这次来说一下multipliedBy的用法，
// Create views
UIView *topView = [[UIView alloc] init];
topView.backgroundColor = [UIColor redColor];
[self.view addSubview:topView];
UIView *topInnerView = [[UIView alloc] init];
topInnerView.backgroundColor = [UIColor greenColor];
[topView addSubview:topInnerView];
UIView *bottomView = [[UIView alloc] init];
bottomView.backgroundColor = [UIColor blackColor];
[self.view addSubview:bottomView];
UIView *bottomInnerView = [[UIView alloc] init];
bottomInnerView.backgroundColor = [UIColor blueColor];
[bottomView addSubview:bottomInnerView];
[topView mas_makeConstraints:^(MASConstraintMaker *make) {
make.left.right.top.mas_equalTo(0);
make.height.mas_equalTo(bottomView);
}];
// width = height / 3
[topInnerView mas_makeConstraints:^(MASConstraintMaker *make) {
make.left.right.mas_equalTo(topView);
//这是最关键的代码，使用multipliedBy必须是对同一个控件本身，比如，上面的代码中，我们都是对bottomInnerView.mas_width本身的，如果修改成相对于其它控件，会出问题。
make.width.mas_equalTo(topInnerView.mas_height).multipliedBy(3);
make.center.mas_equalTo(topView);
// 设置优先级
make.width.height.mas_equalTo(topView).priorityLow();
make.width.height.lessThanOrEqualTo(topView);
}];
[bottomView mas_makeConstraints:^(MASConstraintMaker *make) {
make.left.right.bottom.mas_equalTo(0);
make.height.mas_equalTo(topView);
make.top.mas_equalTo(topView.mas_bottom);
}];
// width/height比为1/3.0，要求是同一个控件的属性比例
[bottomInnerView mas_makeConstraints:^(MASConstraintMaker *make) {
make.top.bottom.mas_equalTo(bottomView);
make.center.mas_equalTo(bottomView);
// 注意，这个multipliedBy的使用只能是设置同一个控件的，比如这里的bottomInnerView，
// 设置高/宽为3:1
make.height.mas_equalTo(bottomInnerView.mas_width).multipliedBy(3);
make.width.height.mas_equalTo(bottomView).priorityLow();
make.width.height.lessThanOrEqualTo(bottomView);
}];
这里用到这个其实就是设置宽高比，就比如 例子中的 bottomInnerView ，我们想让它的高宽比为3：1，那我们首先设置其top和bottom与父视图一致且始终在父视图中居中显示，然后我们通过make.width.height.lessThanOrEqualTo设置了宽、高的最大值与父视图相同，然后设置了宽和高与父视图相等，但是优先级为最低，以保证子视图的宽高不超过父视图，最后，我们设置了bottomInnerView的高为宽的3倍。 make.height.mas_equalTo(bottomInnerView.mas_width).multipliedBy(3)
