自定义控件导致 Xamarin 表单框架中的空引用异常

Question

我已经构建了一个自定义 SVG 按钮，这样我就可以拥有一个带有文本和 SVG 图像的 Xamarin Forms 按钮。我让它运行良好，然后我将我的项目迁移到 .NET Standard 2.0。

现在，当更改控件上的文本时，我在 ListDictionaryInternal 上的 Xamarin Forms Framework 的 StackLayout 的 LayoutChildIntoBoundingRegion 方法中得到空引用异常。

这里是控件。这很简单。

using System.Windows.Input;
using FFImageLoading.Svg.Forms;
using Core.Interfaceses;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace CustomControls
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public class SVGImageButton : ContentView
    {
        #region Member Variables

        private readonly Button _button;
        private readonly SvgCachedImage _svgImage;

        #endregion

        #region Bindible Properties

        public static BindableProperty ButtonPressedCommandProperty = BindableProperty.Create(nameof(ButtonPressedCommand), typeof(ICommand), typeof(SVGImageButton));
        public ICommand ButtonPressedCommand
        {
            get
            {
                return (ICommand)GetValue(ButtonPressedCommandProperty);
            }
            set
            {
                SetValue(ButtonPressedCommandProperty, value);
            }
        }

        public static BindableProperty CommandParameterProperty = BindableProperty.Create(nameof(CommandParameter), typeof(object), typeof(SVGImageButton));
        public object CommandParameter
        {
            get { return GetValue(CommandParameterProperty); }
            set { SetValue(CommandParameterProperty, value); }
        }

        public static BindableProperty ButtonTextProperty = BindableProperty.Create(nameof(ButtonText), typeof(string), typeof(SVGImageButton), string.Empty, BindingMode.TwoWay, propertyChanged: (bindable, oldValue, newValue) =>
        {
            if (newValue == null) return;
            var control = (SVGImageButton)bindable;
            control.ButtonText = newValue.ToString();
        });

        public string ButtonText
        {
            get
            {
                return _button.Text;
            }
            set
            {
                _button.Text = value;
            }
        }

        public static BindableProperty ButtonBackgroundColorProperty = BindableProperty.Create(nameof(ButtonBackgroundColor), typeof(Color), typeof(View), Color.Fuchsia, BindingMode.OneWay, propertyChanged: (bindable, oldValue, newValue) =>
        {
            if (newValue == null) return;
            var control = (SVGImageButton)bindable;
            control.ButtonBackgroundColor = (Color)newValue;
        });

        public Color ButtonBackgroundColor
        {
            get
            {
                return _button.BackgroundColor;
            }
            set
            {
                _button.BackgroundColor = value;
            }
        }

        public static BindableProperty ButtonTextColorProperty = BindableProperty.Create(nameof(ButtonTextColor), typeof(Color), typeof(View), Color.Fuchsia, BindingMode.OneWay, propertyChanged: (bindable, oldValue, newValue) =>
        {
            if (newValue == null) return;
            var control = (SVGImageButton)bindable;
            control.ButtonTextColor = (Color)newValue;
        });

        public Color ButtonTextColor
        {
            get
            {
                return _button.TextColor;
            }
            set
            {
                _button.TextColor = value;
            }
        }

        public static BindableProperty ButtonStyleProperty = BindableProperty.Create(nameof(ButtonBackgroundColor), typeof(Style), typeof(View), null, BindingMode.OneWay, propertyChanged: (bindable, oldValue, newValue) =>
        {
            if (newValue == null) return;
            var control = (SVGImageButton)bindable;
            control.ButtonStyle = (Style)newValue;
        });

        public Style ButtonStyle
        {
            get
            {
                return _button.Style;
            }
            set
            {
                _button.Style = value;
            }
        }

        public static BindableProperty ButtonBorderWidthProperty = BindableProperty.Create(nameof(ButtonBorderWidth), typeof(double), typeof(View), 0.0, BindingMode.OneWay, propertyChanged: (bindable, oldValue, newValue) =>
        {
            if (newValue == null) return;
            var control = (SVGImageButton)bindable;
            control.ButtonBorderWidth = (double)newValue;
        });

        public double ButtonBorderWidth
        {
            get
            {
                return _button.BorderWidth;
            }
            set
            {
                _button.BorderWidth = value;
            }
        }

        public static BindableProperty ButtonBorderColorProperty = BindableProperty.Create(nameof(ButtonBorderColor), typeof(Color), typeof(View), Color.Fuchsia, BindingMode.OneWay, propertyChanged: (bindable, oldValue, newValue) =>
        {
            if (newValue == null) return;
            var control = (SVGImageButton)bindable;
            control.ButtonBorderColor = (Color)newValue;
        });

        public Color ButtonBorderColor
        {
            get
            {
                return _button.BorderColor;
            }
            set
            {
                _button.BorderColor = value;
            }
        }

