db4o Tutorial 中文翻译（八）

6. 继承

到现在为止，我们已经可以操作实体类了。但是如何操作子类或者接口呢？

为了方便讲解，我们将创造不同种类的sensors

Sensor
using System;
namespace Db4objects.Db4o.Tutorial.F1.Chapter4
{   
    public class SensorReadout
    {
        DateTime _time;
        Car _car;
        string _description;
        
        public SensorReadout(DateTime time, Car car, string description)
        {
            _time = time;
            _car = car;
            _description = description;
        }
        
        public Car Car
        {
            get
            {
                return _car;
            }
        }
        
        public DateTime Time
        {
            get
            {
                return _time;
            }
        }
        
        public string Description
        {
            get
            {
                return _description;
            }
        }
        
        override public string ToString()
        {
            return string.Format("{0}:{1}:{2}", _car, _time, _description);
        }
    }
}

TemperatureSensorReadout
using System;
namespace Db4objects.Db4o.Tutorial.F1.Chapter4
{   
    public class TemperatureSensorReadout : SensorReadout
    {
        double _temperature;
        public TemperatureSensorReadout(DateTime time, Car car, string description, double temperature)
            : base(time, car, description)
        {
            _temperature = temperature;
        }
        
        public double Temperature
        {
            get
            {
                return _temperature;
            }
        }
        
        override public string ToString()
        {
            return string.Format("{0} temp: {1}", base.ToString(), _temperature);
        }
    }
}

PressureSensorReadout
using System;
namespace Db4objects.Db4o.Tutorial.F1.Chapter4
{
    public class PressureSensorReadout : SensorReadout
    {
        double _pressure;
        
        public PressureSensorReadout(DateTime time, Car car, string description, double pressure)
            : base(time, car, description)
        {
            _pressure = pressure;
        }
        
        public double Pressure
        {
            get
            {
                return _pressure;
            }
        }
        
        override public string ToString()
        {
            return string.Format("{0} pressure: {1}", base.ToString(), _pressure);
        }
    }
}

Car的snapshot也要跟着变化：

new car
using System;
using System.Collections;
namespace Db4objects.Db4o.Tutorial.F1.Chapter4
{   
    public class Car
    {
        string _model;
        Pilot _pilot;
        IList _history;
        
        public Car(string model)
        {
            _model = model;
            _pilot = null;
            _history = new ArrayList();
        }
        
        public Pilot Pilot
        {
            get
            {
                return _pilot;
            }
            
            set
            {
                _pilot = value;
            }
        }
        
        public string Model
        {
            get
            {
                return _model;
            }
        }
        
        public SensorReadout[] GetHistory()
        {
            SensorReadout[] history = new SensorReadout[_history.Count];
            _history.CopyTo(history, 0);
            return history;
        }
        
        public void Snapshot()
        {
            _history.Add(new TemperatureSensorReadout(DateTime.Now, this, "oil", PollOilTemperature()));
            _history.Add(new TemperatureSensorReadout(DateTime.Now, this, "water", PollWaterTemperature()));
            _history.Add(new PressureSensorReadout(DateTime.Now, this, "oil", PollOilPressure()));
        }
        
        protected double PollOilTemperature()
        {
            return 0.1*_history.Count;
        }
        
        protected double PollWaterTemperature()
        {
            return 0.2*_history.Count;
        }
        
        protected double PollOilPressure()
        {
            return 0.3*_history.Count;
        }
        
        override public string ToString()
        {
            return string.Format("{0}[{1}]/{2}", _model, _pilot, _history.Count);
        }
    }
}

6.1. 存储

代码只是加入了与sensor的交互而已，其他部分没有动：

// storeFirstCar
Car car1 = new Car("Ferrari");
Pilot pilot1 = new Pilot("Michael Schumacher", 100);
car1.Pilot = pilot1;
db.Set(car1);

// storeSecondCar
Pilot pilot2 = new Pilot("Rubens Barrichello", 99);
Car car2 = new Car("BMW");
car2.Pilot = pilot2;
car2.Snapshot();
car2.Snapshot();
db.Set(car2);

