/*

    Copyright (C) 2017 Apple Inc. All Rights Reserved.

    See LICENSE.txt for this sample’s licensing information

    Abstract:

    A view controller that demonstrates how to use a `UICollectionViewController`. A custom `UIView` subclass, `GradientMaskView` is used to mask the contents of the collection view as it scrolls beneath the containing `UINavigationController`'s `UINavigationBar`.

*/

import UIKit

class CollectionViewController: UICollectionViewController {

    // MARK: Properties

    private let items = DataItem.sampleItems

    private let cellComposer = DataItemCellComposer()

    // MARK: UIViewController

    override func viewDidLoad() {

        super.viewDidLoad()

        guard let collectionView = collectionView else { return }

        /*

            Add a gradient mask to the collection view. This will fade out the

            contents of the collection view as it scrolls beneath the transparent

            navigation bar.

        */

        collectionView.mask = GradientMaskView(frame: CGRect(origin: CGPoint.zero, size: collectionView.bounds.size))

    }

    override func viewDidLayoutSubviews() {

        super.viewDidLayoutSubviews()

        guard let collectionView = collectionView,

                let maskView = collectionView.mask as? GradientMaskView,

                let layout = collectionView.collectionViewLayout as? UICollectionViewFlowLayout

        else {

            return

        }

        /*

            Update the mask view to have fully faded out any collection view

            content above the navigation bar's label.

        */

        maskView.maskPosition.end = topLayoutGuide.length * 0.8

        /*

            Update the position from where the collection view's content should

            start to fade out. The size of the fade increases as the collection

            view scrolls to a maximum of half the navigation bar's height.

        */

        let maximumMaskStart = maskView.maskPosition.end + (topLayoutGuide.length * 0.5)

        let verticalScrollPosition = max(0, collectionView.contentOffset.y + collectionView.contentInset.top)

        maskView.maskPosition.start = min(maximumMaskStart, maskView.maskPosition.end + verticalScrollPosition)

        /*

            Position the mask view so that it is always fills the visible area of

            the collection view.

        */

        var rect = CGRect(origin: CGPoint(x: 0, y: collectionView.contentOffset.y), size: collectionView.bounds.size)

        /*

            Increase the width of the mask view so that it doesn't clip focus

            shadows along its edge. Here we are basing the amount to increase the

            frame by on the spacing defined in the collection view's layout.

        */

        rect = rect.insetBy(dx: -layout.minimumInteritemSpacing, dy: 0)

        maskView.frame = rect

    }

    // MARK: UICollectionViewDataSource

    override func numberOfSections(in collectionView: UICollectionView) -> Int {

        // The collection view shows all items in a single section.

        return 1

    }

    override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {

        return items.count

    }

    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {

        // Dequeue a cell from the collection view.

        return collectionView.dequeueReusableCell(withReuseIdentifier: DataItemCollectionViewCell.reuseIdentifier, for: indexPath)

    }

    // MARK: UICollectionViewDelegate

    override func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {

        guard let cell = cell as? DataItemCollectionViewCell else { fatalError("Expected to display a `DataItemCollectionViewCell`.") }

        let item = items[(indexPath as NSIndexPath).row]

        // Configure the cell.

        cellComposer.compose(cell, withDataItem: item)

    }

}