自定义 navigationbar的 title

 UIView *titleView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, 100, 44)];
    UILabel *titleLabel = [[UILabel alloc]initWithFrame:CGRectMake(0, 0, 100, 44)];
    titleLabel.text = @"Exercise";
    titleLabel.textColor = [UIColor whiteColor];
    [titleLabel setFont:[UIFont fontWithName:@"Helvetica-Bold" size:18]];
    titleLabel.textAlignment = NSTextAlignmentCenter;
    [titleView addSubview:titleLabel];
    self.navigationItem.titleView = titleView;

 出来的效果是这样：