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Implementation Notes

Work in progress…

So far we only have notes on the implementation of the Swift runtime. They’re actually pretty close to what goes on in Kotlin, the WorkflowNode and SubtreeManager classes in particular.


The Render loop

Initial pass

workflow rendering sequence diagram

The root of your workflow hierarchy gets put into a WorkflowHost (if you’re using ContainerViewController this is created for you). As part of its initializer, WorkflowHost creates a WorkflowNode that wraps the given root Workflow (and keeps track of the Workflow’s State). It then calls render() on the node:

// WorkflowHost
public init(workflow: WorkflowType, debugger: WorkflowDebugger? = nil) {
    self.debugger = debugger

    self.rootNode = WorkflowNode(workflow: workflow)  // 1. Create the node

    self.mutableRendering = MutableProperty(self.rootNode.render())  // 2. Call render()

WorkflowNode contains a SubtreeManager, whose primary purpose is to manage child workflows (more on this later). When render() gets invoked on the node, it calls render on the SubtreeManager and passes a closure that takes a RenderContext and returns a Rendering for the Workflow associated with the node.

// WorkflowNode
func render() -> WorkflowType.Rendering {
    return subtreeManager.render { context in
        return workflow.render(
            state: state,
            context: context

The SubtreeManager instantiates a RenderContext and invokes the closure that was passed in. This last step generates the Rendering. This Rendering then gets passed back up the call stack until it reaches the WorkflowHost.


In cases where a Workflow has child Workflows, the render sequence is similar. The [tutorial] (../tutorial/building-a-workflow/#the-render-context) goes through this in more detail.

nested workflow rendering sequence diagram

Essentially, a Workflow containing child Workflows calls render(context:key:outputMap:) on each child Workflow and passes in the RenderContext. The context does some bookkeeping for the child Workflow (creating or updating a ChildWorkflow<T>) and then calls render(). ChildWorkflow<T>.render() calls render() on its WorkflowNode and we recurse back to step 2.