Xamarin 布局无法获得焦点

Question

我正在尝试在 Xamarin Forms 中创建一个名为 FormElement 的复合视图组件，它由两个标签和一个条目组成：

<?xml version="1.0" encoding="UTF-8"?>
<StackLayout xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:custom="clr-namespace:Mynamespace;assembly=Mynamespace"
         x:Class="Mynamespace.Components.FormEntry">
    <StackLayout Orientation="Horizontal">
        <Label x:Name="formRequiredStar"
               IsVisible="{Binding IsRequired}"
               Text="*" TextColor="Red"
               FontSize="15"
               FontAttributes="Bold"
               Margin="-12,0,0,0"
               HorizontalOptions="Start" />
        <Label x:Name="formLabel"
               HorizontalOptions="Start"
               Text="{Binding LabelText}"
               TextColor="{Binding LabelTextColor}"
               FontSize="{Binding LabelTextFontSize}"
               FontAttributes="{Binding LabelTextFontStyle}" />
    </StackLayout>
    <Frame BorderColor="Black"
           CornerRadius="7"
           Padding="5,0"
           Margin="0,-3,0,0"
           HasShadow="false">
        <Entry x:Name="mainEntry"
               Keyboard="{Binding KeybdType}"
               Placeholder="{Binding EntryPlaceHolder}"
               TextColor="Black"
               FontSize="Default"
               HeightRequest="{Binding EntryHeight}" />
    </Frame>
</StackLayout>

接下来，当用户点击 DONE 按钮时，我想将焦点从 Entry 切换到“下一个”元素，所以我这样做了：

    namespace Mynamespace.Components
    {
        public partial class FormEntry : StackLayout
        {
            public VisualElement NextFocus
            {
                get { return (VisualElement)GetValue(NextFocusProperty); }
                set { SetValue(NextFocusProperty, value); }
            }

            public static readonly BindableProperty NextFocusProperty =
                BindableProperty.Create(nameof(NextFocus),
                                        typeof(VisualElement),
                                        typeof(FormEntry),
                                        null,
                                     Xamarin.Forms.BindingMode.OneWay);

            public FormEntry()
            {
                InitializeComponent();
                BindingContext = this;

                mainEntry.Completed += (s, e) =>
                {
                    if (NextFocus != null)
                    {
                        NextFocus.Focus();
                    }
                };
            }
        }
    }

接下来，为了让 FormEntry 成为 NextFocus 的目标，我尝试添加

    this.Focused += (s,e) => { mainEntry.Focus(); };

到构造函数，但从不调用处理程序，我也尝试过覆盖

    public new void Focus() {
        mainEntry.Focus();
    }

但是这个方法永远不会被调用。布局类是 VisualElement 的后代，因此它们应该继承 Focused。布局对象有什么我缺少的吗？我可以理解 Layout 对象通常不是焦点目标，但事件处理程序应该在那里，所以我应该能够使用它。

这是我如何在登录屏幕上使用 FormEntry 的示例：

    <!-- Email -->
    <controls:FormEntry x:Name="usernameEntry"
                        Margin="25,40,25,0"
                        IsRequired="true"
                        EntryHeight="40"
                        KeybdType="Email"
                        NextFocus="{x:Reference passwordEntry}"
                        LabelText="{il8n:Translate Emailorusername}"
                        EntryPlaceHolder="{il8n:Translate EnterUsername}">
    </controls:FormEntry>

    <!-- Password -->
    <controls:FormEntry x:Name="passwordEntry"
                        Margin="25,0,25,0"
                        IsRequired="true"
                        EntryHeight="40"
                        LabelText="{il8n:Translate Password}"
                        EntryPlaceHolder="{il8n:Translate EnterPassword}" />

Answer 1

我认为你已经获得了 nextfocus 元素，你可以从 nextfocus 获得 mainEntry，像这样：

 public FormEntry ()
    {
        InitializeComponent ();
        BindingContext = this;

        mainEntry.Completed += (s, e) =>
        {
            if (NextFocus != null)
            {
                FormEntry formentry = (FormEntry)NextFocus;
                Entry entry = formentry.mainEntry;
                entry.Focus();
            }
        };
    }

然后你会发现你会得到焦点。