More good habits

On the forum there was a suggestion that a person should use a container control that had a few controls embedded in it at design time. And then when using that container control that they should skip providing an API that the container presented and just reach inside the container and manipulate the controls it contain directly.

Personally I would recommend against this.

I’d start by saying when you create a container control ALL the controls in it should be private by default to prevent this. And that if you want to expose functionality of the controls on the container you do so by defining methods and events on the container that any code OUTSIDE the container can call or react to just as if the container control was any other single control and not a composite one like it is.

Why would I make such a recommendation ?

  1. good habits
  2. encapsulation
  3. reusability
  4. long term flexibility and maintainability

The first point is just that this is a good habit to get into. And the reason its a good habit is because of points 2, 3 and 4. Properly encapsulating and hiding the details from other bits of your code is a good thing. Code outside the container doesnt need to know HOW the container does what it does. Just that it does what is expected when you call its methods, change its properties and react to the events it exposes. Thats it. It should be a black box like the built in Xojo listbox, pushbutton, or any other built in control is. You dont need to know how those do what they do, just that they do what you expect when you call the methods, set the properties and react to their events.

And the bonus to doing this is that it makes the likelihood you, or others, can reuse your control in more places in your project or in other projects much higher because the control is self contained.

Long term it also lets you do things like completely swap out the implementation of the container for some other means and as long as you dont need to change the API nothing outside the container control even needs to be aware this has happened. This makes your own code easier to maintain since you no longer have to look through all the code outside of the container to know if you also need to alter it because something in the container changed.

These are all good things regardless of whether this code is for your own use, more general distribution or possibly for sale or to give away.

I’d encourage everyone to keep these things in mind when ever they write their own custom controls.

Of bounds and positions

Windows have two sets of properties that all relate to positioning a window.

However, they are not all quite created equally.

There are, of course, the typical top, left, width and height properties. And also the “bounds” property which is a Rect.

If you examine the bounds property for a Document window, and the top, left, width and height you will find that the Bounds.top is not the same as the top property value. Nor is the height. Now why is that ?

In the following image the BOUNDS are the area enclosed by the red rectangle. And the top, left, width and height properties describe the area in light blue.

If you only had the top, left, width and height properties to use to position a window you would have to somehow figure out what the real size of the window was and account for the title bar size and possibly the outer window frame size. There may or may not be one depending on platform and window type.

And if you add a toolbar this further complicates that. The following image is the same window with a toolbar and once again marked with a red rectangle around the bounds and the blue area is the rectangle described by the top, left, width and height properties.

If, for some reason, you want to know the height of the title bar you can use the difference between the bounds top and the windows top property to see how tall it is.

Note that you cant use this difference to know how tall the title bar and toolbar independently. And toolbars dont appear to propertly report their top, left, width, or height at runtime. 🙁

Still the difference between the bounds properties and the windows other properties will let you determine how tall the title bar + any toolbar is.

Careful with those bounds out there.

Generics

One of the things that Xojo lacks is the notion of generics.

So what are these things and why would they be useful ?

In many programming languages you might want to define a class that behaves like a List. But you want to be able to make this generic enough that when you go to use one you can make a List of Strings, a List of Classes, a List of controls etc.

Right now the only way to do this in Xojo is to make all the the parameters and return values be variants in the interface definition. The downside to this is that you lose all compile time type checking and have to rely solely on runtime checks YOU put in the code.

If you could declare a List variable like

Dim myStringList as List<String>

This would indicate that the list should use String as the “generic type” for all the method parameters and return types. The interface declaration might have to change to something like

Interface List<Type>

  Sub AddRow(ParamArray values() as <Type>)
  End Sub
  
  Sub AddRowAt(ParamArray values() as <Type>, zeroBasedInxed as integer)
  End Sub
  
  Sub FirstRowIndex() as integer
  End Sub
  
  Sub LastAddedRowIndex() as integer
  End Sub
  
  Sub LastRowIndex() as integer
  End Sub
  
  Sub RemoveAllRows()
  End Sub
  
  Sub RemoveRowAt(zeroBasedIndex as integer)
  End Sub
  
  Sub RowCount() as integer
  End Sub
  
  Sub RowTag() as Variant
  End Sub
  
  Sub RowTagAt(zeroBasedIndex as integer) as <Type>
  End Sub
  
  Sub RowValue() as <Type>
  End Sub
  
  Sub RowValueAt(zeroBasedIndex as integer) as <Type>
  End Sub
  
  Sub SelectedRowCount() as Integer
  End Sub
  
  Sub SelectedRowIndex() as integer
  End Sub
End Interface

And now we have a generic interface AND a way to define a list that will, at compile time, have a specific and known type so the compiler can detect any incompatible type errors.

This would make interfaces even more useful than they are now.

Good habits when creating custom controls

Suppose you have the need to create a custom control like I did recently

One of the things that you should do so people do not get confused about using your control is to “implement” any events in whatever you use as your base class that should not be exposed to end users of your control. If you dont inplement these events then a user could, and that might end up in surprising behaviour in your carefully crafted control.

If you dont implement the Open event for your custom control a user could put an instance on a layout and implement that event. If this causes problems than you can make it so they cannot implement the Open event simply by adding that event handler to your custom control.

There may be events, like ConstructContextualMenu, DragEnter, DragMove, etc that make no sense for your custom control and so implementing them in your class would make it so users cant.

And this should make your custom controls easier for others to use.