The selectedObjects - is it a container or not?

[Mark Waddingham]

Hi Graham,

On 2017-04-17 21:07, Graham Samuel via use-livecode wrote:
> I have a stack where the user is allowed to select vector objects. For
> this example, let’s assume some objects are already selected.
> 
> I am using LC 8.1.4 rc1 on a Mac running Sierra, and I  have a bit of
> code that says in part:
> 
>     if the selectedObjects is not empty then
>       repeat with i = 1 to the number of lines of the selectedObjects
>       …

Just to clear up precisely what is going on here...

There are a number of pieces of syntax in LiveCode which return control
references (e.g. field 1). As LiveCode represents such things as strings
there is an ambiguity when you do:

   <something text related> of <syntax returning control reference>

e.g. the number of lines of the selectedObject

In this case it could either mean:

   1) the number of lines in the string returned by the selectedObject

   2) the number of lines in the control returned by the selectedObject

The engine tends towards case (2) in most cases (I can't say all cases
without going through all such things and checking) - including this 
one.

> OTOH, if I change the second line to
> 
>      repeat with i = 1 to the number of lines of (the selectedObjects0
> 
> the code works as expected.

The presence of brackets here force evaluation of 'the selectedObjects'
as a string and not as a control.

> If I execute in the message box:
> 
>    put the number of lines of the selectedObjects
> 
> I sometimes get no result and sometimes “source is not a container”,
> but if I change the line to:
> 
>     put the number of lines of (the selectedObjects)
> 
> I always get the right answer.

In the first case, whether or not 'the number of lines of the 
selectedObjects'
will do something sensible depends on:

   1) if there is a selectedObject (otherwise you are trying to get the
      number of lines in no object - which makes no sense)

   2) There is a selected object, but it has no idea of a 'text' property
      so doesn't have anything for 'the number of lines of' to process.

Hope this helps!

Mark.

-- 
Mark Waddingham ~ mark at livecode.com ~ http://www.livecode.com/
LiveCode: Everyone can create apps