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Some docs point to a developer guide in Help -> Help Contents -> Developer Guide. My version of TOS (downloaded a few days ago) doesn't have this in the online help. In my help contents I just have 'Work Bench User Guide' and 'Talend User Documentation'. Neither make reference to a developer guide.
What am I missing here?
That definitely helps. Is that the only developer documentation then? Seems to be focused on component construction.
Yes it is. We've focused the "developer guide" on component creation, because TOS application was designed to be extended with components.
Anything on the core engine/API?
No. Do you plan to bring modification to the core? Give us some details and I will ask the core developers to give you some details (I'm mainly a component developer).