QtWebEngine-同步执行JavaScript以读取函数结果 [英] QtWebEngine - synchronously execute JavaScript to read function result
问题描述
我的一个C ++类(使用QtWebEngine)具有以下方法:
I have the following method in one of my C++ classes (using QtWebEngine):
QString get()
{
QString result;
view->page()->runJavaScript("test();", [this](const QVariant &v)
{
result = v.toString();
});
return result;
}
将执行test() JS函数并返回此调用的结果.
It is to execute test() JS function and return the result of this invocation.
不幸的是，回调是异步的，程序崩溃了.我该如何运作?
Unfortunately, the callback is asynchronous and the program crashes. How can I make it work?
推荐答案
回调是异步的，因为JavaScript执行不仅发生在另一个线程中，而且发生在另一个进程中.因此，无法使其完全同步.
The callback is asynchronous because the JavaScript execution occurs not only in another thread but in another process. So there is no way to make it fully synchronous.
最好的解决方案是迁移C ++代码以使其异步工作.如果您不能这样做，则唯一可行的解决方案是使用QEventLoop，如下所示:
The best possible solution would be to migrate your C++ code to work asynchronously. If you can't do that, the only feasible solution is to use QEventLoop, somewhat like this:
void ranJavaScript()
{
emit notifyRanJavaScript();
}
QString get()
{
QString result;
QEventLoop loop;
QObject::connect(this, SIGNAL(notifyRanJavaScript()), &loop, SLOT(quit()));
view->page()->runJavaScript("test();", [this](const QVariant &v)
{
result = v.toString();
this.ranJavaScript();
});
loop.exec();
return result;
}
但是，请注意，此示例在实际使用中被简化了:您需要确保在事件循环开始之前未运行JavaScript.最合适的方法是实现一个适当的插槽，而不是lambda +将对view->page()->runJavaScript()的调用分解为另一个插槽，该插槽在启动事件循环后将被异步地称为 .这项看似简单的任务需要很多胶水代码，但这就是需要的.这是一个示例:
However, note that this example is oversimplified for a real-world usage: you need to ensure the JavaScript was not ran before the event loop is started. The most proper way to do that would involve implementing a proper slot instead of a lambda + factoring out the call to view->page()->runJavaScript() into another slot which would be called asynchronously after starting the event loop. It is a lot of glue code for such a seemingly simple task but that's what it takes. Here's an example:
MainWindow.h
#ifndef TMP_MAIN_WINDOW_H
#define TMP_MAIN_WINDOW_H
#include <QMainWindow>
#include <QVariant>
class QWebEngineView;
class QPushButton;
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
MainWindow(QWidget * parent = 0);
QString get();
void onScriptEnded(const QVariant & data);
Q_SIGNALS:
void notifyRanJavaScript();
private Q_SLOTS:
void onButtonPressed();
void startScript();
private:
QWebEngineView * m_view;
QPushButton * m_button;
QString m_scriptResult;
};
#endif // TMP_MAIN_WINDOW_H
MainWindow.cpp
#include "MainWindow.h"
#include <QWebEngineView>
#include <QPushButton>
#include <QHBoxLayout>
#include <QVBoxLayout>
#include <QMessageBox>
#include <QEventLoop>
#include <QDebug>
#include <QTimer>
MainWindow::MainWindow(QWidget * parent) :
QMainWindow(parent)
{
m_view = new QWebEngineView;
QWebEnginePage * page = new QWebEnginePage(m_view);
m_view->setPage(page);
QString html = QStringLiteral("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01//EN\""
"\"http://www.w3.org/TR/html4/strict.dtd\"><html>"
"<head><h3>head</h3>\n</head>"
"<script type=\"text/javascript\">function test() { return \"A!\"; }</script>"
"<body>text\n</body></html>");
m_view->page()->setHtml(html);
m_button = new QPushButton;
m_button->setMinimumWidth(35);
m_button->setText(QStringLiteral("Test"));
QObject::connect(m_button, SIGNAL(pressed()), this, SLOT(onButtonPressed()));
QHBoxLayout * buttonLayout = new QHBoxLayout;
buttonLayout->addWidget(m_button);
buttonLayout->addStretch();
QVBoxLayout * viewLayout = new QVBoxLayout;
viewLayout->addLayout(buttonLayout);
viewLayout->addWidget(m_view);
QWidget * widget = new QWidget(this);
widget->setLayout(viewLayout);
setCentralWidget(widget);
}
QString MainWindow::get()
{
QEventLoop loop;
QObject::connect(this, SIGNAL(notifyRanJavaScript()), &loop, SLOT(quit()));
// Schedule the slot to run in 0 seconds but not right now
QTimer::singleShot(0, this, SLOT(startScript()));
// The event loop would block until onScriptEnded slot is executed
loop.exec();
// If we got here, the script has been executed and the result was saved in m_scriptResult
return m_scriptResult;
}
void MainWindow::onScriptEnded(const QVariant & data)
{
qDebug() << QStringLiteral("Script ended: ") << data;
m_scriptResult = data.toString();
emit notifyRanJavaScript();
}
void MainWindow::onButtonPressed()
{
QString str = get();
QMessageBox::information(this, QStringLiteral("Script result"), str,
QMessageBox::StandardButton::Ok);
}
struct Functor
{
Functor(MainWindow & window) : m_window(window) {}
void operator()(const QVariant & data)
{
m_window.onScriptEnded(data);
}
MainWindow & m_window;
};
void MainWindow::startScript()
{
qDebug() << QStringLiteral("Start script");
m_view->page()->runJavaScript(QStringLiteral("test();"), Functor(*this));
}
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