P7 Stream W: TaskFormState + validation (iOS parity)

Pure-function field validators matching iOS TaskFormStates.swift error
strings. TaskFormState container derives errors from fields, exposes
isValid and typed request builders.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

composeApp/src/commonMain/kotlin/com/tt/honeyDue/ui/screens/task/TaskFormState.kt

@@ -0,0 +1,142 @@
+package com.tt.honeyDue.ui.screens.task
+
+import com.tt.honeyDue.models.TaskCreateRequest
+import com.tt.honeyDue.models.TaskUpdateRequest
+import kotlinx.datetime.LocalDate
+
+/**
+ * Immutable snapshot of the create/edit-task form field values.
+ *
+ * Mirrors the iOS `TaskFormState` container in
+ * `iosApp/iosApp/Core/FormStates/TaskFormStates.swift` — kept purely
+ * descriptive so it can be fed into pure validators.
+ *
+ * IDs are [Int] because [TaskCreateRequest]/[TaskUpdateRequest] use [Int].
+ * The public plan spec asked for Long, but matching the existing wire
+ * contract avoids a lossy conversion at the call-site.
+ */
+data class TaskFormFields(
+    val title: String = "",
+    val description: String = "",
+    val priorityId: Int? = null,
+    val categoryId: Int? = null,
+    val frequencyId: Int? = null,
+    val dueDate: LocalDate? = null,
+    val estimatedCost: String = "",
+    val residenceId: Int? = null,
+)
+
+/**
+ * Per-field error snapshot — one nullable string per validated field. A
+ * `null` entry means "no error".
+ */
+data class TaskFormErrors(
+    val title: String? = null,
+    val priorityId: String? = null,
+    val categoryId: String? = null,
+    val frequencyId: String? = null,
+    val estimatedCost: String? = null,
+    val residenceId: String? = null,
+) {
+    /** True when every entry is null — i.e. the form passes validation. */
+    val isEmpty: Boolean
+        get() = title == null &&
+                priorityId == null &&
+                categoryId == null &&
+                frequencyId == null &&
+                estimatedCost == null &&
+                residenceId == null
+}
+
+/**
+ * Mutable container that glues [TaskFormFields] to the pure validators in
+ * [TaskFormValidation] and exposes typed request builders.
+ *
+ * Deliberately not a Compose `State` — keep this plain so common tests can
+ * exercise it without a UI runtime. The Compose call-site can wrap the
+ * container in a `remember { mutableStateOf(...) }` as needed.
+ */
+class TaskFormState(initial: TaskFormFields = TaskFormFields()) {
+
+    var fields: TaskFormFields = initial
+        private set
+
+    /** Derived: recomputed from the current [fields] on every access. */
+    val errors: TaskFormErrors
+        get() = computeErrors(fields)
+
+    /** Derived: `true` iff every validator returns `null`. */
+    val isValid: Boolean
+        get() = errors.isEmpty
+
+    /** Replaces the current field snapshot. Errors/[isValid] update automatically. */
+    fun update(f: TaskFormFields) {
+        fields = f
+    }
+
+    /** Alias for `errors` — returned as an explicit snapshot for callers that prefer imperative style. */
+    fun validate(): TaskFormErrors = errors
+
+    /**
+     * Builds a [TaskCreateRequest] from the current fields, or `null` if the
+     * form does not validate. Blank description / empty cost are mapped to
+     * `null` (matching iOS).
+     */
+    fun toCreateRequest(): TaskCreateRequest? {
+        if (!isValid) return null
+        val f = fields
+        // Non-null asserts are safe: isValid implies all required fields are set.
+        return TaskCreateRequest(
+            residenceId = f.residenceId!!,
+            title = f.title,
+            description = f.description.ifBlank { null },
+            categoryId = f.categoryId,
+            priorityId = f.priorityId,
+            frequencyId = f.frequencyId,
+            dueDate = f.dueDate?.toString(),
+            estimatedCost = f.estimatedCost.ifBlank { null }?.toDoubleOrNull(),
+        )
+    }
+
+    /**
+     * Builds a [TaskUpdateRequest] from the current fields. Unlike create,
+     * edit mode does not require a residence (the task already has one),
+     * so this returns `null` only when the non-residence validators fail.
+     */
+    fun toUpdateRequest(taskId: Int): TaskUpdateRequest? {
+        val errs = errors
+        val editErrorsPresent = errs.title != null ||
+                errs.priorityId != null ||
+                errs.categoryId != null ||
+                errs.frequencyId != null ||
+                errs.estimatedCost != null
+        if (editErrorsPresent) return null
+        val f = fields
+        return TaskUpdateRequest(
+            title = f.title,
+            description = f.description.ifBlank { null },
+            categoryId = f.categoryId,
+            priorityId = f.priorityId,
+            frequencyId = f.frequencyId,
+            dueDate = f.dueDate?.toString(),
+            estimatedCost = f.estimatedCost.ifBlank { null }?.toDoubleOrNull(),
+        )
+    }
+
+    /**
+     * Overload that accepts [Long] for symmetry with screens that pull the
+     * task id from navigation arguments typed as Long. Narrows to [Int]
+     * because the wire model expects Int.
+     */
+    fun toUpdateRequest(taskId: Long): TaskUpdateRequest? =
+        toUpdateRequest(taskId.toInt())
+
+    private fun computeErrors(f: TaskFormFields): TaskFormErrors = TaskFormErrors(
+        title = TaskFormValidation.validateTitle(f.title),
+        priorityId = TaskFormValidation.validatePriorityId(f.priorityId),
+        categoryId = TaskFormValidation.validateCategoryId(f.categoryId),
+        frequencyId = TaskFormValidation.validateFrequencyId(f.frequencyId),
+        estimatedCost = TaskFormValidation.validateEstimatedCost(f.estimatedCost),
+        residenceId = TaskFormValidation.validateResidenceId(f.residenceId),
+    )
+}

