Xamarin: TextView with Placeholder

PlaceholderTextView.cs

namespace Kvandake
{
    using System;

    using CoreGraphics;

    using Foundation;

    using UIKit;

    [Register("PlaceholderTextView")]
    public class PlaceholderTextView : UITextView
    {
        private bool isInitialized;
        private UIColor placeholderColor;
        private string placeholder;
        UIEdgeInsets padding;
        NSLayoutConstraint LeftConstraint;
        NSLayoutConstraint RightConstraint;
        NSLayoutConstraint TopConstraint;
        NSLayoutConstraint BottomConstraint;

        public bool AllowWhiteSpace { get; set; } = false;

        public UILabel PlaceholderLabel { get; set; } = new UILabel { TranslatesAutoresizingMaskIntoConstraints = false };

        public PlaceholderTextView(CGRect frame) : base(frame)
        {
            this.Initialize();
        }

        public PlaceholderTextView(IntPtr ptr) : base(ptr)
        {
            this.Initialize();
        }

        public UIColor PlaceholderColor
        {
            get { return this.placeholderColor; }
            set
            {
                this.placeholderColor = value;
                this.PlaceholderLabel.TextColor = value;
            }
        }

        public string Placeholder
        {
            get { return this.placeholder; }
            set
            {
                this.placeholder = value;
                this.PlaceholderLabel.Text = value;
            }
        }

        private void Initialize()
        {
            this.PlaceholderColor = UIColor.LightGray;
            this.AddSubview(this.PlaceholderLabel);
            this.LeftConstraint = NSLayoutConstraint.Create(this.PlaceholderLabel, NSLayoutAttribute.Left, NSLayoutRelation.Equal, this, NSLayoutAttribute.Left, 1, 0);
            this.RightConstraint = NSLayoutConstraint.Create(this, NSLayoutAttribute.Trailing, NSLayoutRelation.Equal, this.PlaceholderLabel, NSLayoutAttribute.Trailing, 1, 0);
            this.TopConstraint = NSLayoutConstraint.Create(this.PlaceholderLabel, NSLayoutAttribute.Top, NSLayoutRelation.Equal, this, NSLayoutAttribute.Top, 1, 0);
            this.BottomConstraint = NSLayoutConstraint.Create(this, NSLayoutAttribute.Bottom, NSLayoutRelation.Equal, this.PlaceholderLabel, NSLayoutAttribute.Bottom, 1, 0);
            this.AddConstraints(new[] { this.LeftConstraint, this.BottomConstraint, this.RightConstraint, this.TopConstraint });
            this.Padding = new UIEdgeInsets(8, 4, 8, 4);
            this.Started += this.OnStarted;
            this.Ended += this.OnEnded;
        }

        public bool IsEmpty => string.IsNullOrEmpty(this.Text);

        public UIEdgeInsets Padding
        {
            get
            {
                return this.padding;
            }
            set
            {
                this.padding = value;
                if (this.LeftConstraint != null)
                {
                    this.LeftConstraint.Constant = value.Left;
                }
                if (this.RightConstraint != null)
                {
                    this.RightConstraint.Constant = value.Right;
                }
                if (this.TopConstraint != null)
                {
                    this.TopConstraint.Constant = value.Top;
                }
                if (this.BottomConstraint != null)
                {
                    this.BottomConstraint.Constant = value.Bottom;
                }
            }
        }

        public override void LayoutSubviews()
        {
            base.LayoutSubviews();
            if (this.PlaceholderLabel != null)
            {
                this.PlaceholderLabel.PreferredMaxLayoutWidth = this.TextContainer.Size.Width - this.TextContainer.LineFragmentPadding * 2.0f;
            }

            if (!this.isInitialized)
            {
                this.isInitialized = true;
                this.PlaceholderLabel.Font = Font;
                this.PlaceholderLabel.TextAlignment = this.TextAlignment;
                this.PlaceholderLabel.Lines = 0;
                this.PlaceholderLabel.BackgroundColor = UIColor.Clear;
            }
        }

        private void OnStarted(object sender, EventArgs e)
        {
            this.PlaceholderLabel.Hidden = true;
        }

        private void OnEnded(object sender, EventArgs e)
        {
            if (this.AllowWhiteSpace)
            {
                this.PlaceholderLabel.Hidden = !string.IsNullOrEmpty(this.Text);
            }
            else
            {
                this.PlaceholderLabel.Hidden = !string.IsNullOrWhiteSpace(this.Text);
            }
        }
    }
}