摘抄自：http://www.cnblogs.com/kenshincui/p/3890880.html#runProcess
程序的运行过程
在几乎所有的程序开发中程序一般都是从main函数开始运行的，那么IOS程序也不例外，在上图中我们可以看到Xcode为我们生成了一个main.m文件：
//
// main.m
// FirstIOS
//
// Created by Kenshin Cui on 14-2-23.
// Copyright (c) 2014年 Kenshin Cui. All rights reserved.
//
#import <UIKit/UIKit.h>
#import "AppDelegate.h"
int main(int argc, char * argv[]) {
@autoreleasepool {
return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
}
}
这个默认的iOS程序就是从main函数开始执行的，但是在main函数中我们其实只能看到一个方法，这个方法内部是一个消息循环（相当于一个死循 环），因此运行到这个方法UIApplicationMain之后程序不会自动退出，而只有当用户手动关闭程序这个循环才结束。这个方法有四个参数：
- 第一个参数和第二个参数其实就是main函数的参数，分别代表：参数个数、参数内容；
- 第三个参数代表UIApplication类（或子类）字符串，这个参数默认为nil则代表默认为UIApplication类，用户 可以自定义一个类继承于这个类；如果为nil则等价于NSStringFromClass([UIApplication class])，大家可以自己试验，效果完全一样；UIApplication是单例模式，一个应用程序只有一个UIApplication对象或子对 象；
- 第四个参数是UIApplication的代理类字符串，默认生成的是AppDelegate类，这个类主要用于监听整个应用程序生命 周期的各个事件（其实类似于之前我们文章中提到的事件监听代理），当UIApplication运行过程中引发了某个事件之后会调用代理中对应的方法；
小技巧：
其实在Xcode中如果要看一些系统方法的解释或者参数说明，可以直接鼠标放到这个方法上，在Xcode右侧面板中就会给出帮助提示，如下图当我们放到UIApplicationMain上之后：
也就是说当执行UIApplicationMain方法后这个方法会根据第三个参数创建对应的UIApplication对象，这个对象会根据第四 个参数AppDelegate创建并指定此对象为UIApplication的代理；同时UIApplication会开启一个消息循环不断监听应用程序 的各个活动，当应用程序生命周期发生改变UIApplication就会调用代理对应的方法。
既然应用程序UIApplication是通过代理和外部交互的，那么我们就有必要清楚AppDelegate的操作细节，下面是UIApplication详细的代码：
AppDelegate.h
//
// AppDelegate.h
//
//
// Created by Kenshin Cui on 14-2-23.
// Copyright (c) 2014年 Kenshin Cui. All rights reserved.
//
#import <UIKit/UIKit.h>
@interface AppDelegate : UIResponder <UIApplicationDelegate>
@property (strong, nonatomic) UIWindow *window;
@end
AppDelegate.m
//
// AppDelegate.m
//
//
// Created by Kenshin Cui on 14-2-23.
// Copyright (c) 2014年 Kenshin Cui. All rights reserved.
//
#import "AppDelegate.h"
@implementation AppDelegate
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
return YES;
}
- (void)applicationWillResignActive:(UIApplication *)application
{
// Sent when the application is about to move from active to inactive state. This can occur for certain types of temporary interruptions (such as an incoming phone call or SMS message) or when the user quits the application and it begins the transition to the background state.
// Use this method to pause ongoing tasks, disable timers, and throttle down OpenGL ES frame rates. Games should use this method to pause the game.
}
- (void)applicationDidEnterBackground:(UIApplication *)application
{
// Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later.
// If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
}
- (void)applicationWillEnterForeground:(UIApplication *)application
{
// Called as part of the transition from the background to the inactive state; here you can undo many of the changes made on entering the background.
}
- (void)applicationDidBecomeActive:(UIApplication *)application
{
// Restart any tasks that were paused (or not yet started) while the application was inactive. If the application was previously in the background, optionally refresh the user interface.
}
- (void)applicationWillTerminate:(UIApplication *)application
{
// Called when the application is about to terminate. Save data if appropriate. See also applicationDidEnterBackground:.
