我在XIB文件中有一个具有动态高度的自定义单元格,该文件显示带有消息和图像的帖子.因为图像将从我的API异步加载,所以图像的宽高比将随资源一起发送.在updateConstraints方法中,将设置具有适当乘数的宽高比约束.

XIB文件：

细胞类：

//
//  PostTableViewCell.swift
//  Lome
//
//  Created by Tobias Feistmantl on 10/09/15.
//  Copyright (c) 2015 Tobias Feistmantl. All rights reserved.
//

import UIKit
import Alamofire

class PostTableViewCell: UITableViewCell {

    @IBOutlet weak var userProfileImageView: TFImageView!
    @IBOutlet weak var usersNameLabel: UILabel!
    @IBOutlet weak var usernameLabel: UILabel!
    @IBOutlet weak var userProfileButton: TFCellButton!
    @IBOutlet weak var distanceLabel: UILabel!
    @IBOutlet weak var messageLabel: UILabel!
    @IBOutlet weak var postImageView: UIImageView!
    @IBOutlet weak var likeCountLabel: UILabel!
    @IBOutlet weak var timestampLabel: UILabel!
    @IBOutlet weak var likeButton: UIButton!

    @IBOutlet weak var constraintBetweenMessageLabelAndPostImageView: NSLayoutConstraint!

    var post: Post! {
        didSet {
            post.author.profileImage(version: .Thumbnail) { image, _ in
                self.userProfileImageView.image = image
            }

            if let name = post.author.fullName {
                usersNameLabel.text = name
                usernameLabel.text = post.author.username
            } else {
                usersNameLabel.text = post.author.username
                usernameLabel.hidden = true
            }

            if let attributedMessage = post.attributedMessage {
                messageLabel.attributedText = attributedMessage
                messageLabel.hidden = false
                constraintBetweenMessageLabelAndPostImageView.constant = 15
            } else {
                messageLabel.hidden = true
                messageLabel.attributedText = nil
                constraintBetweenMessageLabelAndPostImageView.constant = 0
            }

            post.image { image, _ in
                if let image = image {
                    self.postImageView.image = image
                    self.setNeedsUpdateConstraints()
                }
            }

            timestampLabel.text = "Posted \(post.createdAt.timeAgoSinceNow())"
            distanceLabel.text = post.distanceText
            likeCountLabel.text = "\(post.likesCount) Likes"
            likeButton.setImage(post.likeButtonImage, forState: .Normal)
        }
    }

    override func updateConstraints() {
        if let aspectRatio = post.imageAspectRatio {
            postImageAspectConstraint = NSLayoutConstraint(item: postImageView, attribute: NSLayoutAttribute.Width, relatedBy: NSLayoutRelation.Equal, toItem: postImageView, attribute: NSLayoutAttribute.Height, multiplier: CGFloat(aspectRatio), constant: 0)
        }

        super.updateConstraints()
    }

    func setupUserProfileButton(indexPath: NSIndexPath, viewController: UIViewController) {
        userProfileButton.indexPath = indexPath
        userProfileButton.addTarget(viewController, action: "userProfileButtonDidTouch:", forControlEvents: .TouchUpInside)
    }

    var postImageAspectConstraint: NSLayoutConstraint? {
        didSet {
            if let oldValue = oldValue {
                postImageView.removeConstraint(oldValue)
            }
            if let postImageAspectConstraint = postImageAspectConstraint {
                postImageView.addConstraint(postImageAspectConstraint)
            }
        }
    }

    override func prepareForReuse() {
        super.prepareForReuse()

        postImageAspectConstraint = nil
    }

}

cellForRowAtIndexPath方法：

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCellWithIdentifier("postCell", forIndexPath: indexPath) as! PostTableViewCell

    cell.post = posts[indexPath.row]
    cell.setupUserProfileButton(indexPath, viewController: self)

