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上一篇QT截图程序，可多屏幕截图只是实现了最基本的截图功能，虽然能用但是缺点也有，没办法更改选中的区域，这在实际使用时不太方便。这篇增加了这个功能。先看看效果。

实现代码为：

头文件

#ifndef MASKWIDGET_H
#define MASKWIDGET_H#include <QWidget>
#include "mainwindow.h"
namespace Ui {
class MaskWidget;
}
enum SnapState{NoSnap,Snapped,PreLeftDrag,LeftDrag,PreRightDrag,RightDrag,PreTopDrag,TopDrag,PreBottomDrag,BottomDrag
};class MaskWidget : public QWidget
{Q_OBJECTpublic:explicit MaskWidget(QWidget *parent = nullptr);~MaskWidget();
protected:void mousePressEvent(QMouseEvent *event)override;void mouseReleaseEvent(QMouseEvent *event)override;void mouseMoveEvent(QMouseEvent *event)override;void paintEvent(QPaintEvent *event)override;void keyPressEvent(QKeyEvent *event) override;void showEvent(QShowEvent *event) override;private slots:void ResetSnap();private:QPoint m_pressPos;QPoint m_newPos;QRect m_maskRect{0, 0, 0, 0};QPixmap m_image;bool isPressed{false};MainWindow m;SnapState snapstate{NoSnap};
private:Ui::MaskWidget *ui;};#endif // MASKWIDGET_H

源文件

#include "maskwidget.h"
#include "ui_maskwidget.h"
#include <QMouseEvent>
#include <QRegion>
#include <QScreen>
#include <QPainter>
#include <QGuiApplication>
#include <QPixmap>
#include <QDebug>
#include <QtMath>
#include <QCursor>const int MINSIZE = 10;MaskWidget::MaskWidget(QWidget *parent) :QWidget(parent),ui(new Ui::MaskWidget)
{ui->setupUi(this);setMouseTracking(true);setWindowFlags(Qt::FramelessWindowHint);setWindowOpacity(0.8);QList<QScreen*> screens = QGuiApplication::screens();int width = 0;int height = 0;for (QScreen *screen : screens){width += screen->geometry().width();if (height < screen->geometry().height()){height = screen->geometry().height();}qDebug()<<screen->geometry();}this->setFixedSize(width, height);m.hide();connect(&m, SIGNAL(resetSnap()), this, SLOT(ResetSnap()));
}MaskWidget::~MaskWidget()
{delete ui;
}void MaskWidget::mousePressEvent(QMouseEvent *event)
{if (event->button() == Qt::LeftButton){if (m_maskRect.width() == 0 && m_maskRect.width() == 0){m_pressPos = event->pos();this->setCursor(Qt::CrossCursor);isPressed = true;update();}if (snapstate == PreLeftDrag){snapstate = LeftDrag;}else if (snapstate == PreRightDrag){snapstate = RightDrag;}else if (snapstate == PreTopDrag){snapstate = TopDrag;}else if (snapstate == PreBottomDrag){snapstate = BottomDrag;}}if (event->button() == Qt::RightButton){if (m_maskRect.width() > 0){isPressed = false;QRegion all(0, 0, width(), height());QRegion sub(m_maskRect);setMask(all.subtracted(sub));QPixmap combined(this->width(), this->height());combined.fill(Qt::transparent);QPainter painter(&combined);QList<QScreen*> screens = QGuiApplication::screens();for (QScreen *screen : screens){m_image = screen->grabWindow(0);painter.drawPixmap(screen->geometry().x(), 0, screen->geometry().width(), screen->geometry().height(), m_image);}//            auto gpos = mapToGlobal(event->pos());
//            auto gposStart = mapToGlobal(m_pressPos);
//            qDebug()<<gpos<<gposStart;//m_image = combined.copy(qMin(gpos.x(), gposStart.x()), qMin(gpos.y(), gposStart.y()),//                        qFabs(gpos.x() - gposStart.x()), qFabs(gpos.y() - gposStart.y()));m_image = combined.copy(m_maskRect);this->hide();m.SetImage(m_image);update();m.show();}}QWidget::mousePressEvent(event);
}void MaskWidget::mouseReleaseEvent(QMouseEvent *event)
{qDebug()<<__func__<<" "<<snapstate;if (isPressed){isPressed = false;snapstate = Snapped;this->setCursor(Qt::ArrowCursor);}switch(snapstate){case LeftDrag:{snapstate = Snapped;this->setCursor(Qt::ArrowCursor);}break;case RightDrag:{snapstate = Snapped;this->setCursor(Qt::ArrowCursor);}break;case TopDrag:{snapstate = Snapped;this->setCursor(Qt::ArrowCursor);}break;case BottomDrag:{snapstate = Snapped;this->setCursor(Qt::ArrowCursor);}break;default:break;}return QWidget::mouseReleaseEvent(event);
}void MaskWidget::mouseMoveEvent(QMouseEvent* event)
{if (isPressed){m_newPos = event->pos();QRegion all(0, 0, width(), height());m_maskRect = QRect(qMin(m_pressPos.x(), m_newPos.x()),qMin(m_pressPos.y(), m_newPos.y()),qAbs(m_newPos.x() - m_pressPos.x()),qAbs(m_newPos.y() - m_pressPos.y()));QRegion sub(m_maskRect);setMask(all.subtracted(sub));update();}else{switch(snapstate){case Snapped:{if (m_maskRect.bottom() > event->pos().y() && m_maskRect.top() < event->pos().y()){if (qFabs(m_maskRect.left() - event->pos().x()) < 5){this->setCursor(Qt::SizeHorCursor);this->snapstate = PreLeftDrag;}else if (qFabs(m_maskRect.right() - event->pos().x()) < 5){this->setCursor(Qt::SizeHorCursor);this->snapstate = PreRightDrag;}}else if (m_maskRect.left() < event->pos().x() && m_maskRect.right() > event->pos().y()){if (qFabs(m_maskRect.top() - event->pos().y()) < 5){this->setCursor(Qt::SizeVerCursor);this->snapstate = PreTopDrag;}else if (qFabs(m_maskRect.bottom() - event->pos().y()) < 5){this->setCursor(Qt::SizeVerCursor);this->snapstate = PreBottomDrag;}}}break;case LeftDrag:{if (event->pos().x() + MINSIZE >= m_maskRect.right()){m_maskRect.setLeft(m_maskRect.right() - MINSIZE);}else{m_maskRect.setLeft(event->pos().x() + 2);}QRegion sub(m_maskRect);QRegion all(0, 0, width(), height());setMask(all.subtracted(sub));qDebug()<<m_maskRect;update();}break;case PreLeftDrag:{if (qFabs(m_maskRect.left() - event->pos().x()) >= 5 || event->pos().y() > m_maskRect.bottom() || event->pos().y() < m_maskRect.top()){this->setCursor(Qt::ArrowCursor);this->snapstate = Snapped;}}break;case PreRightDrag:{if (qFabs(event->pos().x() - m_maskRect.right()) >= 5 || event->pos().y() > m_maskRect.bottom() || event->pos().y() < m_maskRect.top()){this->setCursor(Qt::ArrowCursor);this->snapstate = Snapped;}}break;case RightDrag:{if (event->pos().x() - MINSIZE <= m_maskRect.left()){m_maskRect.setRight(m_maskRect.left() + MINSIZE);}else{m_maskRect.setRight(event->pos().x() - 2);}QRegion sub(m_maskRect);QRegion all(0, 0, width(), height());setMask(all.subtracted(sub));qDebug()<<m_maskRect;update();}break;case PreTopDrag:{if (qFabs(event->pos().y() - m_maskRect.top()) >= 5 || event->pos().x() < m_maskRect.left() || event->pos().x() > m_maskRect.right()){this->setCursor(Qt::ArrowCursor);this->snapstate = Snapped;}}break;case TopDrag:{if (event->pos().y() + MINSIZE >= m_maskRect.bottom()){m_maskRect.setTop(m_maskRect.bottom() - MINSIZE);}else{m_maskRect.setTop(event->pos().y() + 2);}QRegion sub(m_maskRect);QRegion all(0, 0, width(), height());setMask(all.subtracted(sub));update();}break;case PreBottomDrag:{if (qFabs(event->pos().y() - m_maskRect.bottom()) >= 5 || event->pos().x() < m_maskRect.left() || event->pos().x() > m_maskRect.right()){this->setCursor(Qt::ArrowCursor);this->snapstate = Snapped;}}break;case BottomDrag:{if (event->pos().y() - MINSIZE <= m_maskRect.top()){m_maskRect.setBottom(m_maskRect.top() + MINSIZE);}else{m_maskRect.setBottom(event->pos().y() - 2);}QRegion sub(m_maskRect);QRegion all(0, 0, width(), height());setMask(all.subtracted(sub));update();}break;default:break;}}return QWidget::mouseMoveEvent(event);
}void MaskWidget::paintEvent(QPaintEvent *)
{QPainter painter(this);painter.setPen(Qt::red);painter.drawRect(m_maskRect.x()-1, m_maskRect.y()-1, m_maskRect.width()+1, m_maskRect.height() + 1);}void MaskWidget::keyPressEvent(QKeyEvent *event)
{if (event->key() == Qt::Key_Escape){close();}else if (event->key() == Qt::Key_Enter){if (isPressed){isPressed = false;snapstate = Snapped;this->setCursor(Qt::ArrowCursor);}}QWidget::keyPressEvent(event);
}void MaskWidget::showEvent(QShowEvent *event)
{QWidget::showEvent(event);
}void MaskWidget::ResetSnap()
{QRegion all(0, 0, width(), height());setMask(all);m_maskRect.setRect(0,0,0,0);snapstate = NoSnap;this->show();
}

