iOS swift : Constraints did not update on screen
<p><p>晚上，</p>
<p>我遇到了 ScrollView 约束的问题。</p>
<p>这是我的代码:</p>
<pre><code>// when the keyboard is shown, move up some elements
@objc func keyboardWillShow(notification: NSNotification) {
if let keyboardSize = (notification.userInfo? as? NSValue)?.cgRectValue {
// Bring up the scrol lview
print(scrollViewBottomConstraint.constant)
self.scrollViewBottomConstraint.constant = keyboardSize.height
print(scrollViewBottomConstraint.constant)
self.view.updateConstraints()
self.view.updateConstraintsIfNeeded()
self.view.needsUpdateConstraints()
self.view.layoutIfNeeded()
// Bring up the next arrow with animation
self.nextButtonBottomConstraint.constant = keyboardSize.height + 8
UIView.animate(
withDuration: 1.0,
delay: 0.0,
usingSpringWithDamping: 0.5,
initialSpringVelocity: 6.0,
options: [.allowUserInteraction,
.curveEaseInOut],
animations: {
self.view.layoutIfNeeded()
}, completion: nil)
}
}
</code></pre>
<p>控制台打印 0 和 253，因此约束常量被更新。但在屏幕上我看不到变化，我有这个日志:</p>
<blockquote>
<p> Snapshotting a view (0x7fa041471bc0,
UIInputSetHostView) that has not been rendered at least once requires
afterScreenUpdates:YES.</p>
</blockquote></p>
<br><hr><h1><strong>Best Answer-推荐答案</ strong></h1><br>
<p><p>很奇怪，更改约束后只需 view.layoutIfNeeded() 就可以了。</p>
<p>尝试删除:</p>
<pre><code>self.view.updateConstraints()
self.view.updateConstraintsIfNeeded()
self.view.needsUpdateConstraints()
</code></pre>
<p>只是:</p>
<pre><code> view.layoutIfNeeded()
</code></pre>
<p>试试这个:</p>
<pre><code>@objc func keyboardWillShow(notification: NSNotification) {
guard let keyboardSize = (notification.userInfo? as? NSValue)?.cgRectValue else{
return
}
scrollViewBottomConstraint.constant = keyboardSize.height
nextButtonBottomConstraint.constant = keyboardSize.height + 8
view.layoutIfNeeded()
}
</code></pre></p>
<p style="font-size: 20px;">关于iOSswift: Constraints did not update on screen，我们在Stack Overflow上找到一个类似的问题：
<a href="https://stackoverflow.com/questions/47157215/" rel="noreferrer noopener nofollow" style="color: red;">
https://stackoverflow.com/questions/47157215/
</a>
</p>
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