预先定义一个约束的全局变量，用来接收需要变化的约束

@property (nonatomic, strong) MASConstraint *bottomViewBottom;

[self.bottomViewBottom uninstall];

        [UIView animateWithDuration:0.3 animations:^{

            [self.bottomViewmas_updateConstraints:^(MASConstraintMaker*make) {

                self.bottomViewBottom= make.bottom.mas_equalTo(-300);

            }];

            [self.viewlayoutIfNeeded];   //一定要加，---父视图---  刷新

        }];