【UI初级 连载二】------做一个100秒倒计时的程序,注意考虑,当程序进入后台时的情况。

    /*
     做一个100秒倒计时的程序,注意考虑,当程序进入后台时的情况。
     */
   

AppDelegate.h文件中:

#import <UIKit/UIKit.h>

@interface AppDelegate : UIResponder <UIApplicationDelegate>

{
    UILabel *_label;
    NSInteger _index;
    NSTimer *_timer;
    NSDate *_backgroundDate;
}

@property (strong, nonatomic) UIWindow *window;

@end

AppDelegate.m文件中:

#import "AppDelegate.h"

@interface AppDelegate ()

@end

@implementation AppDelegate


- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.

    //******* 创建window窗口 *********************
    //拿到屏幕的大小
    CGRect rect = [UIScreen mainScreen].bounds;
    //创建一个window
    self.window = [[UIWindow alloc]initWithFrame:rect];
    //设置窗口颜色
    self.window.backgroundColor = [UIColor greenColor];
    //把当前的window作为程序的主window显示出来
    [self.window makeKeyAndVisible];
   
    _index = 100;
    //创建一个label
     _label = [[UILabel alloc]initWithFrame:CGRectMake(0, 175, 375, 75)];
    //设置label背景色
    _label.backgroundColor = [UIColor grayColor];
    _label.text = @"100";
    //设置文字颜色
    _label.textColor = [UIColor whiteColor];
    //设置文字对齐方式
    _label.textAlignment = NSTextAlignmentCenter;
    //设置字体大小
    _label.font = [UIFont boldSystemFontOfSize:24];
   
   
    //将label添加到window上
    [self.window addSubview:_label];
   
    _timer = [NSTimer scheduledTimerWithTimeInterval:1
                                              target:self
                                            selector:@selector(timeAction:)
                                            userInfo:nil repeats:YES];
   
    return YES;
}

- (void)timeAction:(NSTimer *)timer{

     _index --;
    _label.text = [NSString stringWithFormat:@"%ld",_index];;
    if (_index == 0) {
        [timer invalidate];
        return;
    }
}
//程序已经进入后台
- (void)applicationDidEnterBackground:(UIApplication *)application {
    [_timer invalidate];//停止计时器
    _backgroundDate = [NSDate date];//获得程序进入前台时的时间
}
//程序将要进入前台
- (void)applicationWillEnterForeground:(UIApplication *)application {
    NSDate *foregroundDate = [NSDate date];//获得程序即将进入活动状态时的时间
    NSTimeInterval subTime = [foregroundDate timeIntervalSinceDate:_backgroundDate];//获得程序待在后台的时间
    _index -= subTime;
    _label.text = [NSString stringWithFormat:@"%ld",_index];
    [NSTimer scheduledTimerWithTimeInterval:1
                                     target:self
                                   selector:@selector(timeAction:)
                                   userInfo:nil
                                    repeats:YES];
}

@end