class FirstFragment : Fragment(R.layout.fragment_layout) {

    private lateinit var temp: TextView
    private lateinit var hud: TextView

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        temp = view.findViewById(R.id.temp)
        hud = view.findViewById(R.id.hud)
    }

    fun setDataToView(data: WeatherResponse) {
        temp.text = data.temp
        hud.text = data.hud
    }
}

HomeActivity.kt

class HomeActivity: AppCompatActivity() {

    private val fragment = FirstFragment()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.home_activity)

        addFragment()

        fragment.view?.findViewById<Button>(R.id.button2)?.setOnClickListener {
            getCurrentData()
        }


    }

    private fun addFragment(){
        val fm = supportFragmentManager
        fm.beginTransaction().replace(R.id.container, fragment, "FirstFragment").commit()
    }

    private fun getCurrentData(){
        //Your retrofit code in here. I only show code in onResponse()
        //.....
        @Override
        public void onResponse(@NonNull Call<WeatherResponse> call, @NonNull Response<WeatherResponse> response) {
            if (response.code() == 200) {
                fragment.setDataToView(response)
            }
        }

        //....
    }
}

2。使用 ViewModel

使用 livedata 参数创建一个 SharedViewModel class。
在 activity 中，您在 onCreate() 上创建了一个如下所示的 SharedViewModel 参数：

SharedViewModel viewModel = new SharedViewModel(this).get(SharedViewModel .class);
在片段中，在 onActivityCreated() 上创建一个如下所示的 SharedViewModel 参数：

SharedViewModel viewModel = new SharedViewModel(requireActivity()).get(SharedViewModel .class);
最后，您在 activity 和片段中有相同的 ViewModel 实例，因为它们都使用 activity 上下文。当您从 api 接收数据时，更新 activity 中的 livedata 参数，片段也接收 livedata 参数 onChanged 事件，然后您可以更新文本视图。

Answer 2

在 bundle 集片段 arguments 中设置您要更新的数据并调用 newInstance 然后相应地更新您的视图。

来自Activity

   FirstFragment.newInstance("yourData","yourData");

在你的片段中

if (getArguments() != null) {
        mParam1 = getArguments().getString(ARG_PARAM1);
        mParam2 = getArguments().getString(ARG_PARAM2);
    }

Answer 3

有很多方法可以实现你想要做的事情，我将使用 RxJava。

在您的项目中添加RxJava：

implementation 'io.reactivex.rxjava3:rxandroid:3.0.0'

创建一个 POJO class 来保存您的数据，稍后可以使用 RxJava 总线

public class WeatherReport {

    private String time_zone;
    private String time_field;
    private String current_temp;
    private String current_output;
    private String rise_time;
    private String set_time;
    private String temp_out;
    private String Press_out;
    private String Humid_out;
    private String Ws_out;
    private String Visi_out;

    public String getCurrent_output() {
        return current_output;
    }

    public String getCurrent_temp() {
        return current_temp;
    }

    public String getHumid_out() {
        return Humid_out;
    }

    public String getPress_out() {
        return Press_out;
    }

    public String getRise_time() {
        return rise_time;
    }

    public String getSet_time() {
        return set_time;
    }

    public String getTemp_out() {
        return temp_out;
    }

    public String getTime_field() {
        return time_field;
    }

    public String getTime_zone() {
        return time_zone;
    }

    public String getVisi_out() {
        return Visi_out;
    }

    public String getWs_out() {
        return Ws_out;
    }

    public void setCurrent_output(String current_output) {
        this.current_output = current_output;
    }

    public void setCurrent_temp(String current_temp) {
        this.current_temp = current_temp;
    }

    public void setHumid_out(String humid_out) {
        Humid_out = humid_out;
    }

    public void setPress_out(String press_out) {
        Press_out = press_out;
    }

    public void setRise_time(String rise_time) {
        this.rise_time = rise_time;
    }

    public void setSet_time(String set_time) {
        this.set_time = set_time;
    }

    public void setTemp_out(String temp_out) {
        this.temp_out = temp_out;
    }

    public void setTime_field(String time_field) {
        this.time_field = time_field;
    }

    public void setTime_zone(String time_zone) {
        this.time_zone = time_zone;
    }

    public void setVisi_out(String visi_out) {
        Visi_out = visi_out;
    }

    public void setWs_out(String ws_out) {
        Ws_out = ws_out;
    }
}

创建 RxJava 总线

import io.reactivex.rxjava3.subjects.BehaviorSubject;

public class RxJavaBus {

    private static final BehaviorSubject<WeatherReport> behaviorSubject
            = BehaviorSubject.create();


    public static BehaviorSubject<WeatherReport> getSubject() {
        return behaviorSubject;
    }

}

现在，从您的 activity（或您想要传输数据的任何地方）使用 RxBus，如下所示：

WeatherReport weatherReport = new WeatherReport();
        
weatherReport.setCurrent_output("Some DATA");
weatherReport.setCurrent_temp("Some DATA");
weatherReport.setHumid_out("Some DATA");
weatherReport.setPress_out("Some DATA");
weatherReport.setRise_time("Some DATA");
weatherReport.setSet_time("Some DATA");
weatherReport.setTime_field("Some DATA");
weatherReport.setVisi_out("Some DATA");
weatherReport.setTemp_out("Some DATA");
weatherReport.setWs_out("Some DATA");
weatherReport.setTime_zone("some DATA");

