如何在 UIAlertController 的中心显示活动指示器？答案

【问题标题】：How to display activity indicator in center of UIAlertController?如何在 UIAlertController 的中心显示活动指示器？
【发布时间】：2015-01-17 22:51:24
【问题描述】：

我目前在屏幕上显示UIAlertController。警报视图应仅在警报中心显示 2 个元素，一个标题和一个 UIActivityIndicatorView。下面是显示警报及其元素的函数。

func displaySignUpPendingAlert() -> UIAlertController {
        //Create the UIAlertController
        let pending = UIAlertController(title: "Creating New User", message: nil, preferredStyle: .Alert)
        //Create the activity indicator to display in it.
        let indicator = UIActivityIndicatorView(frame: CGRectMake(pending.view.frame.width / 2.0, pending.view.frame.height / 2.0, 20.0, 20.0))
        indicator.center = CGPointMake(pending.view.frame.width / 2.0, pending.view.frame.height / 2.0)
        //Add the activity indicator to the alert's view
        pending.view.addSubview(indicator)
        //Start animating
        indicator.startAnimating()

        self.presentViewController(pending, animated: true, completion: nil)
        return pending
    }

但是，活动指示器不会显示在视图的中心，实际上它显示在屏幕的右下方，远离视图。这是什么原因？

编辑：我知道我可以对指示器位置的数字进行硬编码，但我希望警报能够在具有多种屏幕尺寸和方向的多台设备上工作。

【问题讨论】：

相关：How do I put a UIActivityIndicatorView in a UIAlertController?

标签： ios swift uiview uiactivityindicatorview uialertcontroller

【解决方案1】：

我必须实现NSLayoutConstraints 将UIActivityIndicatorView 放在UIAlertController 的中心

对于斯威夫特：

let loadingAlertController: UIAlertController = UIAlertController(title: "Loading", message: nil, preferredStyle: .alert)
let activityIndicator: UIActivityIndicatorView = UIActivityIndicatorView(style: .gray)
activityIndicator.translatesAutoresizingMaskIntoConstraints = false
    
loadingAlertController.view.addSubview(activityIndicator)
    
let xConstraint: NSLayoutConstraint = NSLayoutConstraint(item: activityIndicator, attribute: .centerX, relatedBy: .equal, toItem: loadingAlertController.view, attribute: .centerX, multiplier: 1, constant: 0)
let yConstraint: NSLayoutConstraint = NSLayoutConstraint(item: activityIndicator, attribute: .centerY, relatedBy: .equal, toItem: loadingAlertController.view, attribute: .centerY, multiplier: 1.4, constant: 0)
    
NSLayoutConstraint.activate([ xConstraint, yConstraint])
activityIndicator.isUserInteractionEnabled = false
activityIndicator.startAnimating()
    
let height: NSLayoutConstraint = NSLayoutConstraint(item: loadingAlertController.view, attribute: NSLayoutConstraint.Attribute.height, relatedBy: NSLayoutConstraint.Relation.equal, toItem: nil, attribute: NSLayoutConstraint.Attribute.notAnAttribute, multiplier: 1, constant: 80)
loadingAlertController.view.addConstraint(height)

self.present(loadingAlertController, animated: true, completion: nil)

结果：

【讨论】：

【解决方案2】：

tl;博士

所有其他答案都已关闭:) 请参阅文档：

重要

UIAlertController 类旨在按原样使用，而不是支持子类化。此类的视图层次结构是私有的，并且 不得修改。

问题

问题不在于 UIAlertController。这是一个非常简单的 UI，一个或两个堆栈视图，具体取决于您是否希望 UIActivityIndicatorView 留在标题标签或标题下方。演示动画是我们想要的。

以下代码基于 WWDC 会话 A Look Inside Presentation Controllers。

斯威夫特

重新创建演示控制器：

class LOActivityAlertControllerPresentationController: UIPresentationController {
    
    var dimmerView: UIView!
    
