如何通过'on_<属性>'回调将文本插入TextInput

Question

我正在用kivy做一个简单的计算器，想用TextInput来显示屏幕上的内容。我不太确定用TextInput还是Label更好，不过我觉得TextInput可能更合适（也许我错了？）。现在界面基本搭建好了，所有的按钮和它们对应的文本框（'0', '1', '2', '+', '='等等）都已经准备好了。每次用户按下一个按钮，那个按钮上的文本就会被添加到calc_string中，当按下'='按钮时，这个字符串会被计算。基本上，功能都正常，但我的输出全都打印在控制台上，因为我无法把字符串插入到TextInput的显示区域。我尝试在一个回调函数（screen_callback）中设置TextInput的文本，这个回调是通过一个绑定到TextInput的on_property方法触发的，但没有成功。我该怎么做才能实现这个功能？这是可能的吗？

main.py:

#!/usr/bin/env python

import kivy
kivy.require('1.8.0')
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.gridlayout import GridLayout
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.label import Label
from kivy.uix.button import Button
from kivy.uix.textinput import TextInput
from kivy.properties import ListProperty, ObjectProperty, StringProperty, NumericProperty

class Screen(TextInput):
    pass

class Calculator(BoxLayout):

    function_btns = ListProperty(['+', '-', '*', '/', '^', 'Mem <-', 'Mem ->', 'Clear'])
    digit_btns = ListProperty(['7', '8', '9', '4', '5', '6', '1', '2', '3', '0', '.', '+/-'])
    eval_string = StringProperty() # one of these strings is probably unnecessary
    calc_string = StringProperty()
    result = NumericProperty(0)

    def build_calc(self):
        # add screen textinput
        self.add_widget(Screen(text='0', multiline=False, on_calc_string=self.screen_callback))

        #create inner boxlayout to hold 2 button gridlayouts
        main_buttons_box = BoxLayout(orientation='horizontal', size_hint=(1, 0.75))

        # create 2 button gridlayouts
        digit_grid = GridLayout(cols=3, rows=4, size_hint=(0.5, 1))
        function_grid = GridLayout(cols=1, size_hint=(0.5, 1))

        # populate grids with buttons
        for i in self.digit_btns:
            digit_grid.add_widget(Button(text=str(i), on_press=self.button_callback))

        for f in self.function_btns:
            function_grid.add_widget(Button(text=str(f), on_press=self.button_callback))

        # add grids to inner boxlayout
        main_buttons_box.add_widget(digit_grid)
        main_buttons_box.add_widget(function_grid)

        # add inner boxlayout to main grid(root)
        self.add_widget(main_buttons_box)

        # finally, add boxlayout at bottom of main grid and insert equals btn
        equals_box = BoxLayout(size_hint=(1, 0.25))
        equals_box.add_widget(Button(text='=', on_press=self.button_callback))
        self.add_widget(equals_box)

    def button_callback(self, instance):
        value = instance.text
        if value == '=':
            try:
                self.result = eval(self.eval_string)
                self.calc_string = str(self.result)
                self.eval_string = ''
            except SyntaxError:
                self.calc_string = 'Error: Invalid Input'
        else:
            self.eval_string += value
            self.calc_string = self.eval_string

        print self.calc_string

    def screen_callback(self, instance):
        instance.text = self.calc_string # not working
        print 'seeing this!!!' # not working


class CalcApp(App):

    def build(self):
        calc = Calculator()
        calc.build_calc()
        return calc

if __name__ == '__main__':
    CalcApp().run()

kv文件:

#kivy: 1.0

<Screen>:
    size_hint: 1, 0.25

<Calculator>:
    orientation: 'vertical'

用户界面字符串处理回调函数事件绑定界面交互 kivy 计算器应用 textinput

如何通过'on_<属性>'回调将文本插入TextInput

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