How can I get the TextBlock's "text" value after edit?

I want to get the new Value in textBlock after my editing . But when I use this way

I found the key not change.

Whatever How I edit node text,my JSON still can’t change.

So , I want to know how can I get the TextBlock after I editing the node text.
Here is my TEMPLATE

If the user were editing any text value other than the node key, it’s just a matter of making the Binding TwoWay.

But you do not want users modifying any key values, which must be unique within the model.

Thank you for your answer . I have solved my question in your way. Thanks .