java - Android 在 webview 中启用后退按钮

Question

我正在使用以下代码在我的 Android 应用中显示 WebView 。

package com.company.myapp;

import com.google.android.apps.analytics.GoogleAnalyticsTracker;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.Window;
import android.webkit.WebChromeClient;
import android.webkit.WebView;
import android.webkit.WebViewClient;
import android.widget.Toast;

public class ArticlesActivity extends Activity {

    /** Initialize the Google Analytics Tracker */
    GoogleAnalyticsTracker tracker;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        getWindow().requestFeature(Window.FEATURE_PROGRESS);
        getWindow().setFeatureInt(Window.FEATURE_PROGRESS, Window.PROGRESS_VISIBILITY_ON);
        WebView webview = new WebView(this);
        setContentView(webview); 
        setProgressBarVisibility(true); 
        webview.getSettings().setJavaScriptEnabled(true);
        webview.setScrollBarStyle(View.SCROLLBARS_INSIDE_OVERLAY);
        final Activity activity = this;
        tracker = GoogleAnalyticsTracker.getInstance();
        // Start the tracker, updating google every 20 seconds
        tracker.start((String) getText(R.string.analyticsID), 20, this);

        webview.setWebChromeClient(new WebChromeClient() { 
          public void onProgressChanged(WebView view, int progress) { 
            activity.setProgress(progress * 100 ); 
          } 
        }); 

        webview.setWebViewClient(new WebViewClient() { 
            public void onReceivedError(WebView view, int errorCode, String description, String failingUrl) {
                Toast.makeText(activity, "Oh no! " + description, Toast.LENGTH_SHORT).show();
            }
        });
        webview.loadUrl("http://www.google.com");
    }
    @Override
    public void onResume() {
        tracker.trackPageView("ArticlesActivity");
        super.onResume();
    }
    @Override
    protected void onDestroy() {
      super.onDestroy();
      // Stop the tracker when it is no longer needed.
      tracker.stop();
    }
}

如果存在历史记录，我需要启用后退按钮才能后退，而不仅仅是退出 WebView 。

I've tried many different code examples such as this但不能得到任何工作。当按下后退按钮时，应用程序将关闭。

这是我的带有后退按钮代码的代码，但是当按下后退按钮时它只会使应用程序崩溃:

package com.company.myapp;

import com.google.android.apps.analytics.GoogleAnalyticsTracker;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.Window;
import android.webkit.WebChromeClient;
import android.webkit.WebView;
import android.webkit.WebViewClient;
import android.widget.Toast;

public class ArticlesActivity extends Activity {

    WebView webview;

    /** Initialize the Google Analytics Tracker */
    GoogleAnalyticsTracker tracker;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        getWindow().requestFeature(Window.FEATURE_PROGRESS);
        getWindow().setFeatureInt(Window.FEATURE_PROGRESS, Window.PROGRESS_VISIBILITY_ON);
        WebView webview = new WebView(this);
        setContentView(webview); 
        setProgressBarVisibility(true); 
        webview.getSettings().setJavaScriptEnabled(true);
        webview.setScrollBarStyle(View.SCROLLBARS_INSIDE_OVERLAY);
        final Activity activity = this;
        tracker = GoogleAnalyticsTracker.getInstance();
        // Start the tracker, updating google every 20 seconds
        tracker.start((String) getText(R.string.analyticsID), 20, this);

        webview.setWebChromeClient(new WebChromeClient() { 
          public void onProgressChanged(WebView view, int progress) { 
            activity.setProgress(progress * 100 ); 
          } 
        }); 

        webview.setWebViewClient(new WebViewClient() { 
            public void onReceivedError(WebView view, int errorCode, String description, String failingUrl) {
                Toast.makeText(activity, "Oh no! " + description, Toast.LENGTH_SHORT).show();
            }
        });
        webview.loadUrl("http://www.google.com");
    }

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK) {

            webview.goBack();

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

    @Override
    public void onResume() {
        tracker.trackPageView("ArticlesActivity");
        super.onResume();
    }
    @Override
    protected void onDestroy() {
      super.onDestroy();
      // Stop the tracker when it is no longer needed.
      tracker.stop();
    }
}

有人可以帮我解决吗？

Answer 1

明白了!你这一行的问题

WebView webview = new WebView(this);

您没有使用您的成员变量，而是在函数内部创建了一个变量，因此您的成员变量在 onKeyDown 函数内部为 null。

只需将其替换为

webview = new WebView(this);