ios - 如何在 objective-c 中以编程方式添加约束

Question

我想以编程方式添加约束，并使用以下代码添加 TOP 和 LEFT 约束。

NSLayoutConstraint *top = [NSLayoutConstraint constraintWithItem:label1
                                                       attribute:NSLayoutAttributeTop
                                                       relatedBy:NSLayoutRelationEqual
                                                          toItem:self.view
                                                       attribute:NSLayoutAttributeTop
                                                      multiplier:1
                                                        constant:110];


NSLayoutConstraint *left = [NSLayoutConstraint constraintWithItem:label1
                                                        attribute:NSLayoutAttributeLeading
                                                        relatedBy:NSLayoutRelationEqual
                                                           toItem:self.view
                                                        attribute:NSLayoutAttributeLeading
                                                       multiplier:1
                                                         constant:10];

lable1 添加到我的 View Controller 中。当我在像

这样的 View 中添加这两个约束时

[self.view addConstraint:top];
[self.view addConstraint:left];

它给出了控制台中的错误并且约束不影响标签。

2016-02-09 19:36:59.824 testinMRC[99160:313382] 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. 
(
    "<NSLayoutConstraint:0x7fa5c8738610 V:|-(110)-[UILabel:0x7fa5c8626940'Label']   (Names: '|':UIView:0x7fa5c86267b0 )>",
    "<NSIBPrototypingLayoutConstraint:0x7fa5c8628a70 'IB auto generated at build time for view with fixed frame' V:|-(88)-[UILabel:0x7fa5c8626940'Label']   (Names: '|':UIView:0x7fa5c86267b0 )>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x7fa5c8738610 V:|-(110)-[UILabel:0x7fa5c8626940'Label']   (Names: '|':UIView:0x7fa5c86267b0 )>

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.
2016-02-09 19:36:59.889 testinMRC[99160:313382] 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. 
(
    "<NSLayoutConstraint:0x7fa5c87334b0 H:|-(10)-[UILabel:0x7fa5c8626940'Label']   (Names: '|':UIView:0x7fa5c86267b0 )>",
    "<NSIBPrototypingLayoutConstraint:0x7fa5c86285c0 'IB auto generated at build time for view with fixed frame' H:|-(188)-[UILabel:0x7fa5c8626940'Label'](LTR)   (Names: '|':UIView:0x7fa5c86267b0 )>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x7fa5c87334b0 H:|-(10)-[UILabel:0x7fa5c8626940'Label']   (Names: '|':UIView:0x7fa5c86267b0 )>

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.

谁能告诉我为什么会出现这个错误？ 帮我解决短路问题

Answer 1

在您的情况下，错误清楚地表明您的 View 有冲突的约束。可能是因为您试图向已经具有来自 Interface Builder 的 View 添加约束。即使您没有明确设置任何约束，IB 也会在检测到某些约束缺失时为您提供它们。

我在评论中看到你提到你想以编程方式完成所有事情。在这种情况下，请看一段代码:

- (void)viewDidLoad {
    [super viewDidLoad];

    UIView *view = [UIView new];
    view.backgroundColor = [UIColor redColor];

    [view setTranslatesAutoresizingMaskIntoConstraints:NO];

    [self.view addSubview:view];

    NSLayoutConstraint *left = [NSLayoutConstraint constraintWithItem:view attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1 constant:100];
    NSLayoutConstraint *top = [NSLayoutConstraint constraintWithItem:view attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1 constant:100];

    NSLayoutConstraint *height = [NSLayoutConstraint constraintWithItem:view attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:50];
    NSLayoutConstraint *width = [NSLayoutConstraint constraintWithItem:view attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:50];

    [self.view addConstraints:@[left, top]];
    [view addConstraints:@[height, width]];

}

这是不言自明的。如您所见，我必须向 View 添加宽度和高度约束，因为只有左侧和顶部不能完全描述它的位置。

结果:

我鼓励您尝试视觉格式语言。使用更少的代码可以获得相同的结果。此代码导致完全相同的结果:

NSArray *horizontal = [NSLayoutConstraint constraintsWithVisualFormat:@"|-100-[view(50)]" options:0 metrics:nil views:@{@"view" : view}];
NSArray *vertical = [NSLayoutConstraint constraintsWithVisualFormat:@"V:|-100-[view(50)]" options:0 metrics:nil views:@{@"view" : view}];

[self.view addConstraints:vertical];
[self.view addConstraints:horizontal];

让我知道这是否有帮助。干杯。