取消progressDialog以返回另一个活动时出现NullPointerException

Question

有人知道这里发生了什么吗？ 有没有办法让用户取消progressDialog并返回活动A而不会有任何崩溃？

重现此崩溃的步骤：

在一个活动A和我启动一个新的意图活动B tabhost，在progressDialog加载完成之前加载“class1.java”（带有progressDialog的Webview），我快速点击后退按钮返回活动A ，我得到了一个崩溃

在以下情况下，应用不会崩溃：

• 我等到webview加载完成并返回活动A.

• 我取消progressDialog并保持在webview上直到页面加载完成并返回活动A.

第51行： progressDialog.incrementProgressBy（进度）;

Logcat： http ：//pastebin.com/uBw4xcaY

java.lang.NullPointerException
at com.domain.package.ui.class1$class1_$1.onProgressChanged(class1.java:51)
at android.webkit.CallbackProxy.handleMessage(CallbackProxy.java:358)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:130)
at android.app.ActivityThread.main(ActivityThread.java:3835)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:507)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:847)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:605)
at dalvik.system.NativeStart.main(Native Method)

这是我的代码：

[...]

public class class1 extends FragmentActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.fragment_stack);

}

public static class class1_ extends Fragment {

    /** The Fragment's UI **/
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.class_, container, false);

        WebView engine = (WebView) v.findViewById(R.id.web_engine);

        final ProgressDialog progressDialog;
        progressDialog = new ProgressDialog(getActivity());
        progressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
        progressDialog.setMessage("Loading...");
        progressDialog.setCancelable(true);
        progressDialog.setProgress(0);
        progressDialog.show();

        engine.setWebChromeClient(new WebChromeClient() {

            public void onProgressChanged(WebView view, int progress) {

                progressDialog.setProgress(0);
                getActivity().setProgress(progress * 1000);

                progressDialog.incrementProgressBy(progress);

                if (progress == 100 && progressDialog.isShowing())
                    progressDialog.dismiss();
            }

        });

            engine.getSettings().setBuiltInZoomControls(false);
            engine.getSettings().setUseWideViewPort(false);
            engine.getSettings().setSupportZoom(false);
            engine.loadUrl("file:///android_asset/index.html");


        Toast.makeText(getActivity(),
                "Hello this is a toast",
                Toast.LENGTH_LONG).show();

        return v;
    }
  }
}

Answer 1

尝试更改你的onChanged()方法，如下所示：

public void onProgressChanged(WebView view, int progress) {
  if( progressDialog.isShowing() ) {
    progressDialog.setProgress(0);
    getActivity().setProgress(progress * 1000);

    progressDialog.incrementProgressBy(progress);

    if ( progress == 100 )
      progressDialog.dismiss();
  }
}

然后将其添加到片段onPause()方法：

if( progressDialog != null && progressDialog.isShowing() )
  dialog.dismiss();

Answer 2

当您取消Dialog并返回时， Dialog设置为null但您的WebChromeClient回调仍在加载，当进度更改时，在Dialog上设置进度，但Dialog为null因此导致NullPointerException 。 将setWebChromeClient()修改为。

engine.setWebChromeClient(new WebChromeClient() {
      public void onProgressChanged(WebView view, int progress) {
            if(progressDialog != null)
                progressDialog.setProgress(0);
                getActivity().setProgress(progress * 1000);
                progressDialog.incrementProgressBy(progress);
                if (progress == 100 && progressDialog.isShowing())
                    progressDialog.dismiss();
            }
      }
});