Xamarin.Forms: the MultiLineLabel doesn't longer work on Android

问题

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.

So I have a MultiLineLabel control:

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); }
    }
}

I use 2 renderers:

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;

    }
}

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);
        }
    }
}

And I use this MultiLineLabel like this in XAML:

<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>

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

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

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);
    }
}

But I didn't get the expected rendering:

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

回答1:

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

Cause:

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;
    }
}

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).

Solution:

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);

    }
}

来源：https://stackoverflow.com/questions/46042386/xamarin-forms-the-multilinelabel-doesnt-longer-work-on-android

标签

android

xamarin.forms

label

multiline

custom-renderer