QWidget::paintEvent 被调用的场景

一般情况下，若通过Qt实现绘图，需要重写此函数。

Qt助手给出如下解释：

void QWidget::paintEvent ( QPaintEvent * event ) [virtual protected]
This event handler can be reimplemented in a subclass to receive paint events passed in event.


A paint event is a request to repaint all or part of a widget. It can happen for one of the following reasons:


1） repaint() or update() was invoked,   (此时可通过定时器关联update槽，间接实现对paintEvent 函数的调用)

   例子如下：

class OvenTimer : public QWidget

{

    Q_OBJECT

public:

    OvenTimer(QWidget *parent = 0);

    void setDuration(int secs);

    int duration() const;

    void draw(QPainter *painter);

signals:

    void timeout();

protected:

    void paintEvent(QPaintEvent *event);

    void mousePressEvent(QMouseEvent *event);

private:

    QDateTime finishTime;

    QTimer *updateTimer;

    QTimer *finishTimer;

};

    .cpp 

OvenTimer::OvenTimer(QWidget *parent)

    : QWidget(parent)

{

    finishTime = QDateTime::currentDateTime();

    updateTimer = new QTimer(this);

    connect(updateTimer, SIGNAL(timeout()), this, SLOT(update()));

｝

通过上面的信号槽关系，即间接的调用了paintEvent 函数。因此只需要实现paintEvent 函数即可。

2） the widget was obscured and has now been uncovered, or （重新实现一个widget时也会导致对对paintEvent 函数的调用）

     例子如下：

        

class MyWidget : public QWidget

{

  Q_OBJECT

public:

    MyWidget(QWidget *parent=0);

    ~MyWidget();

    void paintEvent(QPaintEvent *event);

};

只需要重新实现paintEvent即可。

3）many other reasons.