There is a tool button and a textctrl in the toolbar. I'm trying to expand the textctrl in the horizontal direction to fill all the remaining space.
wxSizer maybe a good choice but it seems not suitable with toolbar because I can't add tool button directly in a sizer.
There is no built in support for this, you will need to handle wxEVT_SIZE (either in the toolbar itself or in the frame containing it, as the size of the toolbar only changes when the size of the frame does), compute the available size (which is going to be tricky, there is no function to find this out neither so I expect you'd have to do some kind of binary search using wxToolBar::FindToolForPosition()) and resize your text control.
It would definitely be much simpler to put both the toolbar and the text in a sizer instead. But it's true that it wouldn't appear quite the same, so if you really want to have the text-inside-toolbar appearance, you would have to do the above. Good luck!