最近在使用snapkit布局时，竟然发现更新约束会导致崩溃，为什么这样呢？
checkButton.snp.makeConstraints { (make) in
make.left.top.equalToSuperview()
make.height.equalTo(radioListSubviewButtonHeight)
make.width.equalTo(self).multipliedBy(0.5)
}
checkButton.snp.updateConstraints { (make) in
make.width.equalTo(self).multipliedBy(0.7)
}
看起来完全没有问题，但是一运行就崩溃，崩溃位置在activeIfNeeded方法中：
internal func activateIfNeeded(updatingExisting: Bool = false) {
guard let item = self.from.layoutConstraintItem else {
print("WARNING: SnapKit failed to get from item from constraint. Activate will be a no-op.")
return
}
let layoutConstraints = self.layoutConstraints
if updatingExisting {
var existingLayoutConstraints: [LayoutConstraint] = []
for constraint in item.constraints {
existingLayoutConstraints += constraint.layoutConstraints
}
for layoutConstraint in layoutConstraints {
let existingLayoutConstraint = existingLayoutConstraints.first { $ == layoutConstraint }
guard let updateLayoutConstraint = existingLayoutConstraint else {
fatalError("Updated constraint could not find existing matching constraint to update: \(layoutConstraint)")
}
let updateLayoutAttribute = (updateLayoutConstraint.secondAttribute == .notAnAttribute) ? updateLayoutConstraint.firstAttribute : updateLayoutConstraint.secondAttribute
updateLayoutConstraint.constant = self.constant.constraintConstantTargetValueFor(layoutAttribute: updateLayoutAttribute)
}
} else {
NSLayoutConstraint.activate(layoutConstraints)
item.add(constraints: [self])
}
}
fatalerror信息提示找不到因存在的constraint来更新，输出existingLayoutConstraints不为nil，输出existingLayoutConstraint却为nil，很奇怪。
点击进入first方法，发现是取第一个满足谓词条件的值，而不是简单的取第一个值：
/// Returns the first element of the sequence that satisfies the given
/// predicate.
///
/// The following example uses the `first(where:)` method to find the first
/// negative number in an array of integers:
///
/// let numbers = [3, 7, 4, -2, 9, -6, 10, 1]
/// if let firstNegative = numbers.first(where: { $0 < 0 }) {
/// print("The first negative number is \(firstNegative).")
/// }
/// // Prints "The first negative number is -2."
///
/// - Parameter predicate: A closure that takes an element of the sequence as
/// its argument and returns a Boolean value indicating whether the
/// element is a match.
/// - Returns: The first element of the sequence that satisfies `predicate`,
/// or `nil` if there is no element that satisfies `predicate`.
///
/// - Complexity: O(*n*), where *n* is the length of the sequence.
public func first(where predicate: (Element) throws -> Bool) rethrows -> Element?
在此处谓词条件为 $0 == layoutConstraint ,进入LayoutConstraint类中，发现==运算符已被重载：
internal func ==(lhs: LayoutConstraint, rhs: LayoutConstraint) -> Bool {
guard lhs.firstItem === rhs.firstItem &&
lhs.secondItem === rhs.secondItem &&
lhs.firstAttribute == rhs.firstAttribute &&
lhs.secondAttribute == rhs.secondAttribute &&
lhs.relation == rhs.relation &&
lhs.priority == rhs.priority &&
lhs.multiplier == rhs.multiplier else {
return false
}
return true
}
在判断相等时，竟然还判断了multiplier，本例中更新约束前后multiplier不相等，所以找不到对应的约束。
解决方法：
要想修改multiplier，不能使用update，要使用remake