小型支持撤销功能的绘图工具实战：Smalltalk【1】 语言图形绘制案例
Smalltalk 是一种面向对象的编程语言，以其简洁、直观和强大的对象模型而闻名。本文将围绕使用 Smalltalk 语言开发一个支持撤销功能的绘图工具进行实战案例讲解。我们将从设计思路、实现细节到代码示例，逐步展示如何构建这样一个工具。
设计思路
在设计支持撤销功能的绘图工具时，我们需要考虑以下几个关键点：
1. 图形对象管理【2】：我们需要定义图形对象，如点、线、矩形等，并管理这些对象的状态。
2. 操作栈【3】：为了实现撤销功能，我们需要一个操作栈来记录用户的操作历史。
3. 用户界面【4】：提供一个简单的用户界面，允许用户进行绘图操作和撤销操作。
实现细节
1. 图形对象管理
我们定义几个基本的图形对象类：
smalltalk
| Point Line Rectangle |
Point := Object subclass: Point
instanceVariableNames: 'x y'
classVariableNames: ''
poolDictionaries: 'points'
class >> initialize
"Initialize the class"
points := Dictionary new.
instance >> initialize: x: xValue y: yValue
"Initialize the instance"
super initialize.
x := xValue.
y := yValue.
points at: x put: self.
instanceVariable: 'x'
instanceVariable: 'y'
Line := Object subclass: Line
instanceVariableNames: 'start end'
classVariableNames: ''
poolDictionaries: 'lines'
class >> initialize
"Initialize the class"
lines := Dictionary new.
instance >> initialize: start: startPoint end: endPoint
"Initialize the instance"
super initialize.
start := startPoint.
end := endPoint.
lines at: start put: self.
instanceVariable: 'start'
instanceVariable: 'end'
Rectangle := Object subclass: Rectangle
instanceVariableNames: 'topLeft bottomRight'
classVariableNames: ''
poolDictionaries: 'rectangles'
class >> initialize
"Initialize the class"
rectangles := Dictionary new.
instance >> initialize: topLeft: topLeftPoint bottomRight: bottomRightPoint
"Initialize the instance"
super initialize.
topLeft := topLeftPoint.
bottomRight := bottomRightPoint.
rectangles at: topLeft put: self.
instanceVariable: 'topLeft'
instanceVariable: 'bottomRight'
2. 操作栈
接下来，我们实现一个操作栈来记录用户的操作：
smalltalk
OperationStack := Object subclass: OperationStack
instanceVariableNames: 'stack'
class >> initialize
"Initialize the class"
stack := List new.
instance >> initialize
"Initialize the instance"
super initialize.
stack := List new.
instance >> addOperation: operation
"Add an operation to the stack"
stack addLast: operation.
instance >> undo
"Undo the last operation"
stack last ifNotNil: [ :operation |
operation undo.
stack removeLast.
].
instance >> redo
"Redo the last undone operation"
stack ifNotNil: [ :operations |
operations do: [ :operation |
operation execute.
stack addLast: operation.
].
stack := operations.
].
3. 用户界面
我们创建一个简单的用户界面，允许用户进行绘图和撤销操作：
smalltalk
DrawTool := Object subclass: DrawTool
instanceVariableNames: 'canvas operationStack'
class >> initialize
"Initialize the class"
canvas := Canvas new.
operationStack := OperationStack new.
instance >> initialize
"Initialize the instance"
super initialize.
canvas open.
canvas mouseDown: [ :event |
"Start a new operation"
operationStack addOperation: (Operation new start: event position).
].
canvas mouseDragged: [ :event |
"Continue the current operation"
operationStack last ifNotNil: [ :operation |
operation continue: event position.
].
].
canvas mouseUp: [ :event |
"End the current operation"
operationStack last ifNotNil: [ :operation |
operation end: event position.
operation execute.
].
].
canvas keyDown: [ :event |
"Handle key events"
ifTrue: [ operationStack undo ] ifFalse: [ operationStack redo ] if: [ event key = u ].
ifTrue: [ operationStack redo ] ifFalse: [ operationStack undo ] if: [ event key = r ].
].
4. 操作类
我们还需要定义操作类来处理具体的绘图操作：
smalltalk
Operation := Object subclass: Operation
instanceVariableNames: 'start end'
class >> initialize
"Initialize the class"
start := Point new.
end := Point new.
instance >> initialize: start: startValue end: endValue
"Initialize the instance"
super initialize.
start := startValue.
end := endValue.
instance >> undo
"Undo the operation"
"Implementation depends on the specific operation"
instance >> execute
"Execute the operation"
"Implementation depends on the specific operation"
总结
本文通过Smalltalk语言，展示了如何实现一个支持撤销功能的绘图工具。我们定义了图形对象，实现了操作栈，并创建了一个简单的用户界面。这个案例展示了Smalltalk在图形界面编程中的强大能力，同时也为其他面向对象编程语言提供了参考。
由于篇幅限制，本文未能详细展开每个类的具体实现。在实际开发中，您可以根据需要进一步完善这些类，并添加更多的图形对象和操作。希望这个案例能够激发您在Smalltalk编程中的创造力。
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