使用 Visual Format Language 将视图居中在其父视图中 [英] Centering a view in its superview using Visual Format Language

问题描述

我刚开始学习 iOS 版的 AutoLayout，并了解了 Visual Format Language.

I just started learning AutoLayout for iOS and had a look at Visual Format Language.

除了一件事之外，一切都很好:我只是无法将视图置于其超级视图的中心.
这可以通过 VFL 实现还是我需要自己手动创建约束?

It all works fine except for one thing: I just can't get a view to center within its superview.
Is this possible with VFL or do I need to manually create a constraint myself?

推荐答案

目前，不，看起来不可能仅使用 VFL 在父视图中居中视图.然而，使用单个 VFL 字符串和单个额外约束(每个轴)来做到这一点并不困难:

Currently, no, it doesn't look like it is possible to center a view in the superview using only VFL. It is, however, not that difficult to do it using a single VFL string and a single extra constraint (per axis):

VFL:"|-(>=20)-[view]-(>=20)-|"

[NSLayoutConstraint constraintWithItem:view
                             attribute:NSLayoutAttributeCenterX
                             relatedBy:NSLayoutRelationEqual
                                toItem:view.superview
                             attribute:NSLayoutAttributeCenterX
                            multiplier:1.f constant:0.f];

有人会认为你可以做到这一点(这是我看到这个问题时最初的想法和尝试):

One would think that you would simply be able to do this (which is what I initially thought and tried when I saw this question):

[NSLayoutConstraint constraintsWithVisualFormat:@"|-(>=20)-[view(==200)]-(>=20)-|"
                                 options: NSLayoutFormatAlignAllCenterX | NSLayoutFormatAlignAllCenterY
                                 metrics:nil
                                   views:@{@"view" : view}];

我尝试了上面的许多不同变体，试图将其弯曲成我的意愿，但这似乎不适用于超级视图，即使在两个轴上明确具有两个单独的 VFL 字符串时(H:|V:).然后我开始尝试在选项 do 应用于 VFL 时准确地隔离.它们似乎不适用于 VFL 中的超级视图，只会适用于 VFL 字符串中提到的任何显式视图(在某些情况下令人失望).

I tried many different variations of the above trying to bend it to my will, but this does not appear to apply to the superview even when explicitly having two separate VFL strings for both axes (H:|V:). I then started to try and isolate exactly when the options do get applied to the VFL. They appear to not apply to the superview in the VFL and will only apply to any explicit views that are mentioned in the VFL string (which is disappointing in certain cases).

我希望 Apple 将来会添加某种新选项，让 VFL 选项考虑超级视图，即使仅在 VFL 中的超级视图之外只有一个显式视图时才这样做.另一种解决方案可能是传递到 VFL 的另一个选项，其内容类似于:NSLayoutFormatOptionIncludeSuperview.

I hope in the future Apple adds some kind of new option to have the VFL options take into account the superview, even if doing it only when there is only a single explicit view besides the superview in the VFL. Another solution could be another option passed into the VFL that says something like: NSLayoutFormatOptionIncludeSuperview.

不用说，我在尝试回答这个问题时学到了很多关于 VFL 的知识.

Needless to say, I learned a lot about VFL trying to answer this question.

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