Writings from the sticks

There are a LOT of things that you will hav a hard time doing if you only ever add instances of controls from the Library in Xojo to your layouts. And you will spend a LOT of time trying to sort out how to beat them into submission to do what you want with some combination of handling the events, properties and methods on your layout.

Like this fun little request that came in on an off forum chat service.

When you do “CMD-A” in a listbox it selects ALL rows…. but is there a way to select all “but certain” rows… rows that should never be selected?

With just an instance on a layout and the handful of events you have available you could probably make this work – eventually.

But I made a subclass and within 5 minutes (or less) had this

And very quickly altered it to this

The reason I could do this so quickly ? A subclass.

In a subclass you can add methods, properties, events, new events using event definitions and you CAN also add things like Menu Handlers. And therein lies the crucial piece of information.

In an instance placed directly on a window you have no opportunity to add menu handlers so you have to devise some other means to catch the “Select All” keystroke or menu selection and try to do something else.

But in the subclass I CAN add whatever menu handlers I want – I can even add menu handlers for menu items that do not exist yet (yes really !). And this makes is possible for us to add a menu handler for EditSelectAll – the default name for that menu item in a desktop project. If you happen to rename this menu then you would need to update the menu handler implementation to match.

As soon as you do that and place an instance of your custom subclass on a layout you will now be able to handle the Select All menu however you want.

And that makes it possible for you to use all kinds of things that are not normally accessible when you put an instance directly on a layout and try to make it behave specially.

Subclassing is a VERY powerful way to make your customized versions of the normal controls and classes Xojo provides as part of its framework.

A recent discussion lead to an odd question. Why do you need to subclass things in Xojo ? Whats the point since they give you all these great controls and classes ? What would make you WANT to subclass things instead of just implementing the events of the existing controls ?

I was sort of surprised at this. And I can see where this line of thinking has some merit. It’s not often that I will subclass something like a pushbutton. I just use them as is from the library in the Xojo IDE.

And for many things this is true. They are perfectly suited to the task I need to accomplish as is. But, often, they aren’t. Sometimes I can make an instance do that tiny little bit special that I need just by implementing the various events. When I I need something special about how a listbox draws itself I can often just use the various paint events that it provides to do the customization. If I only need to do this customization once then a subclass may be more effort than I really want to invest.

But if I have several places where this same customization is required I’ll create a subclass, write my custom drawing code once in the subclass, and then put instances of this subclass in the many places its needed. It keeps me from repeated copying and pasting the same code over & over into multiple instances all over the place. And I also don’t have to remember to fix each one when I find a bug or need some additional tweaks to how things work. I fix the code once in the subclass and ALL instances now get that behaviour without touching each one.

Some times I need much more than just custom drawing. And that’s when I will choose to subclass a control to add some special behaviour to that subclass. A recent discussion was about how to make the SelectAll menu item behave in a special way for a specific listbox. In a subclass I can add menu handlers to handle Select All as well as many others – something I cannot do in an instance on a layout. There is no way to add a menu handler to an instance.

The other thing that a subclass will let me do is add new events that I can then implement in the instances I put on layouts. I can add new events for all kinds of purposes – even to ASK the outside world for more information about the environment the instance is running in (more on that in another post about events). Or you can add something like a “BeforeKeyPress”, “KeyPress”, “AfterKeyPress” for a control where you need this sort of functionality.

In addition to adding new events you can, in a subclass, hide existing events that the superclass exposes that your do not want your subclass to expose. You simply add the event handler to your subclass and do NOT leave it exposed to the rest of your code. You don’t have to put ANY code in the event handler.

Subclasses not only let you add new events, they let you add properties, methods and just about any other kind of code item you can think of. All of this can be open to the public or for private use only.

And the BEST thing about using subclasses is that your code then becomes much more self contained and reusable. You can alter the subclass code and not worry about whether altering that subclass’ code has somehow altered something else outside of its control. It limits the amount of side effects your code in the subclass can possibly cause.

Subclasses are for taking a more general purpose class and specializing it to do something unique.

Take advantage of that in your own applications.

I asked this on the forums and the post was promptly deleted so I’ll ask here.

This is an unscientific survey.

Answer if you feel inclined to. Emails are optional.

EDIT : updated with stats as they exist up to today – Jan 17, 2020
Amazingly the percentages have remained fairly stable as more replies are added.

