如何使用片段在Web視圖中向後導航？

Question

當我嘗試從我的web視圖向後導航，這是一個片段內部時，我不斷收到錯誤，可能是導致此問題的原因？

我的logcat：

10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime: FATAL EXCEPTION: main 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime: Process: pi.com.ndtc, PID: 28567 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.webkit.WebView.canGoBack()' on a null object reference 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at pi.com.ndtc.SocialActivity.onKeyDown(SocialActivity.java:343) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.KeyEvent.dispatch(KeyEvent.java:2619) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.app.Activity.dispatchKeyEvent(Activity.java:2707) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.support.v7.internal.view.WindowCallbackWrapper.dispatchKeyEvent(WindowCallbackWrapper.java:50) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.support.v7.app.AppCompatDelegateImplBase$AppCompatWindowCallbackBase.dispatchKeyEvent(AppCompatDelegateImplBase.java:224) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.support.v7.internal.view.WindowCallbackWrapper.dispatchKeyEvent(WindowCallbackWrapper.java:50) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:2276) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$ViewPostImeInputStage.processKeyEvent(ViewRootImpl.java:4020) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:3982) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3544) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3597) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3563) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3680) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3571) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:3737) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3544) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3597) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3563) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3571) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3544) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3597) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3563) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3713) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.ViewRootImpl$ImeInputStage.onFinishedInputEvent(ViewRootImpl.java:3874) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.inputmethod.InputMethodManager$PendingEvent.run(InputMethodManager.java:2208) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.inputmethod.InputMethodManager.invokeFinishedInputEventCallback(InputMethodManager.java:1849) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.inputmethod.InputMethodManager.finishedInputEvent(InputMethodManager.java:1840) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.inputmethod.InputMethodManager$ImeInputEventSender.onInputEventFinished(InputMethodManager.java:2185) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.view.InputEventSender.dispatchInputEventFinished(InputEventSender.java:141) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.os.MessageQueue.nativePollOnce(Native Method) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.os.MessageQueue.next(MessageQueue.java:143) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.os.Looper.loop(Looper.java:122) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at android.app.ActivityThread.main(ActivityThread.java:5257) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at java.lang.reflect.Method.invoke(Native Method) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at java.lang.reflect.Method.invoke(Method.java:372) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903) 
10-10 22:42:23.926 28567-28567/pi.com.ndtc E/AndroidRuntime:  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698) 

的logcat的指向此行

at pi.com.ndtc.SocialActivity.onKeyDown(SocialActivity.java:343) 

這是在這裏：

if ((keyCode == KeyEvent.KEYCODE_BACK) && myWebViewInstagram.canGoBack()) 

這是我的方法，我用它來嘗試在網絡視圖中向後導航：

 @Override 
public boolean onKeyDown(int keyCode, KeyEvent event) { 
    WebView myWebViewInstagram = (WebView) findViewById(R.id.webview_instagram); 



    // Check if the key event was the Back button and if there's history 
    if ((keyCode == KeyEvent.KEYCODE_BACK) && myWebViewInstagram.canGoBack()) { 
     myWebViewInstagram.goBack(); 

     return true; 
    } 


    // If it wasn't the Back key or there's no web page history, bubble up to the default 
    // system behavior (probably exit the activity) 
    return super.onKeyDown(keyCode, event); 
} 

我的片段：

public class FragmentThree extends Fragment { 
public FragmentThree(){ 
    //Required empty constructor 
} 

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

@Override 
public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState){ 
    // Inflate the layout for this fragment 
    //return inflater.inflate(R.layout.fragment_two, container, false); 

    //Using the "View" to obtain the WebView 
    View viewInsta = inflater.inflate(R.layout.fragment_three, container, false); 

    //in app WebView 
    WebView myInstagramWebView = (WebView) viewInsta.findViewById(R.id.webview_instagram); 
    WebSettings webSettings = myInstagramWebView.getSettings();//required for enabling javascript 
    webSettings.setJavaScriptEnabled(true); //javascript was disabled by default by webView; i enabled it 
    myInstagramWebView.setWebViewClient(new WebViewClient()); 
    myInstagramWebView.setWebChromeClient(new WebChromeClient()); 
    final ProgressDialog dialog; 
    dialog = new ProgressDialog(getActivity(),R.style.MyDialogTheme); 
    //dialog.setMessage("Loading..."); 
    dialog.show(); 
    myInstagramWebView.loadUrl("https://instagram.com/ndtcjamaica/"); 

    myInstagramWebView.setWebViewClient(new WebViewClient() { 
              public void onPageFinished(WebView myInstagramWebView, String url) { 
               // DISMISS PROGRESS DIALOG HERE 
               dialog.dismiss(); 
              } 
             } 
    ); 

    return viewInsta; 
    } 
} 

Answer 1

使用這一個mWebView應該是你的WebView

您必須聲明它

private WebView mWebView; 

初始化它在onCreate()方法你應該很好走。

mWebView = (WebView) findViewById(R.id.webView); 

然後輸入如下代碼活動

@Override 
public boolean onKeyDown(int keyCode, KeyEvent event) { 
    if (event.getAction() == KeyEvent.ACTION_DOWN) { 
     switch (keyCode) { 
      case KeyEvent.KEYCODE_BACK: 
       if (mWebView.canGoBack()) { 
        mWebView.goBack(); 
       } else { 
        finish(); 
       } 
       return true; 
     } 

    } 
    return super.onKeyDown(keyCode, event); 
} 

如果你使用這一個片段

webView.setOnKeyListener(new OnKeyListener() 
{ 
    @Override 
    public boolean onKey(View v, int keyCode, KeyEvent event) 
    { 
     if(event.getAction() == KeyEvent.ACTION_DOWN) 
     { 
      WebView webView = (WebView) v; 

      switch(keyCode) 
      { 
       case KeyEvent.KEYCODE_BACK: 
        if(webView.canGoBack()) 
        { 
         webView.goBack(); 
         return true; 
        } 
        break; 
      } 
     } 

     return false; 
    } 
}); 

來調用的所有片段用波紋管方法

((YourActivityClassName)getActivity()).yourPublicMethod();