Placeholder.swift
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//
// Placeholder.swift
// Kingfisher
//
// Created by Tieme van Veen on 28/08/2017.
//
// Copyright (c) 2018 Wei Wang <onevcat@gmail.com>
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
#if os(macOS)
import AppKit
#else
import UIKit
#endif
/// Represent a placeholder type which could be set while loading as well as
/// loading finished without getting an image.
public protocol Placeholder {
/// How the placeholder should be added to a given image view.
func add(to imageView: ImageView)
/// How the placeholder should be removed from a given image view.
func remove(from imageView: ImageView)
}
/// Default implementation of an image placeholder. The image will be set or
/// reset directly for `image` property of the image view.
extension Placeholder where Self: Image {
/// How the placeholder should be added to a given image view.
public func add(to imageView: ImageView) { imageView.image = self }
/// How the placeholder should be removed from a given image view.
public func remove(from imageView: ImageView) { imageView.image = nil }
}
extension Image: Placeholder {}
/// Default implementation of an arbitrary view as placeholder. The view will be
/// added as a subview when adding and be removed from its super view when removing.
///
/// To use your customize View type as placeholder, simply let it conforming to
/// `Placeholder` by `extension MyView: Placeholder {}`.
extension Placeholder where Self: View {
/// How the placeholder should be added to a given image view.
public func add(to imageView: ImageView) {
imageView.addSubview(self)
self.translatesAutoresizingMaskIntoConstraints = false
NSLayoutConstraint.activate([
NSLayoutConstraint(item: self, attribute: .centerX, relatedBy: .equal, toItem: imageView, attribute: .centerX, multiplier: 1, constant: 0),
NSLayoutConstraint(item: self, attribute: .centerY, relatedBy: .equal, toItem: imageView, attribute: .centerY, multiplier: 1, constant: 0),
NSLayoutConstraint(item: self, attribute: .height, relatedBy: .equal, toItem: imageView, attribute: .height, multiplier: 1, constant: 0),
NSLayoutConstraint(item: self, attribute: .width, relatedBy: .equal, toItem: imageView, attribute: .width, multiplier: 1, constant: 0)
])
}
/// How the placeholder should be removed from a given image view.
public func remove(from imageView: ImageView) {
self.removeFromSuperview()
}
}