    override init(presentedViewController: UIViewController, presenting presentingViewController: UIViewController?) {
        self.dimmerView = UIView()
        super.init(presentedViewController: presentedViewController, presenting: presentingViewController)
        dimmerView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        dimmerView.backgroundColor = UIColor.init(white: 0, alpha: 0.4)
        
        guard let presentedView = self.presentedView else { return }
        presentedView.layer.cornerRadius = 8.0
        
        let centerXMotionEffect: UIInterpolatingMotionEffect = UIInterpolatingMotionEffect(keyPath: "center.x", type: .tiltAlongHorizontalAxis)
        centerXMotionEffect.minimumRelativeValue = -10.0
        centerXMotionEffect.maximumRelativeValue = 10.0
        
        let centerYMotionEffect: UIInterpolatingMotionEffect = UIInterpolatingMotionEffect(keyPath: "center.y", type: .tiltAlongVerticalAxis)
        centerYMotionEffect.minimumRelativeValue = -10.0
        centerYMotionEffect.maximumRelativeValue = 10.0
        
        let group: UIMotionEffectGroup = UIMotionEffectGroup()
        group.motionEffects = [centerXMotionEffect, centerYMotionEffect]
        
        presentedView.addMotionEffect(group)
    }
    
    override var frameOfPresentedViewInContainerView: CGRect {
        guard let containerView = self.containerView, let presentedView = self.presentedView else { return .zero }
        
        let size = presentedView.systemLayoutSizeFitting(UIView.layoutFittingCompressedSize)
        var frame = CGRect.zero
        
        frame.origin = CGPoint(x: containerView.frame.midX - (size.width / 2.0), y: containerView.frame.midY - (size.height / 2.0))
        
        frame.size = size
        
        return frame
    }
    
    override func presentationTransitionWillBegin() {
        guard let containerView: UIView = self.containerView, let presentedView: UIView = self.presentedView, let dimmerView = self.dimmerView else { return }
        let presentingViewController: UIViewController = self.presentingViewController
        
        dimmerView.alpha = 0.0
        dimmerView.frame = containerView.bounds
        containerView.insertSubview(dimmerView, at: 0)
        
        presentedView.center = containerView.center
        
        guard let transitionCoordinator = presentingViewController.transitionCoordinator else { return }
        
        transitionCoordinator.animate(
            alongsideTransition: { _ in
                dimmerView.alpha = 1.0
            },
            completion: nil
        )
    }
    
    override func containerViewWillLayoutSubviews() {
        super.containerViewWillLayoutSubviews()
        
        guard let containerView: UIView = self.containerView, let presentedView: UIView = self.presentedView, let dimmerView = self.dimmerView else { return }
        
        dimmerView.frame = containerView.bounds
        presentedView.frame = self.frameOfPresentedViewInContainerView
    }
    
    override func dismissalTransitionWillBegin() {
        guard let dimmerView = self.dimmerView, let transitionCoordinator = self.presentingViewController.transitionCoordinator else { return }
        
        transitionCoordinator.animate(
            alongsideTransition: { _ in
                dimmerView.alpha = 0.0
            },
            completion: nil
        )
    }

}

动画过渡：

class LOActivityAlertControllerAnimatedTransitioning: NSObject, UIViewControllerAnimatedTransitioning {
    
    var presentation: Bool
    
    init(presentation: Bool) {
        self.presentation = presentation
    }
    
    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
        let containerView = transitionContext.containerView
        guard let fromView = transitionContext.view(forKey: .from), let toView = transitionContext.view(forKey: .to) else { return }
        if self.presentation {
            containerView.addSubview(toView)
            toView.transform = CGAffineTransform(scaleX: 1.6, y: 1.6)
            toView.alpha = 0.0
            UIView.animate(
                withDuration: 0.2,
                animations: {
                    toView.alpha = 1.0
                    toView.transform = .identity
                },
                completion: { finished in
                    transitionContext.completeTransition(true)
                }
            )
        } else {
            UIView.animate(
                withDuration: 0.2,
                animations: {
                    fromView.alpha = 0.0
                },
                completion: { finished in
                    fromView.removeFromSuperview()
                    transitionContext.completeTransition(true)
                }
            )
        }
    }
    
    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        return 0.2
    }
    
}

样品UIViewController 子类，用 XIB 调味：

class LOActivityAlertController: UIViewController, UIViewControllerTransitioningDelegate {
    
    var activityIndicatorView: UIActivityIndicatorView!
    var titleLabel: UILabel!
    var messageLabel: UILabel!
    
    var alertTitle: String
    var alertMessage: String
    
    init(title: String, message: String) {
        self.alertTitle = title
        self.alertMessage = message
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder: NSCoder) {
        fatalError("Not implemented")
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        self.transitioningDelegate = self
        self.modalPresentationStyle = .custom
        self.titleLabel = UILabel()
        self.messageLabel = UILabel()
        self.titleLabel.text = self.alertTitle
        self.messageLabel.text = self.alertMessage
        
        self.activityIndicatorView = UIActivityIndicatorView(style: .medium)
        
        let currentFrame = self.view.frame
        let alertFrame = CGRect(x: 0, y: 0, width: currentFrame.width / 2.0, height: currentFrame.height / 2.0)
        
        let stackView = UIStackView(frame: alertFrame)
        stackView.backgroundColor = .gray
        stackView.axis = .vertical
        stackView.alignment = .center
        stackView.distribution = .fillProportionally
        stackView.addArrangedSubview(self.titleLabel)
        stackView.addArrangedSubview(self.messageLabel)
        stackView.addArrangedSubview(self.activityIndicatorView)
        
        self.activityIndicatorView.startAnimating()
        
        self.view.addSubview(stackView)
    }
    
    override func viewDidAppear(_ animated: Bool) {
        
    }
    
    func presentationController(forPresented presented: UIViewController, presenting: UIViewController?, source: UIViewController) -> UIPresentationController? {
        let presentationController = LOActivityAlertControllerPresentationController(presentedViewController: presented, presenting: presenting)
        return presentationController
    }
    
    func animationController(forPresented presented: UIViewController, presenting: UIViewController, source: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        let transitioning = LOActivityAlertControllerAnimatedTransitioning(presentation: true)
        return transitioning
    }
    
    func animationController(forDismissed dismissed: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        let transitioning = LOActivityAlertControllerAnimatedTransitioning(presentation: false)
        return transitioning
    }
}

快速版本的致谢：@riciloma

目标-C

重新创建演示控制器：

@interface LOActivityAlertControllerPresentationController : UIPresentationController
@end

@interface LOActivityAlertControllerPresentationController ()
@property (nonatomic) UIView *dimmerView;
@end

@implementation LOActivityAlertControllerPresentationController

- (instancetype)initWithPresentedViewController:(UIViewController *)presentedViewController presentingViewController:(UIViewController *)presentingViewController
{
    self = [super initWithPresentedViewController:presentedViewController presentingViewController:presentingViewController];
    if (self)
    {
        _dimmerView = [[UIView alloc] init];
        _dimmerView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
        _dimmerView.backgroundColor = [UIColor colorWithWhite:0 alpha:0.4];
        
        
        UIView *presentedView = [self presentedView];
        presentedView.layer.cornerRadius = 8.0;
        
        UIInterpolatingMotionEffect *centerXMotionEffect = [[UIInterpolatingMotionEffect alloc] initWithKeyPath:@"center.x" type:UIInterpolatingMotionEffectTypeTiltAlongHorizontalAxis];
        centerXMotionEffect.minimumRelativeValue = @(-10.0);
        centerXMotionEffect.maximumRelativeValue = @(10.0);
        
        UIInterpolatingMotionEffect *centerYMotionEffect = [[UIInterpolatingMotionEffect alloc] initWithKeyPath:@"center.y" type:UIInterpolatingMotionEffectTypeTiltAlongVerticalAxis];
        centerYMotionEffect.minimumRelativeValue = @(-10.0);
        centerYMotionEffect.maximumRelativeValue = @(10.0);
        
        UIMotionEffectGroup *group = [[UIMotionEffectGroup alloc] init];
        group.motionEffects = [NSArray arrayWithObjects:centerXMotionEffect, centerYMotionEffect, nil];
        
        [presentedView addMotionEffect:group];
    }
    return self;
    
