tomorrow presentation

0. abstraction

–TheAPI should abstract the behavior of the module to the extent possible

»Definethe “essence” of the module. (abstraction)

»Avoidunnecessary publishing of implementation details. (information hiding)

»Insteadof using the approach of putting something in the API in case someone may need it, take the approach ofnot putting something in the API unless someone really needs it.

1. Information Hiding

–Designdecisions are “hidden” from other modules.

»Preventsother modules from becoming dependent upon non-essential aspects of yourmodule.

»Allowsthe design decision to be changed with minimal “ripple effect”.

Requires the designer to recognize design decisions that are likely to change or that are not an essential aspect of the module being designe

隐藏/封装：  一个设计得好的模块可以将它所有的实现细节隐藏起来，彻底地将提供给外界的API和自己的实现分隔开来。这样一来，模块与模块之间就可以仅仅通过彼此的API相互通信，而不理会模块内部的工作细节。

advantages：

1.1 使各个子系统之间脱耦，从而允许他们独立地被开发、优化、使用、阅读以及修改。

1.2 虽然信息隐藏本身并不能带来更好的性能，但是它可以使性能的有效调整变得容易，一旦确认某个模块是性能的障碍时，设计人员可以针对这个模块本身进行优化，而不必担心影响到其他的模块。

1.3 信息隐藏可以促进软件的复用。由于每一个模块都不依赖与其他模块而存在，因此每个模块都可以独立地在其他的地方使用。 一个系统的规模越大，信息隐藏就越是重要，其威力也就越明显。

1.4 迪米特法则的主要用意是控制信息的过载。使用时注意：

a) 在类的划分上，应当创建有弱耦合的类。类之间的耦合越弱，就越有利于复用。一旦被修改，不会对有关系的类造成波及。

b) 每一个类都应当尽量降低成员的访问权限。

c) 在类的设计上，只要有可能，一个类应当设计成不变类。

d) 在对其他类的引用上，一个对象对其对象的引用应降到最低。

Separation of Concerns

–separating a program into distinctfeatures that overlap as little as possible 同样是脱耦！--易于阅读、修改。。。

–eases understanding by supporting the ability to “focus” on a feature

2. A basa module contains three major components:

–Makefile_<module>.mk

–API

–Implementation

The API is in a separate subproject.

–Thisallows other projects to pull in the API without pulling in the implementation.

–Thisis especially critical if test code for clients of the module need to “stub out” theAPI for unit testing purposes.

A module’s API is broken into multiple files:

–Configurabletypes

–Callouts: decouple, for the module && by the module???

–Typesshared by all of these

3. void Application_Initialization(void)

{

   SAL_Thread_Attr_T thread_attributes;

   SAL_Config_T      sal_configuration;

   bool              status_ok;

   SAL_Thread_Id_T   thread_id;

   APM_Get_SAL_Config(&sal_configuration);

   status_ok = SAL_Init(&sal_configuration);

  

   SAL_Init_Thread_Attr(&thread_attributes);

   APM_Get_Thread_Attr(&thread_attributes);

   thread_id = SAL_Create_Thread(APM_Task,

                                     NULL,&thread_attributes);

   PBC_Ensure((APM_THREAD_ID == thread_id),

              “Failed to create APM_THREAD_ID”);

 

   SAL_Run();

}

Fora single-process system, this is handled by the “application manager” block(sample code shown to the right)

The application manager is responsible for starting the other threads in the systemin a controlled manner

In a multi-process system, each process would perform a similar sequence

The type SAL_Application_Id_T is declared by the system and is the static list of all “applications” in the system that use XSAL.

4. MOST

4.1Explain the plug-n-playconcept of MOST

MOST is a ring network where nodes can join or leave without disturbing the remainderof the network

4.2 a synchronous network: transport data and clock signal

4.3 FBlock

MOST defines Functional units, called Function Blocks(FBlocks) which are standardized,object-oriented interfaces to the respective applications.

–Variousdevices from different manufacturers must interact in a sophisticated way inmodern multimedia systems.

The Net Block is a special type of Function Block

It provides functions related to the entire device

Every device must have at least twoFunction Blocks, one being the Net Block

A MOST Device may contain multiple components such as AM/FM Tuners, CD player, Amplifier, Tape Player, etc.

Each of these components requires a Function Block appropriate to  that component

A MOST Device will always include at least one Transceiver and a Physical Interface