I have two labels next to each other, but the right one truncates even when I set truncate rules on the left one.
My code:
// Club name
labelFirst = [[UILabel alloc] init];
labelFirst.adjustsFontSizeToFitWidth = NO;
[labelFirst setLineBreakMode:NSLineBreakByTruncatingTail];
[labelFirst setTranslatesAutoresizingMaskIntoConstraints:NO];
[self.contentView addSubview:labelFirst];
labelSecond = [[UILabel alloc] init];
[labelSecond setAdjustsFontSizeToFitWidth:NO];
[labelSecond setTranslatesAutoresizingMaskIntoConstraints:NO];
[labelSecond setText:NSLocalizedString(#"IsCancelled", nil)];
[self.contentView addSubview:labelSecond];
NSLayoutConstraint *constraint = [NSLayoutConstraint constraintsWithVisualFormat:#"H:|-73-[label]-[label2]-10-|" options:0 metrics:nil views:#{#"label": labelFirst, #"label2": labelSecond}]
[self.contentView addConstraints:constraint];
How should I do this?
This is where Content Compression Resistance Priority comes into play. Set these values higher and lower based on which one you want compressed first. Higher for the label that you want take priority in resisting compression. Lower for the label that you want compressed (truncated) first.
You can do it programmatically (default is 750):
[labelFirst setContentCompressionResistancePriority:749
forAxis:UILayoutConstraintAxisHorizontal];
Or in interface builder under the size inspector tab:
Here is another answer going into more detail on how they work.
NSLineBreakByTruncatingTail - it's default LineBreakMode value for labels, change it for labelSecond to have diference
I have seen several questions regarding this topic; however, none of them resolved the issue, and for the life of me I cannot figure out why the constraints are not taking effect. (Maybe because I haven't slept in a while.. I know it's counterproductive).
I am new to IOS development, but I'm hitting it hard and a quick learner. If you could provide an explanation as to why my original code was not working that would be super helpful. Thumbs up to whoever can resolve issue!
Okay, so I'm developing an app that I've actually been working on for quite a while & I did a real sloppy job when I first began. So I'm basically rebuilding the app's code but completely cutting out the Interface Builder (Storyboard). I am trying to pin a UIButton to a UIView locally (I'd rather not declare it globally).
// parentView initialized //
parentView=[[UIView alloc]initWithFrame:CGRectMake(0, 0, screenWidth, screenHeight)];
[parentView setBackgroundColor:[UIColor clearColor]];
[self.view addSubview:parentView];
parentView.tag = 0;
// homeScreenView initialized //
homeScreenView=[[UIView alloc]initWithFrame:CGRectMake(0, homeScreenTitle.frame.size.height, screenWidth, screenHeight-homeScreenTitle.frame.size.height-height)];
[homeScreenView setBackgroundColor:[UIColor greenColor]];
[parentView addSubview:homeScreenView];
homeScreenView.tag = 2;
// chatMenuView initialized //
chatMenuView=[[UIView alloc]initWithFrame:CGRectMake(0, homeScreenTitle.frame.size.height+10, 100, screenHeight-height-10-10-homeScreenTitle.frame.size.height)];
[chatMenuView setBackgroundColor:[UIColor grayColor]];
[parentView addSubview:chatMenuView];
chatMenuView.tag = 3;
// chatMenuButton initialized //
chatMenuButton = [UIButton buttonWithType:UIButtonTypeCustom];
NSString *buttonText = [NSString stringWithFormat:#"CHAT"];
[chatMenuButton setTitle:buttonText forState:UIControlStateNormal];
[parentView addSubview:chatMenuButton];
[chatMenuButton sizeToFit];
chatMenuButton.center = CGPointMake(0,screenHeight/2);
UIImage *chatIcon = [UIImage imageNamed:#"GrayHouse1.png"];
[chatMenuButton setBackgroundImage:chatIcon forState:(UIControlStateNormal)];
chatMenuButton.tag = 5;
// pinChatButton //
NSLayoutConstraint *pinChat = [NSLayoutConstraint constraintWithItem:chatMenuView
attribute:NSLayoutAttributeLeading
relatedBy:NSLayoutRelationEqual
toItem:chatMenuButton
attribute:NSLayoutAttributeTrailing
multiplier:1
constant:0];
[self.view addConstraint: pinChat];
I would like to also add that all of this code is in the viewDidLoad method & all of the other views are declared in the header file (as IBOutlets).Basically when I run the code, the UIView leading margin is at position x = 100, and the button is at position x = 0 which is what it's suppose to be at prior to adding constraints which should also move the button to position x = 100.
