From 44f8980262f47f0f24df40e840fe25bd3d859b13 Mon Sep 17 00:00:00 2001
From: lazy-iOS2 <luffy243077002@163.com>
Date: Mon, 1 Jan 2018 22:38:45 +0800
Subject: [PATCH] Support priority update

Add Support for priority update
---
 Masonry/MASViewConstraint.m | 30 ++++++++++++++++++++++++------
 1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/Masonry/MASViewConstraint.m b/Masonry/MASViewConstraint.m
index af3f0b44..f8740bc1 100644
--- a/Masonry/MASViewConstraint.m
+++ b/Masonry/MASViewConstraint.m
@@ -163,10 +163,17 @@ - (void)setSecondViewAttribute:(id)secondViewAttribute {
 
 - (MASConstraint * (^)(MASLayoutPriority))priority {
     return ^id(MASLayoutPriority priority) {
-        NSAssert(!self.hasBeenInstalled,
-                 @"Cannot modify constraint priority after it has been installed");
-        
-        self.layoutPriority = priority;
+        if (!self.hasBeenInstalled) {
+            self.layoutPriority = priority;
+        } else {
+            NSAssert(priority != UILayoutPriorityRequired
+                     && self.layoutConstraint.priority != UILayoutPriorityRequired,
+                     @"the priority cannot be changed from/to NSLayoutPriorityRequired");
+            if (priority != UILayoutPriorityRequired
+                && self.layoutConstraint.priority != UILayoutPriorityRequired){
+                self.layoutConstraint.priority = priority;
+            }
+        }
         return self;
     };
 }
@@ -359,8 +366,20 @@ - (void)install {
         existingConstraint = [self layoutConstraintSimilarTo:layoutConstraint];
     }
     if (existingConstraint) {
-        // just update the constant
+        // update the constant
         existingConstraint.constant = layoutConstraint.constant;
+        
+        // update the priority if possible
+        if (existingConstraint.priority != layoutConstraint.priority) {
+            NSAssert(existingConstraint.priority != UILayoutPriorityRequired
+                     && layoutConstraint.priority != UILayoutPriorityRequired,
+                     @"the priority cannot be changed from/to NSLayoutPriorityRequired");
+            if (existingConstraint.priority != UILayoutPriorityRequired
+                && layoutConstraint.priority != UILayoutPriorityRequired){
+                existingConstraint.priority = layoutConstraint.priority;
+            }
+        }
+        
         self.layoutConstraint = existingConstraint;
     } else {
         [self.installedView addConstraint:layoutConstraint];
@@ -382,7 +401,6 @@ - (MASLayoutConstraint *)layoutConstraintSimilarTo:(MASLayoutConstraint *)layout
         if (existingConstraint.secondAttribute != layoutConstraint.secondAttribute) continue;
         if (existingConstraint.relation != layoutConstraint.relation) continue;
         if (existingConstraint.multiplier != layoutConstraint.multiplier) continue;
-        if (existingConstraint.priority != layoutConstraint.priority) continue;
 
         return (id)existingConstraint;
     }