CompactConstraint

A simple NSLayoutConstraint expression parser for more readable autolayout code. By Marco Arment and released under the MIT license (see LICENSE file).

Apple's visual format syntax is helpful, but it cannot express all types of autolayout constraints, and it's not ideal for simple values. Creating NSLayoutConstraints manually is more powerful and covers all possibilities, but is extremely verbose and hard to read.

CompactConstraint brings compact, readable syntax to creating NSLayoutConstraints manually. In short, rather than writing Cocoa autolayout constraints like this:

[self.view addConstraint:[NSLayoutConstraint constraintWithItem:emailLabel attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationGreaterThanOrEqual toItem:emailField attribute:NSLayoutAttributeLeft multiplier:1.0f constant:0.0f]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:spinner attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:logInButton attribute:NSLayoutAttributeRight multiplier:1.0f constant:10.0f]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:preview attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:preview attribute:NSLayoutAttributeWidth multiplier:0.625f constant:0.0f]];

CompactConstraint lets you write them like this:

[self.view addCompactConstraint:@"emailLabel.left >= emailField.left" metrics:metrics views:views];
[self.view addCompactConstraint:@"spinner.left = logInButton.right + 10" metrics:metrics views:views];
[self.view addCompactConstraint:@"preview.height = preview.width / 1.6" metrics:metrics views:views];

Or this:

[self.view addCompactConstraints:@[
    @"emailLabel.left >= emailField.left",
    @"spinner.left = logInButton.right + 10",
    @"preview.height = preview.width / 1.6"
] metrics:metrics views:views];

Syntax

CompactConstraint syntax maps directly to the paramters passed to NSLayoutConstraint's built-in constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant: method:

leftItem.attribute [= == <= >=] rightItem.attribute [* /] multiplier [+ -] constant @ priority # identifier

leftItem and rightItem are keys from the supplied views dictionary that map to UIViews, just like with Apple's visual-format-language calls.

rightItem may be "super", which is interpreted as leftItem's superview, or "safe", which is interpreted as leftItem's superview's safeAreaLayoutGuide. If you specify @"super" or @"safe" keys in views, your supplied values will be used instead.

multiplier, constant, priority, and identifier are optional. Additionally, rightItem.attribute, multiplier, constant, and priority can all optionally be replaced by entries in the supplied metrics dictionary, mapping to NSNumbers.

In iOS 7+ anything following the # (or the format string itself if no # is present) is set as the constraint’s identifier for easier debugging. To prevent any identifier being set, end your format string with a bare #.

Valid attributes are simply the ends of the NSLayoutAttribute definitions with initial lowercase letters:

left
right
top
bottom
leading
trailing
width
height
centerX
centerY
baseline

For example, given these dictionaries as the views and metrics parameters:

NSDictionary *views = @{
    @"button" : self.button,
    @"label" : self.label,
    @"spinner" : activitySpinner,
};

NSDictionary *metrics = @{
    @"buttonHeight" : @(36),
    @"spacing" : @(15),
    @"scale" : @(1.25),
    @"highPriority", : @(UILayoutPriorityDefaultHigh),
};

These would all be valid CompactConstraint syntax:

button.centerX = super.centerX # horizontally-center button
label.width = button.width * scale + 40
spinner.left = button.right + 10 @ highPriority
button.height = buttonHeight
spinner.height = 40 @ 100

Installation

Just include the handful of files in the CompactConstraint directory in your project, and wherever you'd like to use this (or in the precompiled header): #import "CompactConstraint.h"

Is it fast?

It's fast enough for me. Try it and see if it's fast enough for you.