ViewPager容器中从Activity到Fragment的子片段的通信

Question

我遇到的一个问题是，我很难抓住MainActivity Viewpager片段的子片段。我正在使用View Pager在选项卡式活动中实现主从视图。

在ListView中单击一个项目时，HandleCallback被触发，并尝试从MainActivity调用该片段的子片段，该子片段返回一个空引用。

下面的代码块返回空值。

    @Override
    public void loadImage(int imageSrc) {
        //        Returns null. How to get a reference of a fragment child fragment?
        ImageDetailFragment imageDetailFragment = (ImageDetailFragment) getSupportFragmentManager().findFragmentById(R.id.image_detail_fragment);

        if(imageDetailFragment != null){
            imageDetailFragment.setImageView(imageSrc);
        }
    }

请在下面找到完整的实现。非常感谢您的帮助。我仍在学习android，它是一个全新的领域。

MainActivity。

import android.support.design.widget.TabLayout;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;

import android.widget.TextView;

public class MainActivity extends AppCompatActivity implements HandleCallBack{

    /**
     * The {@link android.support.v4.view.PagerAdapter} that will provide
     * fragments for each of the sections. We use a
     * {@link FragmentPagerAdapter} derivative, which will keep every
     * loaded fragment in memory. If this becomes too memory intensive, it
     * may be best to switch to a
     * {@link android.support.v4.app.FragmentStatePagerAdapter}.
     */
    private SectionsPagerAdapter mSectionsPagerAdapter;

    /**
     * The {@link ViewPager} that will host the section contents.
     */
    private ViewPager mViewPager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        // Create the adapter that will return a fragment for each of the three
        // primary sections of the activity.
        mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

        // Set up the ViewPager with the sections adapter.
        mViewPager = (ViewPager) findViewById(R.id.container);
        mViewPager.setAdapter(mSectionsPagerAdapter);

        TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs);

        mViewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout));
        tabLayout.addOnTabSelectedListener(new TabLayout.ViewPagerOnTabSelectedListener(mViewPager));
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, 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.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @Override
    public void loadImage(int imageSrc) {
//        Returns null. How to get a reference of a fragment child fragment?
        ImageDetailFragment imageDetailFragment = (ImageDetailFragment) getSupportFragmentManager().findFragmentById(R.id.image_detail_fragment);

        if(imageDetailFragment != null){
            imageDetailFragment.setImageView(imageSrc);
        }
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {
        /**
         * The fragment argument representing the section number for this
         * fragment.
         */
        private static final String ARG_SECTION_NUMBER = "section_number";

        public PlaceholderFragment() {
        }

        /**
         * Returns a new instance of this fragment for the given section
         * number.
         */
        public static PlaceholderFragment newInstance(int sectionNumber) {
            PlaceholderFragment fragment = new PlaceholderFragment();
            Bundle args = new Bundle();
            args.putInt(ARG_SECTION_NUMBER, sectionNumber);
            fragment.setArguments(args);
            return fragment;
        }


    }

    /**
     * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
     * one of the sections/tabs/pages.
     */
    public class SectionsPagerAdapter extends FragmentPagerAdapter {

        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            switch (position){
                case 0: {
                    Tab1 tab1 = new Tab1();
                    return tab1;
                }
                case 1: {
                    Tab2 tab2 = new Tab2();
                    return tab2;
                }
                case 2: {
                    MasterDetailFragment masterDetailFragment = new MasterDetailFragment();
                    return masterDetailFragment;
                }
            }
            return null;
        }

        @Override
        public int getCount() {
            // Show 3 total pages.
            return 3;
        }
    }
}

MasterDetailFragment.java

import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class MasterDetailFragment extends Fragment{

    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.master_detailfragment, container, false);

        return view;
    }

}

ImageListFragment.java

import android.content.Context;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;

public class ImageListFragment extends Fragment {

    private int[] imageSrc = new int[]{
            R.drawable.pic1,
            R.drawable.pic2,
            R.drawable.pic3,
            R.drawable.pic4,
            R.drawable.pic5,
            R.drawable.pic6,
            R.drawable.pic7
    };

    private String[] imageNames = new String[]{
            "pic1",
            "pic2",
            "pic3",
            "pic4",
            "pic5",
            "pic6",
            "pic7"
    };

    private HandleCallBack handleCallBack;

