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);

    }
}