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为什么以下代码不适用于将活动指示器定位到其超级视图的中心：
UIActivityIndicatorView *activityIndicator = [[UIActivityIndicatorView alloc]
initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
[self.mysuperview addSubview:activityIndicator];
[activityIndicator addConstraints:[NSLayoutConstraint
constraintsWithVisualFormat:@"|-(>=20)-[view(==100)]-(>=20)-|"
options:NSLayoutFormatAlignAllCenterX | NSLayoutFormatAlignAllCenterY
metrics:nil
views:@{@"view" : self.mysuperview}]];
活动指示器位于左上角的某个地方，绝对不在中心。
==================更新：找到的解决方案：我必须在创建指标后关闭自动调整大小约束，然后关闭所有给定的解决方案：
[activityIndicator setTranslatesAutoresizingMaskIntoConstraints:NO];
我在@Vignesh给出的链接上找到了它，所以我接受他/她的回答。
以下四个 Swift 5/iOS 12 代码示例展示了如何使用自动布局将UIActivityIndicatorView居中置于UIViewController UIView内。
所有样品产生相同的结果，但根据您的需求和口味，您可以选择其中一种。
如果你UIActivityIndicatorView的超级视图不是self.view，你只需要用你自己的（解开包装的）superview替换每个self.view调用。
1. NSLayoutConstraint初始值设定项样式
import UIKit
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
let indicatorView = UIActivityIndicatorView(style: .gray)
indicatorView.isHidden = false
indicatorView.translatesAutoresizingMaskIntoConstraints = false
self.view.addSubview(indicatorView)
// Auto layout
let horizontalConstraint = NSLayoutConstraint(item: indicatorView,
attribute: .centerX,
relatedBy: .equal,
toItem: self.view,
attribute: .centerX,
multiplier: 1,
constant: 0)
let verticalConstraint = NSLayoutConstraint(item: indicatorView,
attribute: .centerY,
relatedBy: .equal,
toItem: self.view,
attribute: .centerY,
multiplier: 1,
constant: 0)
NSLayoutConstraint.activate([horizontalConstraint, verticalConstraint])
/*
// You can replace NSLayoutConstraint activate(_:) call with the following lines:
self.view.addConstraint(horizontalConstraint)
self.view.addConstraint(verticalConstraint)
*/
}
}
2. UIViewAutoresizing风格
弹簧和支柱将在运行时转换为相应的自动布局约束。
import UIKit
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
let indicatorView = UIActivityIndicatorView(style: .gray)
indicatorView.isHidden = false
indicatorView.translatesAutoresizingMaskIntoConstraints = true // default is true
self.view.addSubview(indicatorView)
// Springs and struts
indicatorView.center = CGPoint(x: self.view.bounds.midX, y: self.view.bounds.midY)
indicatorView.autoresizingMask = [
.flexibleLeftMargin,
.flexibleRightMargin,
.flexibleTopMargin,
.flexibleBottomMargin
]
}
}
3. 视觉格式语言风格
import UIKit
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
let indicatorView = UIActivityIndicatorView(style: .gray)
indicatorView.isHidden = false
indicatorView.translatesAutoresizingMaskIntoConstraints = false
self.view.addSubview(indicatorView)
// Auto layout
let views = ["superview": self.view!, "indicatorView": indicatorView]
let horizontalConstraints = NSLayoutConstraint
.constraints(withVisualFormat: "H:[superview]-(<=0)-[indicatorView]",
options: .alignAllCenterY,
metrics: nil,
views: views)
let verticalConstraints = NSLayoutConstraint
.constraints(withVisualFormat: "V:[superview]-(<=0)-[indicatorView]",
options: .alignAllCenterX,
metrics: nil,
views: views)
self.view.addConstraints(horizontalConstraints)
self.view.addConstraints(verticalConstraints)
}
}
4. NSLayoutAnchor风格（需要 iOS 9）
import UIKit
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
let indicatorView = UIActivityIndicatorView(style: .gray)
indicatorView.isHidden = false
indicatorView.translatesAutoresizingMaskIntoConstraints = false
self.view.addSubview(indicatorView)
// Auto layout
let horizontalConstraint = indicatorView
.centerXAnchor.constraint(equalTo: self.view.centerXAnchor)
let verticalConstraint = indicatorView
.centerYAnchor.constraint(equalTo: self.view.centerYAnchor)
NSLayoutConstraint.activate([horizontalConstraint, verticalConstraint])
/*
// You can replace NSLayoutConstraint activate(_:) call with the following lines:
self.view.addConstraint(horizontalConstraint)
self.view.addConstraint(verticalConstraint)
*/
}
}
它通过
处于角落来满足它的要求，因为你并不是说每边的间隙必须相同。 试试这个：
[self.superview addConstraint:[NSLayoutConstraint constraintWithItem:activityIndicator
attribute:NSLayoutAttributeCenterX
relatedBy:NSLayoutRelationEqual
toItem:self.superview
attribute:NSLayoutAttributeCenterX
multiplier:1.0
constant:0.0]];
要垂直居中，也要这样做：
[self.superview addConstraint:[NSLayoutConstraint constraintWithItem:activityIndicator
attribute:NSLayoutAttributeCenterY
relatedBy:NSLayoutRelationEqual
toItem:self.superview
attribute:NSLayoutAttributeCenterY
multiplier:1.0
constant:0.0]];
或者，我强烈建议使用 FLKAutoLayout 项目来简化所有这些：
https://github.com/dkduck/FLKAutoLayout
然后你可以做：
[activityIndicator alignCenterXWithView:self.superview predicate:nil];
[activityIndicator alignCenterYWithView:self.superview predicate:nil];
这是一个很好的:)
你可以
试试这个，
UIView *superview = self.mysuperview;
NSDictionary *variables = NSDictionaryOfVariableBindings(activityIndicator, superview);
NSArray *constraints =
[NSLayoutConstraint constraintsWithVisualFormat:@"V:[superview]-(<=1)-[activityIndicator]"
options: NSLayoutFormatAlignAllCenterX
metrics:nil
views:variables];
[self.view addConstraints:constraints];
constraints =
[NSLayoutConstraint constraintsWithVisualFormat:@"H:[superview]-(<=1)-[activityIndicator]"
options: NSLayoutFormatAlignAllCenterY
metrics:nil
views:variables];
[self.view addConstraints:constraints];
取自这里。
我通过将
活动指标添加到界面生成器中，alpha 为零（在我的视图控制器类中为它的 IBOutlet）将活动指标居中。 然后，我添加了约束以使其在 X 轴和 Y 轴上居中。 最后，我为其添加了宽度和高度约束，以消除自动布局错误。
在我的视图控制器的 startActivityIndicator 方法中，我将活动指示器的 alpha 设置为 1，并对其调用 startAnimating 方法。 在我的 stopActivityIndicator 方法中，我对其调用 stopAnimating 方法，并将活动指标的 alpha 设置回零。