So I'm basically rebuilding the app's code but completely cutting out
the Interface Builder (Storyboard)
First off - you're really swimming against the tide here. Everybody uses Interface Builder for basic layout, because visualizing the layout, and seeing the constraints is much easier in a visual editor. You should really save code-generated constraints for when you're trying to do something clever. Not to mention all that code to set the tag, etc, etc.
Having gotten that out of the way, Your constraint doesn't make a whole lot of sense to me - you're constraining the leading space on chatMenuView to be equal to the trailing space of chatMenuButton. I can imagine scenarios in which this'd be useful, but what you probably want for this is something more like:
NSLayoutConstraint *pinChat = [NSLayoutConstraint constraintWithItem:chatMenuButton
attribute:NSLayoutAttributeLeading
relatedBy:NSLayoutRelationEqual
toItem:chatMenuView
attribute:NSLayoutAttributeLeading
multiplier:1
constant:100];
Finally, even if you are sure you want to create constraints in code, consider using the visual format, which is at least somewhat readable.
I adding views dynamically in to scrollview with layout constraint by programatically, For text view component I wanted to set the height constraint according to text set in textview, so I did created class extending UITextView. Inside text view class I have written following code to adding height constraint.
#import "CETextView.h"
#implementation CETextView
- (void)layoutSubviews
{
[super layoutSubviews];
if (!self.heightConstraint)
{
self.heightConstraint = [NSLayoutConstraint constraintWithItem:self attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:100];
[self addConstraint:self.heightConstraint];
}
CGRect lRect = [self contentSizeRect];
CGSize descriptionSize = lRect.size;
self.heightConstraint.constant = descriptionSize.height;
}
- (CGRect)contentSizeRect
{
NSTextContainer* textContainer = [self textContainer];
NSLayoutManager* layoutManager = [self layoutManager];
[layoutManager ensureLayoutForTextContainer: textContainer];
CGRect lRect = CGRectMake(0, 0,320, 500);
lRect.size = self.contentSize;
lRect.size.height = lRect.size.height + 5;
return lRect;
}
#end
This code gives correct height in iOS 8.0 but gives wrong height in iOS 9.0. I Checked the apple doc for new release of iOS 9.0, there are some of the changes related auto layout.
Any help is appreciated.
Not sure if you solved this or not, but I beat my head against the wall for a few hours until I made sure that enable scrolling was set to YES for the UITextViews I was trying to resize. Not sure if this will solve your issue, it seems slightly different.
iOS 8 resized the height just fine with enable scrolling set to NO, but it appears iOS 9 will not resize the height constraint if enable scrolling is NO. I just checked the box in the story board on all of my text views and everything resized after that.
contentSize is not trustable since it updates until next runloop. You should use "sizeThatFits"
i'm trying to set a constraint in code and don't get the expected result.
I have a UIView container with 3 buttons (sub views) and i'm trying to set one's leading space to be the average of the other two's leading spaces (so it'll be in the middle, horizontally).
The numbers i get seem to be right when compared to the numbers i see on the storyboard when i place the 3 buttons in the position i want.
I'm getting the leading space by their frame's x value (i've double checked that aligmentRectForFrame: gives the same results), and i average them.
I use:
NSLayoutConstraint *twitterConstraint = [NSLayoutConstraint constraintWithItem:middleButton attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEqual toItem:containerView attribute:NSLayoutAttributeLeading multiplier:1.0 constant:average];
[twitterConstraint setPriority:UILayoutPriorityRequired];
[self.view setTranslatesAutoresizingMaskIntoConstraints:NO];
[NSLayoutConstraint activateConstraints:#[twitterConstraint]];
the basic functionality works i.e. if i put a number instead of the average i see results. I'm getting unexpected results with the current code. specifically, i'm getting the "middle button" on the right hand side of the other 2 buttons.
help!
idan
Expanding on why #Ken Thomases answer in comments works: Auto Layout first goes from subview-up to collect information, and then goes superview-down to set frames. So this code is sampling values of its subview frames, but at the time it is executed (in updateConstraints or updateViewConstraints or somewhere else) those views' frames haven't yet been set to their auto-layout-approved values. Unpredictable results can happen.