Xojo supports the use of exceptions and the error handling machnisms that makes possible.

in order to use exceptions effectively you need to use a language feature called a Try block. It has several parts to it. First there is the initial TRY statement which opens a new scope level AND delimits where the TRY block starts. Its entirely possible to simply have code like

TRY
   // some code enclosed by the block
END TRY

You are not forced to have a CATCH statement in a try block. This style can be useful if, for instance, in the try portion you assign a value to a variable when there is no exception raised and then check that variable after the block and behave accordingly. Perhaps something like

dim f as Folderitem
TRY
   f = SpecialFolder.ApplicationData.Child("my.app.bundleID")
END TRY

if f is nil then
   f.CreateAsFolder
end if

But not that this code _could_ have an issue of for some reason SpecialFolder.ApplicationData is nil. We didnt check that and should before just blindly using it. There are other ways to accomplish much the same task by dealing with whatever exceptions are raised. For instance you might rewrite the above code to

dim f as Folderitem
TRY
   f = SpecialFolder.ApplicationData
   f = f.Child("my.app.bundleID")
CATCH nilError as NILObjectException
   // hmmm this means SpecialFolder.ApplicationData is nil ?
   // we might show an error dialog to the user or something to 
   // let them know that the folder we were trying to create cant be 
   // created in the SpecialFolder.ApplicationData location 
   // and we might return here rather than try to carry on
END TRY

if f is nil then
   f.CreateAsFolder
end if

This style at least makes it so we have checked whether or not SpecialFolder.ApplicationData was nil – its unusual that it might be but can happen.

You can have as many CATCH statements as you want and they can be written in many different styles. Personally I always use the form

Catch ErrorParameter As ErrorType

with a name for the error parameter; which actually ends up being a local variable in the CATCH code you write, and a specific type. In my code you might see

TRY
CATCH noExc as NilObjectException
  // code to deal with the Nil Object Exception
CATCH ioExc as IOEXception
  // code to deal with the IO Exception
END TRY

and specific code where noted by the comments to deal with the situation. I do not recommends using a single catch with a long list of select statements using introspection etc to try and handle the various exceptions. The only time I might recommend that is when its the App.Unhandled exception event and you are trying to create a report that a user can send you. Even there you have to be careful of which exceptions you catch because a normally Quit application uses an exception to Quit normally.

The last part of a TRY CATCH block is the FINALLY block. This is code that runs after all the code in any TRY or CATCH sections executes. Some times there is code that you want to execute as part of clean up that needs to release memory, mutexes, semaphores etc.

However, some assume that it will ALWAYS execute regardless of what code you have in the TRY and CATCH portions. In some languages the code in the FINALLY block is ALWAYS executed. This is NOT the case in Xojo and, if you’re not careful, it CAN easily be skipped.

For instance, if you use RETURN as the last line of the TRY or a CATCH block that is executed the FINALLY block WILL be skipped.

Try
  Raise New NilObjectException
Catch
  Return
Finally
  Break
End Try

If you execute this code you will see that the BREAK in the finally block is NOT executed. And if you changed this to

Try
  Return
Catch
Finally
  Break
End Try

Again you will see the break in the finally clause is not executed. So be careful about the use of return in a TRY CATCH block IF you expect the FINALLY clause to always be executed.

Recently I re-read The Pragmatic Programmer mostly while on a break in Mexico. I also did my fair share of sitting in the sun, eating and event drinking but I read a LOT as well.

And I keep learning, or at least reminding myself, of several things I think are really useful to all of us.

In terms of writing OOP code the Law Of Demeter is one that is really useful. Basically in this Law you should only access :

– items (properties, methods, constants, etc) of the of the object you are part of
– items (properties, methods, constants, etc) you are passed as parameters
– items (properties, methods, constants, etc) of objects you create in your code

This limits the knowledge your code needs of the internals of other objects.

Suppose we have a set up something like :

Global Dim someGlobalVar as integer

Class FooBar
   Dim i as integer
   Sub fubaz() as string
   End Sub
End Class

Class Bar
   Dim fubar as FooBar
   Sub Constructor()
     fubar = new FooBar
   End Sub 
 
   Sub Baz()
   End Sub
End Class

Class Foo
  Dim BarInstance as Bar

  Sub Constructor
    self.BarInstance = new Bar
  End Sub

  Function HasInstance() as Boolean
    // this one is OK !!!!!
    return (BarInstance is nil) = false   
  End Function

  Sub CallBazOfBar()
    // this one is OK !!!!!
    BarInstance.Baz()
  End function

  Function CallBazOfPassedInBar(b as Bar)
    // this one is OK !!!!!
    b.Baz()
  End Function

  Function CreateAndCallBazOfBar()
    // this one is OK !!!!!
    dim b as new Bar
    b.Baz()
  End Function

  Function ReachesThroughBarToFooBar() as integer
    return BarInstance.fubar.i
  End function

  Function ReliesOnGlobal() as integer
    return someGlobalVar
  End function

  
End Class 

Then for us to adhere to this principle the following new methods on our Class Foo would NOT be ok :

Function ReachesThroughBarToFooBar() as integer
   return BarInstance.fubar.i
End function

This one is NOT ok because it requires the Foo class to know more about the internals and implementation of the Bar class AND FooBar class than it should. Instead the Bar class should have a method on it that can be invoked to get details of its properties or classes it owns.