        #endregion

        #region Properties

        public string SVGImageName { get; set; }

        #endregion

        #region Life Cycle

        protected override void OnParentSet()
        {
            base.OnParentSet();

            _button.Text = ButtonText;
            _svgImage.Source = SvgImageSource.FromResource(SVGImageName);
        }

        protected override SizeRequest OnMeasure(double widthConstraint, double heightConstraint)
        {
            _button.HorizontalOptions = (Parent as View).HorizontalOptions;
            _button.VerticalOptions = (Parent as View).VerticalOptions;

            if (_button.Text != null)
            {
                var size = DependencyService.Get<ITextMeter>().MeasureTextSize(_button.Text.TrimEnd(), 200, _button.FontSize);
                WidthRequest = size.Width + 60 + _button.Height;
            }

            return base.OnMeasure(widthConstraint, heightConstraint);
        }

        public SVGImageButton()
        {
            var content = new RelativeLayout();

            _button = new Button { BackgroundColor = Color.Gray, TextColor = Color.Black };
            _svgImage = new SvgCachedImage { IsEnabled = false };

            content.Children.Add(_button,
                                 Constraint.RelativeToParent((parent) =>
                                 {
                                     return parent.X;
                                 }),
                                 Constraint.RelativeToParent((parent) =>
                                 {
                                     return parent.Y;
                                 }),
                                 Constraint.RelativeToParent((parent) =>
                                 {
                                     return parent.Width;
                                 }),
                                 Constraint.RelativeToParent((parent) =>
                                 {
                                     return parent.Height;
                                 }));

            content.Children.Add(_svgImage,
            Constraint.RelativeToParent((parent) =>
            {
                return parent.Width - (parent.Height / 2) - (parent.Height / 4);
            }),
            Constraint.RelativeToParent((parent) =>
            {
                return parent.Height - (parent.Height / 2) - (parent.Height / 4);
            }),
            Constraint.RelativeToParent((parent) =>
            {
                return parent.Height / 2;
            }),
            Constraint.RelativeToParent((parent) =>
            {
                return parent.Height / 2;
            }));

            Content = content;

            _button.Clicked += (sender, e) =>
            {
                ButtonPressedCommand?.Execute(CommandParameter);
            };
        }

        #endregion

    }
}

这是该控件在 XAML 中的用法。

<customcontrols:SVGImageButton
               x:Name="TypeSelectionButton" 
               HorizontalOptions="Start" 
               VerticalOptions="Center"
               ButtonPressedCommand="{Binding TypeButtonClickedCommand}"
               SVGImageName="SVGImages.ic_triangle_down.svg"
               CommandParameter="{x:Reference TypeSelectionButton}"
               ButtonText="{Binding SearchTypeButtonText}"
               ButtonBackgroundColor="{Binding ButtonBackgroundColor}"/>

在迁移到 .NET Standard 2.0 之前，当文本更改时，控件会调整大小，现在它会崩溃。

非常感谢我能理解正在发生的事情的任何帮助！

Answer 1

好的，在查看我的代码之后，我能够看到错误。在 OnMeasure 回调中，我正在测量文本，然后设置 SVGButton 的 ContentView 的 WidthRequest。我是这样修改的，现在它可以正常工作了。

protected override SizeRequest OnMeasure(double widthConstraint, double heightConstraint)
{
    _button.HorizontalOptions = (Parent as View).HorizontalOptions;
    _button.VerticalOptions = (Parent as View).VerticalOptions;

    var width = 0d;
    if (_button.Text != null)
    {
        var size = DependencyService.Get<ITextMeter>().MeasureTextSize(_button.Text.TrimEnd(), 200, _button.FontSize);
        width = size.Width + 40 + _button.Height;
    }

    return base.OnMeasure(width, heightConstraint);
}