AbsoluteLayout - 测量标签高度而不在 UI 上放置标签答案

【问题标题】：AbsoluteLayout - Measure Label Height without placing label on UIAbsoluteLayout - 测量标签高度而不在 UI 上放置标签
【发布时间】：2022-01-25 01:32:37
【问题描述】：

我在 AbsoluteLayout 中手动定位标签。要正确执行此操作，我想在将标签放置到 UI 之前知道标签高度。

我找到了这个解决方案，但并非没有实际放置标签：

public double MeasureLabelHeight(string text, double width, double fontSize, double lineHeight, string fontFamily)
    {
        Label label = new Label();
        label.WidthRequest = width;
        label.FontSize = fontSize;
        label.LineHeight = lineHeight;
        label.FontFamily = fontFamily;
        label.LineBreakMode = LineBreakMode.WordWrap;
        label.Text = text;
        MyAbsoluteLayout.Children.Add(view: label, position: new Point(0, Height)); //place out of sight
        var sizeRequest = label.Measure(widthConstraint: width, heightConstraint: double.MaxValue, flags: MeasureFlags.None);
        var labelTextHeight = sizeRequest.Request.Height;
        MyAbsoluteLayout.Children.Remove(label);
        return labelTextHeight;
    }

此解决方案适用于 UWP，我仍需在 Android 和 iOS 上进行测试。

不过，我想改进它。如果没有将其实际放置在 AbsoluteLayout 中（看不见），我无法获得正确的高度值，并且有点担心这可能会导致额外重绘的开销。

我发现了一个old piece of code，它看似使用本机代码来执行此操作，但实际上并未将其放置在 iOS 和 Android 的 UI 中。我想知道是否有不需要平台特定代码的可用解决方案。

【问题讨论】：

标签： xamarin.forms label measure absolutelayout

【解决方案1】：

让 Xamarin Forms 为您测量它们。然后将它们移动到位。

通过继承 AbsoluteLayout 并添加一个页面可以设置的 Action 来实现这一点，以便在您的布局完成时调用 LayoutChildren。

MyAbsoluteLayout.cs：

using System;
using Xamarin.Forms;

namespace XFSOAnswers
{
    public class MyAbsoluteLayout : AbsoluteLayout
    {
        public MyAbsoluteLayout()
        {
        }

        // Containing page will set this, to act on children during LayoutChildren.
        public Action CustomLayoutAction { get; set; }

        private bool _busy;

        protected override void LayoutChildren(double x, double y, double width, double height)
        {
            // Avoid recursed layout calls as CustomLayoutAction moves children.
            if (_busy)
                return;

            // Xamarin measures the children.
            base.LayoutChildren(x, y, width, height);

            _busy = true;
            try
            {
                CustomLayoutAction?.Invoke();
            }
            finally
            {
                _busy = false;
                // Layout again, to position the children, based on adjusted (x,y)s.
                base.LayoutChildren(x, y, width, height);
            }
        }
    }
}

示例用法 - MyAbsoluteLayoutPage.xaml：

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:XFSOAnswers"
             x:Class="XFSOAnswers.MyAbsoluteLayoutPage">
    <ContentPage.Content>
        <local:MyAbsoluteLayout x:Name="TheLayout">
            <!-- Layout positions start (0,0). Adjusted later in PositionLabels. -->
            <Label x:Name="Label1" Text="Welcome" />
            <Label x:Name="Label2" Text="to" />
            <Label x:Name="Label3" Text="Xamarin" />
            <Label x:Name="Label4" Text=".Forms!" />
        </local:MyAbsoluteLayout>
    </ContentPage.Content>
</ContentPage>

MyAbsoluteLayoutPage.xaml.cs：

using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace XFSOAnswers
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class MyAbsoluteLayoutPage : ContentPage
    {
        public MyAbsoluteLayoutPage()
        {
            InitializeComponent();

            TheLayout.CustomLayoutAction = PositionLabels;
        }

        private void PositionLabels()
        {
            // Optional: Set breakpoint after these, to check that the bounds have values.
            var bounds1 = Label1.Bounds;
            var bounds2 = Label2.Bounds;
            var bounds3 = Label3.Bounds;
            var bounds4 = Label4.Bounds;

            double x = 10;
            double y = 20;
            MoveAbsoluteChildTo(Label1, x, y);
            x += Label1.Width;
            y += Label1.Height;

            MoveAbsoluteChildTo(Label2, x, y);
            x += Label2.Width;
            y += Label2.Height;

            MoveAbsoluteChildTo(Label3, x, y);
            x += Label3.Width;
            y += Label3.Height;

            MoveAbsoluteChildTo(Label4, x, y);
        }

        private static void MoveAbsoluteChildTo(View child, double x, double y)
        {
            AbsoluteLayout.SetLayoutBounds(child, new Rect(x, y, child.Width, child.Height));
        }
    }
}

结果：

参见ToolmakerSteve - repo XFormsSOAnswers 中的MyAbsoluteLayout 和MyAbsoluteLayoutPage。

【讨论】：