Add foreAlignmentEdge property to TransitionController.

Summary:
If a view controller has a non-zero preferredContentSize, then the foreAlignmentEdge will be used to align the view to either the center of the screen, if nil, or to the specified edge (one of minX, minY, maxX, or maxY).

If preferredContentSize is zero, then the transition's `finalFrame` value will be used instead - this is generally the containerView's bounds.

This API is useful for building modal dialogs and sliding drawers that are presented over the current context with `modalPresentationStyle = .overCurrentContext`.

Reviewers: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, markwei

Reviewed By: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, markwei

Subscribers: markwei

Tags: #material_motion

Differential Revision: http://codereview.cc/D3067

examples/ModalDialogExample.swift

@@ -46,7 +46,7 @@ class ModalDialogViewController: UIViewController {
     super.init(nibName: nibNameOrNil, bundle: nibBundleOrNil)
 
     transitionController.transitionType = ModalDialogTransition.self
-
+    preferredContentSize = .init(width: 200, height: 200)
     modalPresentationStyle = .overCurrentContext
   }
 
@@ -64,8 +64,6 @@ class ModalDialogViewController: UIViewController {
     view.layer.shadowRadius = 5
     view.layer.shadowOpacity = 1
     view.layer.shadowOffset = .init(width: 0, height: 2)
-
-    preferredContentSize = .init(width: 200, height: 200)
   }
 }
 
@@ -74,15 +72,10 @@ class ModalDialogTransition: SelfDismissingTransition {
   required init() {}
 
   func willBeginTransition(withContext ctx: TransitionContext, runtime: MotionRuntime) -> [Stateful] {
-    let size = ctx.fore.preferredContentSize
-
-    if ctx.direction == .forward {
-      ctx.fore.view.bounds = CGRect(origin: .zero, size: size)
-    }
-
+    let size = ctx.fore.view.frame.size
     let bounds = ctx.containerView().bounds
     let backPosition = CGPoint(x: bounds.midX, y: bounds.maxY + size.height * 3 / 4)
-    let forePosition = CGPoint(x: bounds.midX, y: bounds.midY)
+    let forePosition = ctx.fore.view.layer.position
 
     let reactiveForeLayer = runtime.get(ctx.fore.view.layer)
     let position = reactiveForeLayer.position

src/transitions/TransitionContext.swift

@@ -84,6 +84,8 @@ public final class TransitionContext: NSObject {
    */
   public let fore: UIViewController
 
+  public let foreAlignmentEdge: CGRectEdge?
+
   /** The set of gesture recognizers associated with this transition. */
   public let gestureRecognizers: Set<UIGestureRecognizer>
 
@@ -96,12 +98,14 @@ public final class TransitionContext: NSObject {
        direction: TransitionDirection,
        back: UIViewController,
        fore: UIViewController,
-       gestureRecognizers: Set<UIGestureRecognizer>) {
+       gestureRecognizers: Set<UIGestureRecognizer>,
+       foreAlignmentEdge: CGRectEdge?) {
     self.direction = createProperty("Transition.direction", withInitialValue: direction)
     self.initialDirection = direction
     self.back = back
     self.fore = fore
     self.gestureRecognizers = gestureRecognizers
+    self.foreAlignmentEdge = foreAlignmentEdge
     self.window = TransitionTimeWindow(duration: TransitionContext.defaultDuration)
 
     // TODO: Create a Timeline.
@@ -140,6 +144,30 @@ extension TransitionContext: UIViewControllerInteractiveTransitioning {
   }
 }
 
+private func preferredFrame(for viewController: UIViewController,
+                            inBounds bounds: CGRect,
+                            alignmentEdge: CGRectEdge?) -> CGRect? {
+  guard viewController.preferredContentSize != .zero() else {
+    return nil
+  }
+
+  let size = viewController.preferredContentSize
+  let origin: CGPoint
+  switch alignmentEdge {
+  case nil: // Centered
+    origin = .init(x: bounds.midX - size.width / 2, y: bounds.midY - size.height / 2)
+  case .minXEdge?:
+    origin = .init(x: bounds.minX, y: bounds.midY - size.height / 2)
+  case .minYEdge?:
+    origin = .init(x: bounds.midX - size.width / 2, y: bounds.minY)
+  case .maxXEdge?:
+    origin = .init(x: bounds.maxX - size.width, y: bounds.midY - size.height / 2)
+  case .maxYEdge?:
+    origin = .init(x: bounds.midX - size.width / 2, y: bounds.maxY - size.height)
+  }
+  return .init(origin: origin, size: size)
+}
+
 extension TransitionContext {
   fileprivate func initiateTransition() {
     if let from = context.viewController(forKey: .from) {
@@ -150,7 +178,15 @@ extension TransitionContext {
     }
 
     if let to = context.viewController(forKey: .to) {
-      let finalFrame = context.finalFrame(for: to)
+      let finalFrame: CGRect
+
+      if let preferredFrame = preferredFrame(for: to,
+                                             inBounds: context.containerView.bounds,
+                                             alignmentEdge: (direction.value == .forward) ? foreAlignmentEdge : nil) {
+        finalFrame = preferredFrame
+      } else {
+        finalFrame = context.finalFrame(for: to)
+      }
       if !finalFrame.isEmpty {
         to.view.frame = finalFrame
       }

src/transitions/TransitionController.swift

@@ -109,6 +109,11 @@ public final class TransitionController {
     get { return _transitioningDelegate.gestureDelegate.gestureRecognizers }
   }
 
+  public var foreAlignmentEdge: CGRectEdge? {
+    set { _transitioningDelegate.foreAlignmentEdge = newValue }
+    get { return _transitioningDelegate.foreAlignmentEdge }
+  }
+
   /**
    The transitioning delegate managed by this controller.
 
@@ -152,6 +157,7 @@ private final class TransitioningDelegate: NSObject, UIViewControllerTransitioni
     self.dismisser.delegate = self
   }
 
+  var foreAlignmentEdge: CGRectEdge?
   var ctx: TransitionContext?
   var transitionType: Transition.Type?
 
@@ -180,7 +186,8 @@ private final class TransitioningDelegate: NSObject, UIViewControllerTransitioni
                               direction: direction,
                               back: back,
                               fore: fore,
-                              gestureRecognizers: gestureDelegate.gestureRecognizers)
+                              gestureRecognizers: gestureDelegate.gestureRecognizers,
+                              foreAlignmentEdge: foreAlignmentEdge)
       ctx?.delegate = self
     }
   }