#Masonry

Masonry is a light-weight layout framework which wraps AutoLayout with a nicer syntax. Masonry has its own layout DSL which provides a chainable way of describing your NSLayoutConstraints which results in layout code that is more concise and readable. Masonry supports iOS and Mac OS X.

For examples take a look at the Masonry iOS Examples project in the Masonry workspace. You will need to run pod install after downloading.

What's wrong with NSLayoutConstraints?

Under the hood Auto Layout is a powerful and flexible way of organising and laying out your views. However creating constraints from code is verbose and not very descriptive. Imagine a simple example in which you want to have a view fill its superview but inset by 10 pixels on every side

UIView *superview = self;

UIView *view1 = [[UIView alloc] init];
view1.translatesAutoresizingMaskIntoConstraints = NO;
view1.backgroundColor = [UIColor greenColor];
[superview addSubview:view1];

UIEdgeInsets padding = UIEdgeInsetsMake(10, 10, 10, 10);

[superview addConstraints:@[

    //view1 constraints
    [NSLayoutConstraint constraintWithItem:view1
                                 attribute:NSLayoutAttributeTop
                                 relatedBy:NSLayoutRelationEqual
                                    toItem:superview
                                 attribute:NSLayoutAttributeTop
                                multiplier:1.0
                                  constant:padding.top],

    [NSLayoutConstraint constraintWithItem:view1
                                 attribute:NSLayoutAttributeLeft
                                 relatedBy:NSLayoutRelationEqual
                                    toItem:superview
                                 attribute:NSLayoutAttributeLeft
                                multiplier:1.0
                                  constant:padding.left],

    [NSLayoutConstraint constraintWithItem:view1
                                 attribute:NSLayoutAttributeBottom
                                 relatedBy:NSLayoutRelationEqual
                                    toItem:superview
                                 attribute:NSLayoutAttributeBottom
                                multiplier:1.0
                                  constant:-padding.bottom],

    [NSLayoutConstraint constraintWithItem:view1
                                 attribute:NSLayoutAttributeRight
                                 relatedBy:NSLayoutRelationEqual
                                    toItem:superview
                                 attribute:NSLayoutAttributeRight
                                multiplier:1
                                  constant:-padding.right],

 ]];

Even with such a simple example the code needed is quite verbose and quickly becomes unreadable when you have more than 2 or 3 views. Another option is to use Visual Format Language (VFL), which is a bit less long winded. However the ASCII type syntax has its own pitfalls and its also a bit harder to animate as NSLayoutConstraint constraintsWithVisualFormat: returns an array.

Prepare to meet your Maker!

Heres the same constraints created using MASConstraintMaker

UIEdgeInsets padding = UIEdgeInsetsMake(10, 10, 10, 10);

[view1 mas_makeConstraints:^(MASConstraintMaker *make) {
    make.top.equalTo(superview.mas_top).with.offset(padding.top); //with is an optional semantic filler
    make.left.equalTo(superview.mas_left).with.offset(padding.left);
    make.bottom.equalTo(superview.mas_bottom).with.offset(-padding.bottom);
    make.right.equalTo(superview.mas_right).with.offset(-padding.right);
}];

Or even shorter

[view1 mas_makeConstraints:^(MASConstraintMaker *make) {
    make.edges.equalTo(superview).with.insets(padding);
}];

Also note in the first example we had to add the constraints to the superview [superview addConstraints:.... Masonry however will automagically add constraints to the appropriate view.

Masonry will also call view1.translatesAutoresizingMaskIntoConstraints = NO; for you.

Not all things are created equal

.equalTo equivalent to NSLayoutRelationEqual

.lessThanOrEqualTo equivalent to NSLayoutRelationLessThanOrEqual

.greaterThanOrEqualTo equivalent to NSLayoutRelationGreaterThanOrEqual

These three equality constraints accept one argument which can be any of the following:

1. MASViewAttribute

make.centerX.lessThanOrEqualTo(view2.mas_left);

MASViewAttribute	NSLayoutAttribute
view.mas_left	NSLayoutAttributeLeft
view.mas_right	NSLayoutAttributeRight
view.mas_top	NSLayoutAttributeTop
view.mas_bottom	NSLayoutAttributeBottom
view.mas_leading	NSLayoutAttributeLeading
view.mas_trailing	NSLayoutAttributeTrailing
view.mas_width	NSLayoutAttributeWidth
view.mas_height	NSLayoutAttributeHeight
view.mas_centerX	NSLayoutAttributeCenterX
view.mas_centerY	NSLayoutAttributeCenterY
view.mas_baseline	NSLayoutAttributeBaseline

