Kivy 滚动视图中的背景

Question

测试应用的总结：我正在写一个Kivy应用，这个应用有一个可以滚动的视图（叫做Scroller），里面有很多个字段（叫做Field）供查看。这些字段有时候很难区分，所以我决定给每个字段使用交替的背景颜色，以帮助彼此区分。我的测试应用使用了20个独立的字段，每个字段的背景颜色在深灰色和更深的灰色之间交替。

测试过程：启动应用后，程序看起来很不错。交替的背景颜色显示得很好。即使我向下滚动，应用也看起来没问题。然而，当我向上滚动时，应用就变得奇怪了。文本跟着应用一起滚动，但背景却不动。更糟糕的是，文本开始和邻近的背景混在一起，变得模糊不清。这个问题在我再次向下滚动（超过了向上滚动的最高点）时似乎就消失了。

问题简述：在向上滚动时，Field的“背景颜色”导致了应用出现问题。

附注：我还注意到，滚动太多后，应用变得有点卡顿。我对Kivy的绘制周期不是很熟悉，但背景的处理不应该导致过多的延迟。

测试应用：

import kivy
kivy.require('1.0.7')

from kivy.app import App
from kivy.uix.gridlayout import GridLayout
from kivy.uix.scrollview import ScrollView
from kivy.uix.label import Label
from kivy.uix.button import Button
from kivy.graphics import Color, Rectangle

class Main(App):
    def build(self):
        self.root = GridLayout(rows = 1)
        self.root.add_widget(Scroller())
        return self.root

class Scroller(ScrollView):
    def __init__(self):
        ScrollView.__init__(self)
        self.view = GridLayout(cols = 1, size_hint = (1, None))
        self.add_widget(self.view)
        self.view.bind(minimum_height = self.view.setter('height'))

        for i in range(20):
            self.view.add_widget(Field('Test field {}'.format(i),i%2 is 0))

class Field(GridLayout):
    def __init__(self, name, bg):
        assert isinstance(name, str)
        assert isinstance(bg, bool)
        self.bg = bg
        GridLayout.__init__(self,
                            rows = 1,
                            padding = 10,
                            size = (0, 60),
                            size_hint = (1, None))
        self.add_widget(Label(text = name))
        self.add_widget(Button(text = 'Test button',
                               size = (200, 0),
                               size_hint = (None, 1)))
        self.bind(pos = self.change_background)
        self.bind(size = self.change_background)

    def change_background(self, *args):
        with self.canvas.before:
            if self.bg:
                Color(0.2, 0.2, 0.2, mode = 'rgb')
            else:
                Color(0.1, 0.1, 0.1, mode = 'rgb')
            Rectangle(pos = self.pos, size = self.size)

if __name__ in ('__main__', '__android__'):
    app = Main()
    app.run()

Answer 1

def change_background(self, *args):
        self.canvas.before.clear()#<- clear previous instructions
        with self.canvas.before:
            if self.bg:
                Color(0.2, 0.2, 0.2, mode = 'rgb')
            else:
                Color(0.1, 0.1, 0.1, mode = 'rgb')
            Rectangle(pos = self.pos, size = self.size)

每次当字段的位置或大小变化时，你都在画布上添加新的指令，但没有清除之前的指令。

你还应该考虑使用kv，因为对于超过小片段的代码，这样做能节省你很多时间。你可以像这样将你的代码转换为kv格式：

import kivy
kivy.require('1.0.7')

from kivy.app import App
from kivy.uix.gridlayout import GridLayout
from kivy.uix.scrollview import ScrollView
from kivy.properties import ObjectProperty, BooleanProperty
from kivy.lang import Builder

Builder.load_string('''
<Scroller>
    # root is Scroller here
    # create a new ObjectProperty in kv that holds the ref to Gridlayout
    # so you can access the instance in python code
    view: glayout
    GridLayout:
        id: glayout
        cols: 1
        size_hint: (1, None)
        height: self.minimum_height

<Field>
    canvas.before:
        Color:
            rgba: (0.2, 0.2, 0.2, 1) if self.bg else (0.1, 0.1, 0.1, 1)
        Rectangle:
            # binding properties is done implicitly and instructions aren't
            # piled up while doing that.
            pos: self.pos
            # self here refers to Field as `self` is supposed to refer to the
            # Widget not the drawing instruction
            size: self.size
    rows: 1
    padding: 10
    size: (0, 60)
    size_hint: (1, None)
    Label:
        text: root.name
    Button:
        text: 'test button'
        size: (200, 0)
        size_hint: (None, 1)
''')


class Main(App):

    def build(self):
        self.root = GridLayout(rows = 1)
        self.root.add_widget(Scroller())
        return self.root


class Scroller(ScrollView):
    def __init__(self, **kwargs):
        super(Scroller, self).__init__(**kwargs)
        for i in range(20):
            # access self.view that was set in kv
            self.view.add_widget(
                                Field(
                                    name = 'Test field {}'.format(i),
                                    bg = i%2 is 0))

class Field(GridLayout):

    # use  kivy's Properties so it becomes easier to observe and apply changes
    # as a plus these can also be directly used in kv. As a advantage of using this now
    # you can change name and bg dynamically and the changes should be reflected on
    # screen
    name = ObjectProperty('Test field uninitialized')

    bg = BooleanProperty(False)


if __name__ in ('__main__', '__android__'):
    app = Main()
    app.run()