Xamarin表单-如果为空，则验证条目文本框

Entry entry = new Entry
{
    Placeholder = "请输入内容",
    IsRequired = true
};

Entry entry = new Entry
{
    Placeholder = "请输入内容"
};

entry.SetBinding(Entry.TextProperty, new Binding("EntryText", BindingMode.TwoWay));
entry.BindingContext = this;

// 定义验证规则
entry.Behaviors.Add(new RequiredValidationBehavior { ErrorMessage = "内容不能为空" });

// 验证规则类
public class RequiredValidationBehavior : Behavior<Entry>
{
    public string ErrorMessage { get; set; }

    protected override void OnAttachedTo(Entry entry)
    {
        entry.TextChanged += OnEntryTextChanged;
        base.OnAttachedTo(entry);
    }

    protected override void OnDetachingFrom(Entry entry)
    {
        entry.TextChanged -= OnEntryTextChanged;
        base.OnDetachingFrom(entry);
    }

    void OnEntryTextChanged(object sender, TextChangedEventArgs e)
    {
        bool isValid = !string.IsNullOrEmpty(e.NewTextValue);
        ((Entry)sender).TextColor = isValid ? Color.Default : Color.Red;
    }
}