一，工程结构，如下图所示：

二，代码

PassWordInputWindow.h

#import <UIKit/UIKit.h>

@interface PasswordInputWindow : UIWindow

+(PasswordInputWindow *)shareInstance;
-(void)show;

@end

PasswordInputWindow.m

#import "PasswordInputWindow.h"

@implementation PasswordInputWindow
{
    UITextField *_textField;
}

+(PasswordInputWindow *)shareInstance
{
    static id shareInstance=nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken,^{
        
        shareInstance=[[self alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    });
    return shareInstance;
}
-(id)initWithFrame:(CGRect)frame
{
    self=[super initWithFrame:frame];
    if (self) {
        
        UILabel *label=[[UILabel alloc]initWithFrame:CGRectMake(10, 50, 200, 20)];
        label.text=@"请输入密码";
        [self addSubview:label];
        
        UITextField *textField=[[UITextField alloc]initWithFrame:CGRectMake(10, 80, 200, 20)];
        textField.backgroundColor=[UIColor whiteColor];
        textField.secureTextEntry=YES;
        [self addSubview:textField];
        
        
        UIButton *button=[[UIButton alloc] initWithFrame:CGRectMake(10, 110, 200, 44)];
        [button setBackgroundColor:[UIColor blueColor]];
        button.titleLabel.textColor=[UIColor blackColor];
        [button setTitle:@"确定" forState:UIControlStateNormal];
        [button addTarget:self action:@selector(completeButtonPRessed:) forControlEvents:UIControlEventTouchUpInside];
        [self addSubview:button];
        
        
        self.backgroundColor=[UIColor yellowColor];
        _textField=textField;
    }
    return self;
}
-(void)show
{
    [self makeKeyAndVisible];
    self.hidden=NO;
}
-(void)completeButtonPressed:(id)sender{
    if ([_textField.text isEqualToString:@"abcd"]) {
        [_textField resignFirstResponder];
        [self resignFirstResponder];
        self.hidden=YES;
    }else{
        [self showErrorAlertView];
    }
}
-(void)showErrorAlertView
{
    UIAlertView *alertView=[[UIAlertView alloc]initWithTitle:nil message:@"密码错误，正确密码是abcd" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil, nil];
    [alertView show];
}
@end

三，运行效果。

参考资料 ：

《iOS开发进阶》-唐巧