以编程方式设置动态uiimageview时出现LayoutConstraint错误

Question

我有一个依赖于 iAdBanner 出现的 UIImageView 位置。我收到错误：

"<NSLayoutConstraint:0x17409eb40 UIImageView:0x1741e9200.bottom == UIView:0x17418d270.bottom - 100>",
"<NSLayoutConstraint:0x174281e00 UIImageView:0x1741e9200.bottom == UIView:0x17418d270.bottom - 50>"

关于造成这种情况的任何想法？下面是设置 uiimageview 的代码，根据 iAd 是否加载，设置它的高度。

    UIImageView *imgView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"sunshine.png"]];
    CGRect frame = imgView.frame;
    frame.size.width = 180;
    frame.size.height = 30;
    imgView.frame = frame;
    imgView.translatesAutoresizingMaskIntoConstraints = NO;

    [imgView setContentMode:UIViewContentModeScaleAspectFit];
    [imgView sizeToFit];
    [self.view addSubview:imgView];

    [self.view addConstraint:[NSLayoutConstraint
                              constraintWithItem:imgView
                              attribute:NSLayoutAttributeTrailing
                              relatedBy:NSLayoutRelationEqual
                              toItem:self.view
                              attribute:NSLayoutAttributeTrailing
                              multiplier:1.0
                              constant:6.0]];
    _imgView = imgView;

}

-(void)bannerViewDidLoadAd:(ADBannerView *)banner {
    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:1];
    [banner setAlpha:1];
    [UIView commitAnimations];

        [self.view addConstraint:[NSLayoutConstraint
                                  constraintWithItem:_imgView
                                  attribute:NSLayoutAttributeBottom
                                  relatedBy:NSLayoutRelationEqual
                                  toItem:self.view
                                  attribute:NSLayoutAttributeBottom
                                  multiplier:1.0
                                  constant:-100.0]];

}

-(void)bannerView:(ADBannerView *)banner didFailToReceiveAdWithError:(NSError *)error {
    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:1];
    [banner setAlpha:0];
    [UIView commitAnimations];

        [self.view addConstraint:[NSLayoutConstraint
                                  constraintWithItem:_imgView
                                  attribute:NSLayoutAttributeBottom
                                  relatedBy:NSLayoutRelationEqual
                                  toItem:self.view
                                  attribute:NSLayoutAttributeBottom
                                  multiplier:1.0
                                  constant:-50.0]];
}

Answer 1

据我了解，您的约束之间存在冲突，您应该删除两个约束之一。

Answer 2

问题是您添加了 2 次底部约束，一次在 -(void)bannerViewDidLoadAd:(ADBannerView *)banner 中，一次在 (void)bannerView:(ADBannerView *)banner didFailToReceiveAdWithError:(NSError *)error 方法中。约束具有不同的常量值（-100 和 -50），因此它们会崩溃。

这样做的正确方法是只添加一次约束，然后在 2 种方法中更改它的值。

类似这样的：

在添加尾随约束的第一种方法中，还要添加底部约束并保留对它的引用：

self.bottomOffsetConstraint = [NSLayoutConstraint
                                  constraintWithItem:_imgView
                                  attribute:NSLayoutAttributeBottom
                                  relatedBy:NSLayoutRelationEqual
                                  toItem:self.view
                                  attribute:NSLayoutAttributeBottom
                                  multiplier:1.0
                                  constant:-100.0];
[self.view addConstraint:self.bottomOffsetConstraint];

然后在 2 种方法中更改常量：

-(void)bannerViewDidLoadAd:(ADBannerView *)banner {
    [self.bottomOffsetConstraint setConstant:-50];
    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:1];
    [banner setAlpha:1];
    // You need to call this in the animation block to trigger the constraints update
    [self.view layoutIfNeeded]; 
    [UIView commitAnimations];
}

-(void)bannerView:(ADBannerView *)banner didFailToReceiveAdWithError:(NSError *)error {

    [self.bottomOffsetConstraint setConstant:-50];
    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:1];
    [banner setAlpha:0];
    [self.view layoutIfNeeded];
    [UIView commitAnimations];
}

这将修复崩溃的约束并为布局更改设置动画。

告诉我进展如何！