Hi Alain,

thank you so much for your time and testing my problem with your heart and soul and solving it for the first instance!
I understand all of your arguments but getting headache when I realize that this workaround is needed to make this lwc work.

On the one hand - without criticizing your solution - the parameter "" in the lightning app builder looks really ugly.
And on the other hand I swear the component works before without this hack and didn't understand why this is needed now!


I also changed the format for the lightning app builder parameter "Column Names" to the following:
[{ "fieldName": "Title__c" },{ "fieldName": "Description__c" },{ "fieldName": "Link__c", "type": "url", "fixedWidth": 200 },{ "fieldName": "Date__c", "type": "date", "fixedWidth": 100 }]
It was not needed to hold the label information for each column because this information is extracted in the function _prepareTableColumns().

I am really, really happy to have this dont with you and find a solution. I couldn't imagine a better start to a working week. Yaaaay!


During ideas to optimize this solution I think about the possibility to use "lightning/uiRecordApi" (exactly: https://developer.salesforce.com/docs/component-library/documentation/lwc/lwc.reference_lightning_ui_api_record) to remove the apex controller. But it seems that this wire adapter only returns one record instead of a list. This idea was only a mental thinking.
I also changed the function wiredFilteredRssFeeds to a two parameter function and added the variable listOfApiName to delegate the selected fields from lightning app builder to select needed fields in apex.
This last change is useful, if any other fields should be displayed / added / removed from the object to NOT change the code and redeploy it.
To extract the needed api name I created a new function called _extractApiNames(); Nothing else is changed now. I swear!

Best regards,
Christian


For completeness - My code looks now like the following:


rssFeed.js-meta.xml

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata" fqn="RssFeed">
    <apiVersion>46.0</apiVersion>
    <isExposed>true</isExposed>
    <targets>
        <target>lightning__RecordPage</target>
    </targets>
    <targetConfigs>
        <targetConfig targets="lightning__RecordPage">
            <property default="%5B%7B%20%22fieldName%22:%20%22Title__c%22%20%7D,%7B%20%22fieldName%22:%20%22Description__c%22%20%7D,%7B%20%22fieldName%22:%20%22Link__c%22,%20%22type%22:%20%22url%22,%20%22fixedWidth%22:%20200%20%7D,%7B%20%22fieldName%22:%20%22Date__c%22,%20%22type%22:%20%22date%22,%20%22fixedWidth%22:%20100%20%7D%5D" description="Specifies which columns are displayed." label="Column Names" name="columnNames" required="true" type="String"/>
            <objects>
                <object>Brand__c</object>
            </objects>
        </targetConfig>
    </targetConfigs>
</LightningComponentBundle>

rssFeed.html:

<template>
    <div class={flexipageRegionWidth}>
        <div class="slds-m-top_medium slds-m-bottom_x-large">

            <!-- Simple -->
            <template if:true={data}>
                <div class="slds-p-around_medium lgc-bg">
                    <lightning-datatable
                        key-field="id"
                        data={data}
                        columns={columns}
                        is-loading={tableLoadingState}
                        hide-checkbox-column={hideCheckboxColumn}>
                    </lightning-datatable>
                </div>
            </template>
        </div>
        
        <!-- Data failed to load -->
        <template if:false={data}>
            <div class="slds-text-color_error">
                An error occurred while loading.
            </div>
        </template>
    </div>
</template>

rssFeed.js:

import { LightningElement, wire, track, api } from 'lwc';

import { getObjectInfo } from 'lightning/uiObjectInfoApi'; // https://developer.salesforce.com/docs/component-library/documentation/lwc/lwc.reference_wire_adapters_object_info
import RSSFEED_OBJECT from '@salesforce/schema/RSSFeed__c';

import getFilteredRssFeeds from '@salesforce/apex/RssFeedController.getFilteredRssFeeds';


export default class RssFeed extends LightningElement {
    @api flexipageRegionWidth; // https://developer.salesforce.com/docs/component-library/documentation/lwc/use_width_aware
    @api recordId;// Public property. Id of brand-record.
    @api columnNames = undefined; // Public property. Set by lightning app builder.
    
    objectInfo = undefined; // https://developer.salesforce.com/docs/component-library/documentation/lwc/lwc.reference_wire_adapters_object_info
    @track listOfApiName = [];

    @track data;
    @track error;

    @track columns;
    @track tableLoadingState = true;
    hideCheckboxColumn = true; // If true, the checkbox column for row selection is hidden.


