UIScrollView使用Masonry设置contentSi

场景1：scrollview上有3个子视图，子视图高度确定，通过masonry设置scrollview的contentsize高度。

方法：第一个子视图的顶部设置好与scrollview的顶部的值，最后一个子视图的底部设置好距离scrollview的底部的值，其他子视图正常设置。

代码：

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    UIScrollView *scrollView = [UIScrollView new];
    [self.view addSubview:scrollView];
    [scrollView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.edges.equalTo(self.view).mas_equalTo(0);
    }];
    
    UIView *v1 = [UIView new];
    [scrollView addSubview:v1];
    v1.backgroundColor = [UIColor redColor];
    [v1 mas_makeConstraints:^(MASConstraintMaker *make) {
        make.top.equalTo(scrollView).mas_equalTo(0);
        make.centerX.equalTo(scrollView).mas_equalTo(0);
        make.width.mas_equalTo(300);
        make.height.mas_equalTo(200);
    }];
    
    UIView *v2 = [UIView new];
    [scrollView addSubview:v2];
    v2.backgroundColor = [UIColor orangeColor];
    [v2 mas_makeConstraints:^(MASConstraintMaker *make) {
        make.top.equalTo(v1.mas_bottom).mas_equalTo(10);
        make.centerX.equalTo(scrollView).mas_equalTo(0);
        make.width.mas_equalTo(300);
        make.height.mas_equalTo(400);
    }];
    
    UIView *v3 = [UIView new];
    [scrollView addSubview:v3];
    v3.backgroundColor = [UIColor redColor];
    [v3 mas_makeConstraints:^(MASConstraintMaker *make) {
        make.top.equalTo(v2.mas_bottom).mas_equalTo(10);
        make.centerX.equalTo(scrollView).mas_equalTo(0);
        make.width.mas_equalTo(300);
        make.height.mas_equalTo(300);
        make.bottom.equalTo(scrollView).mas_equalTo(-50);
    }];
}