adding new mas_updateConstraints method

Masonry/MASCompositeConstraint.m

@@ -18,6 +18,7 @@ @interface MASCompositeConstraint () <MASConstraintDelegate>
 @implementation MASCompositeConstraint
 
 @synthesize delegate = _delegate;
+@synthesize updateExisting = _updateExisting;
 
 - (id)initWithChildren:(NSArray *)children {
     self = [super init];
@@ -181,6 +182,7 @@ - (void)constraint:(id<MASConstraint>)constraint shouldBeReplacedWithConstraint:
 
 - (void)install {
     for (id<MASConstraint> constraint in self.childConstraints) {
+        constraint.updateExisting = self.updateExisting;
         [constraint install];
     }
 }

Masonry/MASConstraint.h

@@ -113,6 +113,11 @@
  */
 @property (nonatomic, copy, readonly) id<MASConstraint> (^key)(id key);
 
+/**
+ *  Whether or not to check for an existing constraint instead of adding constraint
+ */
+@property (nonatomic, assign) BOOL updateExisting;
+
 /**
  *	Creates a NSLayoutConstraint. The constraint is installed to the first view or the or the closest common superview of the first and second view. 
  */

Masonry/MASConstraintMaker.h

@@ -53,6 +53,11 @@
  */
 @property (nonatomic, strong, readonly) id<MASConstraint> center;
 
+/**
+ *  Whether or not to check for an existing constraint instead of adding constraint
+ */
+@property (nonatomic, assign) BOOL updateExisting;
+
 /**
  *	initialises the maker with a default view
  *

Masonry/MASConstraintMaker.m

@@ -34,6 +34,7 @@ - (id)initWithView:(MAS_VIEW *)view {
 - (NSArray *)install {
     NSArray *constraints = self.constraints.copy;
     for (id<MASConstraint> constraint in constraints) {
+        constraint.updateExisting = self.updateExisting;
         [constraint install];
     }
     [self.constraints removeAllObjects];

Masonry/MASViewConstraint.m

@@ -28,6 +28,7 @@ @interface MASViewConstraint ()
 @implementation MASViewConstraint
 
 @synthesize delegate = _delegate;
+@synthesize updateExisting = _updateExisting;
 
 - (id)initWithFirstViewAttribute:(MASViewAttribute *)firstViewAttribute {
     self = [super init];
@@ -295,7 +296,11 @@ - (void)install {
         self.installedView = firstLayoutItem;
     }
 
-    MASLayoutConstraint *existingConstraint = [self layoutConstraintSimiliarTo:layoutConstraint];
+
+    MASLayoutConstraint *existingConstraint = nil;
+    if (self.updateExisting) {
+        existingConstraint = [self layoutConstraintSimiliarTo:layoutConstraint];
+    }
     if (existingConstraint) {
         // just update the constant
         existingConstraint.constant = layoutConstraint.constant;

Masonry/View+MASAdditions.h

@@ -55,4 +55,15 @@
  */
 - (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *make))block;
 
+/**
+ *  Creates a MASConstraintMaker with the callee view.
+ *  Any constraints defined are added to the view or the appropriate superview once the block has finished executing.
+ *  If an existing constraint exists then it will be updated instead.
+ *
+ *  @param block scope within which you can build up the constraints which you wish to apply to the view.
+ *
+ *  @return Array of created/updated MASConstraints
+ */
+- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block;
+
 @end

Masonry/View+MASAdditions.m

@@ -18,6 +18,14 @@ - (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *))block {
     return [constraintMaker install];
 }
 
+- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *))block {
+    self.translatesAutoresizingMaskIntoConstraints = NO;
+    MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];
+    block(constraintMaker);
+    constraintMaker.updateExisting = YES;
+    return [constraintMaker install];
+}
+
 #pragma mark - NSLayoutAttribute properties
 
 - (MASViewAttribute *)mas_left {

Masonry/View+MASShorthandAdditions.h

@@ -29,6 +29,7 @@
 @property (nonatomic, strong, readonly) MASViewAttribute *baseline;
 
 - (NSArray *)makeConstraints:(void(^)(MASConstraintMaker *make))block;
+- (NSArray *)updateConstraints:(void(^)(MASConstraintMaker *make))block;
 
 @end
 
@@ -55,6 +56,10 @@ MAS_ATTR_FORWARD(baseline);
     return [self mas_makeConstraints:block];
 }
 
+- (NSArray *)updateConstraints:(void(^)(MASConstraintMaker *))block {
+    return [self mas_updateConstraints:block];
+}
+
 @end
 
 #endif