一、UIWindow的作用？
- A UIScreen object that identifies physical screen connected to the device.
- A UIWindow object that provides drawing support for the screen.
- A set of UIView objects to perform the drawing. These objects are attached to the window and draw their contents when the window asks them to.(不太好翻译)
二、系统如何自动创建Window和window的根控制器？
- 在main函数中，UIApplicationMain会加在info.plist文件，并判断有没有指定Main Interface, 系统默认的Main Interface指向Main.storyboard，然后自动创建window，并加在Main.storyboard中的启动控制器（initial View Controller）。
三、如何用代码方式创建window和window的根控制器？
1.首先删除Main Interface中的内容
删除Main Interface指定的入口
2.AppDelegate中代码创建如下：
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
UIViewController *VC = [[UIViewController alloc] init];
// VC.view.backgroundColor = [UIColor orangeColor];
self.window.rootViewController = VC;
[self.window makeKeyAndVisible];
return YES;
}
四、如何从自定义的storyboard中创建window的根控制器
1.创建一个storyboard, 命名为"XJStoryboard"
创建storyboard
2.storyboard中拖一个viewController进去，并设置为这个storyboard的initial viewController
3.AppDelegate中代码创建如下：
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"XJStoryboard" bundle:nil];
self.window.rootViewController = storyboard.instantiateInitialViewController;
[self.window makeKeyAndVisible];
return YES;
}
- 注意：上面的代码其实不写也可以，直接在Main Interface 中指定你需要加载的storyboard，系统会自动创建window和根控制器。
五、如何从自定义的xib中创建window的根控制器
1.xib的创建
xib文件的创建
2.上面创建的是一个空的xib，现在拖一个UIView进去，设置这个xib的File's Owner为ViewController，并把ViewController的View指向刚刚拖进区的UIView,如下图所示，
关联ViewController
File's Owner的view 连线到刚刚拖进去的UIView
3.代码部分
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
// 这里的ViewController借用了工程创建时，Xcode自动创建的类
// 这里完全可以直接创建一个自定义的带xib的Controller
// 这里主要展示如何自己创建一个空的xib，然后关联已存在的类
UIViewController *VC = [[ViewController alloc] initWithNibName:@"XJXib" bundle:nil];
self.window.rootViewController = VC;
[self.window makeKeyAndVisible];
return YES;
}
