Lua Scripting with REAPER 5 - Part. 9: Preventing Bugs

Source Article and Exercice:
http://extremraym.com/en/reascript-video-preventing-bugs

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HI, this is X-Raym, for the 8th part
of our ReaScript Tutorial.

Scripts can define new values
to REAPER objects.

But you have to be careful.

Because these values
can be invalid.

No error message may appear,

but the value inserted in the project
will be invalid.

This is not fatal, be reassured,

it cannot corrupt a project
and make it unable to be open.

We will take back our script
which modify fade-out length

In order to show you
how to prevent this kind of error.

In order to ease
the understanding of the phenomenon,

I will simplify our script.

To do that, I will not
run the Main function

but a new function that
we will call Test.

I declare this function
before it's execution.

Function Test()

And this function will only act
on the first selected item.

0 for the current project,

0 for the first item.

I check that there is at least
one selected item.

If there is one item selected

Our script will execute a custom function,

I call it SetltemFadeOut

She will have as parameter

an item

But also an item length

and a new value of fade out length.

We will get the item length
thanks to this function here.

I replace item by
first_sel_item.

We will write the new value here...

and it will be 0.

Now I declare this function

It has 3 parameters

The first parameter is an item.

The 2nd will be a length
I will name it item_len

the 3rd parameter
a new fade-out length value.

This function will define the
fade-out length of the first selected item.

I copy this function here.

I replace the variables by
the parameters of our function.

I add UpdateArrange

A define a fade on our item
and if all happen as expected,

The fade-out length value of the first
selected item should be equal to 0.

I run the script.

All happen as expected.

Now imagine the following thing.

If fade_out_new_value is
equal to 100

Knowing that this value is in seconds
and that the item length is only 1 second.

Let's see what happens.

As you can see the fade-out length
seems to match the item length.

However, if I display the item properties, I can see
that its fade-out length value is equal to 100 seconds.

And you can see that the mouse doesn't behave
like usual, because of this invalid value.

Usually, you can see the fade icon-cursor
being displayed hover the fade.

But in our case, there is a bug
because of an invalid value.

So, we will use this function here
to prevent this kind of error.

We will write the following thing.

If the fade length is bigger than
the item length,

then the fade-out length
will be equal to the item length.

I run the script.

Nothing change visually,
but it seems that it is corrected.

I check the item properties.

The fade-out is not
bigger than the item length.

Let's see a more complex case:
what if there was a fade-in?

I run the script.

It seems to be good

but as you can see
we can feel a problem.

If I check the item properties I can see
that the fade-out as an invalid value

because it is as long as the item, and this
is not possible as there is a fade-in.

So, I will write the following thing.

Here I will get
the fade-in length value.

The parameter is "D_FADEINLENGTH"

and I write the following condition:

If fade-out length is bigger
than item_length minus the fade-in length

then the fade-out length will be equal
to item length minus fade-in length.

Rather that making the calculation twice
I will write a max_value variable

and I place there here.

Oops, small mistake here

I run the script.

Nothing has visually change,
but if I check the properties,

We see that the fade-out length
has a vlaid value.

I can now use this function here

in our Main function.

Rather than setting a new fade-out length
value directly here,

I use our function

which take as first parameter: the item

2nd parameter: the item length

which has already been calculated here

and as new value, the fade-out length
of the item udner mouse

It was here.

I don't need the test function anymore
so I delete it.

I replace the execution of the
Test function by Main.

Main works on item in time selection
on selected tracks.

I define a big fade-out length
on our reference item.

I make these items sshorter.

I pute some fades-in.

I propagate the length value of this fade-out
on these items and check if there is no errors.

As you can see, all seems to be fine.

I check.

It seems to be good,
the sums seems to be good.

Voilà, with simple conditions, we have prevented
invalid values insertions in our project.

In general, the rule is simple:

determine the minimum and maximum value
that a parameter can have,

compare this to the new value
we want to set,

and write some conditions to avoid this value to be
lower than the minimum or greater than the maximum.

In the next video, we will see
how to use tables. See ya!