【描述】外觀模式通過在對必需的邏輯和方法的集合前建立簡單的外觀介面，隱藏來自呼叫的複雜性。

【UML圖】

圖1 外觀模式UML圖

(1) AirCondition、Fan、Light電器類定義了一個on和off的方法；

(2) Facade類定義了on和off的方法，並呼叫了AirCondition、Fan、Light類的on和off方法，隱藏呼叫的複雜性；

(3) 與組合模式的不同在於Facade和AirCondition、Fan、Light電器類是關聯的關係，而不是組合的關係。

【示例程式碼】

aircondition.h

#ifndef AIRCONDITION_H
#define AIRCONDITION_H

class AirCondition
{
public:
    AirCondition();
    ~AirCondition();

public:
    void on();
    void off();
};

#endif // AIRCONDITION_H
 

aircondition.cpp

#include <QDebug>
#include "aircondition.h"

AirCondition::AirCondition()
{
    qDebug()<<"construct AirCondition";
}

AirCondition::~AirCondition()
{
    qDebug()<<"destruct AirCondition";
}

void AirCondition::on()
{
    qDebug()<<"AirCondition on";
}

void AirCondition::off()
{
    qDebug()<<"AirCondition off";
}
 

fan.h

#ifndef FAN_H
#define FAN_H

class Fan
{
public:
    Fan();
    ~Fan();

public:
    void on();
    void off();
};

#endif // FAN_H

fan.cpp

#include <QDebug>
#include "fan.h"

Fan::Fan()
{
    qDebug()<<"construct Fan";
}

Fan::~Fan()
{
    qDebug()<<"destruct Fan";
}

void Fan::on()
{
    qDebug()<<"Fan on";
}

void Fan::off()
{
    qDebug()<<"Fan off";
}
 

light.h

#ifndef LIGHT_H
#define LIGHT_H

class Light
{
public:
    Light();
    ~Light();

public:
    void on();
    void off();
};

#endif // LIGHT_H

light.cpp

#include <QDebug>
#include "light.h"

Light::Light()
{
    qDebug()<<"construct Light";
}

Light::~Light()
{
    qDebug()<<"destruct Light";
}

void Light::on()
{
    qDebug()<<"Light on";
}

void Light::off()
{
    qDebug()<<"Light off";
}

facade.h

#ifndef FACADE_H
#define FACADE_H

#include "aircondition.h"
#include "fan.h"
#include "light.h"

class Facade
{
public:
    Facade();
    ~Facade();

private:
    AirCondition *_airCondition;
    Fan *_fan;
    Light *_light;

public:
    void on();
    void off();
};

#endif // FACADE_H

facade.cpp

#include <QDebug>
#include "facade.h"

Facade::Facade()
{
    qDebug()<<"construct Facade";

    _airCondition = new AirCondition;
    _fan = new Fan;
    _light = new Light;
}

Facade::~Facade()
{
    qDebug()<<"destruct Facade";

    delete _airCondition;
    delete _fan;
    delete _light;
}

void Facade::on()
{
    qDebug()<<"Facade::on";

    _airCondition->on();
    _fan->on();
    _light->on();
}

void Facade::off()
{
    qDebug()<<"Facade::off";

    _airCondition->off();
    _fan->off();
    _light->off();
}

main.cpp

#include "facade.h"

int main(void)
{
    Facade *facade = new Facade;
    facade->on();
    facade->off();

    return 0;
}

【執行結果】

construct Facade 
construct AirCondition 
construct Fan 
construct Light 
Facade::on 
AirCondition on 
Fan on 
Light on 
Facade::off 
AirCondition off 
Fan off 
Light off

【結果分析】
Facade的on方法為電器類on方法的集合提供了介面，可以根據需要定義不同的on方法。

【例項剖析】

外觀模式是我們常用的設計模式，有時候不知不覺就用到了。看下面一個登陸介面的設計例項。

先看UML圖，

圖2

(1) LoginDlg繼承了QDialog類，實現使用者登入鑑權；

(2) LoginDlg呼叫了QLabel、QLineEdit進行介面設計，介面設計的程式碼單獨分離，構建了外觀方法，提供給建構函式呼叫；

(3) 提供了cancel方法，cancel呼叫了QLineEdit相關物件的clear方法，實質上是一個外觀打包。

【程式碼清單】

LoginDlg::LoginDlg()
{
    //qDebug()<<"construct_login";
    initDlg();
    initConnect();
}

LoginDlg::~LoginDlg()
{
    delete label_username;
    delete label_passwd;
    delete lineEdit_username;
    delete lineEdit_passwd;
    delete pushButton_ok;
    delete pushButton_cancel;

    //qDebug()<<"desstruct_login";
}

void LoginDlg::initDlg()
{
    setWindowFlags(Qt::FramelessWindowHint);
    label_username = new QLabel;
    label_username->setText(tr("用 戶 名:"));
    label_passwd = new QLabel;
    label_passwd->setText(tr("使用者密碼:"));
    lineEdit_username = new QLineEdit;
    lineEdit_username->setFocus();
    lineEdit_passwd = new QLineEdit;
    lineEdit_passwd->setEchoMode(QLineEdit::Password);

    pushButton_ok = new QPushButton;
    pushButton_ok->setText(tr("確定"));
    pushButton_ok->setIcon(style()->standardIcon(QStyle::SP_DialogOkButton));
    pushButton_cancel = new QPushButton;
    pushButton_cancel->setText(tr("取消"));
    pushButton_cancel->setIcon(style()->standardIcon(QStyle::SP_DialogCancelButton));

    QGridLayout *gridLayout = new QGridLayout;
    gridLayout->addWidget(label_username,0,0);
    gridLayout->addWidget(lineEdit_username,0,1);
    gridLayout->addWidget(label_passwd,1,0);
    gridLayout->addWidget(lineEdit_passwd,1,1);

    QHBoxLayout *buttonLayout = new QHBoxLayout;
    buttonLayout->addStretch();
    buttonLayout->addWidget(pushButton_ok);
    buttonLayout->addStretch();
    buttonLayout->addWidget(pushButton_cancel);
    buttonLayout->addStretch();

    QVBoxLayout *mainLayout = new QVBoxLayout;
    mainLayout->addLayout(gridLayout);
    mainLayout->addLayout(buttonLayout);
    mainLayout->addStretch();

    setLayout(mainLayout);
}

void LoginDlg::initConnect()
{
    connect(pushButton_cancel,SIGNAL(clicked()),this,SLOT(cancel()));
    connect(pushButton_ok,SIGNAL(clicked()),this,SLOT(Login()));
}

void LoginDlg::cancel()
{
    lineEdit_username->clear();
    lineEdit_passwd->clear();

    close();
}

LoginDlg類中initDlg、cancel方法，應用了外觀模式。cancel方法運用了多型特性，覆蓋了LoginDlg的父類QDialog的cancel方法，是LoginDlg類的一個私有槽函式。外觀模式的應用使得建構函式更為清晰，initConnect方法更為簡單。這時可以利用：

connect(pushButton_cancel,SIGNAL(clicked()),this,SLOT(cancel()));

來取代

connect(pushButton_cancel,SIGNAL(clicked()),lineEdit_username,SLOT(clear()));
connect(pushButton_cancel,SIGNAL(clicked()),lineEdit_passwd,SLOT(clear()));
connect(pushButton_cancel,SIGNAL(clicked()),this,SLOT(close()));

將介面設計的程式碼從建構函式中，單獨分離，也是一種好的設計方式。

*對外觀模式有更好的見解，歡迎留言，交流學習。

【原始碼下載】

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