from tkinter import *

# Trial 3 - OptionMenu with a trace (auto-updating label)
# Purpose: Learn how to make the UI respond instantly when a dropdown
#          value changes, WITHOUT needing a button press.
#          A "trace" calls a function every time the StringVar changes.

UNITS = ["km", "m", "cm", "mm"]


class DropdownTrial:

    def __init__(self):

        self.frame = Frame(padx=10, pady=10)
        self.frame.grid()

        self.heading = Label(self.frame,
                             text="Dropdown Trial - v3 (Auto-Update Trace)",
                             font=("Arial", "16", "bold"))
        self.heading.grid(row=0)

        instructions = ("Change either dropdown and watch the label "
                        "update automatically - no button needed!")
        Label(self.frame, text=instructions,
              wraplength=300, font=("Arial", "11"),
              fg="#666666").grid(row=1, pady=5)

        # --- From unit ---
        from_frame = Frame(self.frame)
        from_frame.grid(row=2, pady=5)

        Label(from_frame, text="Convert from:",
              font=("Arial", "12")).grid(row=0, column=0, padx=(0, 10))

        self.from_var = StringVar(value=UNITS[0])
        # trace_add fires whenever the variable is written to
        self.from_var.trace_add("write", self.update_label)

        OptionMenu(from_frame, self.from_var, *UNITS).grid(row=0, column=1)

        # --- To unit ---
        to_frame = Frame(self.frame)
        to_frame.grid(row=3, pady=5)

        Label(to_frame, text="Convert to:    ",
              font=("Arial", "12")).grid(row=0, column=0, padx=(0, 10))

        self.to_var = StringVar(value=UNITS[1])
        self.to_var.trace_add("write", self.update_label)

        OptionMenu(to_frame, self.to_var, *UNITS).grid(row=0, column=1)

        # --- Auto-updating result label ---
        self.result_label = Label(self.frame,
                                  text="",
                                  font=("Arial", "13", "bold"),
                                  fg="#004C99")
        self.result_label.grid(row=4, pady=15)

        # Call once at startup so the label shows something immediately
        self.update_label()

    def update_label(self, *args):
        """
        Called automatically whenever from_var or to_var changes.
        *args is required because trace passes extra arguments we don't need.
        """
        from_unit = self.from_var.get()
        to_unit = self.to_var.get()

        if from_unit == to_unit:
            self.result_label.config(
                text="Please choose two different units",
                fg="#9C0000"
            )
        else:
            self.result_label.config(
                text=f"Ready to convert: {from_unit}  \u2192  {to_unit}",
                fg="#004C99"
            )


# main routine
if __name__ == "__main__":
    root = Tk()
    root.title("Dropdown Trial v3")
    DropdownTrial()
    root.mainloop()