[self.view addSubview:siftView];

// siftView中含有子view, 此時在屏幕外

[siftView mas_makeConstraints:^(MASConstraintMaker *make) {

make.top.mas_equalTo(weakSelf.view.mas_top);

make.left.mas_equalTo(weakSelf.view.mas_left);

make.bottom.mas_equalTo(weakSelf.view.mas_bottom);

make.right.mas_equalTo(weakSelf.view.mas_right);

}];

// 動畫開始前強制刷新

[siftView layoutIfNeeded];

// 更新約束

[siftView.siftView mas_updateConstraints:^(MASConstraintMaker *make) {

make.right.mas_equalTo(siftView.mas_right);

}];

// 在UIView的動畫中調用強制刷新產生動畫效果

[UIView animateWithDuration:0.2 animations:^{

[siftView layoutIfNeeded];

}];