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        if(context instanceof HandleCallBack){
            handleCallBack = (HandleCallBack) context;
        }
    }

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        ViewGroup viewGroup = (ViewGroup) inflater.inflate(R.layout.image_listfragment, container, false);

        ListView imageListView = (ListView) viewGroup.findViewById(R.id.imagelist);

        ArrayAdapter<String> adapter = new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1, imageNames);
        imageListView.setAdapter(adapter);


        imageListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                handleCallBack.loadImage(imageSrc[position]);
            }
        });

        return viewGroup;
    }

}

ImageDetailFragment.java

import android.media.Image;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;

public class ImageDetailFragment extends Fragment {

    private ImageView imageView;

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        ViewGroup viewGroup = (ViewGroup) inflater.inflate(R.layout.image_detailfragment, container, false);

        imageView = (ImageView) viewGroup.findViewById(R.id.mainimage);

        return viewGroup;
    }

    public void setImageView(int imgResId) {
        imageView.setImageResource(imgResId);
    }
}

HandleCallback.java

public interface HandleCallBack {

    public abstract void loadImage(int imageSrc);
}

activity_main.xml

    <?xml version="1.0" encoding="utf-8"?>
    <android.support.design.widget.CoordinatorLayout 
    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/main_content"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context=".MainActivity">

    <android.support.design.widget.AppBarLayout
        android:id="@+id/appbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:paddingTop="@dimen/appbar_padding_top"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:layout_weight="1"
            android:background="?attr/colorPrimary"
            app:layout_scrollFlags="scroll|enterAlways"
            app:popupTheme="@style/AppTheme.PopupOverlay"
            app:title="@string/app_name">

        </android.support.v7.widget.Toolbar>

        <android.support.design.widget.TabLayout
            android:id="@+id/tabs"
            android:layout_width="match_parent"
            android:layout_height="wrap_content">

            <android.support.design.widget.TabItem
                android:id="@+id/tabItem"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="@string/tab_text_1" />

            <android.support.design.widget.TabItem
                android:id="@+id/tabItem2"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="@string/tab_text_2" />

            <android.support.design.widget.TabItem
                android:id="@+id/tabItem3"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="@string/tab_text_3" />

        </android.support.design.widget.TabLayout>
    </android.support.design.widget.AppBarLayout>

    <android.support.v4.view.ViewPager
        android:id="@+id/container"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior" />

</android.support.design.widget.CoordinatorLayout>

master_detailFragment.xml（纵向）

    <?xml version="1.0" encoding="utf-8"?>
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:id="@+id/masterDetailFragment">

    <fragment
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/image_list_fragment"
        android:name="ca.agnition.tabbedlayoutmasterdetail.ImageListFragment" 
         />


</LinearLayout>

master_detailfragment.xml（横向）

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal"
    android:baselineAligned="false"
    android:id="@+id/masterDetailFragment">

    <fragment
        android:layout_width="0dp"
        android:layout_weight="1"
        android:layout_height="match_parent"
        android:id="@+id/image_list_fragment"
        android:name="ca.agnition.tabbedlayoutmasterdetail.ImageListFragment" />

    <fragment
        android:layout_width="0dp"
        android:layout_weight="2"
        android:layout_height="match_parent"
        android:id="@+id/image_detail_fragment"
        android:name="ca.agnition.tabbedlayoutmasterdetail.ImageDetailFragment" />


</LinearLayout>

image_detailfragment.xml

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

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/img_name"
        android:padding="10sp"
        android:background="@android:color/black"
        android:textColor="@android:color/white"
        android:gravity="center"
        android:layout_alignParentTop="true"
        android:id="@+id/title"/>

    <ImageView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/mainimage"
        android:contentDescription="@string/app_name"
        android:layout_below="@+id/title"
        android:src="@drawable/pic1"/>

