Add AutoLayoutSizeSource (#110)

A SizeSource that calculates a view size based on its auto-layout constraints.

Update AutoLayoutSizeSource

It now supports updating the view with a given closure, and customization of the fittingPriority for layout size calculations.

Fixed code linting

Sources/SizeSource/AutoLayoutSizeSource.swift

@@ -0,0 +1,28 @@
+import UIKit
+
+public typealias ClosureViewUpdateFn<Data, View> = (View, Data, Int) -> Void
+
+open class AutoLayoutSizeSource<Data, View: UIView>: SizeSource<Data> {
+  private let dummyView: View
+  private let dummyViewUpdater: ClosureViewUpdateFn<Data, View>
+  private let horizontalFittingPriority: UILayoutPriority
+  private let verticalFittingPriority: UILayoutPriority
+   public init(dummyView: View.Type, 
+              horizontalFittingPriority: UILayoutPriority = .defaultHigh, 
+              verticalFittingPriority: UILayoutPriority = .defaultLow, 
+              viewUpdater: @escaping ClosureViewUpdateFn<Data, View>) {
+
+    self.dummyView = View()
+    self.dummyViewUpdater = viewUpdater
+    self.horizontalFittingPriority = horizontalFittingPriority
+    self.verticalFittingPriority = verticalFittingPriority
+  }
+   open override func size(at index: Int, data: Data, collectionSize: CGSize) -> CGSize {
+    self.dummyViewUpdater(self.dummyView, data, index)
+
+    return self.dummyView.systemLayoutSizeFitting(
+      collectionSize, 
+      withHorizontalFittingPriority: self.horizontalFittingPriority, 
+      verticalFittingPriority: self.verticalFittingPriority)
+  }
+}