WPF, RichTextBox在光标位置获得正确文本属性的问题

I'm building a simple editor using the wpf richtextbox. This editor has some sort of toggle buttons for Bold, Italic, Underlined, etc. which are 'pressed' when the selected text or the text at the cursor has the approptiate property. I did it like this:I'm building a simple editor using the wpf rich