Calling -layoutIfNeeded on this view forces the Auto Layout engine to do this for the subviews--actaully do the layout work. So then sampling those subviews can work.
Unfortunately in this method, problems are created both sampling the frames to get information and calling layoutIfNeeded, which duplicates the expensive layout operations. As noted, it requires the buttons to be all the same size. Regardless, it's probably fine for this use case.
The way to set items to be evenly spaced using the native Auto Layout system (and allowing different-sized items) is spacer views. It's inelegant, but necessary. It can be done manually in IB, and here's how to do it with Visual Format Language:
NSMutableDictionary *autoLayoutDict = [[NSMutableDictionary alloc] init];
NSDictionary *setDictionary = #{
#"leftLabel":self.leftLabel,
#"middleLabel":self.middleLabel,
#"rightLabel":self.rightLabel
};
[autoLayoutDict setValuesForKeysWithDictionary:setDictionary];
int numberOfSpacerViewsRequired = 2;
for (int i = 0; i < numberOfSpacerViewsRequired; i++) {
UIView *spacerViewX = [[UIView alloc] init];
spacerViewX.backgroundColor = [UIColor clearColor];
spacerViewX.userInteractionEnabled = NO;
spacerViewX.translatesAutoresizingMaskIntoConstraints = NO;
NSString *spacerViewKey = [NSString stringWithFormat:#"spacerView%i", i];
[autoLayoutDict setObject:spacerViewX forKey:spacerViewKey];
[self.view addSubview:spacerViewX];
}
NSArray *constraintsToAdd = [NSLayoutConstraint constraintsWithVisualFormat:#"H:[leftLabel]-0-[spacerView0(spacerView1)]-0-[middleLabel]-0-[spacerView1(spacerView0)]-0-[rightLabel]"
options:NSLayoutFormatAlignAllCenterY
metrics:0
views:autoLayoutDict];
[self.view addConstraints:constraintsToAdd];
We use auto layout constraints selectively, primarily to position labels in relation to editable field elements (UITextView, UITextField, typically). However, since implementing auto layout for these fields, we're seeing a nasty exception and crash whenever we're unloading views, deallocating, etc. The exceptions are happening as it's attempting to remove the constraints from a view before unloading it.
Our view/controller hierarchy is as such:
UITableViewController (plain style, but with cell appearance to mimic grouped style)
--> UITableViewCell
----> UIViewController (container for editable form)
------> UICollectionViewController (editable form)
--------> UICollectionViewCell
-----------> UIViewController (editable field)
--------------> UILabel (field label) **HAS CONSTRAINTS**
--------------> UITextView / UITextField (field value) **HAS CONSTRAINTS**
Many times when the upper level table cells are being deallocated/replaced/reloaded, we see a huge exception and then crash as it's trying to deallocate/unload the view hierarchy within.
I've attempted to mitigate the crash by catching the exception (no help) and also by forcefully removing all of the constraints on the affected view and all of the subviews prior to deallocation/unload (in viewWillDisappear:) and it doesn't seem to help. I've even tried to remove these constraints one by one to see if there's one in particular that's causing the trouble but all of them are blowing up when we call removeConstraint: or removeConstraints: on a container in preparation for disappearing.
I'm baffled! Here's a snippet of our exception -- roughly about 3000 lines have been chopped out of it, so if you need more, just ask.
Exception while deallocating view: { Rows:
0x18911270.posErrorMarker == 4 + 1*0x18911270.negError + 1*0x189112f0.marker + -1*0x189113f0.negError + 1*0x189113f0.posErrorMarker + 1*0x18911a60.marker + -0.5*0x1892dae0.negError + 0.5*0x1892dae0.posErrorMarker + 1*0x18951520.negError + -1*0x18951520.posErrorMarker + -0.5*0x18958090.negError + 0.5*0x18958090.posErrorMarker
0x189112b0.negError == 12 + 1*0x189112b0.posErrorMarker + -1*0x189112f0.marker + 1*0x189113f0.negError + -1*0x189113f0.posErrorMarker + -1*0x18911a60.marker + 1*0x18925530.marker + 0.5*0x1892dae0.negError + -0.5*0x1892dae0.posErrorMarker + 1*0x1893e080.marker + 0.5*0x18958090.negError + -0.5*0x18958090.posErrorMarker + 1*0x18963640.marker
0x18911370.negError == 9 + -1*0x189112f0.marker + 1*0x18911370.posErrorMarker + 1*0x18925530.marker + 1*0x1892dae0.negError + -1*0x1892dae0.posErrorMarker + 1*0x1893e080.marker + 1*0x18963640.marker
0x189113b0.slackMarker == 2 + -1*0x189107d0.marker + 1*0x18910b90.negError + -1*0x18910b90.posErrorMarker +
........ EXPLETIVES DELETED .........
UITableView:0xca2b000.contentHeight == 36 + 1*0xc221c00.marker
UITableView:0xca2b000.contentWidth == 704 + 1*0xc239470.marker
UITableView:0xca2b000.minX == 0 + 1*0xc2a23f0.marker + -0.5*0xc2a2590.marker
UITableView:0xca2b000.minY == 0 + 1*0xc2a25d0.marker + -0.5*0xc2a2630.marker
UITableViewCellContentView:0x18ab13d0.Height == 174 + 1*0x18abd4f0.marker
UITableViewCellContentView:0x18ab13d0.Width == 704 + 1*0x18abd470.marker
........ EXPLETIVES DELETED .........
<NSAutoresizingMaskLayoutConstraint:0x18988bc0 h=-&- v=-&- UIView:0x18911e50.midY == UIView:0x1892d0c0.midY> Marker:0x18988bc0.marker
<NSAutoresizingMaskLayoutConstraint:0x18994b40 h=-&- v=-&- UIView:0xc4a6fb0.midX == UIView:0xc4b4990.midX> Marker:0x18994b40.marker
<NSAutoresizingMaskLayoutConstraint:0x18998480 h=-&- v=-&- UIView:0x18915180.width == UIView:0xc4c5970.width> Marker:0x18998480.marker
<NSAutoresizingMaskLayoutConstraint:0x18aae320 h=--& v=--& TapSectionalTableViewCell:0x18a3d270.midX == + 352> Marker:0x18aae320.marker
<NSAutoresizingMaskLayoutConstraint:0x18aae410 h=--& v=--& H:[TapSectionalTableViewCell:0x18a3d270(704)]> Marker:0x18aae410.marker
<NSAutoresizingMaskLayoutConstraint:0x18aae450 h=--& v=--& TapSectionalTableViewCell:0x18a3d270.midY == + 144> Marker:0x18aae450.marker
........ EXPLETIVES DELETED .........
<NSAutoresizingMaskLayoutConstraint:0xc2de2f0 h=--& v=--& TapGenericCollectionCell:0xc2ac500.midX == + 499> Marker:0xc2de2f0.marker
<NSAutoresizingMaskLayoutConstraint:0xc2de3b0 h=--& v=--& V:[TapGenericCollectionCell:0xc2ac500(34)]> Marker:0xc2de3b0.marker
<NSAutoresizingMaskLayoutConstraint:0xc2de430 h=-&- v=-&- UIView:0x18953f80.height == UIView:0xc2acb20.height> Marker:0xc2de430.marker
<NSAutoresizingMaskLayoutConstraint:0xc2de520 h=-&- v=-&- UIView:0x18923af0.height == UIView:0xc2ae570.height> Marker:0xc2de520.marker
<NSAutoresizingMaskLayoutConstraint:0xc2de560 h=--& v=--& H:[TapGenericCollectionCell:0xc2ac500(280)]> Marker:0xc2de560.marker
........ EXPLETIVES DELETED .........
<NSContentSizeLayoutConstraint:0xc2f5730 H:[_UIBaselineLayoutStrut:0x18994a30(0)] Hug:250 CompressionResistance:750> Marker:0xc2f5730.posErrorMarker
<NSContentSizeLayoutConstraint:0xc2f5730 H:[_UIBaselineLayoutStrut:0x18994a30(0)] Hug:250 CompressionResistance:750> Marker:0xc2f5730.posErrorMarker
<NSContentSizeLayoutConstraint:0xc2f5770 V:[_UIBaselineLayoutStrut:0x18994a30(18)] Hug:250 CompressionResistance:750> Marker:0xc2f5770.posErrorMarker
internal error. Cannot find an outgoing row head for incoming head UIView:0x189712b0.Width, which should never happen.'
/**** BEGIN Individual Field Controller - This code is from the base individual field controller used in our editable form collection *****/
- (void)viewDidLoad {
[super viewDidLoad];
self.view.clipsToBounds = YES;
self.view.opaque = YES;
CGRect viewFrame = self.view.frame;
viewFrame.size = [self defaultFieldSize];
self.view.frame = viewFrame;
if (self.backgroundColor) {
self.view.backgroundColor = self.backgroundColor;
}
else {
self.view.backgroundColor = [UIColor whiteColor];
}
[self createLabelAndField];
[self setLabelAndFieldContraints];
[self.view addConstraints:self.labelValueConstraints];
[self.view setNeedsUpdateConstraints];
}
- (void)createLabelAndField {
[self removeLabelAndField];
UILabel *label = [[UILabel alloc] init];
label.font = self.labelFont;
label.textColor = self.labelColor;
label.lineBreakMode = NSLineBreakByWordWrapping;
label.textAlignment = NSTextAlignmentLeft;
label.adjustsFontSizeToFitWidth = NO;
label.numberOfLines = 0;
if (self.backgroundColor) {
label.backgroundColor = self.backgroundColor;
}
else {
label.backgroundColor = [UIColor whiteColor];
}
[self.view addSubview:label];
self.label = label;
/// EXAMPLE valueView initialization from a subclass that handles long text
TapEditableTextView *textView = [[TapEditableTextView alloc] init];
if (self.hasLabelOverValue) {
textView.shouldMimicTextField = NO;
}
else {
textView.shouldMimicTextField = YES;
}
textView.delegate = self;
textView.keyboardType = UIKeyboardTypeDefault;
textView.font = self.valueFont;
textView.textColor = self.valueColor;
textView.textAlignment = NSTextAlignmentLeft;
textView.normalBackgroundColor = self.backgroundColor;
textView.editable = NO;
textView.textLines = self.textLines;
self.valueTextView = textView;
self.valueView = textView;
[self.view addSubview:textView];
}
- (void)removeLabelAndField {
[self clearConstraints];
if (self.label) {
[self.label removeFromSuperview];
self.label = nil;
}
if (self.valueView) {
[self.valueView removeFromSuperview];
self.valueView = nil;
}
}
- (void)clearConstraints {
if (self.isViewLoaded && self.labelValueConstraints) {
[self.view removeConstraints:self.labelValueConstraints];
}
self.labelValueConstraints = nil;
self.labelToValueHorizConstraint = nil;
self.valueWidthConstraint = nil;
}
// This is called in our field's viewDidLoad, after we've created our label and valueView (UITextField, UITextView, etc)
- (void)setLabelAndFieldContraints {
[self clearConstraints];
self.labelValueConstraints = [NSMutableArray array];
self.label.translatesAutoresizingMaskIntoConstraints = NO;
self.valueView.translatesAutoresizingMaskIntoConstraints = NO;
NSLayoutConstraint *constraint = nil;
constraint = [NSLayoutConstraint
constraintWithItem:self.label attribute:NSLayoutAttributeLeft
relatedBy:NSLayoutRelationEqual
toItem:self.view attribute:NSLayoutAttributeLeft
multiplier:1.0f constant:self.labelValueGap];
constraint.priority = UILayoutPriorityRequired;
[self.labelValueConstraints addObject:constraint];
constraint = [NSLayoutConstraint
constraintWithItem:self.label attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationEqual
toItem:self.view attribute:NSLayoutAttributeTop
multiplier:1.0f constant:0];
constraint.priority = 550;
[self.labelValueConstraints addObject:constraint];
constraint = [NSLayoutConstraint
constraintWithItem:self.label attribute:NSLayoutAttributeBottom
relatedBy:NSLayoutRelationEqual
toItem:self.view attribute:NSLayoutAttributeBottom
multiplier:1.0f constant:0];
constraint.priority = 400;
[self.labelValueConstraints addObject:constraint];
constraint = [NSLayoutConstraint
constraintWithItem:self.valueView attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationEqual
toItem:self.view attribute:NSLayoutAttributeTop
multiplier:1.0f constant:0];
constraint.priority = UILayoutPriorityRequired;
[self.labelValueConstraints addObject:constraint];
constraint = [NSLayoutConstraint
constraintWithItem:self.valueView attribute:NSLayoutAttributeBottom
relatedBy:NSLayoutRelationEqual
toItem:self.view attribute:NSLayoutAttributeBottom
multiplier:1.0f constant:0];
constraint.priority = 499;
[self.labelValueConstraints addObject:constraint];
constraint = [NSLayoutConstraint
constraintWithItem:self.valueView attribute:NSLayoutAttributeRight
relatedBy:NSLayoutRelationEqual
toItem:self.view attribute:NSLayoutAttributeRight
multiplier:1.0f constant: -(kDisclosureWidth + self.labelValueGap) ];
constraint.priority = 901;
[self.labelValueConstraints addObject:constraint];
constraint = [NSLayoutConstraint
constraintWithItem:self.valueView attribute:NSLayoutAttributeLeading
relatedBy:NSLayoutRelationGreaterThanOrEqual
toItem:self.label attribute:NSLayoutAttributeTrailing
multiplier:1.0f constant:self.labelValueGap];
constraint.priority = UILayoutPriorityDefaultHigh + 1;
[self.labelValueConstraints addObject:constraint];
self.labelToValueHorizConstraint = constraint;
constraint = [NSLayoutConstraint
constraintWithItem:self.label attribute:NSLayoutAttributeBaseline
relatedBy:NSLayoutRelationEqual
toItem:self.valueView attribute:NSLayoutAttributeBaseline
multiplier:1.0f constant:0.f];
constraint.priority = 600;
[self.labelValueConstraints addObject:constraint];
constraint = [NSLayoutConstraint
constraintWithItem:self.valueView attribute:NSLayoutAttributeWidth
relatedBy:NSLayoutRelationEqual
toItem:self.view attribute:NSLayoutAttributeWidth
multiplier:(1.f - self.labelWidthPercentage) constant:0];
constraint.priority = 305;
[self.labelValueConstraints addObject:constraint];
self.valueWidthConstraint = constraint;
[self setCompressionAndHuggingForLabelView:self.label];
[self setCompressionAndHuggingForValueView:self.valueView];
}
- (void)setCompressionAndHuggingForLabelView:(UILabel *)labelView {
if (!labelView) {
return;
}
[labelView setContentCompressionResistancePriority:510 forAxis:UILayoutConstraintAxisHorizontal];
[labelView setContentCompressionResistancePriority:UILayoutPriorityDefaultHigh forAxis:UILayoutConstraintAxisVertical];
[labelView setContentHuggingPriority:450 forAxis:UILayoutConstraintAxisHorizontal];
[labelView setContentHuggingPriority:UILayoutPriorityDefaultHigh forAxis:UILayoutConstraintAxisVertical];
}
- (void)setCompressionAndHuggingForValueView:(UIView *)valueView {
if (!valueView) {
return;
}
[valueView setContentCompressionResistancePriority:509 forAxis:UILayoutConstraintAxisHorizontal];
[valueView setContentCompressionResistancePriority:UILayoutPriorityDefaultHigh forAxis:UILayoutConstraintAxisVertical];
[valueView setContentHuggingPriority:300 forAxis:UILayoutConstraintAxisHorizontal];
[valueView setContentHuggingPriority:650 forAxis:UILayoutConstraintAxisVertical];
}
/****** END Individual Field Controller ******/
I had an (extensive) conversation with an Apple engineer about this crash.
Here are the two most probable causes:
You have an invalid constraint, such as view1.left = view2.left + 20 where view2 is unexpectedly nil, or has a multiplier of 0. Be sure to double (and triple) check your constraints to make sure that they are correct. Here are 2 examples of problematic constraints:
// The first constraint would be a problem if view2 were nil
[NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:view2 attribute:NSLayoutAttributeBottom multiplier:1 constant:20];
// The second constraint is a problem because the 0 multiplier causes view2 to be "lost"
[NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:view2 attribute:NSLayoutAttributeBottom multiplier:0 constant:5];
You're hitting a bug in the internal Foundation auto layout engine related to accumulated loss of floating point precision. When you've crashed, the way you can know that this is the case is to search through the (large) exception log in the console for a very small (nearly zero) floating point number such as this:
<505:-7.45058e-08>*PWPlotLegendEntryView:0x600000582be0.Height{id: 34609} +
(Search for e- in the console output to find small numbers like this.) This number (-7.45058e-08 in this case) represents the coefficient at this particular point in time while the internal engine is solving constraints. In this case, the number is supposed to be exactly 0, but due to the way the auto layout engine does calculations with floating point numbers it has become an extremely tiny negative number instead which blows everything up. If you can find such a number in the output, you know that you've hit this bug.
How can you work around this issue?
Changing the order that you add (activate) constraints can end up changing the order of calculations in the internal engine, which as a result can cause this issue to disappear as the math is done without any problematic loss of precision.
