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Advanced Object Oriented Systems

(CM0318)

Lecture 9

(Last modified Thursday 21st February 2002)

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In this lecture we shall learn ...

• How to debug Smalltalk code effectively

• How the model-view-controller paradigm is realised in Smalltalk

• How to discover the structure of a currently-executing application

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Debugging

• To find out how your code is working in Smalltalk is a bit different from in most systems, because:– you are running the debugger within a fully functioning

Smalltalk system

– because of the way many methods, particularly iteration methods, are implemented you often need to step through system code as well as your own

– you can normally trace through code that does screen updates with limited success: often the wrong bits of the screen get updated!

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Example

• Consider:

#(3 4 5) do: [:i|

Transcript show:

i printString.

Transcript cr]

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When executed ...

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Debugger

• Can either proceed, abandon execution or enter the debugger at this point. If you select Debug, confronted with a debugger comprising:– call stack– code pane– panes for inspecting the receiver’s instance vars (left

bottom)– panes for inspecting the temporary variables of a

method (right bottom)

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Debugger (ctd.)

• E.g. selecting UndefinedObject>>DoIt causes the code that was executed in the workspace to be displayed. You can ‘step’ through methods (execute to completion) or ‘send’ a message (trace execution through the method involved)

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9Stepping through to where do: is sent

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‘Sending’ the do: message ...

• End up tracing through the code implementing do:. E.g. just before executing the block for the first time ...

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If we now send the message ...

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Tracing code

• So to trace code productively using Smalltalk, need to be able to understand the system code too

• It’s a fundamental tenet of Smalltalk that you are expected to be a good code reader!

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The Model-View-Controller paradigm

View Controller

Model

Dependent

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MVC (ctd.)

• Models contain data to be displayed• Views display the data• Controllers handle user gestures

• Some Views are pluggable. They:– automatically create a suitable controller to handle user

input

– specify in their creation methods the messages to send to the model to retrieve the data to be displayed, etc.

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MVC (ctd.)

• A top-level window is represented by StandardSystemView/StandardSystemController. A StandardSystemView will have sub-views, one for each part of the window.

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Example• Code for PluggableTest to create a single list pane. Class

method:newExample

| model listView1 topView |

model ← self new initialize.

listView1 ←

PluggableListView on: model

list: #extraList

selected: #extraListSelection

changeSelected: #extraListSelection:.

topView ← StandardSystemView new

label: 'Pluggable Test';

minimumSize: 300@200;

borderWidth: 1;

addSubView: listView1.

topView borderWidth: 1.

topView controller open.

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New PluggableTest methods

extraList

^#('hello' 'world')

extraListSelection

^1

extraListSelection: aNumber

extraIndex ← aNumber

NB: need to define extra instance variable extraIndex

19Screen dump

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Exploring interactive applications

• To find out how System Browsers work, for example, one starting point is the code;

• another is to break into execution and explore the structure of the System Browser created. <ALT><.>

• This will allow us to inspect the corresponding views and look at super/subviews, at models, etc.

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