VB2008从入门到精通(PDF格式英文版)-第58部分
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198 CH AP T E R 8 ■ L E A R N IN G AB OU T CO M P O N E N TO R IE N T E D AR C HI TE CT U R E
ponents are implemented at the technical level using interfaces and classes that subclass
the interfaces。 The interface represents an idea； and the classes represent the implementation
of an idea。 A class could implement multiple interfaces； where each interface represents a
unique characteristic of the class implementation。 Ideas and interfaces also represent contracts
or standards。 The kernel defines a standard； and the ponent has the responsibility of imple
menting that standard。
Implementing a kernel is like being a coach。 A coach thinks of where to place players in the
field and develops strategies that the players need to implement。 But on game day； the players
will do what they think is best； and the coach is powerless。 A coach can teach the players； but
whether they apply that knowledge is up to the individual players。
In programming terms； the kernel is the coach； and the external implementations to
manipulate are the players。 When designing interfaces that other pieces of functionality will
implement； you cannot watch over the shoulders of the programmers and make sure they do
things correctly。 You need to trust； but you also need to implement a mode of programming
where you assume the programmers are going to do things incorrectly。 This has nothing to do
with the personal abilities of the other programmers。 It has to do with the ability to make sure
your kernel continues functioning； even when someone makes a mistake。
Remember that when implementing a kernel； you are implementing a controller； and you
are devising the strategy of the application。 You are just not doing all of the implementations。
When you write production code； and you have been put in charge of writing a kernel； count
yourself lucky。 But remember with the job of building a kernel es great responsibility。 If
your kernel is buggy or badly designed； then the external implementations will also be buggy
and potentially badly designed。 The kernel is the rock and foundation of the application。
Organizing the Lighting Application
For the moment； imagine we are not developing software； but actually building a house。 And
in this house； we are going to add a central lighting system。 The make of light bulbs； lamps； and
lighting controller are not identical。 This is interesting in that multiple panies make prod
ucts that work with each other’s device； because all panies adhere to a particular standard。
The process of standardization is all around us； and is as simple as the electricity that es
out of our electrical outlets。 However； what is a standard in one country is not necessarily the
standard in another country。 Different countries will have different standard voltages。 In terms
of software； the kernel represents a standard that allows the integration of ponents。
The lighting application will include these features：
o A room where the light can be controlled using either a simple on/off switch or a gradual
light…intensity mechanism。
o The controller represents a building； and the rooms within the building can be grouped；
making it simpler to perform group operations。
o Rooms are associated with identifiers that allow each room to be isolated individually。
o Rooms can be associated with a set of attributes that indicate the behavior they do and
do not support。
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C H AP TE R 8 ■ L E AR N IN G AB O U T CO M P O N E N T O R IE N TE D A R CH I TE C TU R E 199
The source code will implement the controller as a library project named LibLightingSystem。
This library project will also define the interfaces that the ponents will implement。 To
demonstrate building a plete working application； two other projects implement the inter
faces and represent ponents： Museum and Home。 The key characteristics of a museum are that
some rooms are made dark at night and are pletely managed by the controller； while other
rooms are controlled individually in the room itself and also can be managed by the controller。
The key characteristics of a home are that all of the rooms are individually controlled； but they
have sensors that allow automatic control。 Some of the lighting associated with the house depends
on certain prerequisites being fulfilled。 For example； night…lights might turn on depending on
the time of year； or room lights might be turned on and off automatically when no one is home。
As usual； we will use a testing console application； named TestLightingSystem； to test the three
assemblies （it is the startup project）。 Figure 8…1 shows the project structure。
Figure 8…1。 Lighting system application project structure
Building the Kernel
The lighting system is a two…part implementation。 The first part is that we must write code that
works properly。 The second part is to integrate code that someone else has written and make
sure that if that code does not work properly for some reason； those problems will not affect the
code that we wrote。
This application is plicated by the fact that we are dealing with the unknown。 In the
previous examples； we had control of every class； interface； and definition。 This time； we are
not in control； and thus we need to use a defensive style of programming。 This means we need
to write many tests and keep certain information private。
Defining the Interfaces
The core of the lighting controller is to control the lighting in a room that is part of the building。
We can define and organize the rooms by using interfaces。 We need four interfaces：
o IRoom： A placeholder interface for the idea of a room
o INoRemoteControlRoom： An interface for rooms that should not be controlled by the
lighting controller
o IRemoteControlRoom： An interface for rooms that should be pletely controlled by the
lighting controller
o ISensorRoom： An interface for rooms whose control is based on state （whether or not a
person is in the room）
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200 CH AP T E R 8 ■ L E A R N IN G AB OU T CO M P O N E N TO R IE N T E D AR C HI TE CT U R E
The interfaces for the rooms where lighting may be controlled； IRemoteControlRoom and
ISensorRoom； will depend on certain pieces of logic。 The interfaces will need to provide input
data and accept output data。 The logic might also seek input from other sources； such as time
of day or amount of sunlight outdoors。 It boils down to defining some type of logic that the kernel
implements。 This is key； and it relates to the children…and…parent issue。 While you accept your
children as being intelligent beings that can make decisions； at the end of the day； it is usually
the parent who makes the final decisions。 Likewise； while your kernel might accept input and
potential decisions； the kernel makes the final decisions。
Defining IRoom； a Placeholder Interface
For design purposes； the simplest and base idea is the room itself； which can be defined as
follows （in the controller library LibLightingSystem）：
Public Interface IRoom
End Interface
The interface does not have any methods or properties。 It is called a placeholder interface。
A placeholder type serves no other purpose than identifying that the implementation is of a
certain type。 Placeholder interfaces make it simpler to group objects that have certain capabilities。
Imagine defining objects without a placeholder interface； something like this：
Class Type1
End Class
Class Type2
End Class
Looking at Type1 and Type2； you cannot see any correlation between the two types； there
is no way to say that Type1 and Type2 have anything in mon。 （Well； technically there is a
correlation in that both types are derived from Object； but that type of correlation is like saying
that all people are humans。） Using a placeholder interface； Type1 and Type2 can be correlated；
as follows：
Class Type1
Implements IRoom
End Class
Class Type2
Implements IRoom
End Class
。 。 。
Dim rooms As IRoom（） = New IRoom（） ｛New Type1（）； New Type2（）｝
Having Type1 and Type2 implement the IRoom interface； which means do nothing other
than subclass IRoom； establishes a correlation between Type1 and Type2。 The correlation is that
both Type1 and Type2 are rooms。 We have no idea what kind of rooms； and we have no idea if
the rooms are in the same building。 We only know that they are rooms。
The use of placeholder interfaces is very important in kernel design。 Placeho
