Flet 教程：正确显示 AlertDialog 弹窗的异步方法

# 错误的尝试
import flet as ft

class TodoApp(ft.UserControl):
    def build(self):
        return ft.Column(
            controls=[
                ft.Row(
                    [
                        ft.ElevatedButton(text="button", on_click=self.route_change),
                    ],
                    alignment=ft.MainAxisAlignment.SPACE_BETWEEN,
                ),
            ],
        )
    async def route_change(self, e):
        dlg_modal = ft.AlertDialog(
            title=ft.Text("login"),
            content=ft.Text("asasasas"),
            actions=[
                ft.TextButton("Yes"),
                ft.TextButton("No"),
            ],
            actions_alignment=ft.MainAxisAlignment.END,
        )
        self.dialog = dlg_modal # 尝试将弹窗赋值给当前控件的属性
        dlg_modal.open = True   # 尝试直接设置弹窗的open属性
        self.update()           # 尝试更新当前控件
        # ... (后续代码)

import flet as ft

class TodoApp(ft.UserControl):
    def build(self):
        return ft.Column(
            controls=[
                ft.Row(
                    [
                        ft.ElevatedButton(text="点击显示弹窗", on_click=self.show_alert_dialog),
                    ],
                    alignment=ft.MainAxisAlignment.CENTER, # 居中对齐按钮
                ),
            ],
            horizontal_alignment=ft.CrossAxisAlignment.CENTER, # 居中对齐列内容
        )

    async def show_alert_dialog(self, e):
        """
        按钮点击事件处理函数，用于显示 AlertDialog。
        """
        dlg_modal = ft.AlertDialog(
            modal=True, # 设置为模态，阻止与背景交互
            title=ft.Text("登录提示"),
            content=ft.Text("这是一个示例登录弹窗内容。"),
            actions=[
                ft.TextButton("确认", on_click=lambda _: self.close_dialog(e, "确认")),
                ft.TextButton("取消", on_click=lambda _: self.close_dialog(e, "取消")),
            ],
            actions_alignment=ft.MainAxisAlignment.END,
            on_dismiss=lambda _: print("弹窗被关闭 (点击外部或Esc)"), # 弹窗关闭事件
        )
        # 核心：使用 e.page.show_dialog_async() 显示弹窗
        await e.page.show_dialog_async(dlg_modal)

    async def close_dialog(self, e, action_taken):
        """
        关闭弹窗的辅助函数。
        """
        print(f"用户点击了: {action_taken}")
        # 通过设置页面的dialog属性的open为False来关闭弹窗
        e.page.dialog.open = False
        # 必须更新页面以反映dialog状态的变化
        await e.page.update_async()

async def main(page: ft.Page):
    page.title = "Flet AlertDialog 示例"
    page.vertical_alignment = ft.MainAxisAlignment.CENTER
    page.horizontal_alignment = ft.CrossAxisAlignment.CENTER
    page.scroll = ft.ScrollMode.ADAPTIVE
    await page.add_async(TodoApp())

if __name__ == "__main__":
    ft.app(target=main)