java - 当从 fragment 更改我的 Activity 标题时会导致崩溃

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 5 年前。

我有一个名为 WebActivity 的 viewPager Activity 。viewPager 使用 webView Fragment 的实例(该 fragment 内部包含一个 webview)。我将 WebActivity 的标题从 viewPager 内的 fragment 更改为“Loading..”指示 webView 正在加载。它不会引起任何问题，但是当我频繁浏览页面时，我会遇到崩溃，日志如下所示:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.app.FragmentActivity.setTitle(java.lang.CharSequence)' on a null object reference
  at com.example.android.gametalks.Fragments.WebViewFragment$1.onPageFinished(WebViewFragment.java:91)
  at com.android.webview.chromium.WebViewContentsClientAdapter.onPageFinished(WebViewContentsClientAdapter.java:199)
  at org.chromium.android_webview.AwContentsClientCallbackHelper$MyHandler.handleMessage(AwContentsClientCallbackHelper.java:72)
  at android.os.Handler.dispatchMessage(Handler.java:102)
  at android.os.Looper.loop(Looper.java:168)
  at android.app.ActivityThread.main(ActivityThread.java:5885)
  at java.lang.reflect.Method.invoke(Native Method)
  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:797)
  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:687)

这是我的 fragment (相关部分):

public class WebViewFragment extends Fragment {

    WebView mWebView;
    String currentUrl;

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        currentUrl = getArguments().getString("currentUrl");
        super.onCreate(savedInstanceState);
    }

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


    @Override
    public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        getActivity().setTitle("Web View");

        setRetainInstance(true);
        mWebView = (WebView) view.findViewById(R.id.webvieww);

        mWebView.setWebViewClient(new WebViewClient() {
            public void onReceivedError(WebView view, int errorCode, String description, String failingUrl) {
                Toast.makeText(getActivity(), description, Toast.LENGTH_SHORT).show();


            }

            @Override
            public void onPageStarted(WebView view, String url, Bitmap favicon)
            {
                getActivity().setTitle("Loading...");
            }


            @Override
            public void onPageFinished(WebView view, String url) {

                super.onPageFinished(view, url);
                getActivity().setTitle("Web View");
            }
        });


        if (savedInstanceState == null)
        {
            mWebView.loadUrl(currentUrl);
        }

    }

Answer 1

简短回答:检查 getActivity() 是否因为访问它而返回 null。

if(getActivity() != null) {
    getActivity().setTitle("New title");
}

长答案:最好使用某种监听器来通知 Activity fragment 内的更改，而不是直接使用 getActivity()。 Google Developer docs have a pretty good walkthrough如何使用接口(interface)来完成此任务。