</RelativeLayout>

image_listfragment.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <ListView
        android:id="@+id/imagelist"
        android:layout_width="match_parent"
        android:layout_height="match_parent"></ListView>

</LinearLayout>

是的！所以我终于弄清楚了我的问题。而且我对代码进行了一些更改。

在ImageListFragment.java中，我更改了onAttach方法，并指出它要检查HandleCallBack是ParentFragment的实例，该实例是MasterDetailFragment，如果是，则将其初始化。

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        if(getParentFragment() instanceof HandleCallBack){
            handleCallBack = (HandleCallBack) getParentFragment();
        }
    }

在MasterDetailFragment.java中，在覆盖loadImage方法中，我使用getChildFragment来获取image_detail_fragment鞭子，就像魔术一样。

在此之后，现在我还有另一个问题需要以纵向模式加载细节视图。当用户单击列表上的项目时，其详细信息视图应以一个单独的片段打开。为此，我创建了一个新的片段，其布局如下。

ImageListDetailFragment.java

import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.support.v7.app.ActionBar;
import android.support.v7.app.AppCompatActivity;
import android.view.LayoutInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;

public class ImageListDetailFragment extends Fragment {


    public static ImageListDetailFragment newInstance(int position) {
        ImageListDetailFragment fragment = new ImageListDetailFragment();
//        fragment.position = position;
        return fragment;
    }

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        ViewGroup viewGroup = (ViewGroup) inflater.inflate(R.layout.image_listdetailfragment, container, true);

        ImageDetailFragment imageDetailFragment = (ImageDetailFragment) getFragmentManager().findFragmentById(R.id.image_detail_fragment);

        ActionBar actionBar = ((AppCompatActivity)getActivity()).getSupportActionBar();
        actionBar.setDisplayHomeAsUpEnabled(true);
        actionBar.setTitle("back");

        Bundle bundle = this.getArguments();

        if(bundle != null){
            imageDetailFragment.setImageView(bundle.getInt(MasterDetailFragment.IMAGE_ID));
        }

        return viewGroup;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        if(item.getItemId() == android.R.id.home){
            ((AppCompatActivity)getActivity()).finish();
        }
        return super.onOptionsItemSelected(item);
    }
}

image_listdetailfragment.java

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <fragment
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/image_list_detail_fragment"
        android:name="ca.agnition.tabbedlayoutmasterdetail.ImageListDetailFragment" />


</LinearLayout>

然后在MasterDetailFragment中，我单击列表上的一个项目，首先，它检查设备是否处于横向模式，然后初始化image_detail_fragment，如果不是，则加载image_listdetailfragment以在单独的片段中显示细节。下面是方法。

    @Override
    public void loadImage(int imageSrc) {
        if (getChildFragmentManager().findFragmentById(R.id.image_detail_fragment) != null) {
            ImageDetailFragment imageDetailFragment = (ImageDetailFragment) getChildFragmentManager().findFragmentById(R.id.image_detail_fragment);
            imageDetailFragment.setImageView(imageSrc);
        } else {
            ImageListDetailFragment ImageListDetailFragment = (ImageListDetailFragment) getFragmentManager().findFragmentById(R.id.image_list_detail_fragment);
            Bundle bundle = new Bundle();
            bundle.putInt(IMAGE_ID, imageSrc);
            ImageListDetailFragment.setArguments(bundle);
        }
    }

但是同样，ImageListDetailFragment从getFragmentManager（）。findFragmentById（R.id.image_list_detail_fragment）;获取空引用

我也尝试过使用以下方法。

    ImageListDetailFragment imageListDetailFragment = new ImageListDetailFragment();
    FragmentManager fm = getFragmentManager();
    fm.beginTransaction().replace(R.id.container, imageListDetailFragment).commit();

但是这也不起作用。我不确定这是否是正确的方法。因此，我们非常感谢您的帮助，如果您有更好的方法在ViewPager内实现Master-Detail视图或可以参考的工作示例，请提出建议。

谢谢！