}

- (CGRect)frameOfPresentedViewInContainerView
{
    UIView *containerView = [self containerView];
    UIView *presentedView = [self presentedView];
    
    CGSize size = [presentedView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize];
    CGRect frame = CGRectZero;
    frame.origin = CGPointMake(CGRectGetMidX([containerView frame]) - (size.width / 2.0),
                               CGRectGetMidY([containerView frame]) - (size.height / 2.0));
    frame.size = size;
    
    return frame;
}

- (void)presentationTransitionWillBegin
{
    UIViewController *presentingViewController = [self presentingViewController];
    UIView *containerView = [self containerView];
    UIView *presentedView = [self presentedView];
    UIView *dimmerView = [self dimmerView];
    
    dimmerView.alpha = 0.0;
    dimmerView.frame = [containerView bounds];
    [containerView insertSubview:dimmerView atIndex:0];
    
    presentedView.center = [containerView center];
    
    [[presentingViewController transitionCoordinator] animateAlongsideTransition:^(id<UIViewControllerTransitionCoordinatorContext> context) {
        
        dimmerView.alpha = 1.0;
        
    } completion:NULL];
}

- (void)containerViewWillLayoutSubviews
{
    [super containerViewWillLayoutSubviews];
    
    UIView *containerView = [self containerView];
    UIView *presentedView = [self presentedView];
    UIView *dimmerView = [self dimmerView];
    
    dimmerView.frame = [containerView bounds];
    presentedView.frame = [self frameOfPresentedViewInContainerView];
}

- (void)dismissalTransitionWillBegin
{
    UIViewController *presentingViewController = [self presentingViewController];
    UIView *dimmerView = [self dimmerView];
    
    [[presentingViewController transitionCoordinator] animateAlongsideTransition:^(id<UIViewControllerTransitionCoordinatorContext> context) {
        
        dimmerView.alpha = 0.0;
        
    } completion:NULL];
}


@end

动画过渡：

@interface LOActivityAlertControllerAnimatedTransitioning : NSObject <UIViewControllerAnimatedTransitioning>

@property (getter=isPresentation) BOOL presentation;

@end

@implementation LOActivityAlertControllerAnimatedTransitioning

- (void)animateTransition:(nonnull id<UIViewControllerContextTransitioning>)transitionContext
{
    UIView *containerView = [transitionContext containerView];
    UIView *fromView = [transitionContext viewForKey:UITransitionContextFromViewKey];
    UIView *toView = [transitionContext viewForKey:UITransitionContextToViewKey];
    if (_presentation)
    {
        [containerView addSubview:toView];
        toView.transform = CGAffineTransformMakeScale(1.6, 1.6);
        toView.alpha = 0.0;
        [UIView animateWithDuration:0.2 animations:^{
            
            toView.alpha = 1.0;
            toView.transform = CGAffineTransformIdentity;
            
        } completion:^(BOOL finished) {
            
            [transitionContext completeTransition:YES];
            
        }];
    }
    else
    {
        [UIView animateWithDuration:0.2 animations:^{
            
            fromView.alpha = 0.0;
            
        } completion:^(BOOL finished) {
            
            [fromView removeFromSuperview];
            [transitionContext completeTransition:YES];
            
        }];
    }
}

- (NSTimeInterval)transitionDuration:(nullable id<UIViewControllerContextTransitioning>)transitionContext
{
    return 0.2;
}

@end

UIViewController 子类样品，用 XIB 调味：

@interface LOActivityAlertController : UIViewController <UIViewControllerTransitioningDelegate>

@property (nonatomic, strong) IBOutlet UIActivityIndicatorView *activityIndicatorView;
@property (nonatomic, strong) IBOutlet UILabel *titleLabel;

@end

@implementation LOActivityAlertController

@dynamic title;

+ (instancetype)alertControllerWithTitle:(NSString *)title
{
    LOActivityAlertController *alert = [LOActivityAlertController new];
    alert.title = title;
    return alert;
}

- (instancetype)init
{
    self = [super init];
    if (self)
    {
        self.transitioningDelegate = self;
        self.modalPresentationStyle = UIModalPresentationCustom;
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    self.titleLabel.text = self.title;
}

