问题描述
我将VS2015与Xamarin一起使用来创建一个多平台项目,该项目可以显示启动画面,然后在Webview中加载网页.这是我的项目结构.我正在使用如下PCL项目类型:
I am using VS2015 with Xamarin to create a multi-platform project that can show a splashscreen and then load a webpage in a webview. Here is my project structure. I am using a PCL project type as below:
TheApp(便携式)
-WebPageHoster.Xaml //Contains a WebView control
-WebPageHoster.Xaml.cs //sets the WebView controls source property to load a webpage
-App.Xaml
-App.Xaml.cs
TheApp.Droid
/Resources/drawable/splashlogo.png
/Resources/drawable/icon3.png
/Resources/values/Styles.xml
-MainActivity.cs
-SplashActivity.cs
TheApp.iOS
TheApp.WindowsPhone(Windows Phone 8.1)
这是Styles.xml
<?xml version="1.0" encoding="utf-8" ?>
<resources>
<style name="Theme.Splash" parent="android:Theme">
<item name="android:windowBackground">@drawable/splashlogo</item>
<item name="android:windowNoTitle">true</item>
</style>
</resources>
SplashActivity.cs
[Activity(MainLauncher = true, NoHistory = true, Theme = "@style/Theme.Splash", Icon = "@drawable/icon3")]
public class SplashActivity : AppCompatActivity
{
protected override void OnCreate(Bundle savedInstanceState)
{
base.OnCreate(savedInstanceState);
var intent = new Intent(this, typeof(MainActivity));
StartActivity(intent);
Finish();
}
}
MainActivity.cs
[Activity(Label = "Splash App", ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsApplicationActivity
{
protected override void OnCreate(Bundle bundle)
{
base.OnCreate(bundle);
global::Xamarin.Forms.Forms.Init(this, bundle);
LoadApplication(new App()); // << Problem here
}
上述OnCreate
方法中的LoadApplication
方法调用会加载App.Xaml.cs类的应用程序构造函数,该类将运行以下代码
The LoadApplication
method call in the OnCreate
method above loads the app constructor of the App.Xaml.cs class which runs the following code
公共应用程序() { InitializeComponent(); MainPage = new NavigationPage(new WebPageHoster(){标题=加载网页"}); }
public App() { InitializeComponent(); MainPage = new NavigationPage(new WebPageHoster() { Title = "Load Web-Page" }); }
这将显示初始屏幕,并在设置WebView的URL后返回到OnCreate方法并给出此错误
This shows the splash screen and after setting the WebView's url reaches back to the OnCreate method and gives this error
System.NullReferenceException: Object reference not set to an instance of an object
我找不到导致此错误的原因.这是Android Manifest文件.
I am unable to find what is causing this error. Here is the Android Manifest file.
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" package="TheApp.TheApp" android:installLocation="auto" android:versionCode="1">
<uses-sdk android:minSdkVersion="15" />
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.CHANGE_NETWORK_STATE" />
<application android:icon="@drawable/icon3" android:theme="@style/Theme.AppCompat" android:label="Splash Web Host"></application>
</manifest>
推荐答案
我测试了您的源代码,并且您的MainActivity
是从FormsAppCompatActivity
派生的.在这种情况下,您必须提供使用AppCompat
的主题.
I tested your source code and your MainActivity
derives from FormsAppCompatActivity
.In that case you must provide a theme that uses AppCompat
.
为解决这个问题,我在您的样式中添加了AppCompat
主题:
To solve that, I added an AppCompat
Theme to your styles:
<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
<item name="windowNoTitle">true</item>
<item name="windowActionBar">false</item>
<item name="windowActionModeOverlay">true</item>
</style>
然后在MainActivity
上使用它:
[Activity(Label = "Southern Travel", Theme = "@style/AppTheme", ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
这篇关于Xamarin表单MainActivity OnCreate LoadApplication System.NullReferenceException:对象引用未设置为对象的实例的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持!