前言

在之前的部落格《如何在 pyqt 中實現平滑捲動的 QScrollArea》中，我們使用定時器和佇列實現了平滑捲動。但是實現程式碼還是有一點複雜，所以這篇部落格將使用 Qt 的動畫框架 QPropertyAnimation 來實現相同的功能。

實現過程

SmoothScrollBar

捲動過程其實就是改變 QScrollBar 的 value() 的過程，Qt 自帶的 QScrollArea 之所以無法平滑捲動，就是因為捲動時在 QScrollBar 的兩個 value() 之間進行跳變。如果我們能在兩個捲動值之間進行插值，就能實現平滑捲動了，這裡通過重寫 setValue() 函數來啟動捲動動畫。

class SmoothScrollBar(QScrollBar):
    """ Smooth scroll bar """

    scrollFinished = pyqtSignal()

    def __init__(self, parent=None):
        QScrollBar.__init__(self, parent)
        self.ani = QPropertyAnimation()
        self.ani.setTargetObject(self)
        self.ani.setPropertyName(b"value")
        self.ani.setEasingCurve(QEasingCurve.OutCubic)
        self.ani.setDuration(500)
        self.ani.finished.connect(self.scrollFinished)

    def setValue(self, value: int):
        if value == self.value():
            return

        # stop running animation
        self.ani.stop()
        self.scrollFinished.emit()

        self.ani.setStartValue(self.value())
        self.ani.setEndValue(value)
        self.ani.start()

    def scrollValue(self, value: int):
        """ scroll the specified distance """
        value += self.value()
        self.scrollTo(value)

    def scrollTo(self, value: int):
        """ scroll to the specified position """
        value = min(self.maximum(), max(self.minimum(), value))
        self.setValue(value)

    def mousePressEvent(self, e):
        self.ani.stop()
        super().mousePressEvent(e)

    def mouseReleaseEvent(self, e):
        self.ani.stop()
        super().mouseReleaseEvent(e)

    def mouseMoveEvent(self, e):
        self.ani.stop()
        super().mouseMoveEvent(e)

SmoothScrollArea

最後需要將 QScrollArea 的預設卷軸替換為平滑捲動的 SmoothScrollBar：

class SmoothScrollArea(QScrollArea):
    """ Smooth scroll area """

    def __init__(self, parent=None):
        super().__init__(parent)
        self.hScrollBar = SmoothScrollBar()
        self.vScrollBar = SmoothScrollBar()
        self.hScrollBar.setOrientation(Qt.Horizontal)
        self.vScrollBar.setOrientation(Qt.Vertical)
        self.setVerticalScrollBar(self.vScrollBar)
        self.setHorizontalScrollBar(self.hScrollBar)

    def setScrollAnimation(self, orient, duration, easing=QEasingCurve.OutCubic):
        """ set scroll animation

        Parameters
        ----------
        orient: Orient
            scroll orientation

        duration: int
            scroll duration

        easing: QEasingCurve
            animation type
        """
        bar = self.hScrollBar if orient == Qt.Horizontal else self.vScrollBar
        bar.ani.setDuration(duration)
        bar.ani.setEasingCurve(easing)

    def wheelEvent(self, e):
        if e.modifiers() == Qt.NoModifier:
            self.vScrollBar.scrollValue(-e.angleDelta().y())
        else:
            self.hScrollBar.scrollValue(-e.angleDelta().x())

測試

下面是一個簡單的圖片檢視器測試程式：

# coding:utf-8
import sys
from PyQt5.QtCore import QEasingCurve, Qt
from PyQt5.QtGui import QPixmap
from PyQt5.QtWidgets import QApplication, QLabel


class Demo(SmoothScrollArea):

    def __init__(self):
        super().__init__()
        self.label = QLabel(self)
        self.label.setPixmap(QPixmap("shoko.jpg"))

        # customize scroll animation
        self.setScrollAnimation(Qt.Vertical, 400, QEasingCurve.OutQuint)
        self.setScrollAnimation(Qt.Horizontal, 400, QEasingCurve.OutQuint)

        self.horizontalScrollBar().setValue(1900)
        self.setWidget(self.label)
        self.resize(1200, 800)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    w = Demo()
    w.show()
    app.exec_()

測試用的圖片如下（硝子醬真可愛