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
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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;