iOS以编程方式在UITableviewcell中打破了宽高比约束

Question

I am trying to set an aspect ratio to an image inside a tableviewcell and it is getting broken because iOS can't accomplish accomplish constraints for width or height with decimals that are different from .5 or .0.

There is anyway to make that a constraint doesn't fail/break if it is close to its value, or it gets rounded to a value erasing the decimal?

I know that the problem is because of the decimal after many tries, I will show you with the following example:

With the following code

postImage.heightAnchor.constraint(equalToConstant: 88.9).isActive = true

I get the following error:

    Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x6000036067b0 UIImageView:0x7fb76fe397e0.height == 88.9   (active)>

And the view view hierarchy the size of the image is 89

It happens to me when I set a height that is different from .5 or .0 it gets rounded to the nearest value .5 or .0 (If I set 88.6 it gets rounded to 88.5 throwing the error), but If I set any value that ends in .0 or .5 it works smooth!

(I know I can do it setting a lower priority but it can give me more unexpected behaviors)

(I am doing it programmatically, doing it through a Xib is not an option :) )

EDIT: Whole log:

    (
        "<NSLayoutConstraint:0x600000facd20 UIImageView:0x7fc98362fe80.height == 86.9   (active)>",
        "<NSLayoutConstraint:0x600000facd70 V:|-(0)-[UIImageView:0x7fc98362fe80]   (active, names: '|':UITableViewCellContentView:0x7fc98362f940 )>",
        "<NSLayoutConstraint:0x600000face10 UIImageView:0x7fc98362fe80.bottom == UITableViewCellContentView:0x7fc98362f940.bottom - 8   (active)>",
        "<NSLayoutConstraint:0x600000fad680 'UIView-Encapsulated-Layout-Height' UITableViewCellContentView:0x7fc98362f940.height == 95   (active)>"
    )

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x600000facd20 UIImageView:0x7fc98362fe80.height == 86.9   (active)>

Answer 1

I believe you must reduce the priority on your aspect-ratio constraint to avoid the broken constraint messages.

And, I think you're not-quite-correct about nn.5 values working.

For example... With a 420:360 ratio:

setting the width of the view to 59 results in an auto-layout height calculation of

59.0 * 360.0 / 420.0 == 50.571428571428569

Auto-layout is not going to give you an actual view height of 50.571428571428569 ... it's going to round it to the nearest .5 which is 50.5 and that is less than requested so you get the error.

With:

59.4 * 360.0 / 420.0 == 50.914285714285718

it rounds to 51 , which is greater than requested, and you do not get the error.

A few more examples:

58.5 * 360.0 / 420.0 == 50.142857142857146  // rounds down -- error
58.4 * 360.0 / 420.0 == 50.057142857142857  // rounds down -- error
58.3 * 360.0 / 420.0 == 49.971428571428568  // rounds up -- NO ERROR

Changing your aspect-ratio constraint to Priority: 999 gives the same sizing results (rounds to the nearest .5 ) but does not throw the broken constraint error.

Since you will get the same actual size, changing the priority will simply allow auto-layout to "do the right thing" and shouldn't have any other effect on your layout.