android - 防止 Android WebView 在 Xamarin 环境中重新加载页面旋转

Question

我意识到有很多与此相关的问题，但我已经尝试了所有本地开发解决方案，但无法让它们在 Xamarin 下工作。

我有一个 webview，它加载了一个 URL，其中包含用于绘图的图形 UI。当设备旋转时，webview 重新加载页面，用户的工作丢失。

下面的博文给出了最好的解决方案，并解释了广泛记录的解决方案是不够的，原因如下:

http://www.devahead.com/blog/2012/01/preserving-the-state-of-an-android-webview-on-screen-orientation-change/

The main problem with this implementation is that, whenever you rotate the screen, the >WebView is created again because the activity is destroyed and its saveState method doesn’t >save the full state, but only a part of it like the URL of the page that was loaded and the >browsing history. So it happens that for example the zoom and the scroll position are not >preserved after the screen orientation change and sometimes the page is reloaded from the web.

所以我已经实现了他们的解决方案，将其从 Java 移植到 Xamarin C#。它看起来很有希望，但每当我的 Activity 重新加载并调用 InitUi 时，web_view 类变量始终为空，而该示例依赖于此类变量在设备旋转之间保持其值。

有人知道为什么 web_view 失去了它的值(value)吗？

这是我的代码，它与文章完全一样，但移植到了 Xamarin C#。

public class EbookViewerActivity : ActionBarActivity
{
    protected WebView web_view;
    protected FrameLayout webViewPlaceholder;

    Button loadButton;
    Button downloadButton;
    EditText testUrlText;
    private string viewerPath;

    protected override void OnCreate(Bundle savedInstanceState)
    {
        #region Set up activity and action bar

        //Create the activity screen and initialise the action bar
        base.OnCreate(savedInstanceState);
        RequestWindowFeature(WindowFeatures.NoTitle);
        SetContentView(Resource.Layout.Viewer);
        InitializeActionBar();

        //Set action bar button delegates
        ActionBar
            .AddLeftAction(new DelegateAction(Finish, Resource.Drawable.CloseIcon))
            .SetTitle("Ebook Annotator");

        //Set action bar logo
        ActionBar.SetHomeLogo(Resource.Drawable.AcmeTrainingLogo);
        #endregion

        InitUi();
    }

    private void InitUi()
    {
        Logger logger = Logger.Instance;
        // Retrieve UI elements
        webViewPlaceholder = FindViewById<FrameLayout>(Resource.Id.webViewPlaceholder);
        // Initialize the WebView if necessary
        if (web_view == null)
        {
            web_view = new WebView(this);
            web_view.Id = Resource.Id.ebookDynamicWebView;

            //web_view = FindViewById<WebView>(Resource.Id.ebookWebview);
            web_view.Settings.JavaScriptEnabled = true;
            web_view.AddJavascriptInterface(new AnnotationApiProxy(this), "AnnotationApi");
            string ebookId = Intent.GetStringExtra("ebookId");
            string userEmail = Intent.GetStringExtra("userEmail");
            Ebook ebook = EbookManager.GetEbook(int.Parse(ebookId));

            GlobalVariableHolder.Instance.EbookToOpen = int.Parse(ebookId);
            viewerPath = "file:///android_asset/Annotator/annotator.html";

            web_view.Settings.AllowFileAccess = true;
            logger.WriteToLog("Loading test harness with ebook id: " + ebookId, LogEntryLevel.Message);
            web_view.SetWebChromeClient(new EbookWebViewClient() { });
           // web_view.SetWebViewClient(new WebViewClient());
            AnnotationDownloader annotationDownloader = new AnnotationDownloader();
           // annotationDownloader.XmlDownloaded += (sender, args) => RunOnUiThread(() =>
         //   {
                web_view.LoadUrl(viewerPath);
           // });
            annotationDownloader.GetLatestEWorkBookXml(ebook.ID, userEmail);
        }

        // Attach the WebView to its placeholder
        webViewPlaceholder.AddView(web_view);
    }

    protected override void OnSaveInstanceState(Bundle outState)
    {
        base.OnSaveInstanceState(outState);
        // Save the state of the WebView
        web_view.SaveState(outState);
    }

    protected override void OnRestoreInstanceState(Bundle savedInstanceState)
    {
        base.OnRestoreInstanceState(savedInstanceState);
        // Restore the state of the WebView
        web_view.RestoreState(savedInstanceState);
    }

    public override void OnConfigurationChanged(Configuration newConfig)
    {
        if (web_view != null)
        {
            // Remove the WebView from the old placeholder
            webViewPlaceholder.RemoveView(web_view);
        }
        base.OnConfigurationChanged(newConfig);
        // Load the layout resource for the new configuration
        SetContentView(Resource.Layout.Viewer);
        // Reinitialize the UI
        InitUi();
    }
}

Answer 1

不要更改 AndroidManifest.xml 并使用自定义属性。

[Activity(ConfigurationChanges=ConfigChanges.Orientation | ConfigChanges.ScreenSize)] 
    public partial class MyActivity : Activity { ...

这个解决方案对我很有效。