6.2. 检索

db4o提供所有的给定类型的对象。在检索给定类型的对象时候，不管他们是否是子类或者其他，只要给出一个原型就可以。

// retrieveTemperatureReadoutsQBE
SensorReadout proto = new TemperatureSensorReadout(DateTime.MinValue, null, null, 0.0);
IObjectSet result = db.Get(proto);
ListResult(result);

// retrieveAllSensorReadoutsQBE
SensorReadout proto = new SensorReadout(DateTime.MinValue, null, null);
IObjectSet result = db.Get(proto);
ListResult(result);

当在如下情况时QBE就不合适了：如果给定的类是一个接口或者抽象类。有一个小诀窍：只要获取对象的类型就可以了

// retrieveAllSensorReadoutsQBEAlternative
IObjectSet result = db.Get(typeof(SensorReadout));
ListResult(result);

当然，还有SODA API:

// retrieveAllSensorReadoutsQuery
IQuery query = db.Query();
query.Constrain(typeof(SensorReadout));
IObjectSet result = query.Execute();
ListResult(result);

6.3. 更新和删除

不管他们在继承树的哪里，他们都和正常的对象一样。.

就像刚刚我们的检索过程, 我们可以删除所有的对象，好进行下一章的讲解。

// deleteAll
IObjectSet result = db.Get(typeof(Object));
foreach (object item in result)
{
    db.Delete(item);
}

6.4. 总结

现在我们已经知道了所有的OO特征如何在db4o里面操作。我们将在下一章完成预排工作，下一章将可以看到深层次的探险，包括循环结构。

6.5. 完整代码

 

锘縰sing System;
using System.IO;
using Db4objects.Db4o;
using Db4objects.Db4o.Query;
namespace Db4objects.Db4o.Tutorial.F1.Chapter4
{   
    public class InheritanceExample : Util
    {        
        public static void Main(string[] args)
        {
            File.Delete(Util.YapFileName);          
            IObjectContainer db = Db4oFactory.OpenFile(Util.YapFileName);
            try
            {
                StoreFirstCar(db);
                StoreSecondCar(db);
                RetrieveTemperatureReadoutsQBE(db);
                RetrieveAllSensorReadoutsQBE(db);
                RetrieveAllSensorReadoutsQBEAlternative(db);
                RetrieveAllSensorReadoutsQuery(db);
                RetrieveAllObjects(db);
            }
            finally
            {
                db.Close();
            }
        }
        
        public static void StoreFirstCar(IObjectContainer db)
        {
            Car car1 = new Car("Ferrari");
            Pilot pilot1 = new Pilot("Michael Schumacher", 100);
            car1.Pilot = pilot1;
            db.Set(car1);
        }
        
        public static void StoreSecondCar(IObjectContainer db)
        {
            Pilot pilot2 = new Pilot("Rubens Barrichello", 99);
            Car car2 = new Car("BMW");
            car2.Pilot = pilot2;
            car2.Snapshot();
            car2.Snapshot();
            db.Set(car2);
        }
        
        public static void RetrieveAllSensorReadoutsQBE(IObjectContainer db)
        {
            SensorReadout proto = new SensorReadout(DateTime.MinValue, null, null);
            IObjectSet result = db.Get(proto);
            ListResult(result);
        }
        
        public static void RetrieveTemperatureReadoutsQBE(IObjectContainer db)
        {
            SensorReadout proto = new TemperatureSensorReadout(DateTime.MinValue, null, null, 0.0);
            IObjectSet result = db.Get(proto);
            ListResult(result);
        }
        
        public static void RetrieveAllSensorReadoutsQBEAlternative(IObjectContainer db)
        {
            IObjectSet result = db.Get(typeof(SensorReadout));
            ListResult(result);
        }
        
        public static void RetrieveAllSensorReadoutsQuery(IObjectContainer db)
        {
            IQuery query = db.Query();
            query.Constrain(typeof(SensorReadout));
            IObjectSet result = query.Execute();
            ListResult(result);
        }
        
        public static void RetrieveAllObjects(IObjectContainer db)
        {
            IObjectSet result = db.Get(new object());
            ListResult(result);
        }
    }
}