简而言之，您不能使用UIImageView来实现这一点。
一种解决方案是将包含UIImageView的UIView子类化，并根据图像大小更改其帧。例如，您可以找到一个版本here。

将UIImageView的底部布局约束优先级设置为最低（即250.第250章他会帮你的

方法是修改UIImageView层的contentsRect。下面的代码来自我的项目（UIImageView的子类），它假设scaleToFill并偏移图像，使其顶部、底部、左侧或右侧对齐，而不是默认的中心对齐。对于aspectFit，这将是一个类似的解决方案。

typedef NS_OPTIONS(NSUInteger, AHTImageAlignmentMode) {
    AHTImageAlignmentModeCenter = 0,
    AHTImageAlignmentModeLeft = 1 << 0,
    AHTImageAlignmentModeRight = 1 << 1,
    AHTImageAlignmentModeTop = 1 << 2,
    AHTImageAlignmentModeBottom = 1 << 3,
    AHTImageAlignmentModeDefault = AHTImageAlignmentModeCenter,
};

- (void)updateImageViewContentsRect {
    CGRect imageViewContentsRect = CGRectMake(0, 0, 1, 1);

    if (self.image.size.height > 0 && self.bounds.size.height > 0) {

        CGRect imageViewBounds = self.bounds;
        CGSize imageSize = self.image.size;

        CGFloat imageViewFactor = imageViewBounds.size.width / imageViewBounds.size.height;
        CGFloat imageFactor = imageSize.width / imageSize.height;

        if (imageFactor > imageViewFactor) {
            //Image is wider than the view, so height will match
            CGFloat scaledImageWidth = imageViewBounds.size.height * imageFactor;
            CGFloat xOffset = 0.0;
            if (BM_CONTAINS_BIT(self.alignmentMode, AHTImageAlignmentModeLeft)) {
                xOffset = -(scaledImageWidth - imageViewBounds.size.width) / 2;
            } else if (BM_CONTAINS_BIT(self.alignmentMode, AHTImageAlignmentModeRight)) {
                xOffset = (scaledImageWidth - imageViewBounds.size.width) / 2;
            }
            imageViewContentsRect.origin.x = (xOffset / scaledImageWidth);
        } else if (imageFactor < imageViewFactor) {
            CGFloat scaledImageHeight = imageViewBounds.size.width / imageFactor;

            CGFloat yOffset = 0.0;
            if (BM_CONTAINS_BIT(self.alignmentMode, AHTImageAlignmentModeTop)) {
                yOffset = -(scaledImageHeight - imageViewBounds.size.height) / 2;
            } else if (BM_CONTAINS_BIT(self.alignmentMode, AHTImageAlignmentModeBottom)) {
                yOffset = (scaledImageHeight - imageViewBounds.size.height) / 2;
            }
            imageViewContentsRect.origin.y = (yOffset / scaledImageHeight);
        }
    }
    self.layer.contentsRect = imageViewContentsRect;
}

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Swift版本

///Works perfectly with AspectFill. Note there's no protection for obscure issues like (say) scaledImageWidth is zero.
class AlignmentImageView: UIImageView {

    enum HorizontalAlignment { case left, center, right }
    enum VerticalAlignment { case top, center, bottom }

    var horizontalAlignment: HorizontalAlignment = .center  { didSet { updateContentsRect() } }
    var verticalAlignment: VerticalAlignment = .center  { didSet { updateContentsRect() } }

    override var image: UIImage? { didSet { updateContentsRect() } }

    override func layoutSubviews() {
        super.layoutSubviews()
        updateContentsRect()
    }

    private func updateContentsRect() {
        var contentsRect = CGRect(origin: .zero, size: CGSize(width: 1, height: 1))

        guard let imageSize = image?.size else {
            layer.contentsRect = contentsRect
            return
        }

        let viewBounds = bounds
        let imageViewFactor = viewBounds.size.width / viewBounds.size.height
        let imageFactor = imageSize.width / imageSize.height

        if imageFactor > imageViewFactor {
            // Image is wider than the view, so height will match
            let scaledImageWidth = viewBounds.size.height * imageFactor
            var xOffset: CGFloat = 0.0

            if case .left = horizontalAlignment {
                xOffset = -(scaledImageWidth - viewBounds.size.width) / 2
            }
            else if case .right = horizontalAlignment {
                xOffset = (scaledImageWidth - viewBounds.size.width) / 2
            }

            contentsRect.origin.x = xOffset / scaledImageWidth
        }
        else {
            let scaledImageHeight = viewBounds.size.width / imageFactor
            var yOffset: CGFloat = 0.0

            if case .top = verticalAlignment {
                yOffset = -(scaledImageHeight - viewBounds.size.height) / 2
            }
            else if case .bottom = verticalAlignment {
                yOffset = (scaledImageHeight - viewBounds.size.height) / 2
            }

            contentsRect.origin.y = yOffset / scaledImageHeight
        }

        layer.contentsRect = contentsRect
    }

}

型

这将使图像填充宽度并且只占用它需要适合图像的高度（从宽度上讲）
swift 4.2：

let image = UIImage(named: "my_image")!
let ratio = image.size.width / image.size.height
cardImageView.widthAnchor
  .constraint(equalTo: cardImageView.heightAnchor, multiplier: ratio).isActive = true

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我也有类似的问题。最简单的方法是创建自己的UIImageView子类。我为子类3添加了属性，所以现在它可以在不知道内部实现的情况下轻松使用：

@property (nonatomic) LDImageVerticalAlignment imageVerticalAlignment;
@property (nonatomic) LDImageHorizontalAlignment imageHorizontalAlignment;
@property (nonatomic) LDImageContentMode imageContentMode;

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您可以在这里查看：https://github.com/LucasssD/LDAlignmentImageView

1.将纵横比约束与图像比例一起添加。
1.不要将UIImageView固定到底部。
1.如果你想动态地改变UIImage，记得更新长宽比约束。

我在Interface Builder中通过设置UIImageView高度的约束解决了这个问题，因为当图像大于屏幕尺寸时，图像总是会被“推”起来。
更具体地说，我将UIImageView设置为与它所在的View相同的高度（通过高度约束），然后在IB中使用间距约束定位UIImageView。这导致UIImageView具有“方面拟合”，其仍然遵守我在IB中设置的顶部间距约束。

如果您能够子类化UIImageView，那么您可以覆盖image var。

override var image: UIImage? {
    didSet {
        self.sizeToFit()
    }
}

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在Objective-C中，你可以通过覆盖setter来做同样的事情。