ScrollView constraint issue

ViewController.m

//
//  ViewController.m
//  scrollveiwtes
//
//  Created by willard on 2017/5/9.
//  Copyright © 2017年 willard. All rights reserved.
//

#import "ViewController.h"

@interface ViewController ()
@property (nonatomic, strong) UIScrollView *scrollView;
@end

@implementation ViewController
@synthesize scrollView;

- (void)viewDidLoad {
    [super viewDidLoad];
    
    scrollView = [[UIScrollView alloc] init];
    scrollView.backgroundColor = [UIColor grayColor];
    scrollView.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:scrollView];
    [scrollView.leftAnchor constraintEqualToAnchor:self.view.leftAnchor constant:20].active = YES;
    [scrollView.rightAnchor constraintEqualToAnchor:self.view.rightAnchor constant:-20].active = YES;
    [scrollView.topAnchor constraintEqualToAnchor:self.view.topAnchor constant:20].active = YES;
    [scrollView.bottomAnchor constraintEqualToAnchor:self.view.bottomAnchor constant:-20].active = YES;
    
    
    
    NSLog(@"%@", NSStringFromCGSize(scrollView.contentSize));
    
    

    UIView *sView = scrollView;

    UIView *view = [[UIView alloc] init];
    view.translatesAutoresizingMaskIntoConstraints = NO;
    [scrollView addSubview:view];
    view.backgroundColor = [UIColor blueColor];
    [view.leftAnchor constraintEqualToAnchor:sView.leftAnchor constant:30].active = YES;
    [view.rightAnchor constraintEqualToAnchor:sView.rightAnchor constant:-30].active = YES;
    [view.topAnchor constraintEqualToAnchor:sView.topAnchor constant:30].active = YES;
    [view.bottomAnchor constraintEqualToAnchor:sView.bottomAnchor constant:-30].active = YES;
    
    
//    [view.widthAnchor constraintEqualToConstant:100].active = YES;
//    [view.heightAnchor constraintEqualToConstant:100].active = YES;
}


- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    //scrollView.contentSize = self.view.frame.size;
}
@end