创建一个 Xamarin Forms 自定义输入控件（不需要本机）

下面是纯 Xamarin Forms 自定义控件的示例。没有为此做自定义渲染，但可以轻松实现，事实上，在我自己的代码中，我使用这个相同的控件以及 Label 和 Entry 的自定义渲染器。

自定义控件是一个 ContentView，其中包含 Label，Entry 和 BoxView，使用 2 个 StackLayouts 固定。我们还定义了多个可绑定属性以及 TextChanged 事件。

自定义可绑定属性的工作原理是它们位于下面，并且控件中的元素（在本例中为 Label 和 Entry）绑定到自定义可绑定属性。关于可绑定属性的一些还需要实现 BindingPropertyChangedDelegate，以使有界元素改变它们的值。

public class InputFieldContentView : ContentView {

    #region Properties

    /// <summary>
    /// Attached to the <c>InputFieldContentView</c>'s <c>ExtendedEntryOnTextChanged()</c> event, but returns the <c>sender</c> as <c>InputFieldContentView</c>.
    /// </summary>
    public event System.EventHandler<TextChangedEventArgs> OnContentViewTextChangedEvent; //In OnContentViewTextChangedEvent() we return our custom InputFieldContentView control as the sender but we could have returned the Entry itself as the sender if we wanted to do that instead.

    public static readonly BindableProperty LabelTextProperty = BindableProperty.Create("LabelText", typeof(string), typeof(InputFieldContentView), string.Empty);

    public string LabelText {
        get { return (string)GetValue(LabelTextProperty); }
        set { SetValue(LabelTextProperty, value); }
    }

    public static readonly BindableProperty LabelColorProperty = BindableProperty.Create("LabelColor", typeof(Color), typeof(InputFieldContentView), Color.Default);

    public Color LabelColor {
        get { return (Color)GetValue(LabelColorProperty); }
        set { SetValue(LabelColorProperty, value); }
    }

    public static readonly BindableProperty EntryTextProperty = BindableProperty.Create("EntryText", typeof(string), typeof(InputFieldContentView), string.Empty, BindingMode.TwoWay, null, OnEntryTextChanged);

    public string EntryText {
        get { return (string)GetValue(EntryTextProperty); }
        set { SetValue(EntryTextProperty, value); }
    }

    public static readonly BindableProperty PlaceholderTextProperty = BindableProperty.Create("PlaceholderText", typeof(string), typeof(InputFieldContentView), string.Empty);

    public string PlaceholderText {
        get { return (string)GetValue(PlaceholderTextProperty); }
        set { SetValue(PlaceholderTextProperty, value); }
    }

    public static readonly BindableProperty UnderlineColorProperty = BindableProperty.Create("UnderlineColor", typeof(Color), typeof(InputFieldContentView), Color.Black, BindingMode.TwoWay, null, UnderlineColorChanged);

    public Color UnderlineColor {
        get { return (Color)GetValue(UnderlineColorProperty); }
        set { SetValue(UnderlineColorProperty, value); }
    }

    private BoxView _underline;

    #endregion

    public InputFieldContentView() {

        BackgroundColor   = Color.Transparent;
        HorizontalOptions = LayoutOptions.FillAndExpand;

        Label label = new Label {
            BindingContext    = this,
            HorizontalOptions = LayoutOptions.StartAndExpand,
            VerticalOptions   = LayoutOptions.Center,
            TextColor         = Color.Black
        };

        label.SetBinding(Label.TextProperty, (InputFieldContentView view) => view.LabelText, BindingMode.TwoWay);
        label.SetBinding(Label.TextColorProperty, (InputFieldContentView view) => view.LabelColor, BindingMode.TwoWay);

        Entry entry = new Entry {
            BindingContext          = this,
            HorizontalOptions       = LayoutOptions.End,
            TextColor               = Color.Black,
            HorizontalTextAlignment = TextAlignment.End
        };

        entry.SetBinding(Entry.PlaceholderProperty, (InputFieldContentView view) => view.PlaceholderText, BindingMode.TwoWay);
        entry.SetBinding(Entry.TextProperty, (InputFieldContentView view) => view.EntryText, BindingMode.TwoWay);

        entry.TextChanged += OnTextChangedEvent;

        _underline = new BoxView {
            BackgroundColor   = Color.Black,
            HeightRequest     = 1,
            HorizontalOptions = LayoutOptions.FillAndExpand
        };

        Content = new StackLayout {
            Spacing           = 0,
            HorizontalOptions = LayoutOptions.FillAndExpand,
            Children          = {
                new StackLayout {
                    Padding           = new Thickness(5, 0),
                    Spacing           = 0,
                    HorizontalOptions = LayoutOptions.FillAndExpand,
                    Orientation       = StackOrientation.Horizontal,
                    Children          = { label, entry }
                }, _underline
            }
        };

        SizeChanged += (sender, args) => entry.WidthRequest = Width * 0.5 - 10;
    }

    private static void OnEntryTextChanged(BindableObject bindable, object oldValue, object newValue) {
        InputFieldContentView contentView = (InputFieldContentView)bindable;
        contentView.EntryText             = (string)newValue;
    }

    private void OnTextChangedEvent(object sender, TextChangedEventArgs args) {
        if(OnContentViewTextChangedEvent != null) { OnContentViewTextChangedEvent(this, new TextChangedEventArgs(args.OldTextValue, args.NewTextValue)); } //Here is where we pass in 'this' (which is the InputFieldContentView) instead of 'sender' (which is the Entry control)
    }

    private static void UnderlineColorChanged(BindableObject bindable, object oldValue, object newValue) {
        InputFieldContentView contentView      = (InputFieldContentView)bindable;
        contentView._underline.BackgroundColor = (Color)newValue;
    }
}

这是 iOS 上最终产品的图片（该图显示了使用 Label 和 Entry 的自定义渲染器时的样子，用于移除 iOS 上的边框并为两个元素指定自定义字体）：

我遇到的一个问题是当 UnderlineColor 发生变化时，BoxView.BackgroundColor 会发生变化。即使在绑定了 BoxView 的 BackgroundColor 属性之后，它也不会改变，直到我添加了 UnderlineColorChanged 代表。