Description

PLVisualAttributeConstraints is small lib that makes it easier to create layout constraints (see: NSLayoutConstraint class).

If you don't know much about the AutoLayout mechanism, I strongly suggest you to go there for more information.

PLVisualAttributeConstraints does not try to replace standard VFL (Visual Format Language) or alter default Apple's mechanisms. It integrates with it seamlessly and greatly improves developer's productivity and code readability.

Example

In a nutshell, having two views...

  UIView *firstViewObj = ...
  UIView *secondViewObj = ...

using this lib you can create layout constraint like...

  NSDictionary *views = @{
          @"firstView" : firstViewObj,
          @"secondView" : secondViewObj
  };

  NSLayoutConstraint *constraint1 = 
    [NSLayoutConstraint attributeConstraintWithVisualFormat:@"secondView.left >= firstView.left * 2 + 10"
                                                                                      views:views];

instead of standard

  NSLayoutConstraint *constraint2 = 
    [NSLayoutConstraint constraintWithItem:secondViewObj
                                 attribute:NSLayoutAttributeLeft
                                 relatedBy:NSLayoutRelationGreaterThanOrEqual
                                    toItem:firstViewObj
                                 attribute:NSLayoutAttributeLeft
                                multiplier:2
                                  constant:10];

Those two constraints (constraint1 and constraint2) are identical to each other.

It's very likely that you'll have lots of constraints (it's usual case). Note, that you need to create dictionary with views only once (f.e. at the very beggining of a constraints-creating method) and then you go on creating them with similar one-liners (like in the example above) or you can take advantage of another method:

  NSArray *constraints = [NSLayoutConstraint attributeConstraintsWithVisualFormatsArray:@[
          @"secondView.left <= firstView.left - 10",
          @"secondView.right >= firstView.right + 10",
          @"secondView.top == firstView.bottom * 2.5 + 5",
  ]                                                                               views:views];

Grammar by example ;)

Valid grammar examples

Instead of introducing formal grammar here, I'm quite convinced that a few examples will be more that enough to get you up and running.

control1.top >= control2.bottom + 20 control1.top <= control2.bottom - 20.5 control1.top == control2.bottom * 2.3 + 4.5 control1.top >= control2.bottom * 2 - 10

A little bit hacky (read on to see why): control1.top >= 100 control1.top == 100.5

Supported attributes are:

top, bottom, left, right, leading, trailing, width, height, centerx, centery, baseline (case insensitive) Those attributes are mapped one-to-one with those enums.

Supported relations:

==, >=, <= (mapped one-to-one with those enums)

Demo

To see the lib in action (and in comparision with standard constraint-creating method), checkout the project. Open it via *.xcworkspace and not *.xcproject file (why? CocoaPods). Yeap, I know that this example is somewhat artificial :) But it'll do. If you have better suggestions on how to prepare the demo, pull requests are welcome :)

Hacks

As I've mentioned above, formats like control1.top >= 100 (without an attribute on the right side, only constant) are supported.

Whole lib is a thin wrapper above this method and as we read:

Constraints are of the form view1.attr1 <relation> view2.attr2 * multiplier + constant. If the constraint you wish to express does not have a second view and attribute, use nil and NSLayoutAttributeNotAnAttribute.

Well... That works for constraints with width or height attribute on the left side (control1.width == 100). In case of any other attribute (top, left...) it throws an exception upon constraint creation... And it somewhat makes sense.

Sometimes you want to create such a constraint anyway. In that case, you can create control1.top == control1.top * 0 + 50 which should behave exactly the same as control1.top == 50 and it does :)

TL;DR

Constraints like control1.top >= 100 are automagically created as control1.top >= control1.top * 0 + 100, which I consider a bit hacky, therefore elaborate on it here.

Installation

Just copy source files under PLVisualAttributeConstraints/PLVisualAttributeConstraints/* into your project. Support for installation via CocoaPods will follow shortly.

Requirements

• iOS 6.0+

Notes:

• to open project, use *.xcworkspace and not *.xcproject file (as I use, love and recommend CocoaPods)
• tests are created using Kiwi framework

Author

Kamil Jaworski ([email protected]), Polidea