FFImageLoading.Drawables.SelfDisposingBitmapDrawable.SetIsDisplayed C# (CSharp) Method

        public virtual void SetIsDisplayed(bool isDisplayed)
        {
            EventHandler handler = null;
            lock (monitor)
            {
                if (isDisplayed && !HasValidBitmap)
                {
                    System.Diagnostics.Debug.WriteLine("Cannot redisplay this drawable, its resources have been disposed.");
                }
                else if (isDisplayed)
                {
                    display_ref_count++;
                    if (display_ref_count == 1)
                    {
                        handler = Displayed;
                    }
                }
                else
                {
                    display_ref_count--;
                }

                if (display_ref_count <= 0)
                {
                    display_ref_count = 0;
                    handler = NoLongerDisplayed;
                }
            }
            if (handler != null)
            {
                handler(this, EventArgs.Empty);
            }
            CheckState();
        }