How to run SequentialAnimation in the background? - qt

Question

I'm designing Qt5.7/QML application. The main idea is that pressing a button will trigger shell command which takes long time. The results of this command should be reflected in widgets.
In the meantime, I need SequentialAnimation to run, to show the user that something is happening in the background. So I start the animation and then call the process which sends the shell command.
But the animation seems to be freezed as the entire GUI until the shell command returns.
Any suggestions?
QML Code:
CircularGauge {
id: speedometer_wr
x: 199
y: 158
value: valueSource.bps
maximumValue: 400
width: height
height: container.height * 0.5
stepSize: 0
anchors {
verticalCenter: parent.verticalCenter
verticalCenterOffset: 46
}
style: DashboardGaugeStyle {}
NumberAnimation {
id: animation_wr
running: false
target: speedometer_wr
property: "value"
easing.type: Easing.InOutSine
from: 0
to: 300
duration: 15000
}
}
Button {
id: button_benchmark
x: 168
y: 26
width: 114
height: 47
text: qsTr("Benchmark")
tooltip: "Execute the performance test"
checkable: true
function handle_becnhmark() {
speedometer_wr.value = 0;
// Uncheck the button
button_benchmark.checked = false;
// Start the animation
animation_wr.start();
// Run the benchmark
var res = backend.run_benchmark();
// Stop the animation
animation_wr.stop();
speedometer_wr.value = parseFloat(res.split(",")[0]);
}
onClicked: {handle_becnhmark()}
}
Cpp Code:
#include <QProcess>
#include <QtDebug>
#include "backend.h"
#include <QRegExp>
BackEnd::BackEnd(QObject *parent) : QObject(parent)
{
}
QString BackEnd::run_benchmark() {
qDebug() << "in run_benchmark";
QProcess proc;
// Execute shell command
proc.start(<LONG_SHELL_COMMAND>);
if (!proc.waitForStarted())
return "";
if (!proc.waitForFinished())
return "";
// Get the results, parse and return values
QString result(proc.readAll());
return result;
}

Answer 1

It is problem of singlethread applications. You need to run your execution of shell command in the another thread. Just look at QThread example of using. Main thing is that you need a signal-slot based communication between you GUI and backend (if you are calling some "hard" functions in backend).
Working with threads is make sence in many areas like working with hard drive (reading/writing to file), requests to server or database. All of this can be done synchrony or asynchrony. Base meaning for you - would it be freezes main process or not (answer - asynchronous methods wouldn't).
In your app are using QProcess to call some external command. QProcess start() method will execute this command asynchronously by itself. It means you no need to use QThreads. But it make sence if you are using waitForFinished(). If you will read QProcess reference you will see that waitForStarted() and waitForFinished() methods is a synchronous. That means they will freeze main process and GUI indeed.
So what you need is
declare pointer to QProcess in class header (it would be better to use QSharedPointer<QProcess>)
class BackEnd : public QObject {
...
private slots:
void handleProcFinished(int exitCode, QProcess::ExitStatus exitStatus);
signals:
void executionEnds(const QString &res);
private:
QProcess* proc;
}
In source you need initialize proc and connect QProcess finished() signal to slot that will push the output of executed by proc command with another signal in BackEnd.
BackEnd::BackEnd(QObject *parent) : QObject(parent) {
proc = new QProcess();
connect(proc, &QProcess::finish, this, &BackEnd::handleProcFinished);
}
void handleProcFinished(int exitCode, QProcess::ExitStatus exitStatus) {
//check code
...
QString result("");
if (all is fine) {
result = proc->readAll();
} else {
//debug or something else
}
emit executionEnds(result);
}
In QML side it will be something like this:
Button {
onClicked: {
speedometer_wr.value = 0;
// Uncheck the button
button_benchmark.checked = false;
// Start the animation
animation_wr.start();
// Run the benchmark
var res = backend.run_benchmark();
}
}
Connections {
target: backend //[name of some contextProperty which will be emiting the signals]
onExecutionEnds: {
animation_wr.stop();
if (res != "") {
speedometer_wr.value = parseFloat(res.split(",")[0]);
}
{
}