如何在Fragment中向自定义ListView Adapter传递Activity上下文

我一眼就看到问题所在了——你在Fragment的成员变量初始化阶段就直接调用this.getActivity()来获取上下文，这时候Fragment还没依附到Activity上（Fragment的onAttach生命周期方法都还没执行），所以getActivity()会返回null！后续用这个空上下文去创建Adapter、ProgressDialog或者Toast，自然会出问题。

1. 移除无效的提前初始化代码 #

先删掉Fragment里这两行错误的上下文初始化：

Context siblingContext = this.getActivity();
Activity context = this.getActivity();

2. 在合适时机获取安全上下文 #

在Fragment生命周期的onActivityCreated、onCreateView之后，或者在需要使用上下文的方法内部，用requireActivity()（获取Activity实例）或requireContext()（获取Context实例）来获取有效上下文。这两个方法比getActivity()更安全——如果上下文为空会直接抛出异常，方便你快速定位问题，而不是默默返回null导致难以排查的崩溃。

3. 优化Adapter的上下文参数（可选但推荐） #

你的Adapter构造函数目前要求传入Activity，其实Adapter只需要Context就足够完成布局加载等操作。把Adapter的构造函数改成接受Context，能让它的复用性更强：

// 修改前的构造函数
// public SiblingsListViewAdapter(Activity context, String[] Uuid, String[] Usize, String[] Ustatus)

// 修改后的构造函数
public SiblingsListViewAdapter(Context context, String[] Uuid, String[] Usize, String[] Ustatus) {
    super();
    this.context = context;
    this.Uuid = Uuid;
    this.Usize = Usize;
    this.Ustatus = Ustatus;
}

4. 替换Fragment中所有上下文调用 #

把Fragment里用到上下文的地方，都换成安全的获取方式：

• ProgressDialog初始化：

progressDialog = ProgressDialog.show(requireActivity(), "Loading Data", null, true, true);

• 创建Adapter：

siblingLVAdapter = new SiblingsListViewAdapter(requireActivity(), Uuid, Usize, Ustatus);
// 或者用requireContext()，两者都可以，取决于是否需要Activity的特定方法
// siblingLVAdapter = new SiblingsListViewAdapter(requireContext(), Uuid, Usize, Ustatus);

• Toast提示：

Toast.makeText(requireContext(), "Error: " + e.toString(), Toast.LENGTH_LONG).show();

public class SiblingUnitFragment extends Fragment {
    ProgressDialog progressDialog;
    ListView siblingLVAdapter;
    String ReadOnly;
    String LexaUser;
    String Password;
    String SearchValue;
    String finalResultSiblings;
    String HttpURLSiblings = "https://[myDataSpot]/getSiblings.php";
    HashMap<String, String> hashMapSiblings = new HashMap<>();
    HttpParse httpParse = new HttpParse();
    String[] Uuid;
    String[] Usize;
    String[] Ustatus;
    String SV;
    String[] svSeparated;
    ListView siblingListView;

    public SiblingUnitFragment() { }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_siblings, container, false);
        siblingListView = (ListView) view.findViewById(R.id.SiblingsList);
        return view;
    }

    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        if (getArguments() != null) {
            SV = getArguments().getString("SearchValue");
            LexaUser = getArguments().getString("LexaUser");
        }
        if (SV.contains("-")) {
            svSeparated = SV.split("-");
            SearchValue = svSeparated[0];
            getSiblings(SearchValue, LexaUser);
        } else {
            SearchValue = SV;
            getSiblings(SearchValue, LexaUser);
        }
    }

    public void getSiblings(String searchInput, String lexaUser) {
        class SiblingsClass extends AsyncTask<String,Void,String> {
            @Override
            protected void onPreExecute() {
                super.onPreExecute();
                progressDialog = ProgressDialog.show(requireActivity(), "Loading Data", null, true, true);
            }

            @Override
            protected void onPostExecute(String httpResponseMsg) {
                super.onPostExecute(httpResponseMsg);
                if (httpResponseMsg != null) {
                    try {
                        JSONArray json = new JSONArray(httpResponseMsg);
                        Uuid = new String[json.length()];
                        Usize = new String[json.length()];
                        Ustatus = new String[json.length()];
                        for (int i = 0; i < json.length(); i++) {
                            JSONObject object = json.getJSONObject(i);
                            Uuid[i] = object.getString("id");
                            Usize[i] = object.getString("size");
                            Ustatus[i] = object.getString("status");
                        }
                        siblingLVAdapter = new SiblingsListViewAdapter(requireActivity(), Uuid, Usize, Ustatus);
                        siblingListView.setAdapter(siblingLVAdapter);
                    } catch (JSONException e) {
                        Log.e("JSONException", "Error: " + e.toString());
                        Toast.makeText(requireContext(), "Error: " + e.toString(), Toast.LENGTH_LONG).show();
                    }
                    progressDialog.dismiss();
                } else {
                    progressDialog.dismiss();
                    Toast.makeText(requireContext(), "HttpResponseMsg is null.", Toast.LENGTH_LONG).show();
                }
            }

            @Override
            protected String doInBackground(String... params) {
                hashMapSiblings.put("searchinput", params[0]);
                hashMapSiblings.put("lexauser", params[1]);
                finalResultSiblings = httpParse.postRequest(hashMapSiblings, HttpURLSiblings);
                return finalResultSiblings;
            }
        }
        SiblingsClass siblingsClass = new SiblingsClass();
        siblingsClass.execute(searchInput, lexaUser);
    }
}

• 尽量避免在Fragment中持有Activity的强引用，除非确实需要Activity的特定方法。做布局加载、显示Toast等基础操作时，用requireContext()就足够了。
• ProgressDialog已经被官方标记为过时，推荐使用ProgressBar或者Jetpack组件中的ProgressIndicator替代。
• 如果你需要监听Activity的生命周期变化，可以用getActivityLifecycleOwner()获取LifecycleOwner，配合LiveData等组件使用。

内容的提问来源于stack exchange，提问作者rdimouro