Kivy不显示组件

Question

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import ObjectProperty, DictProperty
from kivy.network.urlrequest import UrlRequest
import json
from kivy.uix.listview import ListItemButton
from kivy.uix.label import Label
from kivy.factory import Factory

class LocationButton(ListItemButton):
    pass

class WeatherRoot(BoxLayout):

    def show_current_weather(self,location):
        self.clear_widgets()
        current_weather = Factory.CurrentWeather()
        current_weather.location = location
        self.add_widget(current_weather)

    def show_add_location_form(self):
        self.clear_widgets()
        self.add_widget(AddLocationForm())

class AddLocationForm(BoxLayout):

    search_input = ObjectProperty()
    search_results = ObjectProperty()

    def search_location_name(self):
        search_template = 'http://api.openweathermap.org/data/2.5/' + 'find?q={}&type=like'
        search_url = search_template.format(self.search_input.text)
        request = UrlRequest(search_url, self.found_location_name)
        if len(self.search_results.item_strings) == 0:
            self.search_results.item_strings.append('City not found')

    def search_location_coor(self):
        try:
            search_coor = self.search_input.text
            search_input_coordinates = search_coor.split(',')
            search_template_coordinates = 'http://api.openweathermap.org/data/2.5/' + "weather?lat=%s&lon=%s&radius=10" % (search_input_coordinates[0] , search_input_coordinates[1])
            request_coordinates = UrlRequest(search_template_coordinates, self.found_location_coordinates)
        except (IndexError):
            self.search_results.item_strings.append('Coordinates needed are Lattitude and Longtitude!')


    def found_location_coordinates(self,request_coordinates,data_coordinates):
        try:
            data_coordinates = json.loads(data_coordinates.decode()) if not isinstance(data_coordinates, dict) else data_coordinates
            cities_coordinates = ['{} ({})'.format(data_coordinates['name'],data_coordinates['sys']['country'])]
            self.search_results.item_strings = cities_coordinates
            self.search_results.adapter.data[:]
            self.search_results.adapter.data.extend(cities_coordinates)
            self.search_results._trigger_reset_populate()
        except KeyError:
            self.search_results.item_strings.append('Wrong Coordinates')

    def found_location_name(self, request, data):
        data = json.loads(data.decode()) if not isinstance(data, dict) else data
        cities = ['{} ({})'.format(d['name'],d['sys']['country']) for d in data['list']]
        self.search_results.item_strings = cities
        self.search_results.adapter.data[:]
        self.search_results.adapter.data.extend(cities)
        self.search_results._trigger_reset_populate()
        if len(self.search_results.item_strings) == 0:
            self.search_results.item_strings.append('City not found')


class WeatherApp(App):
    pass


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

还有这个kv文件：

#:import main main
#:import ListAdapter kivy.adapters.listadapter.ListAdapter
WeatherRoot:

<WeatherRoot>:
    AddLocationForm:
        orientation: 'vertical'
        search_input: search_box
        search_results: search_results_list
        BoxLayout:
            height: '40dp'
            size_hint_y: None
            TextInput:
                id: search_box
                size_hint_x: .5
            Button:
                text: 'Search'
                size_hint_x: .25
                on_press: self.parent.parent.search_location_name()
            Button:
                text: 'Current Location'
                size_hint_x: .25
                on_press: self.parent.parent.search_location_coor()
        ListView:
            id: search_results_list
            item_strings: []
            adapter:
                 ListAdapter(data=[], cls=main.LocationButton)
<LocationButton>:
    on_press: app.root.show_current_weather(self.text)
<CurrentWeather@BoxLayout>:
    location: ''
    conditions: None
    temp: None
    temp_min: None
    temp_max: None
    orientation: 'vertical'
    Label:
        text: root.location
    BoxLayout:
        size_hint_y: None
        height: '40dp'
        Button:
            text: 'Add Location'
            on_press: app.root.show_add_location_form()
        Button:
            text: 'Forecast'

这个程序能做什么和不能做什么：

可以通过城市名称或坐标来搜索城市
会列出所有符合你搜索条件的城市
你可以选择一个城市
会打开一个黑色的页面，上面有城市的名字和两个按钮（添加位置和天气预报）

如果你点击添加位置的按钮，它应该打开最初的窗口，但我却看到一个黑屏，我是不是漏掉了什么？

这段代码来自书籍《用kivy创建应用程序》

用户界面应用程序开发组件显示 kivy 按钮功能城市搜索 kv文件

Kivy不显示组件

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