三种ViewController跳转的异同

- (void)presentViewController:(UIViewController *)viewControllerToPresent animated:(BOOL)flag completion:(void (^)(void))completion

MainVC *mainVC = [[MainVC alloc] init];

[self presentViewController:mainVC animated:YES completion:nil];

这种方式一般出现在需要使用者完成某件事情，如输入密码、增加资料等操作后，才能(回到跳转前的控制器)继续。例如系统的WIFI连接输入密码提示。默认动画是从下至上。

- (void)pushViewController:(UIViewController *)viewController animated:(BOOL)animated

这种方式一般是使用者浏览资料，继而可以前进到下一个页面或回到上一个页面。默认动画是从右至左。

- (void)addChildViewController:(UIViewController *)childController

这个方法出现在iOS5以后，通过它即使不使用NavigationController也能够实现view hierarchy。有以下优点：

1.页面逻辑很清晰，相应的View对应相应的ViewController。
2.当某个子View没有显示时，将不会被Load，减少了内存的使用。
3.当内存紧张时，没有Load的View将被首先释放，优化了程序的内存释放机制。

#import "ViewController.h"
#import "FirstVC.h"
#import "SecondVC.h"
#import "ThirdVC.h"

@interface ViewController ()
{
    FirstVC *firstVC;
    SecondVC *secondVC;
    ThirdVC *thirdVC;
}

@property (weak, nonatomic) IBOutlet UIView *contentView;

@property (strong, nonatomic) UIViewController *currentVC;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    firstVC = [[FirstVC alloc] init];
    secondVC = [[SecondVC alloc] init];
    thirdVC = [[ThirdVC alloc] init];

    [self addChildViewController:firstVC];
    [self addChildViewController:secondVC];
    [self addChildViewController:thirdVC];

    [self.contentView addSubview:thirdVC.view];

    self.currentVC = thirdVC;
}

- (IBAction)onClick:(id)sender {
    if(self.currentVC==firstVC && [sender tag]==1) {
        return;
    }

    if(self.currentVC==secondVC && [sender tag]==2) {
        return;
    }

    if(self.currentVC==thirdVC && [sender tag]==3) {
        return;
    }

    UIViewController *oldVC = self.currentVC;

    switch ([sender tag]) {
        case 1: {
            [self transitionFromViewController:self.currentVC toViewController:firstVC duration:1 options:UIViewAnimationOptionTransitionCurlUp animations:^{

            } completion:^(BOOL finished) {
                if(finished) {
                    self.currentVC = firstVC;
                }
                else {
                    self.currentVC = oldVC;
                }
            }];
        }
        break;
        case 2: {
            [self transitionFromViewController:self.currentVC toViewController:secondVC duration:1 options:UIViewAnimationOptionTransitionCurlUp animations:^{

            } completion:^(BOOL finished) {
                if(finished) {
                    self.currentVC = secondVC;
                }
                else {
                    self.currentVC = oldVC;
                }
            }];
        }
        break;
        case 3: {
            [self transitionFromViewController:self.currentVC toViewController:thirdVC duration:1 options:UIViewAnimationOptionTransitionCurlUp animations:^{

            } completion:^(BOOL finished) {
                if(finished) {
                    self.currentVC = thirdVC;
                }
                else {
                    self.currentVC = oldVC;
                }
            }];
        }
        break;
        default:
            break;
    }
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end