處理兩種情況

非根控制器展示出來(lái)怪蔑，可能有兩種情況：
modal出來(lái)的
另一種是push

控制器的2個(gè)只讀屬性：`presentedViewController` 和 `presentingViewController`
通過(guò)`當(dāng)前顯示控制器`的`presentingViewController屬性`來(lái)判斷屬于哪種情況：
    如果是`nil`贮配，表示是UINavigationController對(duì)象`push`過(guò)來(lái)的，否則是modal過(guò)來(lái)的


`[A presentViewController:B animated:YES completion:nil];`
`A.presentedViewController —> B;`
`B.presentingViewController —> A;`
有 正在把`我`舉起來(lái)的人

> presentedViewController是顯示出來(lái)的控制器

使用說(shuō)明

• 控制器繼承MGBaseViewController即可
• 首頁(yè)的leftBarButtonItem不用設(shè)置
• 不用設(shè)置push控制器和modal出來(lái)控制器的leftBarButtonItem
• 有的控制器返回按鈕不是pop，而是要dismiss（present推進(jìn)去的）
• 統(tǒng)一處理2種情況的返回

@interface MGBaseViewController ()
@end

@implementation MGBaseViewController
- (void)viewDidLoad {
    [super viewDidLoad];    
    [self setupBackBarButtonItem];
}

/**
 創(chuàng)建返回按鈕
 1. modal出來(lái)的
 2. push進(jìn)來(lái)的
 */
- (void)setupBackBarButtonItem
{  
    // 只有根控制器不加leftBarButtonItem
    if (self.presentingViewController || self.navigationController.childViewControllers.count > 1) {
        UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom];
        [button setTitle:@"返回" forState:UIControlStateNormal];
        [button setImage:[UIImage imageNamed:@"navigationButtonReturn"] forState:UIControlStateNormal];
        [button setImage:[UIImage imageNamed:@"navigationButtonReturnClick"] forState:UIControlStateHighlighted];
        [button setTitleColor:[UIColor blackColor] forState:UIControlStateNormal];
        [button setTitleColor:[UIColor redColor] forState:UIControlStateHighlighted];
        
        [button sizeToFit];
        
        button.contentHorizontalAlignment = UIControlContentHorizontalAlignmentLeft;
        button.contentEdgeInsets = UIEdgeInsetsMake(0, -8, 0, 0);
        [button addTarget:self action:@selector(buttonAction) forControlEvents:UIControlEventTouchUpInside];
        
        self.navigationItem.leftBarButtonItem = [[UIBarButtonItem alloc] initWithCustomView:button];
    }
}

- (void)buttonAction
{
    if (self.presentingViewController) {    // modal出來(lái)的控制器
        // 返回是diss
        [self.navigationController dismissViewControllerAnimated:YES completion:nil];
    } else {    // push進(jìn)來(lái)的
        // 返回是pop
        [self.navigationController popViewControllerAnimated:YES];
    }
}
@end

完整版點(diǎn)擊返回按鈕

-(void)onBackButtonClick
{
    [self.view endEditing:YES];
    if (self.presentingViewController &&
        (!self.navigationController ||self == self.navigationController.viewControllers.firstObject)) {
        [self dismissViewControllerAnimated:YES completion:nil];
    }
    else{
        if (self != self.navigationController.viewControllers.firstObject) {
            [self.navigationController popViewControllerAnimated:YES];
        }
    }
}