黄喉貂,蒙雷亚尔,六个月宝宝吐奶
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？
我要评论 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 { $0 == 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
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