Xamarin.Forms:MultiLineLabel在Android上不再起作用 [英] Xamarin.Forms: the MultiLineLabel doesn't longer work on Android

问题描述

在Xamarin.Forms项目中，我使用 MultiLineLabel 在1或2行上显示标题，具体取决于文本的长度.我基于的是 blog 来实现这一目标.

On my Xamarin.Forms project, I use a MultiLineLabel to display a title on 1 or 2 lines, depending the text length. I'm based on this blog to achieve this.

所以我有一个MultiLineLabel控件:

public class MultiLineLabel : Label
{
    private static int _defaultLineSetting = -1;

    public static readonly BindableProperty LinesProperty = BindableProperty.Create(nameof(Lines), typeof(int), typeof(MultiLineLabel), _defaultLineSetting);
    public int Lines
    {
        get { return (int)GetValue(LinesProperty); }
        set { SetValue(LinesProperty, value); }
    }
}

我使用2个渲染器:

• 在 iOS 上，我保留了给定的渲染器:

• on iOS, I've kept the given renderer:

public class CustomMultiLineLabelRenderer : LabelRenderer
{
    protected override void OnElementChanged(ElementChangedEventArgs<Label> e)
    {
        base.OnElementChanged(e);

        MultiLineLabel multiLineLabel = (MultiLineLabel)Element;

        if (multiLineLabel != null && multiLineLabel.Lines != -1)
            Control.Lines = multiLineLabel.Lines;

    }
}

在 Android 上，我已自定义渲染器:

on Android I've customized the renderer:

public class CustomMultiLineLabelRenderer : LabelRenderer
{
    protected override void OnElementChanged(ElementChangedEventArgs<Label> e)
    {
        base.OnElementChanged(e);

        MultiLineLabel multiLineLabel = (MultiLineLabel)Element;

        if (multiLineLabel != null && multiLineLabel.Lines != -1)
        {
            Control.Ellipsize = TextUtils.TruncateAt.End;
            Control.SetMaxLines(multiLineLabel.Lines);
        }
    }
}

我在 XAML 中像这样使用 MultiLineLabel :

<StackLayout 
    Grid.Row="0"
    Spacing="0">

    <local:MultiLineLabel
        Margin="8,6,8,0"
        TextColor="{ DynamicResource InverseTextColor }"
        Text="{ Binding encart_titre }"
        FontSize="{ artina:OnOrientationDouble 
            Default=16,
            PortraitTablet=20,
            LandscapeTablet=20 }"
        LineBreakMode="TailTruncation"
        Lines="2"
        Grid.Column="0"
        BackgroundColor="Yellow"
    />
</StackLayout>

在我使用Xamarin.Forms v.2.3.4.247之前，它在Android上运行良好:

Until I used Xamarin.Forms v.2.3.4.247, this worked well on Android:

但是在更新到最新版本(Xamarin.Forms v.2.4.0.269-pre2)之后，它不再能按预期工作:

But after having updated to the latest version (Xamarin.Forms v.2.4.0.269-pre2), it doesn't longer work as expected:

我还尝试使用博客上提供的渲染器:

I also tried to use the renderer given on the blog:

protected override void OnElementChanged(ElementChangedEventArgs<Label> e)
{
    base.OnElementChanged(e);

    MultiLineLabel multiLineLabel = (MultiLineLabel)Element;

    if (multiLineLabel != null && multiLineLabel.Lines != -1)
    {
        Control.SetSingleLine(false);
        Control.SetLines(multiLineLabel.Lines);
    }
}

但是我没有得到预期的渲染:

But I didn't get the expected rendering:

您有什么解释吗?还是其他建议?在iOS上，效果很好.

Would you have any explanation? Or another suggestion? On iOS this works well.

推荐答案

但是在更新到最新版本(Xamarin.Forms v.2.4.0.269-pre2)之后，它不再能按预期工作:

But after having updated to the latest version (Xamarin.Forms v.2.4.0.269-pre2), it doesn't longer work as expected:

原因:

我已经检查了Xamarin.Forms v.2.4.0.269-pre2的源代码.在LabelRenderer的OnElementChange事件中，将调用FormsTextView的SetLineBreakMode，其中包含以下代码:

I've checked the source codes of Xamarin.Forms v.2.4.0.269-pre2. In LabelRenderer's OnElementChange event, FormsTextView's SetLineBreakMode will be called which contains following codes:

public static void SetLineBreakMode(this TextView textView, LineBreakMode lineBreakMode)
{
    switch (lineBreakMode)
    {
        case LineBreakMode.NoWrap:
            textView.SetMaxLines(1);
            textView.SetSingleLine(true);
            textView.Ellipsize = null;
            break;
        case LineBreakMode.WordWrap:
            textView.Ellipsize = null;
            textView.SetMaxLines(100);
            textView.SetSingleLine(false);
            break;
        case LineBreakMode.CharacterWrap:
            textView.Ellipsize = null;
            textView.SetMaxLines(100);
            textView.SetSingleLine(false);
            break;
        case LineBreakMode.HeadTruncation:
            textView.SetMaxLines(1);
            textView.SetSingleLine(true);
            textView.Ellipsize = TextUtils.TruncateAt.Start;
            break;
        case LineBreakMode.TailTruncation:
            textView.SetMaxLines(1);
            textView.SetSingleLine(true);
            textView.Ellipsize = TextUtils.TruncateAt.End;
            break;
        case LineBreakMode.MiddleTruncation:
            textView.SetMaxLines(1);
            textView.SetSingleLine(true);
            textView.Ellipsize = TextUtils.TruncateAt.Middle;
            break;
    }
}

如您所见，如果使用LineBreakMode.TailTruncation，则会调用textView.SetMaxLines(1);和textView.SetSingleLine(true);，这将禁用多行功能.(在2.3.4 textView.SetSingleLine(true);中不存在).

As you can see, if you use LineBreakMode.TailTruncation, textView.SetMaxLines(1); and textView.SetSingleLine(true); will be called, which disable the multi line function.(In 2.3.4 textView.SetSingleLine(true); doesn't exist).

解决方案:

要解决此问题，只需在渲染器中添加两行代码:

To fix the problem, you simply need to add two lines of codes in your renderer:

protected override void OnElementChanged(ElementChangedEventArgs<Xamarin.Forms.Label> e)
{
    base.OnElementChanged(e);
    MultiLineLabel multiLineLabel = (MultiLineLabel)Element;

    if (multiLineLabel != null && multiLineLabel.Lines != -1)
    {
        Control.SetSingleLine(false);
        Control.SetMaxLines(multiLineLabel.Lines);
        Control.SetLines(multiLineLabel.Lines);

    }
}

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