整体使用了build模式；单是组织结构混乱；不符合人类思维。

UITableViewDataSource：描述了View的要素个数情况；并担负了builder功能。

UITableViewDelegate：描述了要素的几何属性（组织者）并担负了builder功能。

UITableView

@property (nonatomic, strong, nullable) UIView *tableHeaderView;                           // accessory view for above row content. default is nil. not to be confused with section header

@property (nonatomic, strong, nullable) UIView *tableFooterView;                           // accessory view below content. default is nil. not to be confused with section footer

＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝

this represents the display and behaviour of the cells.

UITableViewDelegate

// Variable height support

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath;

- (CGFloat)tableView:(UITableView *)tableView heightForHeaderInSection:(NSInteger)section;

- (CGFloat)tableView:(UITableView *)tableView heightForFooterInSection:(NSInteger)section;

// Section header & footer information. Views are preferred over title should you decide to provide both

- (nullable UIView *)tableView:(UITableView *)tableView viewForHeaderInSection:(NSInteger)section;   // custom view for header. will be adjusted to default or specified header height

- (nullable UIView *)tableView:(UITableView *)tableView viewForFooterInSection:(NSInteger)section;   // custom view for footer. will be adjusted to default or specified footer height

// Called after the user changes the selection.

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath;

- (void)tableView:(UITableView *)tableView didDeselectRowAtIndexPath:(NSIndexPath *)indexPath NS_AVAILABLE_IOS(3_0);

// this protocol represents the data model object. as such, it supplies no information about appearance (including the cells)

UITableViewDataSource

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView;

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section;

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath;