以编程方式约束目标C

Question

我不知道我在做什么错：我正在创建一个UIView，它占据了所有屏幕（已经有约束），然后以编程方式创建了一个UI Image View：

_panel = [[UIImageView alloc] initWithImage:[self loadImageForKey:@"registerPanel"]];
_panel.frame = CGRectMake(0, 0, 100, 100);
_panel.exclusiveTouch = YES;
_panel.userInteractionEnabled = YES,
[self.scrollView addSubview:_panel];

问题来了：我在创建的面板上添加了约束，但崩溃了（我在ViewWillAppear上这样做）：

NSLayoutConstraint *centreHorizontallyConstraint = [NSLayoutConstraint
                                                    constraintWithItem:_panel
                                                    attribute:NSLayoutAttributeCenterX
                                                    relatedBy:NSLayoutRelationEqual
                                                    toItem:self.view
                                                    attribute:NSLayoutAttributeCenterX
                                                    multiplier:1.0
                                                    constant:0];

NSLayoutConstraint *centreVerticalConstraint = [NSLayoutConstraint
                                                constraintWithItem:_panel
                                                attribute:NSLayoutAttributeCenterX
                                                relatedBy:NSLayoutRelationEqual
                                                toItem:self.view
                                                attribute:NSLayoutAttributeCenterX
                                                multiplier:1.0
                                                constant:0];


[_panel addConstraint:centreHorizontallyConstraint];
[_panel addConstraint:centreVerticalConstraint];

错误消息：添加到视图时，约束的项必须是该视图的后代（或视图本身）。 如果在组装视图层次结构之前需要解决约束，则会崩溃。 中断-[UIView _viewHierarchyUnpreparedForConstraint：]进行调试。

Answer 1

首先，您不能在panel和self.view之间创建约束，因为没有公共父对象，而是要使用scrollview创建它们

NSLayoutConstraint *centreHorizontallyConstraint = [NSLayoutConstraint
                                                    constraintWithItem:_panel
                                                    attribute:NSLayoutAttributeCenterX
                                                    relatedBy:NSLayoutRelationEqual
                                                    toItem:self.scrollView 
                                                    attribute:NSLayoutAttributeCenterX
                                                    multiplier:1.0
                                                    constant:0];

NSLayoutConstraint *centreVerticalConstraint = [NSLayoutConstraint
                                                constraintWithItem:_panel
                                                attribute:NSLayoutAttributeCenterY
                                                relatedBy:NSLayoutRelationEqual
                                                toItem:self.scrollView 
                                                attribute:NSLayoutAttributeCenterY
                                                multiplier:1.0
                                                constant:0];


[_scrollView  addConstraint:centreHorizontallyConstraint];
[_scrollView  addConstraint:centreVerticalConstraint];

另外两个约束的centerX，你也需要宽度和高度，或更好的顶部，领先，落后和底部滚动视图,,,宽度和高度静态或比例self.view

//

另外，对于要以编程方式添加约束的任何视图，都必须设置

[self.scrollView setTranslatesAutoresizingMaskIntoConstraints: NO];
[self.panel setTranslatesAutoresizingMaskIntoConstraints: NO];

Answer 2

您可以将scrollView的子视图约束到scrollView的父视图（在这种情况下为self.view），但这可能不是您想要的。

编辑：为澄清起见，收到错误的原因是因为您初始化了约束：

toItem:self.view

然后尝试添加它们：

[_panel addConstraint:centreHorizontallyConstraint];
[_panel addConstraint:centreVerticalConstraint];

您要将它们添加到toItem对象：

[self.view addConstraint:centreHorizontallyConstraint];
[self.view addConstraint:centreVerticalConstraint];

同样，您可能不想在主视图_panel ，但这将编译并运行：

#import "AddPanelScrollViewController.h"   /// just default .h

@interface AddPanelScrollViewController ()

@property (strong, nonatomic) UIScrollView *scrollView;
@property (strong, nonatomic) UIImageView *panel;

@end

@implementation AddPanelScrollViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    _scrollView = [UIScrollView new];
    _scrollView.translatesAutoresizingMaskIntoConstraints = NO;

    [self.view addSubview:_scrollView];

    [_scrollView.topAnchor constraintEqualToAnchor:self.view.topAnchor constant:20.0].active = YES;
    [_scrollView.bottomAnchor constraintEqualToAnchor:self.view.bottomAnchor constant:-20.0].active = YES;
    [_scrollView.leadingAnchor constraintEqualToAnchor:self.view.leadingAnchor constant:20.0].active = YES;
    [_scrollView.trailingAnchor constraintEqualToAnchor:self.view.trailingAnchor constant:-20.0].active = YES;

    _scrollView.backgroundColor = [UIColor blueColor];

    _panel = [UIImageView new];

    // required
    _panel.translatesAutoresizingMaskIntoConstraints = NO;

    [self.scrollView addSubview:_panel];

    // frame will be ignored when using auto-layout / constraints
//  _panel.frame = CGRectMake(0, 0, 100, 100);

    _panel.exclusiveTouch = YES;
    _panel.userInteractionEnabled = YES;
    _panel.backgroundColor = [UIColor redColor];

    // _panel needs width and height constraints
    [_panel.widthAnchor constraintEqualToConstant:100.0].active = YES;
    [_panel.heightAnchor constraintEqualToConstant:100.0].active = YES;

    NSLayoutConstraint *centreHorizontallyConstraint = [NSLayoutConstraint
                                                        constraintWithItem:_panel
                                                        attribute:NSLayoutAttributeCenterX
                                                        relatedBy:NSLayoutRelationEqual
                                                        toItem:self.view
                                                        attribute:NSLayoutAttributeCenterX
                                                        multiplier:1.0
                                                        constant:0];

    NSLayoutConstraint *centreVerticalConstraint = [NSLayoutConstraint
                                                    constraintWithItem:_panel
                                                    attribute:NSLayoutAttributeCenterY
                                                    relatedBy:NSLayoutRelationEqual
                                                    toItem:self.view
                                                    attribute:NSLayoutAttributeCenterY
                                                    multiplier:1.0
                                                    constant:0];

    // if constraints are releated to "self.view" that's where they need to be added
    [self.view addConstraint:centreHorizontallyConstraint];
    [self.view addConstraint:centreVerticalConstraint];

}