android - 在 WebView 中启用后退按钮

Question

我刚刚开始使用 Android SDK。我想用 WebView 制作一个仅显示本地网页的简单应用程序。我使用了在网上找到的示例来制作它。

该应用程序运行得很好，直到我想添加一个后退按钮。当按下手机后退按钮时，应用程序崩溃。

这是我的 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;
    }

}

这是我的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>

这是我在 logcat 中的错误:

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)

提前致谢:)

Answer 1

覆盖 onKeyDown(params)，检查按下的键是否为后退按钮，检查 webview 是否可以导航到上一页，如果是则导航到上一页。如果没有网页显示，您可以完成 Activity

你可以使用下面的

@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);
}

编辑:

删除 xml 布局中的 ScrollView 。试试下面的

<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>

编辑:2

这是一个有效的示例。在三星 galaxy s3 上测试

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>

主要 Activity

 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);
}
}