无法在片段中检索上下文 [重复]

Question

我试图通过多种资源找到解决方案：

1. Retrieve Context from a fragment
2. Using context in a fragment

但我仍然收到以下错误：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.support.v4.app.FragmentActivity.getApplicationContext()' on a null object reference

任何帮助将不胜感激。谢谢！

---

我当前的代码：

AnalysisActivity.java：

package com.app.component.fragment;

import android.os.Bundle;
import android.support.design.widget.TabLayout;
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.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;

import com.material.components.R;
import com.material.components.fragment.FragmentTabsEvents;
import com.material.components.fragment.FragmentTabsPerformance;
import com.material.components.fragment.FragmentTabsRecommendation;
import com.material.components.utils.Tools;

import java.util.ArrayList;
import java.util.List;

public class AnalysisActivity extends AppCompatActivity {


    private ViewPager view_pager;
    private TabLayout tab_layout;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_analysis);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        getSupportActionBar().setHomeButtonEnabled(true);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        Tools.setSystemBarColor(this,R.color.black);

        view_pager = findViewById(R.id.view_pager);
        setupViewPager(view_pager);

        tab_layout = findViewById(R.id.tab_layout);
        tab_layout.setupWithViewPager(view_pager);
    }

    private void setupViewPager(ViewPager viewPager) {
        SectionsPagerAdapter adapter = new SectionsPagerAdapter(getSupportFragmentManager());
        adapter.addFragment(FragmentTabsPerformance.newInstance(), "PERFORMANCE");
        adapter.addFragment(FragmentTabsRecommendation.newInstance(), "RECOMMENDATION");
        adapter.addFragment(FragmentTabsEvents.newInstance(), "EVENTS RELATED TO YOU");
        viewPager.setAdapter(adapter);
    }

    private class SectionsPagerAdapter extends FragmentPagerAdapter {

        private final List<Fragment> mFragmentList = new ArrayList<>();
        private final List<String> mFragmentTitleList = new ArrayList<>();

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

        @Override
        public Fragment getItem(int position) {
            return mFragmentList.get(position);
        }

        @Override
        public int getCount() {
            return mFragmentList.size();
        }

        public void addFragment(Fragment fragment, String title) {
            mFragmentList.add(fragment);
            mFragmentTitleList.add(title);
        }

        @Override
        public CharSequence getPageTitle(int position) {
            return mFragmentTitleList.get(position);
        }
    }

    @Override
    public boolean onSupportNavigateUp() {
        onBackPressed();
        return  true;
    }
}

FragmentTabsPerformance.java：

package com.app.component.fragment;

import android.content.Context;
import android.graphics.Color;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;

import com.material.components.R;
import com.material.components.utils.Tools;

import java.util.ArrayList;
import java.util.List;

import lecho.lib.hellocharts.model.Line;
import lecho.lib.hellocharts.model.LineChartData;
import lecho.lib.hellocharts.model.PointValue;
import lecho.lib.hellocharts.view.LineChartView;

public class FragmentTabsPerformance extends Fragment {

    private Context context;
    public FragmentTabsPerformance() {
        this.context = getActivity().getApplicationContext();
        List<PointValue> values = new ArrayList<>();
        values.add(new PointValue(0, 2));
        values.add(new PointValue(1, 4));
        values.add(new PointValue(2, 3));
        values.add(new PointValue(3, 4));

        //In most cased you can call data model methods in builder-pattern-like manner.
        Line line = new Line(values).setColor(Color.BLUE).setCubic(true);
        List<Line> lines = new ArrayList<>();
        lines.add(line);

        LineChartData data = new LineChartData();
        data.setLines(lines);

        LineChartView chart = new LineChartView(this.getContext());
        chart.setLineChartData(data);
    }

    public static FragmentTabsPerformance newInstance() {
        FragmentTabsPerformance fragment = new FragmentTabsPerformance();
        return fragment;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View root = inflater.inflate(R.layout.fragment_tabs_performance, container, false);

        Tools.displayImageOriginal(getActivity(), (ImageView) root.findViewById(R.id.image_1), R.drawable.image_8);
        Tools.displayImageOriginal(getActivity(), (ImageView) root.findViewById(R.id.image_2), R.drawable.image_9);
        Tools.displayImageOriginal(getActivity(), (ImageView) root.findViewById(R.id.image_3), R.drawable.image_15);
        Tools.displayImageOriginal(getActivity(), (ImageView) root.findViewById(R.id.image_4), R.drawable.image_14);
        Tools.displayImageOriginal(getActivity(), (ImageView) root.findViewById(R.id.image_5), R.drawable.image_12);
        Tools.displayImageOriginal(getActivity(), (ImageView) root.findViewById(R.id.image_6), R.drawable.image_2);
        Tools.displayImageOriginal(getActivity(), (ImageView) root.findViewById(R.id.image_7), R.drawable.image_5);

        return root;
    }
}

Answer 1

当片段未附加到活动时，您在构造方法中调用 getActivity() 后发生此异常。您可以参考API guides关于带有活动的片段的生命周期方法。在您的情况下，您可以使用 getContext() 而不是 getActivity() 方法来获取片段的上下文。

Answer 2

getActivity() 仅在 Fragment 附加到活动后可用。因此，Fragment 构造函数中的 getActivity() 将返回 null。查看fragment life cycle了解更多详情