}
@end
这个类中定义了应用程序生命周期中各个事件的执行方法：
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions;程序启动之后执行，只有在第一次程序启动后才执行，以后不再执行；
- (void)applicationWillResignActive:(UIApplication *)application;程序将要注销激活时（失去焦点）执行，程序失去焦点用户就不能再对程序进行操作；
- (void)applicationDidEnterBackground:(UIApplication *)application;程序进入后台后执行，注意进入后台时会先失去焦点再进入后台；
- (void)applicationWillEnterForeground:(UIApplication *)application;程序将要进入前台时执行；
- (void)applicationDidBecomeActive:(UIApplication *)application;程序被激活（获得焦点）后执行，注意程序被激活时会先进入前台再被激活；
- (void)applicationWillTerminate:(UIApplication *)application;程序在终止时执行，包括正常终止或异常终止，例如说一个应用程序在后太运行（例如音乐播放软件、社交软件等）占用太多内存这时会意外终止调用此方法；
为了演示程序的生命周期，不妨在每个事件中都输出一段内容，简单调整上面的代码：
AppDelegate.m
//
// AppDelegate.m
// FirstIOS
//
// Created by Kenshin Cui on 14-2-23.
// Copyright (c) 2014年 Kenshin Cui. All rights reserved.
//
#import "AppDelegate.h"
@interface AppDelegate ()
@end
@implementation AppDelegate
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
NSLog(@"程序已经启动...");
return YES;
}
- (void)applicationWillResignActive:(UIApplication *)application {
NSLog(@"程序将要失去焦点...");
// Sent when the application is about to move from active to inactive state. This can occur for certain types of temporary interruptions (such as an incoming phone call or SMS message) or when the user quits the application and it begins the transition to the background state.
// Use this method to pause ongoing tasks, disable timers, and throttle down OpenGL ES frame rates. Games should use this method to pause the game.
}
- (void)applicationDidEnterBackground:(UIApplication *)application {
NSLog(@"程序已经进入后台...");
// Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later.
// If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
}
- (void)applicationWillEnterForeground:(UIApplication *)application {
NSLog(@"程序将要进入前台...");
// Called as part of the transition from the background to the inactive state; here you can undo many of the changes made on entering the background.
}
- (void)applicationDidBecomeActive:(UIApplication *)application {
NSLog(@"程序已经获得焦点...");
// Restart any tasks that were paused (or not yet started) while the application was inactive. If the application was previously in the background, optionally refresh the user interface.
}
- (void)applicationWillTerminate:(UIApplication *)application {
NSLog(@"程序将要终止...");
// Called when the application is about to terminate. Save data if appropriate. See also applicationDidEnterBackground:.
}
-(void)applicationDidReceiveMemoryWarning:(UIApplication *)application{
}
@end
下面是各个不同操作的运行结果：
相信通过上面运行过程大家会对整个运行周期有个大概了解。比较容易混淆的地方就是应用程序进入前台、获得焦点、失去焦点、进入后台，这几个方法大家要清楚。如果一个应用程序失去焦点那么意味着用户当前无法进行交互操作，因此一般会先失去焦点再进入后台防止进入后台过程中用户误操作；如果一个应用程序进入前台也是类似的，会先进入前台再获得焦点，这样进入前台过程中未完全准备好的情况下用户无法操作。另外一般如果应用程序要保存用户数据会在注销激活中进行 （而不是在进入后台方法中进行），因为如果用户双击Home不会进入后台只会注销激活；如果用户恢复应用状态一般在进入激活状态时处理（而不是在进入前台 方法中进行），因为用户可能是从任务栏直接返回应用，此时不会执行进入前台操作。
当然，上面的事件并不是所有AppDelegate事件，而是最常用的一些事件，其他事件大家可以查阅官方文档，例如 -(void)applicationDidReceiveMemoryWarning:(UIApplication *)application;用于在内存占用过多发出内存警告时调用并通知对应的ViewController调用其内存回收方法。这里简单以图形方式描 述一下应用程序的调用过程：
注意：从后台进入前台，不会调用控制器的viewDidLoad方法，也不会调用控制器的viewWillAppear方法。