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：

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(keyboardWillShow:) name:UIKeyboardWillChangeFrameNotification object:nil];

约束

@property (nonatomic,strong) NSLayoutConstraint* keyboardConstraint;

初始为0

self.keyboardConstraint = [NSLayoutConstraint constraintWithItem:self.keyboardView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:0];

- (void)keyboardWillShow:(NSNotification *)notification{
    CGRect kbFrame = [[[notification userInfo] objectForKey:UIKeyboardFrameEndUserInfoKey] CGRectValue];
    self.keyboardConstraint.constant = kbFrame.size.height;
    [self.view addConstraint:self.keyboardConstraint];
    //    [self.inputView needsUpdateConstraints];
    [self.view setNeedsLayout];
    [self.view layoutIfNeeded];
    if (self.messages.count > 0){
    NSIndexPath *idxP = [NSIndexPath indexPathForRow:(self.messages.count - 1) inSection:0];
        [self.tableView scrollToRowAtIndexPath:idxP atScrollPosition:UITableViewScrollPositionBottom animated:YES];}

}

- (void)keyboardWillHidden{

    [self.view endEditing:YES];
    [UIView animateWithDuration:0.25 animations:^{
        //        self.keyboardConstraint.constant = 0;
        self.keyboardConstraint.constant = 0;
        //        [self.view removeConstraint:self.keyboardConstraint];
        [self.view setNeedsLayout];
        [self.view layoutIfNeeded];
    }];

}