loaded the "FirstView" nib but the view outlet was not set.'

这个BUG只发生在，我设置self.window.rootViewController =FirstViewController中，当我的FirstViewController不是rootViewController时，并没有问题。

我在工程中创建了FirstView,由于我的FirstView控件很多，布局很复杂，所以我选择创一个同名的Xib来可视化编程。

文件

代码如下

==========AppDelegate==========
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    
    self.window = [[UIWindow alloc]initWithFrame:[[UIScreen mainScreen]bounds]];
    self.window.backgroundColor = [UIColor whiteColor];
    self.window.rootViewController = [self setRootVC];
    [self.window makeKeyAndVisible];
    return YES;
}
- (UIViewController *)setRootVC{
    UIViewController * c = [NSClassFromString(@"FirstViewController") new];
    return c;
}

==========FirstViewController==========
@interface FirstViewController ()
@property (nonatomic,weak) FirstView *firstView;
@end

@implementation FirstViewController
-(FirstView *)firstView{
    if (!_firstView) {
        FirstView * firstView = [FirstView new];
        [self.view addSubview:firstView];
        _firstView = firstView;
    }
    return _firstView;
}
- (void)viewDidLoad {
    [super viewDidLoad];
    self.firstView.frame = CGRectMake(0, 30, 100, 100);
    self.firstView.backgroundColor =[UIColor lightGrayColor];
}

我还未往FirstView.xib中添加任何控件，也没有加载xib的，然后运行，结果：

Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[UIViewController _loadViewFromNibNamed:bundle:] loaded the "FirstView" nib but the view outlet was not set.'

百思不得其解，应该没有错误啊，然后各种折腾。
再看错误，提示FirstView有问题，于是我创建类secondView来替代FirstView

这回竟然成功了！！！到底是什么情况
······

思来想去，又折腾了一番，仔细再看错误提示：

[UIViewController _loadViewFromNibNamed:bundle:] loaded the "FirstView" nib but the view outlet was not set.'

提示我，加载ViewController时，会触发这个方法_loadViewFromNibNamed:bundle:
这个方法就是用来加载ViewController的View的，所以，由于命名关系，我所创建的FirstView.xib，相当于创建了FirstViewController.xib，所以系统提示我loaded the "FirstView" nib but the view outlet was not set.'

好了，知道原因就好解决了
打开FirstView.xib,指定File's Ower为FirstViewController

然后需要我们手动接连View，并且我为FirstView.xib添加两个label

修改FirstViewController为：

#import "FirstViewController.h"
@interface FirstViewController ()
@end
@implementation FirstViewController
- (void)viewDidLoad {
    [super viewDidLoad];
  
}
@end

再次运行，问题解决。

屏幕快照 2017-11-19 下午4.50.38.png

搞开发将近一年，由于一直都是纯代码Coding，最近才尝试可视化编程。想来这次的坑，我是迟早都会碰到的。其实我一直没搞懂File's ower和View里需要填写的关联类各有什么区别，经多这次的调试，额外的让我明白了File's ower的作用。

以下为File's ower和View里需要填写的关联类的区别：
Xib的使用：设置File's Owner的Class和view的Class的区别