A long while ago, I wrote about a (then overdue post) on .NET/C#: Using IDisposable to restore temporary settrings example: TemporaryCursor class.
I had been using a similar technique in Delphi since before I found out about the [WayBack] TRecall class and thought: I think my TTemporaryCursor is smarter, as it is based on interfaces.
TRecall(and the [WayBack] Vcl.Graphics descendants [WayBack] TBrushRecall, [WayBack] TFontRecall, and [WayBack] TPenRecall) store [WayBack] TPersistent properties using the Assign method. They were introduced in Delphi 6.Too bad there are only [WayBack] very few people using TRecall as lots of TPersistent things warrant sasaving and restoring.
My [WayBack] TTemporaryCursor (now [WayBack] at bitbucket) class only stores an integer, so it cannot derive from
TRecall. Besides it is based onIInterfacewhich got introduced in Delphi 6, but was present asIUnknownsince Delphi 3 (see [WayBack] Interface It! A quick guide to the ins and outs of interfaces in Delphi. By Jimmy Tharpe).This means that
TRecallcould have been based onIInterface, so I wonder why it was not.Note I’m not the first to publish about such a class (Malcolm Grooves wrote [WayBack] TCursorSnapshot, SwissDelphiCenter has [WayBack] TMyCursor, Nick Hodges published about [WayBack] TAutoCursor), it’s just that it has been in my tool box for so long, and written memento classes that you will see 2 articles on it this week.
In the mean time (this works with Delphi 2009 and up), I also wrote a small class that does similar things for any [WayBack] anonymous method. More on that tomorrow.
Back to TRecall: it is an example of [WayBack] the memento pattern in Delphi. The [WayBack] memento pattern allows you to restore state.
SourceMaking.com a.k.a. [WayBack] Design Patterns and Refactoring is a great site about [WayBack] Design Patterns, [WayBack] UML, [WayBack] AntiPatterns and [WayBack] Refactoring.
Most pattern example code is available in all of the C#, C++, Delphi, Java and PHP languages.
Great stuff!
One of the constructs for restoring state is a [WayBack] try … finally … end construct: it allows you to always execute something in the finally block, for instance restoring the state to right before the try block. Read the rest of this entry »
