UIWindow 弹出框及注意事项

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;
}