使用 QStateMachine 实现安装向导，状态机不能切换的解决办法 ，sync

       前言：嵌入式项目需要升级功能，升级范围有应用升级，系统升级，外设固件升级，升级包的下载等，每一项升级都包含以下流程，检测安装包，提示用户是否升级，升级过程中提供进度，提示用户升级完成。
      经过几次尝试后，发现用if多个分支已不能很好的管理这么多选项了，经同事提示考虑使用状态机。的确是个稳妥的方法。但经试验后发现有一个问题，就是 安装包复制完成后 ，本想驱动状态机进入 “提示用户已完成的UI”，但就像程序睡眠了一样，卡住了，经kill -9 pid杀掉后，之后每次升级都不会卡了，联想到startapp.sh中App退出后有一个sync操作。因此在App中每次 FileCopy如果都 强行加入sync，神奇的解决了问题，有大侠知道原因的请指教
简化的代码如下：
mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QWidget>
#include <QState>
#include <QStateMachine>

#include <QAbstractTransition>
#define STATE_WRITING 0
#define STATE_WRITE_FINISH    1

namespace Ui {
class MainWindow;
}


struct BootStateEvent : public QEvent
{
    BootStateEvent(const int &val) :
        QEvent(QEvent::Type(QEvent::User+100)),
      value(val) {

    }
    int value;
 };
class BootStateTransition : public QAbstractTransition
 {
 public:
    BootStateTransition(const int &value)
        : m_value(value) {}
 protected:
    bool eventTest(QEvent *e)
    {
        if (e->type() != QEvent::Type(QEvent::User+100))// BootStateEvent
            return false;
        BootStateEvent *se = static_cast<BootStateEvent*>(e);
        return (m_value == se->value);
    }

    void onTransition(QEvent *) {}
 private:
    int m_value;

 };

class MainWindow : public QWidget
{
    Q_OBJECT

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

private slots:
    void slotWritingFileOnEnter();
    void slotShowFinishOnEnter();
private:
    Ui::MainWindow *ui;
    void doCopyFile();//do file copy
    void doSync();
    void initMachine();
    QState *writingFile ;
    QState *showFinish ;
    QStateMachine *machine ;
};

#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QDebug>
#include <QProcess>
MainWindow::MainWindow(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    initMachine();
    machine->start();

}

MainWindow::~MainWindow()
{
    delete ui;
}
void MainWindow::slotWritingFileOnEnter(){
    doCopyFile();
    doSync();
    machine->postEvent(new BootStateEvent(STATE_WRITE_FINISH));//这一句之前加doSync()，否则转入不到s lotShowFinishOnEnter

}

void MainWindow::doCopyFile()
{
    qDebug()<<"FileCopy...";
    qDebug()<<"FileCopy finished";

}

void MainWindow::doSync()
{
    qDebug()<<"doSync...";
    QProcess p;

    QEventLoop loop;
    QObject::connect(&p,SIGNAL(finished(int)),&loop,SLOT(quit()));

    QString programPath = "/sbin/sync";
    QStringList args;
    p.start(programPath,args);
    p.waitForFinished( -1 );
}

void MainWindow::initMachine()
{
    writingFile = new QState();
    showFinish = new QState();
    machine = new QStateMachine();


    machine->addState( writingFile );
    machine->addState( showFinish );
    machine->setInitialState( writingFile );

    BootStateTransition *writingFinishTran = new BootStateTransition(STATE_WRITE_FINISH);

    writingFinishTran->setTargetState( showFinish );
    writingFile->addTransition( writingFinishTran );

    connect(writingFile,SIGNAL(entered()),this,SLOT(slotWritingFileOnEnter()));
    connect(showFinish,SIGNAL(entered()),this,SLOT(slotShowFinishOnEnter()));

}

void MainWindow::slotShowFinishOnEnter(){
    //show finish info
     qDebug()<<"slotShowFinishOnEnter";
}