意外 NSAutoresizingMaskLayoutConstraint 的 AutoLayout 约束问题

In this example, I am creating a very simple view with 2 UI elements: an image view called imageThumbnail and a label called labelName with some constraints:

"H:|-padding-[_imageThumbnail(==imageWidth)]-[_labelName]";
"V:|-padding-[_imageThumbnail(==imageHeight)]-padding-|";
"V:|-padding-[_labelName]";

On both elements I set the AutoresizingMaskIntoConstraints to NO.

And I am getting the following exception:

Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
    "<NSLayoutConstraint:0xa6e4f90 V:[UIImageView:0xa6e4340]-(10)-|   (Names: '|':UITableViewCellContentView:0xa6e4150 )>",
    "<NSLayoutConstraint:0xa6e4f10 V:[UIImageView:0xa6e4340(80)]>",
    "<NSLayoutConstraint:0xa6e4ed0 V:|-(10)-[UIImageView:0xa6e4340]   (Names: '|':UITableViewCellContentView:0xa6e4150 )>",
    "<NSAutoresizingMaskLayoutConstraint:0xa6e4ac0 h=--& v=--& V:[UITableViewCellContentView:0xa6e4150(99)]>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0xa6e4f90 V:[UIImageView:0xa6e4340]-(10)-|   (Names: '|':UITableViewCellContentView:0xa6e4150 )>

I know the last constraint is related to the content view but I am unclear to properly remove it (Setting
AutoresizingMaskIntoConstraints to NO on the contentView raises an error and in the SO link below, it messes up the entire layout):

<NSAutoresizingMaskLayoutConstraint:0xa6e4ac0 h=--& v=--& V:[UITableViewCellContentView:0xa6e4150(99)]>

I've seen the answers at: Auto layout constraints issue on iOS7 in UITableViewCell but none of them seem to be working for me here.

I believe that the constraints I define are valid and pretty straightforward but can't seem to figure out what's going on. And I'm seeing the exception being raised both in iOS 6.1 and iOS 7.

Any idea what I am doing wrong here?

Thanks, Nicolas

I've put a corrected version of your code at https://github.com/mattneub/AutoLayoutCellWarning. Works perfectly. This line was the main cause of your trouble:

NSString *const kImageVertical = @"V:|-padding-[_imageThumbnail(==imageHeight)]-padding-|";

Change that to

NSString *const kImageVertical = @"V:|-padding-[_imageThumbnail]-padding-|";

And all will be well.

The main reason you were having trouble is that by assigning an absolute height to the image view, you were making it impossible to also assign a height to the cell. Floating point is not exact, so you need to allow some room for the cell to grow / shrink. If we take away the absolute height, the image view gets its height from its intrinsic content size, at a lower priority, so there is no conflict.

I have some other critiques of your code. In trying to do dynamic setting of the cell's height while using auto layout, you were giving layout and constraint update commands you should never be giving, and you are giving them at wrong times. It is possible to do dynamic row heights based on constraints, but the way you're doing it is not the way. All you have to do is call systemLayoutSizeFittingSize to find out the correct cell height. Also, there is absolutely no need to put your "hidden" cell into the interface. Don't do that; it just confuses things. One of the things you'll notice when you look at my version of the code is that it is much simpler than yours, because of those differences.

For a working method, see my example at https://github.com/mattneub/Programming-iOS-Book-Examples/blob/master/bk2ch08p424variableHeights/ch21p722variableHeights/RootViewController.m

And see the discussion of this issue in my book.

EDIT (May 2014): Unfortunately my answer above fails to point out one of the key causes of this problem, namely, that the cell separator has height (if it hasn't been set to None). Therefore if you assign the cell a height that doesn't take the separator height into account, the auto layout constraints, if absolute, cannot be resolved into that height. (See this answer, which really made the lightbulb come on inside my head.)

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