UIScrollView UITableView UIColle

方法1：监听contentSize，contentSize发生了变化，重新布局

[collectionView addObserver:self forKeyPath:@"contentSize" options:NSKeyValueObservingOptionNew context:nil];

- (void)observeValueForKeyPath:(NSString*)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void*)context{

    [collectionView mas_remakeConstraints:^(MASConstraintMaker *make) {

        make.left.right.equalTo(_imgBack);

        make.top.equalTo(_imgBack.mas_bottom).offset(20);

        make.height.equalTo(@(_specCollectionView.contentSize.height));

    }];

}