#pragma mark Properties

- (void)setTitle:(NSString *)title
{
    [super setTitle:title];
    
    self.titleLabel.text = title;
}

#pragma mark UIViewControllerTransitioningDelegate

- (UIPresentationController *)presentationControllerForPresentedViewController:(UIViewController *)presented
                                                      presentingViewController:(UIViewController *)presenting
                                                          sourceViewController:(UIViewController *)source
{
    LOActivityAlertControllerPresentationController *myPresentation = nil;
    myPresentation = [[LOActivityAlertControllerPresentationController alloc]
                      initWithPresentedViewController:presented presentingViewController:presenting];
    
    return myPresentation;
}

- (id <UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source;
{
    LOActivityAlertControllerAnimatedTransitioning *transitioning = [LOActivityAlertControllerAnimatedTransitioning new];
    transitioning.presentation = YES;
    return transitioning;
}

- (id <UIViewControllerAnimatedTransitioning>)animationControllerForDismissedController:(UIViewController *)dismissed
{
    LOActivityAlertControllerAnimatedTransitioning *transitioning = [LOActivityAlertControllerAnimatedTransitioning new];
    return transitioning;
}

@end

屏幕录制

错误报告器

rdar://37433306：使 UIAlertController 呈现控制器和转换委托公共 API 以启用重用。

【讨论】：

这是唯一正确的答案。不要破解私人视图层次结构。有趣的是，SO 现在是一个标记为“已接受”的错误答案，然后是 Swift 和 Objective-C 的不同版本中的 500 个“我也是”克隆，但只有 @catlan 说明了这些黑客将如何在未来 UIKit 的版本。
@bpapa 更有趣的是，你用与 OP 要求的不同的编程语言来赞美答案。
我已经用 Swift 翻译了这段代码 here
Swift 代码是错误的，您将看到的只是一个暗淡的视图。 guard 语句应该分开，每个语句都应该在 isPresentation is else 内。 fromView 视图总是 nil isPresentation，反之亦然。
感谢您的回答！似乎这是一个好方法❤️但是当前的示例（Swift）对我不起作用。决定用UIActivityIndicatorView创建一个简单的UIViewController，并使用默认的过渡风格。

【解决方案3】：

Swift 5.0 解决方案

let alert = UIAlertController(title: "Sender ...", message: nil, preferredStyle: .alert)
let activityIndicator = UIActivityIndicatorView(style: .gray)
activityIndicator.translatesAutoresizingMaskIntoConstraints = false
activityIndicator.isUserInteractionEnabled = false
activityIndicator.startAnimating()

alert.view.addSubview(activityIndicator)
alert.view.heightAnchor.constraint(equalToConstant: 95).isActive = true

activityIndicator.centerXAnchor.constraint(equalTo: alert.view.centerXAnchor, constant: 0).isActive = true
activityIndicator.bottomAnchor.constraint(equalTo: alert.view.bottomAnchor, constant: -20).isActive = true

present(alert, animated: true)

【讨论】：

【解决方案4】：

对于 Swift 3 及更高版本如何：

func showActivityIndiactorViewController(title: String) -> UIAlertController {
    let pending = UIAlertController(title: "", message: nil, preferredStyle: .alert)
    let heightConstraint:NSLayoutConstraint = NSLayoutConstraint(item: pending.view, attribute: NSLayoutAttribute.height, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: self.view.frame.height * 0.10)
    pending.view.addConstraint(heightConstraint)

    let label = UILabel()
    label.text = title
    label.textColor = UIColor.black
    label.sizeToFit()

    let space = UIView(frame: CGRect(x: 0, y: 0, width: 8, height: 8))

    let indicator = UIActivityIndicatorView(activityIndicatorStyle: .gray)
    indicator.isUserInteractionEnabled = false
    indicator.startAnimating()

    let width = Int(label.frame.size.width + indicator.frame.size.width + space.frame.size.width)

