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Pods/Kingfisher/Sources/Image/Placeholder.swift

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+//
+//  Placeholder.swift
+//  Kingfisher
+//
+//  Created by Tieme van Veen on 28/08/2017.
+//
+//  Copyright (c) 2019 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(watchOS)
+
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
+import AppKit
+#endif
+
+#if canImport(UIKit)
+import UIKit
+#endif
+
+/// Represents 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: KFCrossPlatformImageView)
+    
+    /// How the placeholder should be removed from a given image view.
+    func remove(from imageView: KFCrossPlatformImageView)
+}
+
+/// Default implementation of an image placeholder. The image will be set or
+/// reset directly for `image` property of the image view.
+extension KFCrossPlatformImage: Placeholder {
+    /// How the placeholder should be added to a given image view.
+    public func add(to imageView: KFCrossPlatformImageView) { imageView.image = self }
+
+    /// How the placeholder should be removed from a given image view.
+    public func remove(from imageView: KFCrossPlatformImageView) { imageView.image = nil }
+}
+
+/// 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: KFCrossPlatformView {
+    
+    /// How the placeholder should be added to a given image view.
+    public func add(to imageView: KFCrossPlatformImageView) {
+        imageView.addSubview(self)
+        translatesAutoresizingMaskIntoConstraints = false
+
+        centerXAnchor.constraint(equalTo: imageView.centerXAnchor).isActive = true
+        centerYAnchor.constraint(equalTo: imageView.centerYAnchor).isActive = true
+        heightAnchor.constraint(equalTo: imageView.heightAnchor).isActive = true
+        widthAnchor.constraint(equalTo: imageView.widthAnchor).isActive = true
+    }
+
+    /// How the placeholder should be removed from a given image view.
+    public func remove(from imageView: KFCrossPlatformImageView) {
+        removeFromSuperview()
+    }
+}
+
+#endif