Let's tackle both of your problems one by one—they're common pitfalls when combining Tkinter with screen capture tools, so you're not alone!

1. Fixing the ValueError: id must be a valid identifier #

The error pops up because on the first run of start_quest, self.loop hasn't been assigned a valid ID from after() yet—it's still None. When you call self.after_cancel(self.loop) with an invalid ID, Tkinter throws that error.

Solution: #

• Initialize self.loop to None in your class's __init__ method.
• Add a check to only call after_cancel if self.loop is a valid ID (not None).

2. Fixing GUI Lag #

Tkinter runs on a single main thread—when you call pyautogui.locateCenterOnScreen directly in start_quest, this screen capture operation blocks the main thread, preventing Tkinter from updating the GUI or responding to user input. That's why your app freezes.

Solution: #

Move the screen capture logic to a separate background thread. This way, the main thread stays free to handle GUI events. We'll also use a flag to control the thread's execution, and stick to Tkinter's thread-safety rules by using after() for any GUI-related actions (though PyAutoGUI's clicks are system-level, it's still good practice to keep GUI operations in the main thread).

Modified Working Code #

Here's the updated version of your code with both fixes applied:

import tkinter as tk
import pyautogui
import threading
import time

class QuestGUI(tk.Tk):
    def __init__(self):
        super().__init__()
        # Initialize variables to fix errors and control threads
        self.loop = None
        self.detection_running = False

        # Example UI to start detection
        self.start_btn = tk.Button(self, text="Start Quest Detection", command=self.start_quest)
        self.start_btn.pack(pady=20)

    def start_quest(self):
        # Cancel any existing scheduled task (only if it exists)
        if self.loop is not None:
            self.after_cancel(self.loop)
            self.loop = None

        # Start background detection thread
        self.detection_running = True
        detect_thread = threading.Thread(target=self._detect_quest_loop, daemon=True)
        detect_thread.start()

    def _detect_quest_loop(self):
        while self.detection_running:
            # Screen capture runs in the background, no GUI block
            quest = pyautogui.locateCenterOnScreen('quest.jpg', confidence=0.9)
            if quest:
                # Use after() to execute click in main thread (thread-safe)
                self.after(0, lambda q=quest: pyautogui.click(q[0] - 100, q[1]))
                # Optional: Stop detection after first success, or keep running
                # self.detection_running = False
                # break

            # Wait 1 second before next check (avoids spamming captures)
            time.sleep(1)

        # Optional: Restart detection after success
        # self.after(1000, self.start_quest)

# Launch the GUI
if __name__ == "__main__":
    app = QuestGUI()
    app.mainloop()

Key Changes Explained: #

• self.loop Initialization: We set self.loop = None in __init__ and validate it before calling after_cancel, eliminating the ValueError entirely.
• Background Thread: The _detect_quest_loop function runs in a separate thread, so screen capture doesn't block the main GUI thread anymore.
• Thread Control: The self.detection_running flag lets us safely start/stop the detection loop without thread-related crashes.
• time.sleep(1) in Thread: Instead of using self.after to schedule the next check, we use time.sleep in the background thread—this keeps the main thread free for GUI updates and user interactions.

内容的提问来源于stack exchange，提问作者user14913979