    let view = UIStackView(arrangedSubviews: [indicator, space, label])
    view.axis = .horizontal
    view.frame = CGRect(x: 20, y: 0, width: width, height: Int(heightConstraint.constant))
    pending.view.addSubview(view)

    let widthConstraint:NSLayoutConstraint = NSLayoutConstraint(item: pending.view, attribute: NSLayoutAttribute.width, relatedBy: NSLayoutRelation.greaterThanOrEqual, toItem: view, attribute: NSLayoutAttribute.width, multiplier: 1, constant: CGFloat(width))
    pending.view.addConstraint(widthConstraint)

    self.present(pending, animated: true, completion: nil)

    return pending
}

【讨论】：

【解决方案5】：

将@petesalt 的答案转换为 Swift 3：

let pending = UIAlertController(title: "Saving, please wait...", message: nil, preferredStyle: .alert)

let indicator = UIActivityIndicatorView()
indicator.translatesAutoresizingMaskIntoConstraints = false
pending.view.addSubview(indicator)

let views = ["pending" : pending.view, "indicator" : indicator]

var constraints = NSLayoutConstraint.constraints(withVisualFormat: "V:[indicator]-(-50)-|", options: NSLayoutFormatOptions.alignAllCenterY, metrics: nil, views: views)
constraints += NSLayoutConstraint.constraints(withVisualFormat: "H:|[indicator]|", options: NSLayoutFormatOptions.alignAllCenterX, metrics: nil, views: views)
pending.view.addConstraints(constraints)

indicator.isUserInteractionEnabled = false
indicator.startAnimating()

self.present(pending, animated: true, completion: nil)

【讨论】：

【解决方案6】：

请务必在创建视图时设置 frame 属性。

func displaySignUpPendingAlert() -> UIAlertController {
        //create an alert controller
        let pending = UIAlertController(title: "Creating New User", message: nil, preferredStyle: .Alert)

        //create an activity indicator
        let indicator = UIActivityIndicatorView(frame: pending.view.bounds)
        indicator.autoresizingMask = [.flexibleWidth, .flexibleHeight]

        //add the activity indicator as a subview of the alert controller's view
        pending.view.addSubview(indicator)
        indicator.isUserInteractionEnabled = false // required otherwise if there buttons in the UIAlertController you will not be able to press them
        indicator.startAnimating()

        self.presentViewController(pending, animated: true, completion: nil)

        return pending
}

致@62Shark：

let pending = UIAlertController(title: "Creating New User", message: nil, preferredStyle: .Alert)

let indicator = UIActivityIndicatorView()
indicator.setTranslatesAutoresizingMaskIntoConstraints(false)
pending.view.addSubview(indicator)

let views = ["pending" : pending.view, "indicator" : indicator]
var constraints = NSLayoutConstraint.constraintsWithVisualFormat("V:[indicator]-(-50)-|", options: nil, metrics: nil, views: views)
constraints += NSLayoutConstraint.constraintsWithVisualFormat("H:|[indicator]|", options: nil, metrics: nil, views: views)
pending.view.addConstraints(constraints)

indicator.userInteractionEnabled = false
indicator.startAnimating()

self.presentViewController(pending, animated: true, completion: nil)

【讨论】：

感谢您的帮助。但是我遇到了麻烦，您的激活器位于中心，并带有这样的警告消息标题 -> puu.sh/ilFpD/d8e1f98d8f.png。请问有什么方法可以将激活器放在警报标题的下方位置吗？谢谢。
嗨@62Shark，我更新了我的答案，希望这会对你有所帮助。
Swipesight，它完全符合我现在想要的样子，先生。谢谢您的回答。
如果您在警报控制器的消息末尾添加几个 \n 换行符，它将扩大它的高度，以使微调器不再遮挡文本，而是显示在标题下方并且应有的消息。
这是一个糟糕的答案，根本无法证明未来。按照@catlan 的建议去做：stackoverflow.com/a/48730050/543

