The layouts module is generating inconsistent output

[sunerphey]

Other modules like ui_patterns_(views|fieldgroup|ds) are setting "#multiple_sources" to TRUE in the render array, while the layouts module does not. In the same time having multiple entity fields pointing to a single pattern field is a valid and well used option. Having different render array gives the potential of errors in the implementation of the patterns as the developer may have assumptions of the data type. For example I have run into an implementation that assumed array of renderable items as it was used initially with the layouts module, but it ultimately failed when I tried to use the pattern from the views module.
However I can see a problem with backwards compatibility from implementations that also made some assumptions if we simply just try to set "#multiple_sources" to TRUE in the layouts module.<
I personally don't see much benefit of the multiple_sources feature as there look to be cleaner ways of achieving its purpose.

[sunerphey]

Reproduction

• Create pattern with field which may be provided with array value
• In the pattern template try to iterate and render its items

{% if field is iterable %}
{% for item in field %}

{{ item }}

{% endfor %}
{% endif %}`

• Use it with ui_patterns_layout module by selecting an entity display mode to use this pattern and provide multiple entity fields in the region which represents this pattern field. This shall work fine and the entity will be displayed with this pattern.
• Setup view where the row style would be pattern and select multiple fields to go to the "field" argument of the pattern.
  You will receive:

Object of type Drupal\ui_patterns\Element\PatternContext cannot be printed.

[ademarco]

@sunerphey I'm noticing some similarities with what was reported in #193, would you agree that the two issues are related? We could maybe remove #multiple_sources all together by forcing a pattern field to become an array of render arrays, even if one render array is passed to it. This might have backward compatibility implications though.

[ademarco]

Or rather, we make layout behave like the other 3 modules, meaning we force there too for #multiple_sources to be TRUE. The thing is: we cannot assume anything about the data structure that will end up in a field, if could be an array of arrays for all we know, and be still a "single values".

The only way to differentiate this is for the calling context to know if what is passing to the field is #multiple_sources or not. For the moment I'm more inclined to have #multiple_sources set to TRUE in the layout module as well.

What do you think guys? \cc @sunerphey @DuaelFr

[ademarco]

Reminder to self: write extensive documentation about #multiple_sources after we solve this.

[sunerphey]

I am not fully convinced that there shall be no predictability about the data type, as I think having an iterable parameter to a pattern makes sense, while having a drupal renderable array is not so useful, especially if we want to have the patterns more portable. But this is a discussion about the necessity of the multiple_sources feature while we already have it and I guess it won't be a safe thing to remove.

[DuaelFr]

I'm all for consistency.
I'd love to always have #sources and #context even when there is only one value. I'm not sure about the usefulness of the #multiple_sources key in this case but I don't have every use case in mind.

In a perfect world, I'd love that:

1. calling {{ my_field }} would output the values rendered by the basic multiple sources template (with more suggestions from the #context values maybe) ;
2. calling {% for value in my_field %} would automatically loop over the #sources content without having to specify (better for patterns interoperability).

I suppose that my_field should be an iterable object with a __toString() magic method to handle the first case but I'm not sure it is easy to implement within Drupal.

(I'd also love to be able to automagically split a multiple field values into multiple #sources but that's another problem :) )