在 android 中了解 WebView 的 onBackPressed 方法

Question

请高人帮我解惑。我正在使用 android studio 构建一个简单的 WebView 应用程序，并具有覆盖方法 onBackPressed() 以在有人触摸后退按钮时使功能返回。不幸的是，我的应用程序因错误 "App has stopped working" 而关闭，同时 android studio 也显示了一些运行时间错误。我的代码如下：- 这都不起作用：

@Override
public void onBackPressed() 
{
    if (view.canGoBack())
    {
        view.goBack();
    } else
    {
        //otherwise exit my app
        super.onBackPressed();
    }
}

这也行不通：

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_BACK) {
    view.goBack();
    return true;
}

return super.onKeyDown(keyCode, event);
}

不知道哪里不对，请大神指点。

更新： 当我运行我的应用程序在模拟器中时它工作正常但是当我点击我设备上的后退按钮时它应该返回到上一页而不是它停止工作设备出现错误 "Unfortunately, appname has stopped working" 并且 android studio 中的错误消息如下：-

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: pk.shippingexpress.shippingexpress, PID: 1996
              java.lang.NullPointerException
                  at pk.shippingexpress.shippingexpress.MainActivity.onBackPressed(MainActivity.java:42)
                  at android.app.Activity.onKeyUp(Activity.java:2193)
                  at android.view.KeyEvent.dispatch(KeyEvent.java:2664)
                  at android.app.Activity.dispatchKeyEvent(Activity.java:2423)
                  at android.support.v7.app.AppCompatActivity.dispatchKeyEvent(AppCompatActivity.java:547)
                  at android.support.v7.view.WindowCallbackWrapper.dispatchKeyEvent(WindowCallbackWrapper.java:59)
                  at android.support.v7.app.AppCompatDelegateImplBase$AppCompatWindowCallbackBase.dispatchKeyEvent(AppCompatDelegateImplBase.java:319)
                  at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:1962)
                  at android.view.ViewRootImpl$ViewPostImeInputStage.processKeyEvent(ViewRootImpl.java:3852)
                  at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:3826)
                  at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3399)
                  at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3449)
                  at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3418)
                  at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3525)
                  at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3426)
                  at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:3582)
                  at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3399)
                  at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3449)
                  at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3418)
                  at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3426)
                  at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3399)
                  at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3449)
                  at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3418)
                  at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3558)
                  at android.view.ViewRootImpl$ImeInputStage.onFinishedInputEvent(ViewRootImpl.java:3718)
                  at android.view.inputmethod.InputMethodManager$PendingEvent.run(InputMethodManager.java:2010)
                  at android.view.inputmethod.InputMethodManager.invokeFinishedInputEventCallback(InputMethodManager.java:1704)
                  at android.view.inputmethod.InputMethodManager.finishedInputEvent(InputMethodManager.java:1695)
                  at android.view.inputmethod.InputMethodManager$ImeInputEventSender.onInputEventFinished(InputMethodManager.java:1987)
                  at android.view.InputEventSender.dispatchInputEventFinished(InputEventSender.java:141)
                  at android.os.MessageQueue.nativePollOnce(Native Method)
                  at android.os.MessageQueue.next(MessageQueue.java:138)
                  at android.os.Looper.loop(Looper.java:123)
                  at android.app.ActivityThread.main(ActivityThread.java:5017)
                  at java.lang.reflect.Method.invokeNative(Native Method)
                  at java.lang.reflect.Method.invoke(Method.java:515)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
                  at dalvik.system.NativeStart.main(Native Method)

我的MainActivity.java的完整代码如下：-

package pk.shippingexpress.shippingexpress;

import android.net.Uri;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.KeyEvent;
import android.webkit.WebView;
import android.webkit.WebViewClient;

import com.google.android.gms.appindexing.Action;
import com.google.android.gms.appindexing.AppIndex;
import com.google.android.gms.appindexing.Thing;
import com.google.android.gms.common.api.GoogleApiClient;

public class MainActivity extends AppCompatActivity {

    public WebView view;
    /**
     * ATTENTION: This was auto-generated to implement the App Indexing API.
     * See https://g.co/AppIndexing/AndroidStudio for more information.
     */
    private GoogleApiClient client;

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

        String url = "http://easyonlineconverter.com";
        WebView view = (WebView) this.findViewById(R.id.express);
        view.getSettings().setJavaScriptEnabled(true);
        view.loadUrl(url);
        view.setWebViewClient(new WebViewClient());

        // ATTENTION: This was auto-generated to implement the App Indexing API.
        // See https://g.co/AppIndexing/AndroidStudio for more information.
        client = new GoogleApiClient.Builder(this).addApi(AppIndex.API).build();
    }
    @Override
public void onBackPressed() 
{
    if (view.canGoBack())
    {
        view.goBack();
    } else 

    {
        //otherwise exit my app
        super.onBackPressed();
    }
}


    /**
     * ATTENTION: This was auto-generated to implement the App Indexing API.
     * See https://g.co/AppIndexing/AndroidStudio for more information.
     */
    public Action getIndexApiAction() {
        Thing object = new Thing.Builder()
                .setName("Main Page") // TODO: Define a title for the content shown.
                // TODO: Make sure this auto-generated URL is correct.
                .setUrl(Uri.parse("http://[ENTER-YOUR-URL-HERE]"))
                .build();
        return new Action.Builder(Action.TYPE_VIEW)
                .setObject(object)
                .setActionStatus(Action.STATUS_TYPE_COMPLETED)
                .build();
    }

    @Override
    public void onStart() {
        super.onStart();

        // ATTENTION: This was auto-generated to implement the App Indexing API.
        // See https://g.co/AppIndexing/AndroidStudio for more information.
        client.connect();
        AppIndex.AppIndexApi.start(client, getIndexApiAction());
    }

    @Override
    public void onStop() {
        super.onStop();

        // ATTENTION: This was auto-generated to implement the App Indexing API.
        // See https://g.co/AppIndexing/AndroidStudio for more information.
        AppIndex.AppIndexApi.end(client, getIndexApiAction());
        client.disconnect();
    }
}

Answer 1

java.lang.NullPointerException
                  at pk.shippingexpress.shippingexpress.MainActivity.onBackPressed(MainActivity.java:42)

再次检查您在这一行中的内容。我正在使用完全相同的代码并且没有任何问题。

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

Answer 2

首先声明一个 class 变量：

    public WebView view;

但是随后您初始化了一个局部变量，该变量无意中隐藏了 class 变量：

    WebView view = (WebView) this.findViewById(R.id.express);

所以当你到达这条线时

    if (view.canGoBack())

view 为空。

将其他行更改为

    view = (WebView) this.findViewById(R.id.express);

获得正确的操作。

在 android 中了解 WebView 的 onBackPressed 方法

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