7

I am just starting out with the Android SDK. I wanted to make a simple app with WebView that displays local web-pages only. I have used examples I have found on the net to make it.

The app works just great, until I want to add a back-button. When the phones back-button is pressed the app crashes.

Here is my MainActivity.java

package com.hspd.avhor;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.webkit.WebView;
import android.webkit.WebViewClient;

public class MainActivity extends Activity {

    WebView wv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        WebView wv = (WebView)findViewById(R.id.hspd_webview);
        wv.setWebViewClient(new WebViewClient() {
            @Override
            public boolean shouldOverrideUrlLoading(WebView view, String url){
                view.loadUrl(url);
                return true;
            }
        });
        wv.loadUrl("file:///android_asset/one.htm");
    }

    @Override
    public void onBackPressed()
    {
        if(wv.canGoBack())
            wv.goBack();
        else
            super.onBackPressed();
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

}

And here is my activity_main.xml:

<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fillViewport="true"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <WebView
        android:id="@+id/hspd_webview"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

</ScrollView>

Here are my errors in logcat:

04-22 17:48:06.953: W/dalvikvm(799): threadid=1: thread exiting with uncaught exception (group=0x40a71930)
04-22 17:48:06.972: E/AndroidRuntime(799): FATAL EXCEPTION: main
04-22 17:48:06.972: E/AndroidRuntime(799): java.lang.NullPointerException
04-22 17:48:06.972: E/AndroidRuntime(799):  at com.hspd.avhor.MainActivity.onKeyDown(MainActivity.java:41)
04-22 17:48:06.972: E/AndroidRuntime(799):  at android.view.KeyEvent.dispatch(KeyEvent.java:2609)
04-22 17:48:06.972: E/AndroidRuntime(799):  at android.app.Activity.dispatchKeyEvent(Activity.java:2375)
04-22 17:48:06.972: E/AndroidRuntime(799):  at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:1847)
04-22 17:48:06.972: E/AndroidRuntime(799):  at android.view.ViewRootImpl.deliverKeyEventPostIme(ViewRootImpl.java:3701)
04-22 17:48:06.972: E/AndroidRuntime(799):  at android.view.ViewRootImpl.handleImeFinishedEvent(ViewRootImpl.java:3651)
04-22 17:48:06.972: E/AndroidRuntime(799):  at android.view.ViewRootImpl$ViewRootHandler.handleMessage(ViewRootImpl.java:2818)
04-22 17:48:06.972: E/AndroidRuntime(799):  at android.os.Handler.dispatchMessage(Handler.java:99)
04-22 17:48:06.972: E/AndroidRuntime(799):  at android.os.Looper.loop(Looper.java:137)
04-22 17:48:06.972: E/AndroidRuntime(799):  at android.app.ActivityThread.main(ActivityThread.java:5041)
04-22 17:48:06.972: E/AndroidRuntime(799):  at java.lang.reflect.Method.invokeNative(Native Method)
04-22 17:48:06.972: E/AndroidRuntime(799):  at java.lang.reflect.Method.invoke(Method.java:511)
04-22 17:48:06.972: E/AndroidRuntime(799):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
04-22 17:48:06.972: E/AndroidRuntime(799):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
04-22 17:48:06.972: E/AndroidRuntime(799):  at dalvik.system.NativeStart.main(Native Method)

Thanks in advance :)

4

1 回答 1

15

Override onKeyDown(params), check if the key pressed is back button ,check if the webview can navigate to previous page, if so naviagate to previous page. If there is no web page to display you can finish the actiivty

You can use the below 

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if ((keyCode == KeyEvent.KEYCODE_BACK) && webView.canGoBack()) { 
            //if Back key pressed and webview can navigate to previous page
        webView.goBack();
            // go back to previous page
        return true;
    }
    else
    {
        finish();
           // finish the activity
    }
    return super.onKeyDown(keyCode, event);
}

Edit:

Remove the scrollview in xml layout. Try the below 

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity" >

<WebView
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:id="@+id/wv"></WebView>
</LinearLayout>

Edit:2

Here's a sample that works. Tested on samsung galaxy s3

activtiy_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity" >

<WebView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:layout_centerHorizontal="true"
android:layout_centerVertical="true"
android:id="@+id/wv"/>

</RelativeLayout>

MainActivity

 public class MainActivity extends Activity {
 private WebView webView;

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


        setContentView(R.layout.activity_main);
        webView = (WebView) findViewById(R.id.wv);
        webView.getSettings().setJavaScriptEnabled(true);
        webView.setWebChromeClient(new WebChromeClient() {
               public void onProgressChanged(WebView view, int progress) {
                MainActivity.this.setProgress(progress * 1000);
               }
             });
             webView.setWebViewClient(new WebViewClient() {
               public void onReceivedError(WebView view, int errorCode, String description, String failingUrl) {
                 Toast.makeText(MainActivity.this, "Oh no! " + description, Toast.LENGTH_SHORT).show();
               }
             });

             webView.loadUrl("http://slashdot.org/");

}   



@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if ((keyCode == KeyEvent.KEYCODE_BACK) && webView.canGoBack()) {
        webView.goBack();
        return true;
    }
    else
    {
        finish();
    }
    return super.onKeyDown(keyCode, event);
}
}
于 2013-04-22T16:54:46.597 回答