ios - 纯代码创建的UIPickerView，数据源和代理方法为何不被调用？

Question

学习UIPickerView后，想尝试用纯代码方式创建一个UIPickerView，关键语句如下：

#import "Masonry.h"
#import "ViewController.h"

@interface ViewController () <UIPickerViewDataSource,UIPickerViewDelegate>
@property (nonatomic,weak) UIView          *bottomView;
@property (nonatomic,weak) UIPickerView    *unitPicker;
@end

- (void)viewDidLoad {
    [super viewDidLoad];
    
    //创建一个底部工具条
    UIView *bottomView = [[UIView alloc] init];
    [bottomView setBackgroundColor:[UIColor colorWithRed:0.133 green:0.959 blue:0.959 alpha:0.600]];
    [self.view addSubview:bottomView];
    
    //创建UIPickerView   
    UIPickerView *unitPicker = [[UIPickerView alloc] init];
    self.unitPicker.dataSource = self;
    self.unitPicker.delegate   = self;
    [bottomView addSubview:unitPicker];
    
    //用Masonry自动布局：
    [unitPicker mas_makeConstraints:^(MASConstraintMaker *make){
        make.left.equalTo(unitLabel.mas_right).offset(3);
        make.height.equalTo(@80);
        make.width.equalTo(@80);
        make.centerY.equalTo(bottomView);
     }];
}  

    //UIPickerView列数
- (NSInteger)numberOfComponentsInPickerView:(UIPickerView *)pickerView{
    NSLog(@"%s",__func__);
    return 1;
}
    //UIPickerView行数
- (NSInteger)pickerView:(UIPickerView *)pickerView numberOfRowsInComponent:(NSInteger)component{
    NSLog(@"%s",__func__);
    return 2;
}
    //UIPickerView标题
- (NSString *)pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger)row forComponent:(NSInteger)component
{
    NSLog(@"%s",__func__);
    return @"a";
}

实际运行的时候没有警告，但是Output窗口一片空白，所有的__func__都没有被调用。UIPickerView布局成功，但除了有三条分割线外全部都是空白，求教是哪里出了问题？

个人已经尝试过：
1、不用Masonry自动布局，改用initWithFrame方法创建UIPickerView，失败；
2、直接在self.view里添加UIPickerView，失败；
3、删除底部工具条里其他的控件，失败。

Answer 1

    UIPickerView *unitPicker = [[UIPickerView alloc] init];
    self.unitPicker.dataSource = self;
    self.unitPicker.delegate   = self;

self.unitPicker还是nil呢…… 你丢了一句self.unitPicker = unitPicker;……