【解决方案7】：

我遇到了同样的问题，使用框架定位对我不起作用。 Yimin Lin 的回答对我来说非常接近，但我只是想提出一种使用非可视格式约束的替代方案：

//...
indicator.setTranslatesAutoresizingMaskIntoConstraints(false)
alert.view.addSubview(indicator)

alert.view.addConstraint(NSLayoutConstraint(item: indicator, attribute: NSLayoutAttribute.CenterX, relatedBy: NSLayoutRelation.Equal, toItem: alert.view, attribute: attribute: NSLayoutAttribute.CenterX, multiplier: 1, constant: 0))
alert.view.addConstraint(NSLayoutConstraint(item: indicator, attribute: NSLayoutAttribute.CenterY, relatedBy: NSLayoutRelation.Equal, toItem: alert.view, attribute: attribute: NSLayoutAttribute.CenterY, multiplier: 1, constant: 0))
//...

【讨论】：

【解决方案8】：

Apple 不鼓励直接将 UIAlertController 子类化，因此我创建了一个显示 UIAlertController 的类，其中 UIActivityIndicator 居中，并使用类协议处理取消条件。

import Foundation
import UIKit

protocol BusyAlertDelegate {
    func didCancelBusyAlert()
}


class BusyAlert {

   var busyAlertController: UIAlertController?
   var presentingViewController: UIViewController?
   var activityIndicator: UIActivityIndicatorView?
   var delegate:BusyAlertDelegate?

   init (title:String, message:String, presentingViewController: UIViewController) {
       busyAlertController = UIAlertController(title: title, message: message, preferredStyle: UIAlertControllerStyle.Alert)
       busyAlertController!.addAction(UIAlertAction(title: NSLocalizedString("Cancel", comment: "Cancel Button"), style: UIAlertActionStyle.Cancel, handler:{(alert: UIAlertAction!) in
            delegate?.didCancelBusyAlert()
    }))
        self.presentingViewController = presentingViewController
        activityIndicator = UIActivityIndicatorView(activityIndicatorStyle: UIActivityIndicatorViewStyle.Gray)
        busyAlertController!.view.addSubview(activityIndicator!)
    }

    func display() {
        dispatch_async(dispatch_get_main_queue(), {
               self.presentingViewController!.presentViewController(self.busyAlertController!, animated: true, completion: {
            self.activityIndicator!.translatesAutoresizingMaskIntoConstraints = false
               self.busyAlertController!.view.addConstraint(NSLayoutConstraint(item: self.activityIndicator!, attribute: NSLayoutAttribute.CenterX, relatedBy: NSLayoutRelation.Equal, toItem: self.busyAlertController!.view, attribute: NSLayoutAttribute.CenterX, multiplier: 1, constant: 0))
            self.busyAlertController!.view.addConstraint(NSLayoutConstraint(item: self.activityIndicator!, attribute: NSLayoutAttribute.CenterY, relatedBy: NSLayoutRelation.Equal, toItem: self.busyAlertController!.view, attribute: NSLayoutAttribute.CenterY, multiplier: 1, constant: 0))
            self.activityIndicator!.startAnimating()

        })
    })

}

func dismiss() {
    dispatch_async(dispatch_get_main_queue(), {
        self.busyAlertController?.dismissViewControllerAnimated(true, completion: nil)
    })
}

}

我建议使用惰性变量来初始化类。

lazy var busyAlertController: BusyAlert = {
        let busyAlert = BusyAlert(title: "Lengthy Task", message: "Please     wait...", presentingViewController: self)
        busyAlert.delegate = self
        return busyAlert
        }()

这里是示例代码的链接：https://github.com/cgilleeny/BusyAlertExample.git

【讨论】：

【解决方案9】：

迅速：

activityIndicator.center = self.view.center

如果您有工具栏或导航控制器，您可能想要移动点，否则，中心就是中心...

