Do you actualy plan to use a textbox? (besides that On text changed works flawless)
I read this "When we move , and the GPS detects a new street , the text of the variable becomes the name of the new street, for example "Main Street 2""
Till there it is about a changing string, and when the string changed an action is required.
I used a list to simulate a changing string. It does not matter how the string is changing, the principle is the same.
Write previous state of the string in a variable. Compare to the current string. If they are different, do the action & set the previous state of the string to the current state. I think that is pretty clear in the example, no ?