Objective-C中点击按钮无法更新UILabel的centerXAnchor问题

嘿，我帮你排查出几个关键问题，咱们一个个来修复：

核心问题分析 #

你遇到的点击按钮后约束不生效，主要是因为旧约束没有被正确移除，再加上代码里还有几个小错误导致的，具体如下：

1. 旧约束未失效，导致约束冲突 #

你在点击事件里直接给self.MyLblConstraint赋值了新约束，但原来的旧约束还处于active = YES状态——这时候两个约束（旧的centerX multiplier=0.5，新的multiplier=1）会冲突，系统会自动忽略其中一个，自然看不到更新效果。

2. 错误初始化UILabel #

你在.h里声明MyLbl是UILabel，但初始化时写成了self.MyLbl = [UIView new];，这会让它变成普通UIView，根本不是标签，就算约束生效也看不到文本。

3. 按钮添加到错误的视图上 #

MyCustomView是UIView的子类，它没有self.view这个属性，你把按钮添加到self.view会导致按钮无法显示甚至崩溃，应该添加到self（也就是自定义视图本身）。

修复后的完整代码 #

MyCustomView.h #

#import <UIKit/UIKit.h>
@interface MyCustomView : UIView
@property (nonatomic, strong) UILabel *myLbl; // 规范命名：小写开头
@property (nonatomic, strong) NSLayoutConstraint *myLblCenterXConstraint;
- (instancetype)initWithFrame:(CGRect)frame; // 用instancetype代替id更规范
@end

MyCustomView.m #

#import "MyCustomView.h"
@implementation MyCustomView
@synthesize myLbl;
@synthesize myLblCenterXConstraint;

- (instancetype)initWithFrame:(CGRect)frame {
    self = [super initWithFrame:frame];
    if (self) {
        self.translatesAutoresizingMaskIntoConstraints = NO;
        self.backgroundColor = [UIColor whiteColor];
        
        // 正确初始化UILabel并设置文本、字体
        NSString *lblTxtStr = @"This is My Label Text";
        UIFont *lblFont = [UIFont fontWithName:@"Helvetica-Bold" size:18];
        CGSize lblTxtStrSize = [lblTxtStr sizeWithAttributes:@{NSFontAttributeName: lblFont}];
        CGSize lblFrameSize = CGSizeMake(ceilf(lblTxtStrSize.width), ceilf(lblTxtStrSize.height));
        
        self.myLbl = [UILabel new];
        self.myLbl.translatesAutoresizingMaskIntoConstraints = NO;
        self.myLbl.text = lblTxtStr;
        self.myLbl.font = lblFont;
        self.myLbl.backgroundColor = [UIColor lightGrayColor]; // 方便看位置
        [self addSubview:self.myLbl];
        
        // 设置固定宽高和底部约束
        [self.myLbl.widthAnchor constraintEqualToConstant:lblFrameSize.width].active = YES;
        [self.myLbl.bottomAnchor constraintEqualToAnchor:self.bottomAnchor].active = YES;
        [self.myLbl.heightAnchor constraintEqualToConstant:lblFrameSize.height].active = YES;
        
        // 初始化centerX约束
        self.myLblCenterXConstraint = [self.myLbl.centerXAnchor constraintEqualToAnchor:self.centerXAnchor multiplier:0.5];
        self.myLblCenterXConstraint.active = YES;
        
        // 创建按钮并添加到自定义视图（self）
        UIButton *btn = [UIButton buttonWithType:UIButtonTypeRoundedRect];
        btn.translatesAutoresizingMaskIntoConstraints = NO;
        [btn setTitle:@"test 1" forState:UIControlStateNormal];
        [btn addTarget:self action:@selector(myBtnClick:) forControlEvents:UIControlEventTouchUpInside];
        [self addSubview:btn];
        
        // 设置按钮约束（注意这里的视图字典要对应）
        [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[btn]|" options:0 metrics:nil views:@{@"btn": btn}]];
        [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[myLbl]-100-[btn(50)]" options:0 metrics:nil views:@{@"myLbl": self.myLbl, @"btn": btn}]];
    }
    return self;
}

- (void)myBtnClick:(id)sender {
    NSLog(@"My Button Click");
    
    // 关键：先把旧约束设为失效
    self.myLblCenterXConstraint.active = NO;
    
    // 创建并激活新约束
    self.myLblCenterXConstraint = [self.myLbl.centerXAnchor constraintEqualToAnchor:self.centerXAnchor];
    self.myLblCenterXConstraint.active = YES;
    
    // 执行动画更新布局
    [UIView animateWithDuration:0.5 delay:0 usingSpringWithDamping:1 initialSpringVelocity:1 options:UIViewAnimationOptionCurveEaseOut animations:^{
        [self layoutIfNeeded];
    } completion:nil];
}
@end

ViewController.m（保持原样即可） #

#import "ViewController.h"
#import "MyCustomView.h"
@interface ViewController (){
    MyCustomView *myView;
}
@end
@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    myView = [MyCustomView new];
    myView.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:myView];
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[myView]|" options:0 metrics:nil views:@{@"myView" : myView}]];
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[myView(600)]" options:0 metrics:nil views:@{@"myView" : myView}]];
}
@end

额外说明 #

如果以后你需要修改的是约束的constant值（而不是multiplier），可以直接修改现有约束的属性，不用替换约束，比如：

self.myLblCenterXConstraint.constant = 50;
[self layoutIfNeeded];

但multiplier是只读属性，所以必须通过失效旧约束+创建新约束的方式来修改。

内容的提问来源于stack exchange，提问作者Mohammad Ashraf Ali