RxJavaBus.getSubject().onNext(weatherReport);

现在您可以像这样在片段（或任何您想要的地方）中接收数据：

RxJavaBus.getSubject().subscribe(weatherReportFromActivity -> {

            weatherReportFromActivity.getCurrent_output();
            weatherReportFromActivity.getCurrent_temp();
            weatherReportFromActivity.getHumid_out();
            weatherReportFromActivity.getPress_out();
            weatherReportFromActivity.getRise_time();
            weatherReportFromActivity.getSet_time();
            weatherReportFromActivity.getWs_out();
            weatherReportFromActivity.getTime_zone();
            weatherReportFromActivity.getVisi_out();
            weatherReportFromActivity.getTime_field();
            weatherReportFromActivity.getTemp_out();
});

Answer 4

你得到一个 NullPointerException 因为 findViewById(R.id.button2) returns null.

您需要使用片段的视图作为根。 activity.findViewById(...) 调用使用 activity 的视图作为根 - 它正在搜索没有 button2 的 activity_home.xml。您可以使用 fragment.getView().findViewById(R.id._) 访问片段的版本，但是：
您需要确保片段已完成其生命周期中的 onCreateView() 步骤。所以它必须发射和充气。否则，同样，该按钮不存在。

您可以通过在片段生命周期中将其设置为回调来确保在正确的时间调用您的设置并使用视图。

//This class can be anywhere - it can be global, private inner, anonymous
class SetupCallback extends FragmentLifeCycleCallback {
    @Override
    void onFragmentViewCreated(FragmentManager fm, Fragment f, View v, Bundle savedInstanceState) {
        ButtonView button = v.findViewById(R.id.button2);
        if(button != null) {    //This gets called for all fragments so check for null
            button.setOnClickListener(new OnClickListener {...});

        }

        //Repeat for any other elements in the view e.g.
        TextView someText = v.findViewById(R.id.someTextView);
        someText.setText(R.string.message);
    }
}

//In the activity setup - I usually put in onCreate()
SupportFragmentManager.supportFragmentManager = getSupportFragmentManager();
supportFragmentManager.registerFragmentLifecycleCallbacks(new SetupCallback());
//anonymous version:
//supportFragmentManager.registerFragmentLifecycleCallbacks(new FragmentLifecycleCallback() { /*contents of class*/ });

或者，您可以在 Activity 中初始化 Retrofit 和 Weather 服务，使用构造函数或包将它们传递给片段，然后使用它们在 [=] 中设置 onClickListener 16=].

RxJava 和 SharedView 方法是让侦听器卡在所有片段生命周期调用上的替代方法，而不是具有使片段更具反应性的替代体系结构。

这需要 androidx.lifecycle.common 依赖项。如果您使用 gradle 那将是：

dependencies {
    implementation 'androidx.lifecycle:lifecycle-common:2.3.0-rc1'
}

Answer 5

创建一个 class 以获得如下改造客户端

 class RetrofitClient {
    private Retrofit retrofit;
    private String BASE_URL = "";

    public ServiceAPI getRetrofitInstance() {
        if (retrofit == null) {
            retrofit = new Retrofit.Builder()
                    .baseUrl(BASE_URL)
                    .addConverterFactory(GsonConverterFactory.create())
                    .build();
        }
        return retrofit.create(WeatherService.class);
   }

现在你可以像这样在片段中调用 api

WeatherService service = RetrofitClient().getRetrofitInstance()
    
    void getCurrentData() {
                  
                    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(getString(R.string.blank, response.body().getCurrent().getTemp() + " ℃"));
                                current_output.setText(response.body().getCurrent().getWeather().get(0).getDescription());
                                rise_time.setText(getString(R.string.blank, response.body().getCurrent().getSunrise() + " AM"));
                                set_time.setText(getString(R.string.blank, response.body().getCurrent().getSunset() + " PM"));
                                temp_out.setText(getString(R.string.blank, response.body().getCurrent().getTemp() + " ℃"));
                                Press_out.setText(getString(R.string.blank, response.body().getCurrent().getPressure() + " hpa"));
                                Humid_out.setText(getString(R.string.blank, response.body().getCurrent().getHumidity() + " %"));
                                Ws_out.setText(getString(R.string.blank, response.body().getCurrent().getWindSpeed() + " Km/h"));
                                Visi_out.setText(getString(R.string.blank, response.body().getCurrent().getVisibility() + " m"));
                            }
                        }
    
                        @Override
                        public void onFailure(@NonNull Call<WeatherResponse> call, @NonNull Throwable t) {
                        }
                    });
                }
            });
        }

如何从 Activity 调用 Fragment 的 TextView 和 Button？

How to Call my Fragment's TextViews and Button from my Activity?

java

android

nullpointerexception

android-fragments

android-activity