2. UIView/NSView

if you want view.left to be greater than or equal to label.left :

//these two constraints are exactly the same
make.left.greaterThanOrEqualTo(label);
make.left.greaterThanOrEqualTo(label.mas_left);

3. NSNumber

Auto Layout allows width and height to be set to constant values. if you want to set view to have a minimum and maximum width you could pass a number to the equality blocks:

//width >= 200 && width <= 400
make.width.greaterThanOrEqualTo(@200);
make.width.lessThanOrEqualTo(@400)

However Auto Layout does not allow alignment attributes such as left, right, centerY etc to be set to constant values. So if you pass a NSNumber for these attributes Masonry will turn these into constraints relative to the view’s superview ie:

//creates view.left = view.superview.left + 10
make.left.lessThanOrEqualTo(@10)

Instead of using NSNumber, you can use primitives and structs to build your constraints, like so:

make.top.mas_equalTo(42);
make.height.mas_equalTo(20);
make.size.mas_equalTo(CGSizeMake(50, 100));
make.edges.mas_equalTo(UIEdgeInsetsMake(10, 0, 10, 0));
make.left.mas_equalTo(view).mas_offset(UIEdgeInsetsMake(10, 0, 10, 0));

By default, macros which support autoboxing are prefixed with mas_. Unprefixed versions are available by defining MAS_SHORTHAND_GLOBALS before importing Masonry.

4. NSArray

An array of a mixture of any of the previous types

make.height.equalTo(@[view1.mas_height, view2.mas_height]);
make.height.equalTo(@[view1, view2]);
make.left.equalTo(@[view1, @100, view3.right]);

Learn to prioritize

.prority allows you to specify an exact priority

.priorityHigh equivalent to UILayoutPriorityDefaultHigh

.priorityMedium is half way between high and low

.priorityLow equivalent to UILayoutPriorityDefaultLow

Priorities are can be tacked on to the end of a constraint chain like so:

make.left.greaterThanOrEqualTo(label.mas_left).with.priorityLow();

make.top.equalTo(label.mas_top).with.priority(600);

Composition, composition, composition

Masonry also gives you a few convenience methods which create multiple constraints at the same time. These are called MASCompositeConstraints

edges

// make top, left, bottom, right equal view2
make.edges.equalTo(view2);

// make top = superview.top + 5, left = superview.left + 10,
//      bottom = superview.bottom - 15, right = superview.right - 20
make.edges.equalTo(superview).insets(UIEdgeInsetsMake(5, 10, 15, 20))

size

// make width and height greater than or equal to titleLabel
make.size.greaterThanOrEqualTo(titleLabel)

// make width = superview.width + 100, height = superview.height - 50
make.size.equalTo(superview).sizeOffset(CGSizeMake(100, -50))

center

// make centerX and centerY = button1
make.center.equalTo(button1)

// make centerX = superview.centerX - 5, centerY = superview.centerY + 10
make.center.equalTo(superview).centerOffset(CGPointMake(-5, 10))

You can chain view attributes for increased readability:

// All edges but the top should equal those of the superview
make.left.right.and.bottom.equalTo(superview);
make.top.equalTo(otherView);

Hold on for dear life

Sometimes you need modify existing constraints in order to animate or remove/replace constraints. In Masonry there are a few different approaches to updating constraints.

1. References

You can hold on to a reference of a particular constraint by assigning the result of a constraint make expression to a local variable or a class property. You could also reference multiple constraints by storing them away in an array.

// in public/private interface
@property (nonatomic, strong) MASConstraint *topConstraint;

...

