iOS开发——使用Autolayout弹出键盘
参考：
http://segmentfault.com/q/1010000002420050
http://blog.csdn.net/qq448631961/article/details/40345653
思路：
在整个View下面塞进一个高度为0的视图(使用低优先级约束)，当键盘改变时改变该View的高度即可。
constraint 有一个唯一可以修改的属性 constant，我承认它的名字确实很具有迷惑性。。。
以题主提到的高度问题为例，可以保存这个高度 constraint 的引用，在键盘出现和收起时
- (void)keyboardDidShowNotification
{
self.viewHeightConstraint.constant = 100.f;
[self.myView layoutIfNeed];
}
- (void)keyboardDidHideNotification
{
self.viewHeightConstraint.constant = 300.f;
[self.myView layoutIfNeed];
}
我的代码：
注册通知：
1: [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(keyboardWillShow:) name:UIKeyboardWillChangeFrameNotification object:nil];
约束
@property (nonatomic,strong) NSLayoutConstraint* keyboardConstraint;
初始为0
1: self.keyboardConstraint = [NSLayoutConstraint constraintWithItem:self.keyboardView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:0];
1: - (void)keyboardWillShow:(NSNotification *)notification{
2: CGRect kbFrame = [[[notification userInfo] objectForKey:UIKeyboardFrameEndUserInfoKey] CGRectValue];
3: self.keyboardConstraint.constant = kbFrame.size.height;
4: [self.view addConstraint:self.keyboardConstraint];
5: // [self.inputView needsUpdateConstraints];
6: [self.view setNeedsLayout];
7: [self.view layoutIfNeeded];
8: if (self.messages.count > 0){
9: NSIndexPath *idxP = [NSIndexPath indexPathForRow:(self.messages.count - 1) inSection:0];
10: [self.tableView scrollToRowAtIndexPath:idxP atScrollPosition:UITableViewScrollPositionBottom animated:YES];}
11:
12: }
13:
14: - (void)keyboardWillHidden{
15:
16: [self.view endEditing:YES];
17: [UIView animateWithDuration:0.25 animations:^{
18: // self.keyboardConstraint.constant = 0;
19: self.keyboardConstraint.constant = 0;
20: // [self.view removeConstraint:self.keyboardConstraint];
21: [self.view setNeedsLayout];
22: [self.view layoutIfNeeded];
23: }];
24:
25: }
