Teratail_53681_Qt

anotherthreadworker.cpp

#include "anotherthreadworker.h"
#include <QPixmap>
#include <mainwindow.h>
#include <QDebug>
#include <QFile>

AnotherThreadWorker::AnotherThreadWorker(QObject *parent) : QObject(parent)
{

}

void AnotherThreadWorker::doDirectCall()
{
    // it will crash
    qDebug() << __FUNCTION__ << QThread::currentThreadId();
    auto pixmap = mainWindow->getMainWindowWidgetPixmap();
}

void AnotherThreadWorker::doSignalCall()
{
    qDebug() << __FUNCTION__ << QThread::currentThreadId();
    emit requestGrabWidget();
}

void AnotherThreadWorker::onGrabRecieved(QPixmap pixmap)
{
    qDebug() << __FUNCTION__ << QThread::currentThreadId();
    QFile file("another_thread.png");
    file.open(QIODevice::WriteOnly);
    pixmap.save(&file, "PNG");
}

anotherthreadworker.h

#ifndef ANOTHERTHREADWORKER_H
#define ANOTHERTHREADWORKER_H

#include <QObject>
#include <QThread>
#include <mainwindow.h>

class AnotherThreadWorker : public QObject
{
    Q_OBJECT
public:
    explicit AnotherThreadWorker(QObject *parent = 0);

    void doDirectCall();
    void doSignalCall();
    void setMainWindow(MainWindow *window) {
        mainWindow = window;
    }

signals:
    void requestGrabWidget();

public slots:
    void onGrabRecieved(QPixmap pixmap);

private:
    MainWindow* mainWindow;
};

#endif // ANOTHERTHREADWORKER_H

main.cpp

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QVBoxLayout>
#include <QPushButton>
#include <QDebug>
#include <QThread>
#include "anotherthreadworker.h"
#include <QFile>

void MainWindow::grabMainWindowWidget() {
    auto pix = this->grab(this->rect());
    emit onFinishedGrabWidget(pix);
}

void MainWindow::onRecievedGrabRequest()
{
    qDebug() << __FUNCTION__ << QThread::currentThreadId();
    grabMainWindowWidget();
}

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    auto workerObject = new AnotherThreadWorker();
    auto thread = new QThread();
    workerObject->moveToThread(thread);
    thread->start();
    qDebug() << "Curernt thread id is " << QThread::currentThreadId();
    auto buttonMain = new QPushButton("Call main thread button");
    connect(buttonMain, &QPushButton::clicked, [=](bool checked)
    {
        qDebug() << "Clicked on "  << QThread::currentThreadId();

        // This call on UI thread, so success
        auto pixmap = getMainWindowWidgetPixmap();
        QFile file("main_thread.png");
        file.open(QIODevice::WriteOnly);
        pixmap.save(&file, "PNG");
    });

    auto buttonThread = new QPushButton("Call another thread button");
    connect(buttonThread, &QPushButton::clicked, [=](bool checked){
        workerObject->doDirectCall();
    });

    auto buttonSignal = new QPushButton("Call another thread with signal button");
    connect(buttonSignal, &QPushButton::clicked, [=](bool checked){
        workerObject->doSignalCall();
    });

    connect(workerObject, SIGNAL(requestGrabWidget()) ,this, SLOT(onRecievedGrabRequest()));
    connect(this, SIGNAL(onFinishedGrabWidget(QPixmap)), workerObject, SLOT(onGrabRecieved(QPixmap)));
    auto layout = new QVBoxLayout(this);
    layout->addWidget(buttonMain);
    layout->addWidget(buttonThread);
    layout->addWidget(buttonSignal);
    ui->centralWidget->setLayout(layout);
}

MainWindow::~MainWindow()
{
    delete ui;
}

QPixmap MainWindow::getMainWindowWidgetPixmap()
{
    return this->grab(this->rect());
}

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

    QPixmap getMainWindowWidgetPixmap();

private:
    void grabMainWindowWidget();

public slots:
    void onRecievedGrabRequest();

signals:
    void onFinishedGrabWidget(QPixmap pixmap);

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

mainwindow.ui

<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow" >
  <property name="geometry" >
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle" >
   <string>MainWindow</string>
  </property>
  <widget class="QMenuBar" name="menuBar" />
  <widget class="QToolBar" name="mainToolBar" />
  <widget class="QWidget" name="centralWidget" />
  <widget class="QStatusBar" name="statusBar" />
 </widget>
 <layoutDefault spacing="6" margin="11" />
 <pixmapfunction></pixmapfunction>
 <resources/>
 <connections/>
</ui>

Teratail_53681_Qt.pro

#-------------------------------------------------
#
# Project created by QtCreator 2016-11-02T15:59:39
#
#-------------------------------------------------

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = Teratail_53681_Qt
TEMPLATE = app


SOURCES += main.cpp\
        mainwindow.cpp \
    anotherthreadworker.cpp

HEADERS  += mainwindow.h \
    anotherthreadworker.h

FORMS    += mainwindow.ui