Question

@JavascriptInterface
public void switchView() {
    //sync the BottomBar Icons since a different Thread is running
    Handler refresh = new Handler(Looper.getMainLooper());
    refresh.post(new Runnable() {
        public void run()
        {
            MapFragment mapFragment = new MapFragment();
            FragmentManager fragmentManager = ((MainActivity) mContext).getFragmentManager();
            FragmentTransaction transaction = fragmentManager.beginTransaction();
            transaction.replace(R.id.content, mapFragment);
            transaction.commit();
        }
    });
}

当我运行此代码时，一切都很好，但是当我添加行

时

mapFragment.setUrl("www.examplestuff.com");

应用程序崩溃尝试在空对象引用上调用虚拟方法'void android.webkit.WebView.loadUrl（java.lang.String）'

My Fragment类看起来像这样

public WebView mapView;
private String thisURL;

public void setUrl(String url) {
    thisURL = url;
    mapView.loadUrl(thisURL);
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

    View view = inflater.inflate(R.layout.fragment_map,container, false);

    mapView = (WebView)view.findViewById(R.id.mapView);
    this.setUrl("file:///android_asset/www/MapView.html");

    mapView.setWebViewClient(new WebViewClient());

    WebSettings webSettings = mapView.getSettings();
    webSettings.setJavaScriptEnabled(true);
    //allow cross origin - like jsonP
    webSettings.setAllowUniversalAccessFromFileURLs(true);

    return view;
}

同样调用方法this.setURL()并正常工作。

我做错了什么？ FragmentManager是否无法访问片段???的实例WebView

Answer 1

这是因为当你调用setUrl时，它会调用这个方法：

public void setUrl(String url) {
    thisURL = url;
    mapView.loadUrl(thisURL);
}

第mapView.loadUrl(thisURL);行访问mapView。但是，您可能会在Android系统调用setUrl之前调用onCreateView，因此mapView为空，从而导致崩溃。

    public void setUrl(String url) {
        thisURL = url;
        if(mapView != null) {
            mapView.loadUrl(thisURL);
        } 
    }

和

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

     View view = inflater.inflate(R.layout.fragment_map,container, false);

     mapView = (WebView)view.findViewById(R.id.mapView);
     if(thisUrl != null) {
        mapView.loadUrl(thisURL);
     }
     ... other code

然后mapFragment.setUrl("www.examplestuff.com");会起作用

更好的解决方案是了解更多活动＆amp;当setUrl处于无效状态时，片段生命周期并且不调用Fragment :-)当片段为时，您可能正在调用setUrl，而实际上您应该将Url作为意图额外传递创建。 https://developer.android.com/training/basics/fragments/communicating.html

Android Fragment Manager Null对象参考

1 个答案: