'NSInvalidArgumentException', reason: 'Unable to parse constraint format'

Question

I have a subview that I want to keep stops during rotating screen, so I decided to put the NSLayoutConstraint type: Trailing Space to Superview Top Space to Superview Button Space to Superview I'm in a subclass of UITableViewCell. I wrote the code but I get the following error: ``` 'NSInvalidArgumentException', reason: 'Unable to parse constraint format: self is not a key in the views dictionary. H:[self.arrows]-5-| ``` My code in CustomCell.m is: ``` self.arrows = [[Arrows alloc]initWithFrame:CGRectMake(self.contentView.bounds.size.width-30, self.bounds.origin.y+4, 30, self.contentView.bounds.size.height-4)]; NSDictionary *viewsDictionary = NSDictionaryOfVariableBindings(self.arrows, self.contentView); NSMutableArray * constraint=[[NSMutableArray alloc]init]; [constraint addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H: [self.arrows]-5-|" options:0 metrics:nil views:viewsDictionary]]; [constraint addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualForm…

Accepted Answer

It looks like that the autolayout visual format parsing engine is interpreting the "`.`" in your VFL constraint to be a keyPath instead of a key like it's using `valueForKeyPath:`.

`NSDictionaryOfVariableBindings(...)` will take whatever your parameter is in the parenthesis and translate it into a literal key with the object as the value (in your case: `@{"self.arrow" : self.arrow}`). In the case of the VFL, autolayout is thinking that you have a key named `self` in your view dictionary with a subdictionary (or subobject) that has a key of `arrow`,

```
@{
   @"self" : @{ @"arrow" : self.arrow }
}
```

when you literally wanted the system to interpret your key as "`self.arrow`".

Usually, when I'm using a instance variables getter like this, I typically end up creating my own dictionary instead of using `NSDictionaryOfVariableBindings(...)` like so:

```
NSDictionary *views = @{ @"arrowView" : self.arrow }
```

or

```
NSDictionary *views = NSDictionaryOfVariableBindings(_arrow);
```

Which would allow you to use the view in your VFL without the self and you still know what you're talking about:

```
NSArray *arrowHorizConstraints = [NSLayoutConstraint constraintsWithVisualFormat:@"H:[arrowView]-5-|" options:0 metrics:nil views];
```

or

```
NSArray *arrowHorizConstraints = [NSLayoutConstraint constraintsWithVisualFormat:@"H:[_arrow]-5-|" options:0 metrics:nil views];
```

As a general rule, I've learned not to have dictionary keys with a dot (`.`) in them to avoid any system confusion or debugging nightmares.

'NSInvalidArgumentException', reason: 'Unable to parse constraint format'

Solution

Problem