In object oriented languages a constant it not necessarily declared at application launch and it can be subject to object scope just as a variable can but luckily that is not something we need to bother with in FileMaker Pro.
So what is the whole point of setting a named value that can never change? One big advantage is readability and reliability in comparisons and when passing parameters. The value is readable like a string but can carry a numeric value safely used in comparisons or as parameters.
An example is sort order. We can sort records ascending or descending using a script and a script parameter tells us which order to use. The most common practice is to send string parameter such as "asc" or "ascending" to the script which of course works just fine. However what we just did was to hardcode a value that possibly can change inside the script in the future. There is also a risk of a miss-typing sending a parameter value to the script the will not work.
If you instead declare constants for sortorders such as SORT_ASC and SORT_DESC you will not need to pass them as strings and FileMaker will instantly tell you of you typed a letter incorrect. You will also be using the same constants for this action throughout you entire solution and consistency is a very important aspect when programmering!
The two constants SORT_ASC and SORT_DESC can represent simple values as 1 and 2.
Inside the script you will use the constants again. Most lightly you will have a IF-statement checking which sortorder was passed to the script. The value passed is just a simple 1 or 2 but because you want to sustain readability you will us your constants again. Look at the two screenshots below, the one on the left is the parameters passed to the script and the one on the right is the comparison inside the script. Please note that there are no quotes surrounding the parameters, the are NOT strings and thus NOT hardcoded!
The advantage of coding with constants as shown above is that it is both readable, safer, adds consistency and theoretically is better in a performance point of view.
In the FileMaker community very few developers make use of the easy way given to us to declare constants. Simply create a custom function named to whatever your constants name should be. Then simply make it return a simple value, preferably a numeric value. In my example SORT_ASC return 1 and SORT_DESC returns 2.
For some reason constants are often written in capital case letters which is a good practice to use in FileMaker as well.
To give you one more example when working with constants is a good ide we can look at ExecuteSQL. When using ExecuteSQL we are required to specify a separator for records and a separator for fields. To use simple one character separators like linebreak and semicolon is not very safe since the probability that these characters exist inside the returned fields is high. Instead try to use a more complex separator is is unlikely to exist inside any field, something like #%@@%# .
This is a great time to use constants. Declare one constant as a record separator and one as row separator.
Example:
COL_SEPARATOR = #%@@%#
ROW_SEPARATOR = #%@!@%#
This way things get much more readable and you can be confidant you will always use the same separators in every query. This also makes it easier to safely extract values from the query result.
So, don´t be afraid to use custom functions as constants. It can be a great tool when developing with FileMaker Pro!
No comments:
Post a Comment