This issue seems to come up more frequently when you have changed the content compression resistance or content hugging priorities for views, so try commenting out any code that does that to see if it's causing this bug to happen, or re-ordering it to happen earlier or later in your constraint setup code.
More details about my specific case:
I ran into this crash on iOS. The steps to reproduce it were quite interesting:
A view controller containing a table view was pushed on screen (in a navigation controller).
The table view had to contain enough cells so they didn't all fit in the visible area, then it had to be scrolled to the last cell and then back up a bit (presumably, this was causing cells to be reused, which was triggering this issue).
Then, when the view controller containing the table view was popped off the navigation stack, immediately after the pop animation completed the app would crash at the point where the view controller's view was removed from the view hierarchy.
After a lot of trial and error I was able to isolate the issue to one specific thing: setting the content compression resistance & hugging priorities for a UIImageView in each of the table view cells. In this case, the image view is being positioned using Auto Layout inside the cell, and to achieve the correct layout the image view needs to be exactly its intrinsic content size (the size of its image).
This was the problematic code:
// Inside of the UITableViewCell's updateConstraints method...
[self.imageView setContentCompressionResistancePriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisHorizontal];
[self.imageView setContentCompressionResistancePriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisVertical];
[self.imageView setContentHuggingPriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisHorizontal];
[self.imageView setContentHuggingPriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisVertical];
Removing the above code and replacing it with 2 constraints (at Required priority) to fix the width & height of the image view to the image's size achieved the same result, but avoided the crash. Here's the replacement code (using PureLayout):
[self.imageView autoSetDimensionsToSize:self.imageView.image.size];
I also found that just moving the problematic 4 lines to a different place in my constraint setup code resolved the issue, presumably because this changed the order of calculations sufficiently to prevent the problematic loss of precision.
Deallocation problem — one possibility
Your code that works with auto-layout may well run on the main thread but one of the blocks that's run in background and uses your view (perhaps indirectly), may hold a strong reference the view or one of its owner like view controller (that's the default behavior of Objective-C blocks). When such a block is run and deallocated on a background queue, strong references that it captures are released on that same queue and you may face a well known deallocation problem.
In your view controller, make sure you are using weak reference to self in all blocks that do not need a strong reference (and may run in background). You can declare it like this: __weak typeof(self) weakSelf = self; before the block — and use weakSelf inside the block.
Same goes for any local variables that hold references to your views — make sure their values are captured as weak refs.
Another possibility
In my work, I've encountered a similar issue on iOS 6 when hidden view participated in layout. Removing the view from hierarchy (-[UIView removeFromSuperview]) instead of setting hidden property to YES fixed the issue for me.
Had the same issue, solved it by deleting constraints one at a time in IB until the crash was solved. This narrowed it down to the offending constraint. I then reinstated said constraint, but reversed the items:
You may be as lucky and be able to solve your AL problems as easily.
To make #smileyborg's awesome answer more actionable:
This can happen if you have any constraints with multipliers that might suffer from floating point precision issues.
To solve:
Go over all your constraints that have multipliers (in layout code or by manually editing a storyboard/nib and searching for multiplier=).
If multiplier isn't a "pretty" power of two float, turn it to the nearest one (you can use a floating point calculator)
To easy way to do 2 is to enter the number you value you want and the calculator and then switch off lower precision bits in the mantissa until the value matches the rounded decimal value at bottom of the calculator.
For anyone encountering this issue in any iOS version > 8.0, the Apple docs state to use the "active" property on the NSLayoutConstraint rather than the removeConstraint/addConstraint functions on UIView. Apple Docs addConstraint reference
I am getting this crash when I call removeConstraints: with a nil argument.
In my case it was a proportional width constraint with 8:9 multiplier. I changed it to 7:9 and it worked.
BTW the easiest way to find a constraint is to start removing views from view controller. Do it using binary algorithm :) removing half views, then half of the half that makes app crash, etc.
For me the problem was that I was removing a constraint at a time that suited me, after calling dequeueReusableCellWithReuseIdentifier while setting properties of my UICollectionViewCell. The solution was to instead call:
[_myUICollectionViewCell setNeedsUpdateConstraints];
and override:
-(void)updateConstraints
and do my messing there. Seems that you can't just remove constraints when you like.