    @wire(getObjectInfo, { objectApiName: RSSFEED_OBJECT })
    wireObjectInfo({error, data}){
        console.log('>>>wireObjectInfo called.');

        if (data) {
            //console.log('>>>data: ' + JSON.stringify(data, null, '\t'));
            this.objectInfo = data;
            this.error = undefined;

            this._prepareTableColumns();
        } else if (error) {
            console.error('>>>error: ' + JSON.stringify(error, null, '\t'));

            this.error = error;
            this.objectInfo = undefined;
        }
    }
    
    @wire(getFilteredRssFeeds, { brandId: '$recordId', listOfApiName: '$listOfApiName' })
    wiredFilteredRssFeeds({error, data}){
        console.log('>>>wiredFilteredRssFeeds called.');

        if (data) {
            console.log('>>>' + JSON.stringify(data, null, '\t'));
            this.data = data;
            this.error = undefined;
        } else if (error) {
            console.error('>>>' + JSON.stringify(error, null, '\t'));
            this.error = error;
            this.data = undefined;
        }
    }

    _prepareTableColumns(){
        console.log('>>>_prepareTableColumns called.');

        this.columns.forEach(column => {
            let columnLabel = ((this.objectInfo) ? this.objectInfo.fields[column.fieldName].label : '');
            column.label = columnLabel;
            console.log('>>>column: ' + JSON.stringify(column, null, '\t'));
        });

        console.log('>>>this.listOfApiName: ' + JSON.stringify(this.listOfApiName, null, '\t'));
    }

    _extractApiNames(){
        console.log('>>>_extractApiNames called.');

        this.columns.forEach(column => {
            this.listOfApiName.push(column.fieldName); // Exctract apiName for selected fields.
        });
    }

    /**
    * https://developer.salesforce.com/docs/component-library/documentation/lwc/create_lifecycle_hooks_dom.html
    * The connectedCallback() lifecycle hook fires when a component is inserted into the DOM.
    */
    async connectedCallback() {
        console.log('>>>async connectedCallback called.');
        this.tableLoadingState = false;
        console.log('>>>this.columnNames: ' + this.columnNames);

        this.columns = JSON.parse(decodeURI(this.columnNames));
        console.log('>>>this.columns: ' + JSON.stringify(this.columns, null, '\t'));
        this._extractApiNames();
    }
}

RssFeedController.cls:

/**
* Is used for lightning web component (lwc) named rssFeed.
* Query relevant rss feed filtered by Id of Brand (Brand__c).
*
* @author: <Christian Schwabe> (Christian.Schwabe@colliers.com)
*
* @history:
* version		    | author				                                | changes
* ====================================================================================
* 0.1 08.05.2019	| Christian Schwabe (Christian.Schwabe@colliers.com)	| initial version.
*/
public with sharing class RssFeedController {

    /**
    * Query RSS-Feed records filtered by Id for Brand (Brand__c).
    *
    * @param  brandId          Id for Brand (Brand__c)
    *
    * @return List<Brand__>
    */
    @AuraEnabled(cacheable=true)
    public static List<RSSFeed__c> getFilteredRssFeeds(Id brandId, List<String> listOfApiName){
        List<RSSFeed__c> listOfRssFeed = new List<RSSFeed__c>();
        String query = 'SELECT ' + String.join(listOfApiName, ', ') + ' FROM RSSFeed__c WHERE Brand__c = :brandId';
        
        for(RSSFeed__c rssFeed : (List<RSSFeed__c>) Database.query(query)){
            rssFeed.Description__c = String.valueOf(rssFeed.Description__c).stripHtmlTags();

            listOfRssFeed.add(rssFeed);
        }

        return listOfRssFeed;
    }
}