Android - 在寻呼机中获取 fragment 的上下文

Question

我有一个使用寻呼机作为导航的安卓应用程序。对于选项卡，我有 3 种布局作为内容。其中一个 fragment 是一个画廊，我想向其中添加图像。为此，我必须设置一个 ImageAdapter，但我需要知道如何访问 fragment 的上下文。

final LayoutInflater factory = getLayoutInflater();
final View view = factory.inflate(R.layout.pictures, null);
Gallery g = (Gallery) view.findViewById(R.id.gallery1);
g.setAdapter(new ImageAdapter(view.getContext()));

我在 onCreate 方法中使用上面的这段代码，从不是内容 View 的布局中获取图库。我必须为 ImageAdapter 提供上下文。但是我必须在那里设置什么上下文？

编辑:这是我的完整代码:

package com.bw2801.uwelugemediathek;

import java.util.Locale;
import android.app.ActionBar;
import android.app.FragmentTransaction;
import android.content.Context;
import android.database.DataSetObserver;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.BaseAdapter;
import android.widget.Gallery;
import android.widget.ImageView;
import android.widget.SpinnerAdapter;
import android.widget.Toast;

public class MainActivity extends FragmentActivity implements
    ActionBar.TabListener {

SectionsPagerAdapter mSectionsPagerAdapter;
ViewPager mViewPager;
PicturesSectionFragment ps = new PicturesSectionFragment();

@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(
            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));
    }

    final LayoutInflater factory = getLayoutInflater();
    final View view = factory.inflate(R.layout.pictures, null);
    Gallery g = (Gallery) view.findViewById(R.id.gallery1);
    g.setAdapter(new ImageAdapter(ps.getActivity()));
}

public class ImageAdapter extends BaseAdapter { 
    private Context mContext;

    private Integer[] mImageIds = {
            R.drawable.image01,
            R.drawable.image02,
            R.drawable.image03,
            R.drawable.image04,
            R.drawable.image05,
            R.drawable.image06,
            R.drawable.image07,
            R.drawable.image08,
    };

    public ImageAdapter(Context c) {
        mContext = c;
    }

    public int getCount() {
        return mImageIds.length;
    }

    public Object getItem(int position) {
        return position;
    }

    public long getItemId(int position) {
        return position;
    }

    public View getView(int position, View convertView, ViewGroup parent) {
        ImageView i = new ImageView(mContext);

        i.setImageResource(mImageIds[position]);
        i.setLayoutParams(new Gallery.LayoutParams(150, 100));
        i.setScaleType(ImageView.ScaleType.FIT_XY);

        return i;
    }
}

@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) {
}

/**
 * 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:
                return new DummySectionFragment();
            case 1:
                return new SoundSectionFragment();
            case 2:
                return ps;
        }
        return new DummySectionFragment();
    }

    @Override
    public int getCount() {
        return 3;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        switch (position) {
        case 0:
            return "Informationen";
        case 1:
            return "Soundboard";
        case 2:
            return "Galerie";
        }
        return null;
    }
}

public static class DummySectionFragment extends Fragment {

    public DummySectionFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.info, container, false);
    }
}

public static class PicturesSectionFragment extends Fragment {

    public PicturesSectionFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.pictures, container, false);
    }
}

public static class SoundSectionFragment extends Fragment {

    public SoundSectionFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.sounds, container, false);
    }
}
}

Answer 1

fragment 没有自己的上下文，它们使用父 Activity 。

• 要获取父 Activity 上下文，请使用 getActivity()
• 要使用应用程序上下文，请使用 getActivity().getApplicationContext()

尽可能选择应用上下文。

更新:

fragment 的

getActivity() 返回一个 Activity 实例当且仅当，所述 fragment 当前附加到一个 Activity。

---

所以，

Fragment f = new MyFragment();  

创建一个 fragment ，但它还没有附加到 Activity 。因此 f.getActivity() 返回 null。

---

添加到 Activity 后:

getFragmentManager().beginTransaction().add(f,"fragment").commit();

现在，getActivity() 将返回一个 Activity 实例。

---

同样，如果我们从 Activity 中分离 fragment :

getFragmentManager().beginTransaction().detach(f).commit()

getActivity() 将再次返回一个 null 值。

---

因此，我们不应该在 Fragment 类之外使用 getActivity()，因为我们无法确定附加状态。因此，我建议您在 fragment 自己的类中的方法中使用 getActivity():onAttach()、onCreate() 或 onActivityCreated()。