// when making constraints
[view1 mas_makeConstraints:^(MASConstraintMaker *make) {
    self.topConstraint = make.top.equalTo(superview.mas_top).with.offset(padding.top);
    make.left.equalTo(superview.mas_left).with.offset(padding.left);
}];

...
// then later you can call
[self.topConstraint uninstall];

2. mas_updateConstraints

Alternatively if you are only updating the constant value of the constraint you can use the convience method mas_updateConstraints instead of mas_makeConstraints

// this is Apple's recommended place for adding/updating constraints
// this method can get called multiple times in response to setNeedsUpdateConstraints
// which can be called by UIKit internally or in your code if you need to trigger an update to your constraints
- (void)updateConstraints {
    [self.growingButton mas_updateConstraints:^(MASConstraintMaker *make) {
        make.center.equalTo(self);
        make.width.equalTo(@(self.buttonSize.width)).priorityLow();
        make.height.equalTo(@(self.buttonSize.height)).priorityLow();
        make.width.lessThanOrEqualTo(self);
        make.height.lessThanOrEqualTo(self);
    }];
    
    //according to apple super should be called at end of method
    [super updateConstraints];
}

3. mas_remakeConstraints

mas_updateConstraints is useful for updating a set of constraints, but doing anything beyond updating constant values can get exhausting. That's where mas_remakeConstraints comes in.

mas_remakeConstraints is similar to mas_updateConstraints, but instead of updating constant values, it will remove all of its contraints before installing them again. This lets you provide different constraints without having to keep around references to ones which you want to remove.

- (void)changeButtonPosition {
    [self.button mas_remakeConstraints:^(MASConstraintMaker *make) {
        make.width.equalTo(@(self.buttonSize.width));
        make.height.equalTo(@(self.buttonSize.height));
        
        if (topLeft) {
        	make.top.equalTo(@10);
        	make.left.equalTo(@10);
        } else {
        	make.bottom.equalTo(self.view.mas_bottom).with.offset(-10);
        	make.right.equalTo(self.view.mas_right).with.offset(-10);
        }
    }];
}

You can find more detailed examples of all three approaches in the Masonry iOS Examples project.

When the ^&*!@ hits the fan!

Laying out your views doesn't always goto plan. So when things literally go pear shaped, you don't want to be looking at console output like this:

Unable to simultaneously satisfy constraints.....blah blah blah....
(
    "<NSLayoutConstraint:0x7189ac0 V:[UILabel:0x7186980(>=5000)]>",
    "<NSAutoresizingMaskLayoutConstraint:0x839ea20 h=--& v=--& V:[MASExampleDebuggingView:0x7186560(416)]>",
    "<NSLayoutConstraint:0x7189c70 UILabel:0x7186980.bottom == MASExampleDebuggingView:0x7186560.bottom - 10>",
    "<NSLayoutConstraint:0x7189560 V:|-(1)-[UILabel:0x7186980]   (Names: '|':MASExampleDebuggingView:0x7186560 )>"
)

Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x7189ac0 V:[UILabel:0x7186980(>=5000)]>

Masonry adds a category to NSLayoutConstraint which overrides the default implementation of - (NSString *)description. Now you can give meaningful names to views and constraints, and also easily pick out the constraints created by Masonry.

which means your console output can now look like this:

Unable to simultaneously satisfy constraints......blah blah blah....
(
    "<NSAutoresizingMaskLayoutConstraint:0x8887740 MASExampleDebuggingView:superview.height == 416>",
    "<MASLayoutConstraint:ConstantConstraint UILabel:messageLabel.height >= 5000>",
    "<MASLayoutConstraint:BottomConstraint UILabel:messageLabel.bottom == MASExampleDebuggingView:superview.bottom - 10>",
    "<MASLayoutConstraint:ConflictingConstraint[0] UILabel:messageLabel.top == MASExampleDebuggingView:superview.top + 1>"
)

Will attempt to recover by breaking constraint
<MASLayoutConstraint:ConstantConstraint UILabel:messageLabel.height >= 5000>

For an example of how to set this up take a look at the Masonry iOS Examples project in the Masonry workspace.

Installation

Use the orsome CocoaPods.

In your Podfile

pod 'Masonry'

If you want to use masonry without all those pesky 'mas_' prefixes. Add #define MAS_SHORTHAND to your prefix.pch before importing Masonry

#define MAS_SHORTHAND

Get busy Masoning

#import "Masonry.h"

Features

• Not limited to subset of Auto Layout. Anything NSLayoutConstraint can do, Masonry can do too!
• Great debug support, give your views and constraints meaningful names.
• Constraints read like sentences.
• No crazy macro magic. Masonry won't pollute the global namespace with macros.
• Not string or dictionary based and hence you get compile time checking.

TODO

• Eye candy
• Mac example project
• More tests and examples