LabVIEW面向对象编程：设计模式解析

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"这篇文档是关于LabVIEW中的GOOP（Graphical Object-Oriented Programming，图形化面向对象编程）的设计模式技术文档，由NI（National Instruments）的工程师Stephen Mercer撰写。文档涵盖了多个经典的设计模式，旨在帮助LabVIEW开发者更好地组织和优化代码结构，提高代码复用性和可维护性。" 在LabVIEW的面向对象编程中，设计模式是程序员在解决特定问题时的标准代码组织方式。它们类似于算法，但与算法不同的是，设计模式不是一次性解决方案，而是在不同场景下需要不断调整和重写以适应不同需求。 1. **Singleton模式**：Singleton模式确保一个类只有一个实例，并提供全局访问点。在LabVIEW中，Singleton常用于管理共享资源或控制整个应用的单一组件。 2. **Factory模式**：Factory模式是一种创建型设计模式，它提供了一种创建对象的最佳方式。在LabVIEW中，Factory可以用于根据不同的条件动态创建不同类型的VI（Virtual Instrument）实例。 3. **Hierarchy Composition**：层次结构组合模式涉及到如何在类的继承层次结构中组合对象。在LabVIEW中，这可能意味着在父类中定义通用行为，而在子类中添加具体实现或扩展功能。 4. **Delegation模式**：委托模式允许一个对象将职责转交给另一个对象，以实现功能的解耦。在LabVIEW中，可以使用这种模式来分离复杂逻辑，使得代码更易于理解和维护。 5. **Aggregation模式**：聚合模式是关联关系的一种，表示整体与部分的关系。在LabVIEW中，这通常表现为一个VI包含多个子VI，每个子VI负责一部分功能。 6. **Specification模式**：规格模式定义了对象必须满足的规则，这些规则可以被用来验证对象是否符合特定条件。在LabVIEW中，它可以用于数据验证或过滤数据流。 7. **Channeling模式**：通道模式涉及如何在对象之间传递消息或数据。在LabVIEW中，这可能是通过事件结构或者队列实现的通信机制。 8. **Visitor模式**：访问者模式允许在不改变类结构的情况下为对象添加新的操作。在LabVIEW中，这可以用于在运行时动态扩展VI的行为。结论部分可能讨论了这些设计模式如何应用于实际的LabVIEW项目，以及它们如何帮助提高代码质量和可维护性。通过理解和应用这些设计模式，LabVIEW开发者能够构建更加灵活、可扩展和易于维护的程序，提升软件工程实践的专业水平。

Singleton Pattern

(adapted from Gang of Four’s Singleton pattern)

Intent

Guarantee that a given class has only a single instance in memory, that no second instance can ever be created,

and that all method calls refer to this single instance.

Motivation

When you create a class, sometimes it is advantageous to guarantee that a program always refers to the same

global instance of the class. Perhaps the class represents a database. It would be unfortunate if some section of

code accidentally instantiated its own database and thus failed to update the global database. Creating a global

means all VIs can access the data. But it does not mean that all VIs will access the global. This pattern describes

a framework of classes that guarantee that single instance of data.

Implementation

See

http://jabberwocky.outriangle.org/SingletonPattern.zip

(An example of this pattern shipped with LV8.2, but that example is flawed, a problem I did not realize before

we released: <labview>\examples\lvoop\SingletonPattern\Design Pattern.lvproj )

In this implementation, we seek to guarantee a single instance of Data.lvclass. We achieve this by putting

Data.lvclass into an owning library, Singleton.lvlib, and making the class private. Now no VI outside the class

can ever drop the class as a control, constant or indicator, so we guarantee that all operations are limited to this

library. Callers can use Singleton.lvlib:Checkout.vi to get the current value of the data, modify it using any of

the operations defined by Singleton.lvlib and then set the new value with Singleton.lvlib:Checkin.vi. The

public functions are the same functions that would normally be on the class itself. They are moved to the library

so that the functionality is exposed without allowing the class itself to be dropped. Checkin.vi and Checkout.vi

use a single-element queue to guarantee only one copy in memory at a time. While the element is checked out

to be modified by some routine, no other operation can proceed, thus guaranteeing serial access to the data.

Editorial Comments

I do not like the implementation that shipped with LV8.2. In the time since LV8.2 released, I have found better

ideas. This “post-release discovery” is one reason why more design patterns are not included with the shipping

product. LabVOOP is too new, and many of the patterns identified during development may be found faulty

when exposed to a wider audience (i.e. real customers). So I’m making this implementation available.

The only hole in the above solution that I can find is VI Server. VI Server could get a reference to the Front

Panel controls of Singleton’s member VIs and use the Value property to effectively manipulate a different

instance of the class than the one in the queue. Would using Subroutine Priority prevent VI Server access? I

think so, but that seems like a pretty odd way to fix these VIs. The fix may not be that necessary – I doubt that

many people are going to attempt such a hack on the class. Anyone trying it is already aware that they’re

working against the design of the class and may break components that depend upon the singleton nature.

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wmslp

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