关键点：
1、要把要显示的window定义为一个static变量。
这样，它不会在方法结束后被销毁。
2、不需要将window add 到某个view或window上去。
只要alloc了一个window,并且window.hiden = NO,则这个window就会显示了。
static UIWindow *__sheetWindow = nil;
@implementation ViewController
-(void)addWindowAction{
UIWindow *window = [[UIWindow alloc] initWithFrame:(CGRect) {{0.f, 0.f}, [[UIScreen mainScreen] bounds].size}];
window.backgroundColor = [UIColor clearColor];
window.windowLevel = UIWindowLevelNormal;
window.alpha = 1.f;
window.hidden = NO;
UIView* view = [[UIView alloc] initWithFrame:CGRectMake(30, 30, 100, 100)];
UIButton* btn = [UIButton buttonWithType:UIButtonTypeDetailDisclosure];
btn.frame = CGRectMake(10, 10, 20, 20);
[view addSubview:btn];
[btn addTarget:self action:@selector(removeWindowAction) forControlEvents:UIControlEventTouchUpInside];
view.backgroundColor = [UIColor redColor];
[window addSubview:view];
__sheetWindow = window;
}
-(void)removeWindowAction{
__sheetWindow.hidden = YES;
__sheetWindow = nil;
}