composeApp/src/commonMain/kotlin/com/tt/honeyDue/ui/screens/task/TaskFormValidation.kt

@@ -0,0 +1,46 @@
+package com.tt.honeyDue.ui.screens.task
+
+/**
+ * Pure-function validators for the task create/edit form.
+ *
+ * Error strings are kept in 1:1 parity with iOS:
+ *  - `iosApp/iosApp/Task/TaskFormView.swift::validateForm()` — the
+ *    authoritative source for the "Please select a..." / "... is required"
+ *    copy that the user actually sees.
+ *  - `iosApp/iosApp/Core/FormStates/TaskFormStates.swift` — container shape.
+ *
+ * Each function returns `null` when the value is valid, or an error message
+ * otherwise. Callers compose these into a [TaskFormErrors] snapshot.
+ */
+object TaskFormValidation {
+
+    fun validateTitle(value: String): String? =
+        if (value.isBlank()) "Title is required" else null
+
+    fun validatePriorityId(value: Int?): String? =
+        if (value == null) "Please select a priority" else null
+
+    fun validateCategoryId(value: Int?): String? =
+        if (value == null) "Please select a category" else null
+
+    fun validateFrequencyId(value: Int?): String? =
+        if (value == null) "Please select a frequency" else null
+
+    /**
+     * Estimated cost is optional — an empty/blank string is valid. A non-empty
+     * value must parse as a [Double]. Matches iOS `estimatedCost.value.asOptionalDouble`
+     * plus the "must be a valid number" phrasing already used for
+     * `customIntervalDays` in `TaskFormView.swift`.
+     */
+    fun validateEstimatedCost(value: String): String? {
+        if (value.isBlank()) return null
+        return if (value.toDoubleOrNull() == null) {
+            "Estimated cost must be a valid number"
+        } else {
+            null
+        }
+    }
+
+    fun validateResidenceId(value: Int?): String? =
+        if (value == null) "Property is required" else null
+}