Function ReliesOnGlobal() as integer
   return someGlobalVar
End function

This one is NOT ok because it requires the Foo class to rely on a global variable that is outside the scope of the instance and the method using it.

Methods in our Foo class should ONLY use :

1. methods in the instance itself (HasInstance is OK)
2. methods of any parameters passed to the methods like CallBazOfPassedInBar(b as Bar)
3. Any objects created/instantiated within the method like CreateAndCallBazOfBar
4. methods in the instances properties (in this case methods of Bar like CallBazOfBar)
5. A global variable, accessible by the instance, that is in the scope of the method (this last one is harder to explain)

I like the Wikipedia summary which is

 For many modern object oriented languages that use a dot as field identifier, the law can be stated simply as “use only one dot”. That is, the code a.b.Method() breaks the law where a.Method() does not.

https://en.wikipedia.org/wiki/Law_of_Demeter

I started up Xojo 2019r3 the other day and changed some colors while tracking down a nifty dark mode bug (see http://feedback.xojo.com/case/58695)

And after that all versions of Xojo would, when starting, immediately show the color picker panel like this :

Now lets be clear – I dont consider this to be a Xojo bug.

It seems that, Apple writes their own keys to the Xojo plist and these may influence whether the color picker panel shows up when you start Xojo.

If you open the Xojo plist in a text editor you may find that you have a bunch of keys like  NSColorPanelMode, NSColorPanelMode.modal, NSColorPanelToolbarIsShown.modal, NSColorPanelVisibleSwatchRows.modal, NSSplitView Subview Frames, NSColorPanelSplitView, NSToolbar Configuration com.apple.NSColorPanel. NSWindow Frame NSColorPanel, and NSWindow Frame NSColorPanel.modal

Apple adds these to the plist on its own (personally I find this less than helpful but there’s really no arguing with Apple about this sort of thing)

What I dont know is if Xojo can/could make sure that the color panel preferences get writtem in a way the color picker does not open on next launch or not.

If they can it would sure be nice.

In the mean time I’ve simply stripped those keys out when/if they appear and things go back to normal.

This is so annoying. I have no bluetooth mouse powered on but I do have a USB one and it works fine. And the mouse control panel says it cannot find it so I cannot change the tracking speed. System Report knows there’s a mouse connected as you can see.

Turn the bluetooth mouse on and the panel finds that and now I can adjust the tracking speed. But it doesn’t affect the USB mouse – which is still connected and working (the bluetooth mouse is literally upside down on my desk untouched)

No. I don’t want a wireless mouse as I’m tired of my Magic mouse and “LOST CONNECTION”. Apparently Apple insists I use a wireless mouse,

Awesome !

A good question on the discord server I participate in prompted a good question. It started with IS and = are just synonyms right?

It turns out that MOST times this may be true. But not always. And that little “but not always” can get you in trouble if you dont know when the difference is important.

IS is always used to determined if one instance of a class refers to the exact same instance as another reference. You might want to reread my prior post about classes, instances and objects

And in many cases, but not all, = will give you the same result as using IS.

So code like

Dim d1 As New Class2
Dim d2 As New Class2

If d1 = d2 Then
  System.debuglog "d1 = d2 => true !"
Else
  System.debuglog "d1 = d2 => FALSE !"
End If

If d1 Is d2 Then
  System.debuglog "d1 IS d2 => true !"
Else
  System.debuglog "d1 IS d2 => FAlSE !"
End If

will usually put out debug log messages that say D1 = D2 => false from both checks.

This assumes that our class, Class2, does NOT implement operator_compare. And that is the caveat here. IF a class implements operator_compare the IS and = operators may not give the same result any longer.

Operator_compare is often used to compare the contents of a class – and not the references (although it doesnt have to compare contents since you can make it do whatever you want).

