Implement calculating the manual sort order

src/models/task.js

@@ -104,6 +104,22 @@ export default class Task {
 		this._modified = this.vtodo.getFirstPropertyValue('last-modified')
 		this._created = this.vtodo.getFirstPropertyValue('created')
 
+		let sortOrder = this.vtodo.getFirstPropertyValue('x-apple-sort-order')
+		if (sortOrder === null) {
+			sortOrder = this._created.subtractDate(
+				new ICAL.Time({
+					year: 2001,
+					month: 1,
+					day: 1,
+					hour: 0,
+					minute: 0,
+					second: 0,
+					isDate: false
+				})
+			).toSeconds()
+		}
+		this._sortOrder = sortOrder
+
 		this._searchQuery = ''
 		this._matchesSearchQuery = true
 	}
@@ -450,6 +466,29 @@ export default class Task {
 		this._created = this.vtodo.getFirstPropertyValue('created')
 	}
 
+	get sortOrder() {
+		return this._sortOrder
+	}
+
+	set sortOrder(sortOrder) {
+		if (sortOrder === null) {
+			sortOrder = this._created.subtractDate(
+				new ICAL.Time({
+					year: 2001,
+					month: 1,
+					day: 1,
+					hour: 0,
+					minute: 0,
+					second: 0,
+					isDate: false
+				})
+			).toSeconds()
+		}
+		this.vtodo.updatePropertyWithValue('x-apple-sort-order', sortOrder)
+		this.updateLastModified()
+		this._sortOrder = sortOrder
+	}
+
 	/**
 	 * Checks if the task matches the search query
 	 *