android - 引用特定 fragment 时 SectionsPagerAdapter 类中的错误

Question

我正在尝试使用 Switch 方法引用特定 fragment ，当引用特定 fragment 时，IDE (Eclipse) 会抛出错误。

具体错误是:类型不匹配:无法从 GuidelineFragment 转换为 Fragment

我的第一个怀疑是我将 GuidelineFragment 的 Activity 扩展到 ListActivity，但是当我扩展到 ListFragment 时抛出更多错误并恢复到 ListActivity 扩展。

我组织了导入并清理了项目，但没有成功。

很遗憾，以下 StackOverflow 问题并未具体回答我的问题:

Android Fragments for Tabbed Menu

Problems with the tabs and the fragment

我做错了什么？

以下是我项目中的文件

SectionsPagerAdapter 类

package com.example.perinatologiev2;

import java.util.Locale;

import com.example.perinatologiev2.R;

import android.content.Context;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;

public class SectionsPagerAdapter extends FragmentPagerAdapter {

    protected Context mContext;

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

    public SectionsPagerAdapter(Context context,FragmentManager fm) {
        super(fm);
        mContext = context;
    }

    @Override
    public Fragment getItem(int position) {
        // getItem is called to instantiate the fragment for the given page..
        switch(position) {
        case 0:
            return new GuidelineFragment(); //error here
        case 1:
            return new CentraFragment(); //error here
        case 2:
            return new UzitecneFragment(); //error here
        }

        return null;

    }

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

    @Override
    public CharSequence getPageTitle(int position) {
        Locale l = Locale.getDefault();
        switch (position) {
        case 0:
            return mContext.getString(R.string.title_section1).toUpperCase(l);
        case 1:
            return mContext.getString(R.string.title_section2).toUpperCase(l);
        case 2:
            return mContext.getString(R.string.title_section3).toUpperCase(l);
        }
        return null;
    }
}

这是 switch 语句引用的 fragment :

指南 fragment

package com.example.perinatologiev2;

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import org.json.JSONTokener;

import com.example.perinatologiev2.R;

import android.app.ListActivity;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;

public class GuidelineFragment extends ListActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_guideline_fragment);
        try
        {
            JSONArray jsonArray = parsePostupyJSON();
            ArrayList<String> fields = new ArrayList<String>();
            for (int index = 0; index < jsonArray.length(); index++) {
                //Set both values into the listview
                JSONObject jsonObject = jsonArray.getJSONObject(index);
                fields.add(jsonObject.getString("title"));
            }

            setListAdapter(new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, fields));
        } catch (FileNotFoundException e) {
            Log.e("jsonFile", "file not found");
        } catch (IOException e) {
            Log.e("jsonFile", "ioerror");
        } catch (JSONException e) {
            e.printStackTrace();
        }
    }


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

    private JSONArray parsePostupyJSON() throws IOException, JSONException {
        //Load File
        BufferedReader jsonReader = new BufferedReader(new InputStreamReader(this.getResources().openRawResource(R.raw.guidelines)));

        StringBuilder jsonBuilder = new StringBuilder();
        for (String line = null; (line = jsonReader.readLine()) != null;) {
        jsonBuilder.append(line).append("\n");
        }

        //Parse Json
        JSONTokener tokener = new JSONTokener(jsonBuilder.toString());
        JSONArray jsonArray = new JSONArray(tokener);
        return jsonArray;
    }


    //The following is the method that I need to modify to listen to the tap and direct to web view

    protected void onListItemClick(ListView l, View v, int position, long id) {
        super.onListItemClick(l, v, position, id);

        try {
            JSONArray jsonArray = parsePostupyJSON();
            JSONObject jsonPost = jsonArray.getJSONObject(position);
            String guidelineUrl = jsonPost.getString("guidelinepath");
            Intent intent = new Intent(this, GuidelineWebActivity.class);
            intent.setData(Uri.parse(guidelineUrl));
            startActivity(intent);
        } 
        catch (JSONException e) {
            e.printStackTrace();
        } 
        catch (IOException e) {
            e.printStackTrace();
        }
    }

}

最后但同样重要的是......主要 Activity

package com.example.perinatologiev2;

import com.example.perinatologiev2.R;

import android.app.ActionBar;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;
import android.view.Menu;

public class MainActivity extends FragmentActivity implements
        ActionBar.TabListener {

    public static final String TAG = MainActivity.class.getSimpleName();

    /**
     * The {@link android.support.v4.view.PagerAdapter} that will provide
     * fragments for each of the sections. We use a
     * {@link android.support.v4.app.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}.
     */

    SectionsPagerAdapter mSectionsPagerAdapter;

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

    ViewPager mViewPager;

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

        // Set up the action bar.
        final ActionBar actionBar = getActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        // Create the adapter that will return a fragment for each of the three
        // primary sections of the app.
        mSectionsPagerAdapter = new SectionsPagerAdapter(this,
                getSupportFragmentManager());

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

        // When swiping between different sections, select the corresponding
        // tab. We can also use ActionBar.Tab#select() to do this if we have
        // a reference to the Tab.
        mViewPager
                .setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
                    @Override
                    public void onPageSelected(int position) {
                        actionBar.setSelectedNavigationItem(position);
                    }
                });

        // For each of the sections in the app, add a tab to the action bar.
        for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
            // Create a tab with text corresponding to the page title defined by
            // the adapter. Also specify this Activity object, which implements
            // the TabListener interface, as the callback (listener) for when
            // this tab is selected.
            actionBar.addTab(actionBar.newTab()
                    .setText(mSectionsPagerAdapter.getPageTitle(i))
                    .setTabListener(this));
        }
    }

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

    @Override
    public void onTabSelected(ActionBar.Tab tab,
            FragmentTransaction fragmentTransaction) {
        // When the given tab is selected, switch to the corresponding page in
        // the ViewPager.
        mViewPager.setCurrentItem(tab.getPosition());
    }

    @Override
    public void onTabUnselected(ActionBar.Tab tab,
            FragmentTransaction fragmentTransaction) {
    }

    @Override
    public void onTabReselected(ActionBar.Tab tab,
            FragmentTransaction fragmentTransaction) {
    }


}

Answer 1

如果您想在 ViewPager 中使用 GuidelineFragment(和其他类)，它们需要扩展 Fragment 而不是 Activity。这两个有不同的方法，你应该重写，这就是为什么你在更改父类(super class)后得到这么多错误的原因，例如在处理 Fragment 而不是 Activity 的 onCreate 时，您通常会使用 onCreateView。