自动布局和可调整大小的视图存在问题

Question

I have issue with autolayout.

I have three different views (could be n views as well). When I pin first one to top of super view and connect one next to the other everything seems fine. But if I want to change height of first one the other two does not follow (they stay on the same position). What I find weird about this is that if I connect only two views this works perfectly.

To have better understanding what I am trying to achieve I made simple, example solution with three views:

http://i.stack.imgur.com/cMwiB.png

So, when I set height of green view to 0 ( can be any other number as well ) and only red view is connected, this works fine, red will move as it should. Blue one is not connected and it remains where it is:

Here is screenshot when I connect blue to red:

What I tried to do:

• connect blue one to green as well ( not working)
• connect blue only to green ( not working )
• "boxing": have boxes with views at the time ( ugly, hard to manage, not working in all cases)

What I would like to know:

• what makes this happen? Can I change this kind of behavior?
• What is the best practice to have flexible length for height (Besides UITableView)

Thanks in advance.

Answer 1

Ok, I think you're falling into the same trap I did, when I investigated autolayout some time ago. It's tricky to explain, but I'll do my best. When you work with autolayouts, you need to forget completely about setting a view's frame at all. And that, my friend, includes changing size and positioning. When you use autolayout, you define some constraints, some "rules", that the view tries to respect when rendering the screen, so the more specific the rules are, the less random will be the behavior of the view. I mean, if you just define the spacing between views, you're implying that the height of the views doesn't matter, so it's possible that some views grow or shrink when the re-layout is called (that is, if you don't specify a constraint for size. You probably want to always specify the size of some views in every layout...)

By the way, you're allowed to violate the constraints by manually changing the frame of an element AFTER a re-layout is called, but when the re-layout method is called again, the constraints will be forced so the size and positioning of the views will change. Quick tip: an easy way to force the re-layout method is to change orientation (command+left / command+right in the simulator).

So after saying that, I have to say that the layout you provided is completely working as intended, at least to me. When you change orientations the constraints you specified are ALWAYS being respected. If you want, you can try to apply some frame-setting in the viewDidAppear method, because this one is called AFTER the autolayout (and thus, you're able to violate the constraints temporally). After doing this, your view will be broken but once you change orientation the constraints should be respected again. Make sure it's that way.

From here I can only wish you luck ;) Oh, and refer to the documentation, it was a life saver to me when I looked at this half a year ago. https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/AutolayoutPG/Introduction/Introduction.html It might seem a classic, but it's a pretty nice doc.

EDIT : one last thing. I have the impression that constraints are not designed to be dynamic. You define them once, and they're there always. Their main purpose is to ensure that subviews are rendered as intended in every screen resolution without fail, so if I were to make an application with moving views, I would leave them outside of the autolayout, or avoid using it at all. Just a personal impression though :)