feat(ioscompanion): port screen-settle stability polling to Go

internal/driver/ioscompanion/settle.go

@@ -0,0 +1,195 @@
+// Package ioscompanion drives an iOS simulator through the native simulator
+// companion. This file ports the screen-settle (stability polling) logic that
+// waits for the on-device UI to stop churning before the runner reads a
+// hierarchy and screenshot pair.
+package ioscompanion
+
+import (
+	"context"
+	"strings"
+	"time"
+
+	"github.com/priyanshujain/sanderling/internal/hierarchy"
+)
+
+// StabilityPollInterval is how long the loop waits between hierarchy probes.
+// It is set wide enough that the companion's hierarchy endpoint is not
+// hammered: tighter intervals back the companion's stream up under load.
+const StabilityPollInterval = 250 * time.Millisecond
+
+// MinStableStreak is how long the tree must stay structurally identical (and
+// non-transitional) before the poll declares settle. Actions whose effect is
+// async (a tap that fires a write which later pops the back stack) leave the
+// UI momentarily stable before the navigation transition fires; requiring an
+// uninterrupted streak of this length means any churn that starts during the
+// window resets the clock instead of being missed.
+const MinStableStreak = 750 * time.Millisecond
+
+// StabilityPollCap bounds total time spent polling so a UI that never settles
+// does not block the runner indefinitely.
+const StabilityPollCap = 2000 * time.Millisecond
+
+// Clock abstracts time so the poll loop can be driven by a fake in tests
+// without sleeping for real.
+type Clock interface {
+	Now() time.Time
+	Sleep(duration time.Duration)
+}
+
+// systemClock is the production Clock backed by the standard library.
+type systemClock struct{}
+
+func (systemClock) Now() time.Time        { return time.Now() }
+func (systemClock) Sleep(d time.Duration) { time.Sleep(d) }
+
+// SystemClock returns a Clock backed by the standard library wall clock.
+func SystemClock() Clock { return systemClock{} }
+
+// PollUntilStable returns once StabilitySnapshot has been non-transitional and
+// equal to itself for an uninterrupted stretch of at least MinStableStreak,
+// capped at StabilityPollCap. fetch returns the current hierarchy tree (nil on
+// fetch failure, treated like a transitional snapshot so the streak resets).
+//
+// The loop mirrors the companion's JVM implementation: it samples a prior
+// snapshot, then on each tick sleeps the poll interval, fetches a fresh
+// snapshot, and grows the streak only while consecutive snapshots are both
+// non-transitional and structurally identical. A transitional snapshot, a
+// changed snapshot, or a fetch failure resets the streak. The function returns
+// when the streak reaches MinStableStreak or the cap elapses, and respects
+// context cancellation.
+func PollUntilStable(ctx context.Context, clock Clock, fetch func() *hierarchy.Tree) {
+	deadline := clock.Now().Add(StabilityPollCap)
+	prior := snapshot(fetch)
+	var streakStart time.Time
+	for clock.Now().Before(deadline) {
+		if ctx.Err() != nil {
+			return
+		}
+		clock.Sleep(StabilityPollInterval)
+		current := snapshot(fetch)
+		now := clock.Now()
+		if prior.valid && current.valid && prior.hash == current.hash {
+			if streakStart.IsZero() {
+				streakStart = now
+			}
+			if now.Sub(streakStart) >= MinStableStreak {
+				return
+			}
+		} else {
+			streakStart = time.Time{}
+		}
+		prior = current
+	}
+}
+
+// stableSnapshot is the result of probing a single hierarchy tree. valid is
+// false when the snapshot is transitional (mid route transition) or the fetch
+// failed, both of which must reset the stable streak.
+type stableSnapshot struct {
+	hash  string
+	valid bool
+}
+
+func snapshot(fetch func() *hierarchy.Tree) stableSnapshot {
+	tree := fetch()
+	if tree == nil {
+		return stableSnapshot{}
+	}
+	return StabilitySnapshot(tree)
+}
+
+// StabilitySnapshot probes a hierarchy tree for both structural shape and route
+// transition state. An empty tree is a valid stable snapshot (the blank-tree
+// case in the companion). A tree with more than one route Screen is
+// transitional and reported invalid: a navigation host keeps both source and
+// destination destinations alive during a cross-fade, and a snapshot taken in
+// that window is unreliable because lazy lists in the incoming screen mount
+// over several frames. Otherwise the snapshot carries the structural hash.
+func StabilitySnapshot(tree *hierarchy.Tree) stableSnapshot {
+	if tree == nil || tree.Root == nil {
+		return stableSnapshot{valid: true}
+	}
+	if CountRouteScreens(tree) > 1 {
+		return stableSnapshot{}
+	}
+	return stableSnapshot{hash: StructuralHash(tree), valid: true}
+}
+
+// routeTagKeys are the attribute keys whose value, when it ends with "Screen",
+// marks a node as a route-level destination. Mirrors the companion's set.
+var routeTagKeys = []string{
+	"resource-id", "resourceId", "testTag",
+	"identifier", "accessibilityIdentifier",
+}
+
+// CountRouteScreens counts nodes that carry a route-level destination tag (a
+// route-tag attribute whose value ends with "Screen"). At most one tag is
+// counted per node, matching the companion which breaks on the first match.
+func CountRouteScreens(tree *hierarchy.Tree) int {
+	if tree == nil || tree.Root == nil {
+		return 0
+	}
+	return countRouteScreens(tree.Root)
+}
+
+func countRouteScreens(node *hierarchy.Node) int {
+	count := 0
+	for _, key := range routeTagKeys {
+		value, ok := node.Attributes[key]
+		if !ok {
+			continue
+		}
+		if strings.HasSuffix(value, "Screen") {
+			count++
+			break
+		}
+	}
+	for _, child := range node.Children {
+		count += countRouteScreens(child)
+	}
+	return count
+}
+
+// stableAttributeKeys are the identity attributes folded into the structural
+// hash, in this exact order. The transient bounds attribute is deliberately
+// excluded so a measure pass that shifts pixels without changing what is on
+// screen does not extend the wait; structure (via tree shape) and text are
+// included so a real content or layout-tree change does reset stability.
+var stableAttributeKeys = []string{
+	"resource-id", "resourceId",
+	"class", "className",
+	"content-desc", "contentDescription", "accessibilityText",
+	"text",
+	"testTag", "identifier", "accessibilityIdentifier",
+}
+
+// StructuralHash walks the tree and concatenates only the stable identity
+// attributes (excluding bounds), in tree order, wrapping each node in
+// parentheses so tree shape is encoded. The result is compared by equality, so
+// it is the hash itself rather than a digest of it.
+func StructuralHash(tree *hierarchy.Tree) string {
+	if tree == nil || tree.Root == nil {
+		return ""
+	}
+	var builder strings.Builder
+	walkForStructuralHash(tree.Root, &builder)
+	return builder.String()
+}
+
+func walkForStructuralHash(node *hierarchy.Node, out *strings.Builder) {
+	out.WriteByte('(')
+	for _, key := range stableAttributeKeys {
+		value, ok := node.Attributes[key]
+		if !ok {
+			continue
+		}
+		out.WriteString(key)
+		out.WriteByte(':')
+		out.WriteString(value)
+		out.WriteByte('|')
+	}
+	for _, child := range node.Children {
+		walkForStructuralHash(child, out)
+	}
+	out.WriteByte(')')
+}