Custom component not rendered "fast enough"?

I have a custom UI component placed within a surrounding box in UI Builder. Initially, it looks like this:

Further, I have a button with an onclick-action. This action shall make the surrounding block visible and then call an action selectImageof my custom component. When clicking on the button, I’m getting this error:

TypeError: Cannot read properties of undefined (reading 'selectImage')

It seems that my component is not rendered/available yet when I try to call the action.

This is my onclick-handler:

If I’m adding a wait-statement, then everything works nicely in sequence as expected:

How can I deal with that? Note, my SurroundingBlock can contain other UI components which I want to have appear all at once when toggling the visibility of SurroundingBlock. So, it’s not enough to toggle the visibility of MyComponent, which works well.


hi @Klaas_Klever

yes, this is expected behaviour, let me explain why it happens:

  1. when you toggle component visibility you just change the component “display” property
  2. changing a component’s properties launches a rerender phase for the component, but it runs an async function to accumulate all the changes and reflects them in UI, for the best performance
  3. when you hide (display=false) a component is still on the stage but it stops rendering all its children, so they are unmounting and you do not have to them anymore, also for best performance.
  4. this is why the “wait” block helps, here is a very similar case with async and re-render Binding issue between custom UI components - #2 by vladimir-upirov

Alright. Works as designed :slightly_smiling_face:
What is the recommended strategy to implement such a scenario? Using the wait block?
I guess, once more an more developers on backendless will use custom UI components, this issue will be met more and more often …


I would recommend running the “Select…” action only when you are sure the component is on the stage, perhaps a custom “On Mounted” handler for the component can be a good place