QPainterPath（组合路径，路径运动）

组合路径

void ComplicatedPathWidget::paintEvent(QPaintEvent *) {
    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing);
    QPainterPath path;
    path.lineTo(100, 0);
    path.cubicTo(200, 0, 100, 50, 200, 100); // 贝塞尔曲线
    path.closeSubpath(); // 画一条线到路径的第一个点，闭合路径
    path.addRect(50, 50, 50, 50); // 加一个矩形
    QPainterPath path2;
    path2.addEllipse(80, 30, 50, 50);
    path = path.subtracted(path2); // 扣掉圆
    painter.translate(20, 20);
    painter.setBrush(Qt::lightGray);
    painter.drawPath(path);
    painter.setBrush(Qt::NoBrush);
    painter.drawRect(path.boundingRect());
}

沿路径运动

QPainterPath::pointAtPercent(qreal t) 是一个很有用的函数，t 的值为 [0, 1.0]，可以取得路径上任意一点的坐标，在动画一节里会使用这个函数来实现动画的插值函数，让物体沿着任意的路径运动，这里没有用 Qt 的动画框架实现了让物体沿着任意的路径运动

#ifndef ANIMATIONALONGPATHWIDGET_H
#define ANIMATIONALONGPATHWIDGET_H
#include 
#include 
class AnimationAlongPathWidget : public QWidget {
    Q_OBJECT
public:
    explicit AnimationAlongPathWidget(QWidget *parent = 0);
    ~AnimationAlongPathWidget();
protected:
    void timerEvent(QTimerEvent *event) Q_DECL_OVERRIDE;
    void paintEvent(QPaintEvent *event) Q_DECL_OVERRIDE;
private:
    QPainterPath path;
    float percent;
    float step;
    int timerId;
};
#endif // ANIMATIONALONGPATHWIDGET_H

#include "AnimationAlongPathWidget.h"
#include 
#include 
#include 
AnimationAlongPathWidget::AnimationAlongPathWidget(QWidget *parent) : QWidget(parent) {
    step = 0.02;
    percent = 0;
    // 构造一个任意的曲线
    path.cubicTo(50, 0, 30, 100, 100, 100);
    path.cubicTo(150, 100, 250, 0, 200, 100);
    path.cubicTo(150, 100, 250, 0, 300, 140);
    path.quadTo(150, 310, 150, 100);
    // 启动定时器
    timerId = startTimer(60);
}
AnimationAlongPathWidget::~AnimationAlongPathWidget() {
}
void AnimationAlongPathWidget::timerEvent(QTimerEvent *event) {
    // 不停的更新 percent 的值，刷新界面，实现动画效果
    if (event->timerId() == timerId) {
        percent += step;
        if (percent < 0 || percent > 1) {
            step = -step; // 运动方向取反
            percent = qMin(percent, 1.0F);
            percent = qMax(percent, 0.0F);
        }
        update();
    }
}
void AnimationAlongPathWidget::paintEvent(QPaintEvent *) {
    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing);
    painter.translate(20, 20);
    painter.drawPath(path);
    painter.setBrush(Qt::red);
    painter.drawEllipse(path.pointAtPercent(percent), 4, 4);
}