一、生成初级event

G4VuserPrimaryGeneratorAction是获得用户自己的实体类的强制类

实体类中必须指定如何生成初级event
初级event实际将由G4VPrimaryGenerator的实体类完成
用户的G4VUserPrimaryGeneratorAction实体类只是安排了初级粒子的生成方式

//
// ExG4PrimaryGeneratorAction01.hh
//
#ifndef ExG4PrimaryGeneratorAction01_h
#define ExG4PrimaryGeneratorAction01_h 1 //G4VUserPrimaryGeneratorAction实体类中构造的主生成器对象必须在析构函数中被删
#include "G4VUserPrimaryGeneratorAction.hh"
#include "G4ThreeVector.hh"
#include "globals.hh"
class G4ParticleGun;
class G4Event;
class ExG4PrimaryGeneratorAction01 : public G4VUserPrimaryGeneratorAction
{
public:
    ExG4PrimaryGeneratorAction01(
            const G4String& particleName = "geantino", //字符&，粒子名称
            G4double energy = 1.*MeV, //粒子能量
            G4ThreeVector position= G4ThreeVector(0,0,0), //初始位置
            G4ThreeVector momentumDirection = G4ThreeVector(0,0,1)); //动量方向
    ~ExG4PrimaryGeneratorAction01();
// methods
    virtual void GeneratePrimaries(G4Event*);
private:
// data members
    G4ParticleGun* fParticleGun; //pointer a to G4 service class
};
#endif

//
// ExG4PrimaryGeneratorAction01.cc
//
#include "ExG4PrimaryGeneratorAction01.hh"
#include "G4Event.hh"
#include "G4ParticleGun.hh"
#include "G4ParticleTable.hh"
#include "G4ParticleDefinition.hh"
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
ExG4PrimaryGeneratorAction01::ExG4PrimaryGeneratorAction01(
        const G4String& particleName,
        G4double energy,
        G4ThreeVector position,
        G4ThreeVector momentumDirection)
        : G4VUserPrimaryGeneratorAction(),
          fParticleGun(0)
{
    G4int nofParticles = 1;
    fParticleGun = new G4ParticleGun(nofParticles);  
// default particle kinematic
    G4ParticleTable* particleTable = G4ParticleTable::GetParticleTable();
    G4ParticleDefinition* particle
            = particleTable->FindParticle(particleName);
    fParticleGun->SetParticleDefinition(particle);
    fParticleGun->SetParticleEnergy(energy);
    fParticleGun->SetParticlePosition(position);
    fParticleGun->SetParticleMomentumDirection(momentumDirection);
}
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
ExG4PrimaryGeneratorAction01::~ExG4PrimaryGeneratorAction01()
{
    delete fParticleGun;
}
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
void ExG4PrimaryGeneratorAction01::GeneratePrimaries(G4Event* anEvent)
{
// this function is called at the beginning of event
    fParticleGun->GeneratePrimaryVertex(anEvent);
}
//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......

1. generator的选择
在G4VUserPrimaryGeneratorAction的构造函数中，要实例化主generator。必要的话，还要为generator设置初始条件

class ExG4PrimaryGeneratorAction01 : public G4VUserPrimaryGeneratorAction

#ifndef ExG4PrimaryGeneratorAction01_h
#define ExG4PrimaryGeneratorAction01_h 1

2. 生成一个event
generatePrimaries()

是G4VUserPrimaryGeneratorAction的纯虚方法
在每个event开始时被调用
必须调用通过generatePrimaryVertex()方法实例化的G4VPrimaryGenerator实体类

二、G4VPrimaryGenerator G4初级发生器

三个G4VPrimaryGenerator实体类：

G4ParticleGun
G4GeneralParticleSource
G4HEPEvtInterface（见3.6 Event Generator Interface）

1. G4ParticleGun

G4ParticleGun类产生具有给定动量和位置的初级粒子，不提供随机化
G4ParticleGun的构造函数取整数，产生一个或多个运动学完全相同的初级粒子
如需随机化能量、动量或位置，可通过G4ParticleGun提供的各种set方法实现（如，可定义X,Y,Z为random变量），在实例化的G4VUserPrimaryGeneratorAction类的generatePrimaries()方法中实现这些调用，随后再调用G4ParticleGun的generatePrimaryVertex()
G4提供的各种分布的随机数生成方法见*3.2 Global Usage Classes

2. Public methods of G4ParticleGun
G4ParticleGun 提供以下方法，可从用户实体化的G4VUserPrimaryGeneratorAction 类中的generatePrimaries()方法调用：

void SetParticleDefinition(G4ParticleDefinition*)
void SetParticleMomentum(G4ParticleMomentum)
void SetParticleMomentumDirection(G4ThreeVector)
void SetParticleEnergy(G4double)
void SetParticleTime(G4double)
void SetParticlePosition(G4ThreeVector)
void SetParticlePolarization(G4ThreeVector)
void SetNumberOfParticles(G4int)

3. G4GeneralParticleSource
若想以更复杂的方式生成初级粒子，可使用G4GeneralParticleSource，称作GEANT4 General Particle Source module (GPS)，见2.7 GEANT4 General Particle Source