addStretch在布局中的作用

今天在使用addStretch，布局的时候，发现addStretch竟然是可以平均分配的，有意思。比如：

#include "drawer.h"

Drawer::Drawer( QWidget *parent, Qt::WindowFlags  f )
    : QToolBox( parent, f )
{
    setWindowTitle(tr("My QQ"));	
	
    QGroupBox *groupBox1 = new QGroupBox();
    
    toolButton1_1 = new QToolButton( );
    toolButton1_1->setText( tr( "beibei" ) );
    toolButton1_1->setIcon( QPixmap(":/images/bb.png") );
    toolButton1_1->setIconSize( QPixmap(":/images/bb.png").size() );
    toolButton1_1->setAutoRaise( TRUE );
    toolButton1_1->setToolButtonStyle( Qt::ToolButtonTextUnderIcon);

    toolButton1_2 = new QToolButton();
    toolButton1_2->setText( tr( "jingjing" ) );
    toolButton1_2->setIcon( QPixmap( ":/images/jj.png") );
    toolButton1_2->setIconSize( QPixmap( ":/images/jj.png").size() );
    toolButton1_2->setAutoRaise( TRUE );
    toolButton1_2->setToolButtonStyle( Qt::ToolButtonTextUnderIcon);
    
    toolButton1_3 = new QToolButton();
    toolButton1_3->setText( tr( "huanhuan" ) );
    toolButton1_3->setIcon( QPixmap( ":/images/hh.png") );
    toolButton1_3->setIconSize( QPixmap( ":/images/hh.png").size() );
    toolButton1_3->setAutoRaise( TRUE );
    toolButton1_3->setToolButtonStyle( Qt::ToolButtonTextUnderIcon);
        
    toolButton1_4 = new QToolButton();
    toolButton1_4->setText( tr( "yingying" ) );
    toolButton1_4->setIcon( QPixmap( ":/images/yy.png") );
    toolButton1_4->setIconSize( QPixmap( ":/images/yy.png").size() );
    toolButton1_4->setAutoRaise( TRUE );
    toolButton1_4->setToolButtonStyle( Qt::ToolButtonTextUnderIcon);
        
    toolButton1_5 = new QToolButton();
    toolButton1_5->setText( tr( "nini" ) );
    toolButton1_5->setIcon( QPixmap( ":/images/nn.png") );
    toolButton1_5->setIconSize( QPixmap( ":/images/nn.png").size() );
    toolButton1_5->setAutoRaise( TRUE );
    toolButton1_5->setToolButtonStyle( Qt::ToolButtonTextUnderIcon);
    
    QVBoxLayout *layout1 = new QVBoxLayout(groupBox1);
    layout1->setMargin(10);
    layout1->setAlignment(Qt::AlignHCenter);  
    layout1->addWidget(toolButton1_1);
    layout1->addWidget(toolButton1_2);
    layout1->addWidget(toolButton1_3);
    layout1->addWidget(toolButton1_4);
    layout1->addWidget(toolButton1_5);  
    //layout1->addStretch();        

    QGroupBox *groupBox2 = new QGroupBox();
    
    toolButton2_1 = new QToolButton( );
    toolButton2_1->setText( tr( "pizicai" ) );
    toolButton2_1->setIcon( QPixmap( ":/images/1.png" ) );
    toolButton2_1->setIconSize( QPixmap( ":/images/1.png").size() );
    toolButton2_1->setAutoRaise( TRUE );
    toolButton2_1->setToolButtonStyle( Qt::ToolButtonTextUnderIcon);
    
    toolButton2_2 = new QToolButton();
    toolButton2_2->setText( tr( "feiyang" ) );
    toolButton2_2->setIcon( QPixmap( ":/images/2.png") );
    toolButton2_2->setIconSize( QPixmap( ":/images/2.png").size() );
    toolButton2_2->setAutoRaise( TRUE );
    toolButton2_2->setToolButtonStyle( Qt::ToolButtonTextUnderIcon);
    
    QVBoxLayout *layout2 = new QVBoxLayout(groupBox2);
    layout2->setMargin(10);
    layout2->setAlignment(Qt::AlignHCenter);    
    layout2->addWidget(toolButton2_1);
    layout2->addWidget(toolButton2_2);
    layout2->addStretch();       
    
    QGroupBox *groupBox3 = new QGroupBox();
    
    toolButton3_1 = new QToolButton( );
    toolButton3_1->setText( tr( "QQ" ) );
    toolButton3_1->setIcon( QPixmap( ":/images/3.png" ) );
    toolButton3_1->setIconSize( QPixmap( ":/images/3.png").size() );
    toolButton3_1->setAutoRaise( TRUE );
    toolButton3_1->setToolButtonStyle( Qt::ToolButtonTextUnderIcon);
    
    toolButton3_2 = new QToolButton();
    toolButton3_2->setText( tr( "angel" ) );
    toolButton3_2->setIcon( QPixmap( ":/images/4.png") );
    toolButton3_2->setIconSize( QPixmap( ":/images/4.png").size() );
    toolButton3_2->setAutoRaise( TRUE );
    toolButton3_2->setToolButtonStyle( Qt::ToolButtonTextUnderIcon);
    
    QVBoxLayout *layout3 = new QVBoxLayout(groupBox3);
    layout3->setMargin(10);
    layout3->setAlignment(Qt::AlignHCenter);    
    layout3->addWidget(toolButton3_1);
    layout3->addWidget(toolButton3_2);     
    layout3->addStretch();         

    this->addItem( ( QWidget* )groupBox1 , tr("my friends" ));
    this->addItem( ( QWidget* )groupBox2 , tr("stranger" ));
    this->addItem( ( QWidget* )groupBox3 , tr("blacklist" ));

}

Drawer::~Drawer()
{
}

addStretch 在拉伸的时候非常有用，可以保持布局

             

左边的用了addStretch   右边的没用