Sometimes when you are building a Qlik Sense extension you need to get access to system data. This is espescially true if you are trying to build something a bit more generic. Perhaps you need a list of fields, or dimensions, or measures. Or you need to work with variables.
If you are new to Qlik Sense development you might look in the API documentation for methods to get the data you need. Don’t do that!! Qlik Sense extension model is based on the idea that you use one Generic Object, described in the extensions initialProperties, and modified by the user in the property panel, and possibly programmatically (but that’s really advanced). So while using the API methods to get additonal data is the approach to use in mashups or Web apps that access Qlik data, you should avoid using them in a masup.
Why you shouldn’t
If you do use these api calls in your extension, you will get problems:
- you might easily get a memory leak, or a ‘Generic Object leak’, where you create lots of Generic Objects
- you can easily lose contral of all callback functions running when the generic objects are revalidated
- if the user makes a snapshot, the API calls will access the latest version of the data, not the one in the snapshot and possibly give the wrong data
- if the user tries to export your extension to PDF or Excel it might break, since the service responsible for those export does not have access to live data, only to the snapshot
If you absolutely must use these calls, you should at least turn export to PDF and Excel of and not allow snapshots of your extension.
What you should do instead
Luckily there are alternatives. These API calls all create Generic Objects, but since the Generic Object is a very flexible structure, you can actually configure the Generic Object behind your extension to provide the data you need. Here is a little table of what you could use:
| You need | Do not use API call | Instead add to initialProperties |
|---|---|---|
| List of fields | app.getList("FieldList") | qFieldListDef |
| List of measures | app.getList("MeasureList") | qMeasureListDef |
| List of dimensions | app.getList("DimensionList") | qDimensionListDef |
| List of variables | app.getList("VariableList") | qVariableListDef |
| Variable value | app.variable.getContent(..) | qStringExpression or qValueExpression |
You find a working example of this in my syslist extension just don’t use it, it’s meant as examples of how to get the data and doesn’t really do anything useful. But do grab the initialProperties part you need for your extension.
It looks like this:
When should you use these API calls
Well, no rule without an exception. While you should avoid using these calls in the API for the rendering part of your extension, you should use them in your property panel, if you for example need to provide a list of fields to the user. And in a mashup, they are definitely very useful. And the API has other calls, which you might want to use, but that vill mainly be when the user does something, like clicking a button etc.
