统一设置导航栏返回按钮

处理两种情况

非根控制器展示出来，可能有两种情况：
modal出来的
另一种是push

控制器的2个只读属性：`presentedViewController` 和 `presentingViewController`
通过`当前显示控制器`的`presentingViewController属性`来判断属于哪种情况：
    如果是`nil`，表示是UINavigationController对象`push`过来的，否则是modal过来的


`[A presentViewController:B animated:YES completion:nil];`
`A.presentedViewController —> B;`
`B.presentingViewController —> A;`
有 正在把`我`举起来的人

> presentedViewController是显示出来的控制器

使用说明

• 控制器继承MGBaseViewController即可
• 首页的leftBarButtonItem不用设置
• 不用设置push控制器和modal出来控制器的leftBarButtonItem
• 有的控制器返回按钮不是pop，而是要dismiss（present推进去的）
• 统一处理2种情况的返回

@interface MGBaseViewController ()
@end

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

/**
 创建返回按钮
 1. modal出来的
 2. push进来的
 */
- (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出来的控制器
        // 返回是diss
        [self.navigationController dismissViewControllerAnimated:YES completion:nil];
    } else {    // push进来的
        // 返回是pop
        [self.navigationController popViewControllerAnimated:YES];
    }
}
@end

完整版点击返回按钮

-(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];
        }
    }
}