If we defined our class, Class2, like

Protected Class Class2
  Sub Constructor(contents as string)
    myContent = contents
  End Sub

  Function Operator_Compare(rhs as Class1) As integer
      If rhs Is Nil Then 
        Return -1
      End If
		  
      If Self.myContent < rhs.myContent Then
        Return -1
      End If
      If Self.myContent > rhs.myContent Then
        Return 1
      End If
		  
    Return 0
		  
  End Function

  Private myContent As String
End Class

Now our code above will give very different results because Operator_compare is used when the line

If d1 = d2 Then

is run. In this case it will call the custom operator_compare method and that compares the contents of two objects and NOT whether or not two objects references refer to the same object. However, the line

If d1 Is d2 Then

will ALWAYS determine if the two objects references refer to the same instance.

And now you know !

It is my blog and this is something I’ve been working at for months.

I PASSED my on snow certifications for the Canadian Ski Patrol and now get to do a few shifts working with more senior patrollers before they turn me loose and I can patrol on my own !

What an awesome way to spend New Years Day !!!!!!

And I would like to take the time to thank ALL of the instructors that have helped all of the candidates along. They have been really really great !

There was a good question from Markus in a recent comment on a different post of mine.

In children of a window we should keep track of the parent window by using WeakRef.
But what if I have a class cDepartment with a list of employees defined as Employees() as cEmployee – should that then better also be defined as WeakRef? And why or why not?

First I’ll say “it depends” as there’s certainly no single correct answer to such a question. In many cases it will depend on the expected usage.

WeakRefs are a curious thing. In Xojo most times you have a reference to an object its a strong reference. What happens is that the objects reference count is increased and only when that counter gets set to 0 will the object be destroyed.

For instance, if we have code like the following

Class Foo

  Sub Destructor()
    break
  End Sub

End Class


dim firstRef as Foo
dim secondRef as Foo

firstRef = new Foo 
// the object firstRef refers to now has a reference count of 1

secondRef = firstRef 
// since secondRef refers to the SAME object as firstRef
// the object firstRef refers to now has a reference count of 2

firstRef = nil
// the object firstRef refers to now has a reference count of 1

secondRef = nil
// the object firstRef refers to now has a reference count of 0
// and it will be destroyed and the breakpoint in the destructor
// will be encountered

Now what happens if we use a WeakRef instead for secondRef ? If we turn that code into the following

Class Foo

  Sub Destructor()
    break
  End Sub

End Class


Dim firstRef As Foo
dim secondRef as Foo
Dim weakSecondRef As WeakRef // <<<<<<< note the type !

firstRef = New Foo 
// the object firstRef refers to now has a reference count of 1

weakSecondRef = New WeakRef(firstRef)
// since weakSecondRef is a WeakRef the object 
// firstRef refers does NOT increment its reference count

firstRef = Nil
// the object firstRef refers to now has a reference count of 0
// and it will be destroyed and the breakpoint in the destructor
// will be encountered now

// but what about weakSecondRef's contents ???
// lets fetch that value and see whats in there now
secondRef = Foo(weakSecondRef.Value)

break // <<< here you will find that the returned value is NIL !

And if you try to use the object contained by the weakref secondRef you will find that it is nil. A weakref allows an object to be referenced in a way that it does not increment the original objects reference count which CAN let that object be nil’d in other code.

When you use a weakref you MUST check the VALUE of the weakref to see if what you get back is NIL because the object may have been set to nil elsewhere.

Now that we have the basics of weakrefs out of the way lets turn back to Markus’ original and my original reply of “it depends”.

n children of a window we should keep track of the parent window by using WeakRef.

Indeed you may want to have child windows retain a weakref to refer to a parent window. It would allow you to close the parent and not the child, as the weakref will not increment the reference count of the parent window. But, you may want the parent to not be set to nil when there are child windows.

This decision will depend entirely on how your UI is supposed to work. If those child windows should not exist without a parent then a hard reference to the parent may make perfect sense.

But what if I have a class cDepartment with a list of employees defined as Employees() as cEmployee – should that then better also be defined as WeakRef? And why or why not?

Again this will depend entirely on how the cDepartment class should behave. Can cEmployees be added & removed at runtime and should the cDepartment class be able to deal with this ? Or should cDepartment need to be reconstituted when such changes of its employees are made ? The correct behaviour isnt dictated by the underlying code and whether you should use weakrefs but by the needs of the application and how cDepartment should behave.

You could always use weakrefs but this could have unintended consequences for the cDepartment class as this would allows its cEmployees to be removed at runtime. If you had used a FOR EACH loop with an iterator to perform some function you would need to handle the exceptions that could be encountered. Iterators will be invalidated by removal of an employee while iterating over the list and will raise an exception of this occurs. Weakrefs would permit this. Hard references would not.