    return cell
}

在应用程序启动时,一切都按预期工作,但如果我稍微晃动,约束开始破坏,图像的方面不再正确或其他一些奇怪的事情如下：

错误：

2015-09-28 11:58:27.546 Lome[32812:528936] Unable to simultaneously satisfy constraints.
    Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
    "<NSLayoutConstraint:0x7f8a008bf6f0 V:[Lome.TFImageView:0x7f8a008bf470(35)]>",
    "<NSLayoutConstraint:0x7f89f9fbc490 V:|-(15)-[Lome.TFCellButton:0x7f8a0087fde0]   (Names: '|':UITableViewCellContentView:0x7f8a008bf270 )>",
    "<NSLayoutConstraint:0x7f8a00831520 Lome.TFCellButton:0x7f8a0087fde0.top == Lome.TFImageView:0x7f8a008bf470.top>",
    "<NSLayoutConstraint:0x7f8a008315c0 Lome.TFCellButton:0x7f8a0087fde0.bottom == Lome.TFImageView:0x7f8a008bf470.bottom>",
    "<NSLayoutConstraint:0x7f8a00831750 V:[Lome.TFCellButton:0x7f8a0087fde0]-(18)-[UILabel:0x7f8a008806d0'Water']>",
    "<NSLayoutConstraint:0x7f8a00831840 H:|-(0)-[UIImageView:0x7f8a00830910]   (Names: '|':UITableViewCellContentView:0x7f8a008bf270 )>",
    "<NSLayoutConstraint:0x7f8a00831890 H:[UIImageView:0x7f8a00830910]-(0)-|   (Names: '|':UITableViewCellContentView:0x7f8a008bf270 )>",
    "<NSLayoutConstraint:0x7f8a008318e0 V:[UILabel:0x7f8a008806d0'Water']-(15)-[UIImageView:0x7f8a00830910]>",
    "<NSLayoutConstraint:0x7f8a00831930 V:[UIImageView:0x7f8a00830910]-(20)-[UILabel:0x7f8a00830b90'0 Likes']>",
    "<NSLayoutConstraint:0x7f8a00831980 V:[UILabel:0x7f8a00830b90'0 Likes']-(20)-|   (Names: '|':UITableViewCellContentView:0x7f8a008bf270 )>",
    "<NSLayoutConstraint:0x7f8a00860870 UIImageView:0x7f8a00830910.width == 0.6672*UIImageView:0x7f8a00830910.height>",
    "<NSLayoutConstraint:0x7f8a0084e990 'UIView-Encapsulated-Layout-Height' V:[UITableViewCellContentView:0x7f8a008bf270(611)]>",
    "<NSLayoutConstraint:0x7f8a0084f930 'UIView-Encapsulated-Layout-Width' H:[UITableViewCellContentView:0x7f8a008bf270(375)]>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x7f8a008bf6f0 V:[Lome.TFImageView:0x7f8a008bf470(35)]>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.
2015-09-28 11:58:27.547 Lome[32812:528936] Unable to simultaneously satisfy constraints.
    Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
    "<NSLayoutConstraint:0x7f89f9fbc490 V:|-(15)-[Lome.TFCellButton:0x7f8a0087fde0]   (Names: '|':UITableViewCellContentView:0x7f8a008bf270 )>",
    "<NSLayoutConstraint:0x7f8a00831520 Lome.TFCellButton:0x7f8a0087fde0.top == Lome.TFImageView:0x7f8a008bf470.top>",
    "<NSLayoutConstraint:0x7f8a008315c0 Lome.TFCellButton:0x7f8a0087fde0.bottom == Lome.TFImageView:0x7f8a008bf470.bottom>",
    "<NSLayoutConstraint:0x7f8a00831750 V:[Lome.TFCellButton:0x7f8a0087fde0]-(18)-[UILabel:0x7f8a008806d0'Water']>",
    "<NSLayoutConstraint:0x7f8a00831840 H:|-(0)-[UIImageView:0x7f8a00830910]   (Names: '|':UITableViewCellContentView:0x7f8a008bf270 )>",
    "<NSLayoutConstraint:0x7f8a00831890 H:[UIImageView:0x7f8a00830910]-(0)-|   (Names: '|':UITableViewCellContentView:0x7f8a008bf270 )>",
    "<NSLayoutConstraint:0x7f8a008318e0 V:[UILabel:0x7f8a008806d0'Water']-(15)-[UIImageView:0x7f8a00830910]>",
    "<NSLayoutConstraint:0x7f8a00831930 V:[UIImageView:0x7f8a00830910]-(20)-[UILabel:0x7f8a00830b90'0 Likes']>",
    "<NSLayoutConstraint:0x7f8a00831980 V:[UILabel:0x7f8a00830b90'0 Likes']-(20)-|   (Names: '|':UITableViewCellContentView:0x7f8a008bf270 )>",
    "<NSLayoutConstraint:0x7f8a00860870 UIImageView:0x7f8a00830910.width == 0.6672*UIImageView:0x7f8a00830910.height>",
    "<NSLayoutConstraint:0x7f8a0084e990 'UIView-Encapsulated-Layout-Height' V:[UITableViewCellContentView:0x7f8a008bf270(611)]>",
    "<NSLayoutConstraint:0x7f8a0084f930 'UIView-Encapsulated-Layout-Width' H:[UITableViewCellContentView:0x7f8a008bf270(375)]>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x7f8a00860870 UIImageView:0x7f8a00830910.width == 0.6672*UIImageView:0x7f8a00830910.height>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.

我认为它与重用单元格有关,但正如您所看到的那样,约束将在prepareForReuse方法中被删除.

我目前非常沮丧,因为我试图解决这个问题几天,但没有找到解决方案.

我希望有人有解决方案.

提前致谢.

最佳答案 此问题是由应用于ImageView的2个冲突约束引起的.如果你看一下调试器输出

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x7f8a00860870 UIImageView:0x7f8a00830910.width == 0.6672*UIImageView:0x7f8a00830910.height>

您已经将ImageView的水平约束固定到超级视图,间距为0.

现在,在更新宽高比约束时,您尝试将ImageView的宽度(通过保持宽度作为第一项)设置为其高度的比率,这将导致与其的前导/尾随约束冲突.
您需要通过交换第一个和第二个项目并反转宽高比因子来设置此ImageView的高度.

postImageAspectConstraint = NSLayoutConstraint(item: postImageView, attribute: NSLayoutAttribute.Height, relatedBy: NSLayoutRelation.Equal, toItem: postImageView, attribute: NSLayoutAttribute.Width, multiplier: 1/CGFloat(aspectRatio), constant: 0)