如果您仍有问题，也许this 教程会有所帮助。如果您尝试将其置于表格视图控制器中，this 答案可能会有所帮助。

【讨论】：

【解决方案10】：

如果有人感兴趣，我将答案转换为 Objective C：

UIAlertController *pending = [UIAlertController alertControllerWithTitle:nil
                                                               message:@"Please wait...\n\n"
                                                        preferredStyle:UIAlertControllerStyleAlert];
UIActivityIndicatorView* indicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
indicator.color = [UIColor blackColor];
indicator.translatesAutoresizingMaskIntoConstraints=NO;
[pending.view addSubview:indicator];
NSDictionary * views = @{@"pending" : pending.view, @"indicator" : indicator};

NSArray * constraintsVertical = [NSLayoutConstraint constraintsWithVisualFormat:@"V:[indicator]-(20)-|" options:0 metrics:nil views:views];
NSArray * constraintsHorizontal = [NSLayoutConstraint constraintsWithVisualFormat:@"H:|[indicator]|" options:0 metrics:nil views:views];
NSArray * constraints = [constraintsVertical arrayByAddingObjectsFromArray:constraintsHorizontal];
[pending.view addConstraints:constraints];
[indicator setUserInteractionEnabled:NO];
[indicator startAnimating];
[self presentViewController:pending animated:YES completion:nil];

干杯

【讨论】：

谢谢，我喜欢 Swift，但我仍然在Objective-C 中编写大部分应用程序，这很有帮助。
你能解释一下限制吗？
工作完美。此代码在数据加载完成后关闭。 [self dismissViewControllerAnimated:NO completion:nil];

【解决方案11】：

对于像我这样喜欢将UIActivityIndicatorView 对齐在UIAlertController.title 左侧的人，这是我在Swift 中适用于所有设备的解决方案：

let alert = UIAlertController(title: NSLocalizedString("Authenticating...", comment: "Authenticating"), message: nil, preferredStyle: .Alert);
let activityIndicator = UIActivityIndicatorView(activityIndicatorStyle: UIActivityIndicatorViewStyle.Gray)
activityIndicator.frame = activityIndicator.frame.rectByOffsetting(dx: 8, dy: (alert.view.bounds.height - activityIndicator.frame.height)/2);
activityIndicator.autoresizingMask = .FlexibleRightMargin | .FlexibleTopMargin | .FlexibleBottomMargin
activityIndicator.color = themeManager().currentTheme.navigationBarTintColor;
activityIndicator.startAnimating();
alert.view.addSubview(activityIndicator);
self.presentViewController(progressAlert, animated: true, completion: nil);

但是，要在视图中心对齐UIActivityIndicatorView，您可以进行如下更改：

activityIndicator.center = CGPoint(x: (alert.view.bounds.width)/2, y: (alert.view.bounds.height)/2)
activityIndicator.autoresizingMask = .FlexibleLeftMargin | .FlexibleRightMargin | .FlexibleTopMargin | .FlexibleBottomMargin

【讨论】：

【解决方案12】：

试试这个：

activityView.center = CGPointMake(self.view.bounds.size.width/2.0, self.view.bounds.size.height / 2.0)

您还需要检查横向模式以及反向宽度和高度。

if(landscapeMode)activityView.center = CGPointMake(self.view.bounds.size.height/2.0, self.view.bounds.size.width / 2.0)

也许你可以获得警报视图位置？

alert.view.frame.origin.x
alert.view.frame.origin.y

并使用它来动态放置您的活动视图，即使用变量？

当然，您可能还希望将大小除以 2 并将其相加，使其也居中。

alert.view.frame.size.height
alert.view.frame.size.width

【讨论】：

警报显示在中心，但活动指示器靠近屏幕底部，几乎在角落。
哦，对不起，我的错，我以为你的意思是警报视图不在中心。只需将活动视图居中到主屏幕，它就应该在它的顶部。

【解决方案13】：

试试这段代码。

UIAlertController *alert = [UIAlertController alertControllerWithTitle:nil
                                    message:@"Creating new user\n\n\n"
                             preferredStyle:UIAlertControllerStyleAlert];

UIActivityIndicatorView *loader = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
loader.center = CGPointMake(130.5, 65.5);
loader.color = [UIColor blackColor];
[loader startAnimating];
[alert.view loader];
[self presentViewController:alert animated:NO completion:nil];

【讨论】：

我不想这样做的原因是它使用了硬编码的数字，我希望它适用于多种屏幕尺寸
你可以添加这个 [loader setCenter:view.center];
这会编译吗？这行 [alert.view loader] 在做什么？