215 lines
6.9 KiB
Python
215 lines
6.9 KiB
Python
"""
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Stiff Scroll Effect
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===================
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An Effect to be used with ScrollView to prevent scrolling beyond
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the bounds, but politely.
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A ScrollView constructed with StiffScrollEffect,
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eg. ScrollView(effect_cls=StiffScrollEffect), will get harder to
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scroll as you get nearer to its edges. You can scroll all the way to
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the edge if you want to, but it will take more finger-movement than
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usual.
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Unlike DampedScrollEffect, it is impossible to overscroll with
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StiffScrollEffect. That means you cannot push the contents of the
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ScrollView far enough to see what's beneath them. This is appropriate
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if the ScrollView contains, eg., a background image, like a desktop
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wallpaper. Overscrolling may give the impression that there is some
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reason to overscroll, even if just to take a peek beneath, and that
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impression may be misleading.
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StiffScrollEffect was written by Zachary Spector. His other stuff is at:
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https://github.com/LogicalDash/
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He can be reached, and possibly hired, at:
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zacharyspector@gmail.com
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"""
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from time import time
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from kivy.animation import AnimationTransition
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from kivy.effects.kinetic import KineticEffect
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from kivy.properties import NumericProperty, ObjectProperty
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from kivy.uix.widget import Widget
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class StiffScrollEffect(KineticEffect):
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drag_threshold = NumericProperty("20sp")
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"""Minimum distance to travel before the movement is considered as a
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drag.
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:attr:`drag_threshold` is an :class:`~kivy.properties.NumericProperty`
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and defaults to `'20sp'`.
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"""
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min = NumericProperty(0)
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"""Minimum boundary to stop the scrolling at.
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:attr:`min` is an :class:`~kivy.properties.NumericProperty`
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and defaults to `0`.
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"""
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max = NumericProperty(0)
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"""Maximum boundary to stop the scrolling at.
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:attr:`max` is an :class:`~kivy.properties.NumericProperty`
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and defaults to `0`.
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"""
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max_friction = NumericProperty(1)
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"""How hard should it be to scroll, at the worst?
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:attr:`max_friction` is an :class:`~kivy.properties.NumericProperty`
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and defaults to `1`.
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"""
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body = NumericProperty(0.7)
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"""Proportion of the range in which you can scroll unimpeded.
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:attr:`body` is an :class:`~kivy.properties.NumericProperty`
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and defaults to `0.7`.
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"""
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scroll = NumericProperty(0.0)
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"""Computed value for scrolling
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:attr:`scroll` is an :class:`~kivy.properties.NumericProperty`
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and defaults to `0.0`.
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"""
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transition_min = ObjectProperty(AnimationTransition.in_cubic)
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"""The AnimationTransition function to use when adjusting the friction
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near the minimum end of the effect.
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:attr:`transition_min` is an :class:`~kivy.properties.ObjectProperty`
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and defaults to :class:`kivy.animation.AnimationTransition`.
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"""
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transition_max = ObjectProperty(AnimationTransition.in_cubic)
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"""The AnimationTransition function to use when adjusting the friction
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near the maximum end of the effect.
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:attr:`transition_max` is an :class:`~kivy.properties.ObjectProperty`
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and defaults to :class:`kivy.animation.AnimationTransition`.
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"""
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target_widget = ObjectProperty(None, allownone=True, baseclass=Widget)
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"""The widget to apply the effect to.
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:attr:`target_widget` is an :class:`~kivy.properties.ObjectProperty`
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and defaults to ``None``.
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"""
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displacement = NumericProperty(0)
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"""The absolute distance moved in either direction.
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:attr:`displacement` is an :class:`~kivy.properties.NumericProperty`
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and defaults to `0`.
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"""
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def __init__(self, **kwargs):
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"""Set ``self.base_friction`` to the value of ``self.friction`` just
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after instantiation, so that I can reset to that value later.
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"""
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super().__init__(**kwargs)
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self.base_friction = self.friction
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def update_velocity(self, dt):
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"""Before actually updating my velocity, meddle with ``self.friction``
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to make it appropriate to where I'm at, currently.
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"""
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hard_min = self.min
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hard_max = self.max
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if hard_min > hard_max:
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hard_min, hard_max = hard_max, hard_min
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margin = (1.0 - self.body) * (hard_max - hard_min)
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soft_min = hard_min + margin
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soft_max = hard_max - margin
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if self.value < soft_min:
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try:
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prop = (soft_min - self.value) / (soft_min - hard_min)
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self.friction = self.base_friction + abs(
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self.max_friction - self.base_friction
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) * self.transition_min(prop)
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except ZeroDivisionError:
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pass
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elif self.value > soft_max:
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try:
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# normalize how far past soft_max I've gone as a
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# proportion of the distance between soft_max and hard_max
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prop = (self.value - soft_max) / (hard_max - soft_max)
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self.friction = self.base_friction + abs(
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self.max_friction - self.base_friction
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) * self.transition_min(prop)
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except ZeroDivisionError:
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pass
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else:
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self.friction = self.base_friction
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return super().update_velocity(dt)
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def on_value(self, *args):
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"""Prevent moving beyond my bounds, and update ``self.scroll``"""
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if self.value > self.min:
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self.velocity = 0
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self.scroll = self.min
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elif self.value < self.max:
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self.velocity = 0
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self.scroll = self.max
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else:
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self.scroll = self.value
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def start(self, val, t=None):
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"""Start movement with ``self.friction`` = ``self.base_friction``"""
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self.is_manual = True
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t = t or time()
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self.velocity = self.displacement = 0
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self.friction = self.base_friction
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self.history = [(t, val)]
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def update(self, val, t=None):
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"""Reduce the impact of whatever change has been made to me, in
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proportion with my current friction.
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"""
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t = t or time()
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hard_min = self.min
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hard_max = self.max
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if hard_min > hard_max:
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hard_min, hard_max = hard_max, hard_min
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gamut = hard_max - hard_min
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margin = (1.0 - self.body) * gamut
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soft_min = hard_min + margin
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soft_max = hard_max - margin
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distance = val - self.history[-1][1]
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reach = distance + self.value
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if (distance < 0 and reach < soft_min) or (
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distance > 0 and soft_max < reach
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):
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distance -= distance * self.friction
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self.apply_distance(distance)
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self.history.append((t, val))
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if len(self.history) > self.max_history:
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self.history.pop(0)
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self.displacement += abs(distance)
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self.trigger_velocity_update()
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def stop(self, val, t=None):
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"""Work out whether I've been flung."""
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self.is_manual = False
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self.displacement += abs(val - self.history[-1][1])
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if self.displacement <= self.drag_threshold:
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self.velocity = 0
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return super().stop(val, t)
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