Rails检查has_many是否存在(Rails check if has_many exist)

 两种解决方案 
 
 1）从ConfigurationChanges = ConfigChanges.ScreenSize|ConfigChanges.Orientation删除|ConfigChanges.Orientation ，这意味着您的活动将在屏幕旋转时重新创建。 
 
 2）在OnConfigurationChanged方法中，在SetImageToToolbar();之上添加以下代码SetImageToToolbar(); 方法，如下所示： 
 
    public override void OnConfigurationChanged(Configuration newConfig)
    {
        base.OnConfigurationChanged(newConfig);
        ToolbarResource = Resource.Layout.CustomToolbar;
        global::Xamarin.Forms.Forms.Init(this, null);
        LoadApplication(new App());
        SetImageToToolbar();
    }
 
 为什么要添加这些代码？ 
 
 我认为在屏幕旋转时你的代码中的imgView有问题，所以我认为OnCreate()方法中的代码会有所帮助。 
 
 更新： 
 
 第二种解决方案存在问题，如果你有很多页面，它会出错，所以我提供了另一种解决方案，使用自定义的NavigationPageRenderer ： 
 
 1）在您的pcl中添加MyNavigationPage ： 
 
namespace ToolbarTestApp
{
    class MyNavigationPage:NavigationPage
    {
        public MyNavigationPage(Page root) : base(root) {
        }
    }
}
 
 2）替换MainPage = new MyNavigationPage(new TestPage()); with MainPage = new NavigationPage(new TestPage()); 
 
 3）在你的android项目中添加MyNavigationPageRender ： 
 
[assembly: ExportRenderer(typeof(MyNavigationPage), typeof(MyNavigationPageRender))]
namespace ToolbarTestApp.Droid
{
    class MyNavigationPageRender : NavigationPageRenderer
    {
        public MyNavigationPageRender(Context context) : base(context)
        {
        }

        protected override void OnLayout(bool changed, int l, int t, int r, int b)
        {
            base.OnLayout(changed, l, t, r, b);
            (Xamarin.Forms.Forms.Context as MainActivity).SetImageToToolbar();
        }
    }
}
 
 将您的SetImageToToolbar方法更改为public，并且您无需重写OnConfigurationChanged方法，只需将其删除即可。 

Two solutions:
 
1) Remove |ConfigChanges.Orientation from ConfigurationChanges = ConfigChanges.ScreenSize|ConfigChanges.Orientation, that mean your activity will recreate when your screen rotate. 
 
2) Add below code in your OnConfigurationChanged method, above the SetImageToToolbar(); method, like this:
 
    public override void OnConfigurationChanged(Configuration newConfig)
    {
        base.OnConfigurationChanged(newConfig);
        ToolbarResource = Resource.Layout.CustomToolbar;
        global::Xamarin.Forms.Forms.Init(this, null);
        LoadApplication(new App());
        SetImageToToolbar();
    }
 
Why should you add these code?
 
I think there is something wrong in the imgView in your code when screen rotating, so I think the codes in OnCreate() method will help it.
 
Update:
 
There is something wrong in the second solution, if you have many pages, it will go wrong, so I provide another solution, use custom NavigationPageRenderer:
 
1) Add MyNavigationPage in your pcl:
 
namespace ToolbarTestApp
{
    class MyNavigationPage:NavigationPage
    {
        public MyNavigationPage(Page root) : base(root) {
        }
    }
}
 
2) Replace MainPage = new MyNavigationPage(new TestPage()); with MainPage = new NavigationPage(new TestPage());
 
3)Add MyNavigationPageRender in your android project:
 
[assembly: ExportRenderer(typeof(MyNavigationPage), typeof(MyNavigationPageRender))]
namespace ToolbarTestApp.Droid
{
    class MyNavigationPageRender : NavigationPageRenderer
    {
        public MyNavigationPageRender(Context context) : base(context)
        {
        }

        protected override void OnLayout(bool changed, int l, int t, int r, int b)
        {
            base.OnLayout(changed, l, t, r, b);
            (Xamarin.Forms.Forms.Context as MainActivity).SetImageToToolbar();
        }
    }
}
 
Change your SetImageToToolbar method to public, and you needn't override OnConfigurationChanged method, just remove it.