切换 RootViewControll

经常会在主 VC 前某些时候先显示其它的过渡页面，这个时候就要切换根视图

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    
    self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
    
    BOOL isFirstStart = YES;
    self.window.backgroundColor = [UIColor whiteColor];
    if(isFirstStart){
        self.window.rootViewController = [AlertViewController new];
    }else{
        [self swtchToDefaultRootViewController];
    }
    
    [self.window makeKeyAndVisible];
    
    return YES;
}

-(void)swtchToDefaultRootViewController{
    [self.window.rootViewController.view removeFromSuperview];
    self.window.rootViewController = [ViewController new];
}

第一个 VC

//
//  AlertViewController.m
//  mjtable
//
//  Created by ldhonline on 2018/10/26.
//  Copyright © 2018年 aidoutu.com. All rights reserved.
//

#import "AlertViewController.h"
#import "masonry.h"
#import "AppDelegate.h"

@interface AlertViewController ()

@end

@implementation AlertViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.view.backgroundColor = [UIColor lightGrayColor];
    
    UIButton *btn = [[UIButton alloc] init];
    
    [self.view addSubview:btn];
    
    [btn mas_makeConstraints:^(MASConstraintMaker *make) {
        make.center.mas_equalTo(self.view);
        make.width.mas_equalTo(200);
        make.height.mas_equalTo(40);
    }];
    
    btn.backgroundColor = [UIColor grayColor];
    [btn setTitle:@"关闭" forState:UIControlStateNormal];
    [btn addTarget:self action:@selector(dimss) forControlEvents:UIControlEventTouchUpInside];
    
}

- (void)dimss{
    AppDelegate *currentAppDelegate = (AppDelegate *)([UIApplication sharedApplication].delegate);
    [currentAppDelegate swtchToDefaultRootViewController];
}


@end