Drop-down list for columns

[rgaiacs]

For blocks that has column as argument, would be incredible useful for users to have the argument as a drop-down list instead of text box.

[gvwilson]

Strongly agreed, but the implementation is very difficult within the Blockly framework:

1. We can't know the names of available columns without running the pipeline. (For example, if the first block is "load data from URL", we can't know what column names to allow in subsequent blocks until we load the data.)

2. Blockly doesn't give us an event when blocks are clicked together, so we don't have a hook for automatically updating fields of blocks.

We would really like to find a better framework - one that uses modern JavaScript and doesn't rely on string concatenation + eval for execution - but that will only solve problem 2: problem 1 is intrinsic.