.internal/while_loop_start

ComfyUI has no native for/while construct - graphs are acyclic. The workaround is dynamic graph expansion: a node rebuilds a chunk of the graph at runtime, re-executing it with new values until a condition says stop. That's what this pack's loop machinery does, and __y_i_u__while_loop_start is the open bracket of that loop: it declares the loop's initial values and emits a flow token that drives the iteration.

It's an .internal node, hidden from the menu by the pack's front-end code. You'll rarely add it yourself - it lives in saved workflows, most visibly paired with the upscale loop's START/END pair, where the machinery writes loop-carried state (the step index, the working image) back into this node each pass.

The design is deliberately generic: one condition boolean and up to 20 loop-carried values, passed through as initial_value0 through initial_value19. On the first pass, those carry your starting values; on later passes, the loop-end node writes the updated values back in. When the condition is false, the node stops the loop by emitting an execution blocker instead of values, which is how a graph with no native loop terminates cleanly.

Inputs

• condition - BOOLEAN, default true. Whether to run the body.
• initial_value0 … initial_value19 - optional, any type. The loop's carried state.

Outputs

flow - the FLOW_CONTROL token that drives the loop, and value0 … value19, the current iteration's values.

Installing it

Part of yiu_comfy_nodes by leizingyiu:

cd ComfyUI/custom_nodes
git clone https://github.com/leizingyiu/yiu_comfy_nodes

restart ComfyUI. No extra dependencies.

Where people get burned

• It's hidden, and that's fine. Don't hunt for it in the menu. If you're building a loop by hand, you're more likely to use the ready-made yiuUpscaleLoopStart/END pair, which drives this machinery for you.
• Unconnected optional values are just None. Pass only the slots you use; leaving a gap is not an error.
• The condition defaults to true. An unwired condition means "loop forever" - the termination has to come from the loop-end side (the pack's loop always wires it), so don't expect a safety net if you construct this by hand.
• Requires dynamic-graph ComfyUI. If your install predates GraphBuilder support, the loop it powers errors out. Update before debugging anything else.