思路：

第一，要保留住选中的框，选中后停留下来不自动跳转。实现方式为在mouseRelease函数里面不再进行隐藏和跳转，跳转改称点击鼠标右键。

第二，当鼠标移动到边框附近时，鼠标的形状要进行变化，表示可以拖动了。左右边对应的是双向横箭头，上下边对应的是双向竖箭头。这里取值距离5作为触发区域，当距离小于5时可进行拖动。类似途中的红色区域。

第三，为了配合形状变化，用一个枚举来表示不同的状态。

enum SnapState{NoSnap,Snapped,PreLeftDrag,LeftDrag,PreRightDrag,RightDrag,PreTopDrag,TopDrag,PreBottomDrag,BottomDrag
};

NoSnap表示初始状态，没有开始截图的时候。

Snapped表示已经截图了，此时会显示一个矩形方框。

PreLeftDrag表示进入左侧边框可拖动状态，此时鼠标形状变化成左右箭头。

        PreLeftDrag----按下鼠标左键--->LeftDrag（可移动鼠标来拖动边框）

        PreLeftDrag----鼠标距离左边框的距离大于5--->Snapped（鼠标状态恢复正常）

LeftDrag表示进入可拖动状态，可拖动鼠标更改左边框位置。此时松开鼠标则回到PreLeftDrag状态

其他几个状态类似。

根据鼠标的位置和动作，变化不同的状态。这里由于逻辑简单，没有使用状态机。

为了有更好的显示效果，这里限制了拖动区域，不会出现一条边覆盖另一条的情况，可在动图里看出来。

这样修改后，截图工具好用了很多。