python - 如何从 kivy 的 Canvas 中删除线条

Question

我正在创建一个简单的绘图应用程序，我想在其中添加一个撤消按钮。到目前为止我尝试过的是:

class DrawScreen(Screen):
    r = NumericProperty(0)
    g = NumericProperty(0)
    b = NumericProperty(0)
    brush_width = NumericProperty(2)

    def on_touch_down(self, touch):
        self.slider = self.ids.slider
        if self.slider.collide_point(touch.x, touch.y):
            self.brush_width = self.slider.value
        else:
            self.undo = [touch.x, touch.y]
            with self.canvas.before:
                Color(self.r, self.g, self.b)
                touch.ud["line"] = Line(points=self.undo, width=self.brush_width)
        return super(DrawScreen, self).on_touch_down(touch)

    def on_touch_move(self, touch):
        if self.slider.collide_point(touch.x, touch.y):
            self.brush_width = self.slider.value
        else:
            try:
                self.undo += [touch.x, touch.y]
                touch.ud["line"].points = self.undo

            except:
                pass
        return super(DrawScreen, self).on_touch_move(touch)

    def color(self, r, g, b):
        self.r = r
        self.g = g
        self.b = b

    def undo_draw(self):
        self.undo = []

此撤消按钮会清除列表，但不会以任何方式影响 Canvas ，也不会删除任何行。解决这个问题的合适方法是什么？

Answer 1

尝试将它们放在 InstructionGroup 中，然后添加到 canvas，然后使用 canvas.remove(item) 简单地从 Canvas 中删除项目>。

如果你想重做，你可能想保存这些项目。
试试这个例子。我不得不替换 on_touch_down，当我用鼠标垫移动光标时，它会不断创建指令，用子项填充 Canvas :

from kivy.app import App
from kivy.lang import Builder
from kivy.graphics import Line, Color, InstructionGroup
from kivy.uix.widget import Widget


class MyWidget(Widget):

    undolist = []
    objects = []
    drawing = False

    def on_touch_up(self, touch):
        self.drawing = False

    def on_touch_move(self, touch):
        if self.drawing:
            self.points.append(touch.pos)
            self.obj.children[-1].points = self.points
        else:
            self.drawing = True
            self.points = [touch.pos]
            self.obj = InstructionGroup()
            self.obj.add(Color(1,0,0))
            self.obj.add(Line())
            self.objects.append(self.obj)
            self.canvas.add(self.obj)


    def undo(self):
        item = self.objects.pop(-1)
        self.undolist.append(item)
        self.canvas.remove(item)

    def redo(self):
        item = self.undolist.pop(-1)
        self.objects.append(item)
        self.canvas.add(item)


KV = """

BoxLayout:
    MyWidget:
        id: widget
    Button:
        text: "undo"
        on_release:
            widget.undo()
    Button:
        text: "redo"
        on_release:
            widget.redo()


"""


class MyApp(App):

    def build(self):
        root = Builder.load_string(KV)
        return root

MyApp().run()