I just stumbled across the same error under OSX Mavericks with an OSX app I'm developing, but unlike the other answers given, I definitely don't have any other threads interacting with the UI objects, and the view hierarchy in question is definitely visible too. I'm not using blocks either. Bizarrely the problem went away when I removed a vertical constraint on an NSTextField.
FWIW the problematic view whose removal from its superview causes the "internal error. Cannot find an outgoing row head for incoming head" error is one of many side-panel controls which together present the properties of objects in the main view which can be cut, copied, created etc. This means that the user can be pasting new objects into the main view quite rapidly, meaning the side-panel's controls are being destroyed and new ones created very rapidly, too. Of course, with everything in the main thread, this should not make a difference, but it seems to.
The exact constraint causing problems was
[self addConstraint:
[NSLayoutConstraint
constraintWithItem:control
attribute:NSLayoutAttributeHeight
relatedBy:NSLayoutRelationEqual
toItem:other
attribute:NSLayoutAttributeHeight
multiplier:1.4
constant:0.0]];
where control is the (editable) NSTextField causing problems, and 'other' is another (non-editable) NSTextField label.
I got this problem with MZFormSheetController pod: https://github.com/m1entus/MZFormSheetController/issues/78
This code crashes:
[formSheetController.view addSubview:self.sharePanel];
// ...
[self.sharePanel removeFromSuperview]; // <-- CRASHES HERE
My solution is very strange but it works:
[self.sharePanel removeFromSuperview]; // <-- This line helps to avoid crash
[formSheetController.view addSubview:self.sharePanel];
// ...
[self.sharePanel removeFromSuperview];
And here is sharePanel property declaration:
#property (weak, nonatomic) IBOutlet UIView *sharePanel;
I got this crash when I still have a missing constraint in wAnyhAny mode, fixing this removed the error.
As the other answers in this thread indicate this is somehow an invalid autolayout/contraint problem, though it appears to be very finicky about what qualifies as "invalid".
Luckily I hadn't made many changes since my last commit and was able to track down the offending changes. For me having a view of 10 horizontal UIImageView with equal width and fixed 2:3 aspect ratio was the issue.
The crash only seemed to occur after leaving the UIViewController that contained this image row. Each UIImageView was set to UIViewContentModeScaleAspectFill. Removing this content mode change (which was done before the UIImages were set) seemed to fix my problem but wasn't an acceptable solution. I ended up removing the aspect ratio restriction and just using a fixed width and height for each image.
Why this was crashing my application I don't know... The crash could also ONLY be reproduce on an iPhone 4s running iOS 7.1.2. I tried to reproduce the same crash on an iPhone 4s simulator running iOS 9.1 without success. It also wouldn't crash when running on a phsyical iPhone 5 running iOS 9.1.
Hope that helps someone out there
According to Apple Documentation:
When developing for iOS 8.0 or later, set the constraint’s active property to YES instead of calling the addConstraint: method directly. The active property automatically adds and removes the constraint from the correct view.
In my case I had to modify the width constraint
for var constraint in self.navigationBar.constraints {
if constraint.identifier == "theProgressWidth" {
let sizeWidth = self.navigationBar.frame.size.width
constraint = NSLayoutConstraint(item: progress!, attribute: .Width, relatedBy: .Equal, toItem: self.navigationBar, attribute: .Width, multiplier: ((sizeWidth * (level / 100)) / sizeWidth), constant: 0)
constraint.active = true
}
}
I just spent the weekend trying to figure out the reason our app crashed with the debug console being filled with rows saying:
Failed to rebuild layout engine without detectable loss of precision. This should never happen. Performance and correctness may suffer.
This post led me in the right direciton, as the error turned out to be caused by having several constraints with multipliers set to arbitrary floating points with bad precision, as #yonix explained in his answer.
In our case the multipliers was dynamicaly set by the user draging a shape across the screen, ranging from 0 to 1.0, so naturally there would be cases where the multipliers required lots of bits set in the mantissa to be well defined.
By aligning the multipliers to a multiple of 1/2^n, the multipliers can be precisely defined with less bits in the mantissa, and the issue is averted.
We figured 9 bits (1/2^9) would be sufficient for us, and aligning the multiplier can be done by just multiplying by the factor, truncating and the dividing by the factor again.
let factor = 512.0 // 9-bits alignment
let alignedMultiplier = (multiplier * factor).rounded() / factor
Ensuring that all user defined multipliers are aligned before using them in our constraints, solved our problem.
So I add to this post for future reference, in case anyone else stumbles across the same issue.