iOS: Simple AutoLayout with 1 Label and 3 Buttons - ios

Question

I'm playing around with AutoLayout and am really banging my head against the wall for what seems like it ought to be an extremely simple solution.
I've got a vertical column of controls: 1 label and 3 buttons.
I want the label to be 40 pixels(points) tall and auto-size its width based on the width of its superview (standard spacing on left, top and right).
I want the 3 buttons to line up vertically below that label.
I would like their widths to auto-size just like the label.
I would like their spacing to be standard (aqua?) spacing (8 points, right?).
I would like the 3 buttons to be the same height.
I can get what I want to happen to work, but I keep getting errors in the console at runtime, and I'd like to figure out WHY I'm getting them and HOW to avoid getting them. I've watched the WWDC videos on AutoLayout, and here's what I've tried so far:
UILabel *label = [self Label];
MMGlossyButton *button1 = ...
MMGlossyButton *button2 = ...
MMGlossyButton *button3 = ...
[[self view] addConstraints:
[NSLayoutConstraint constraintsWithVisualFormat:#"H:|-[label]-|"
options:0
metrics:nil
views:NSDictionaryOfVariableBindings(label)]];
[[self view] addConstraints:
[NSLayoutConstraint constraintsWithVisualFormat:#"H:|-[button1]-|"
options:0
metrics:nil
views:NSDictionaryOfVariableBindings(new)]];
[[self view] addConstraints:
[NSLayoutConstraint constraintsWithVisualFormat:#"H:|-[button2]-|"
options:0
metrics:nil
views:NSDictionaryOfVariableBindings(existing)]];
[[self view] addConstraints:
[NSLayoutConstraint constraintsWithVisualFormat:#"H:|-[button3]-|"
options:0
metrics:nil
views:NSDictionaryOfVariableBindings(provider)]];
[[self view] addConstraints:
[NSLayoutConstraint constraintsWithVisualFormat:#"V:|-[label(40)]-[button1(>=25)]-[button2(==button1)]-[button3(==button1)]-|"
options:0
metrics:nil
views:NSDictionaryOfVariableBindings(label, button1, button2, button3)]];
So, this works for displaying the view in a dynamically-sized way, but the following error pops up in the console:
Unable to simultaneously satisfy constraints.
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)
// A whole bunch of constraint stuff that doesn't appear to be important...
Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x7554c40 V:[MMGlossyButton:0x7554b40(99)]>
So, the last bit appears to indicate that the first button I have on the view is statically sized to 99 points tall.
Which is the size it has on the view.
Which is completely arbitrary.
Which I don't want assigned, but can't figure out a way to un-assign it.
Although I'm getting what I want (sort of, eventually), it seems to be a REALLY roundabout way to accomplish something that's pretty simple. Am I missing something basic about AutoLayout, or does its power require such complexity?

Answer 1

You are encountering errors because you are mixing and matching constraints generated in code with constraints added by interface builder. Interface builder doesn't let you generate an ambiguous layout, so almost by definition if you add additional constraints, you will get an "Unable to simultaneously satisfy" error, which is the downfall of many a marriage.
To resolve this you either need to define all the constraints you need in interface builder, or you need to mark specific ones as outlets and remove them in code before adding your own.
In your case, the constraints are simple enough to create in IB.
You can pin to a specific height by using this button in IB while your label is selected:
The one in the middle, that looks like a girder. This gives you the following useful menu:
Choosing one of these allows you to create a new constraint against the label, which you can then edit by selecting it:
You can then add your buttons, select all three of them and, using the same menu, create an equal heights constraint.
The constraints created in IB aren't particularly flexible, so if you do decide you need to create or modify them in code, it is best to create outlets to the particular constraints, and then either remove and re-create them, or modify the constant value at run time.