［通俗易懂］理解“委托”

委托的意义在于实现多态；在于让对象能够在程序运行时满足外界对其的改变。

（1）一个对象属性、动作，如果在编译时就能确定，可以在这个对象的类里面来实现。

（2）一个对象的属性、动作，如果在运行时才能确定，则只能通过这个对象的委托来实现。

换句话说：类，满足编译时对对象的设置和要求。委托，用于满足运行时对对象的设置和要求。

举个例子：UITableView 的一个实例对象 tableView。

当我们在使用tableView时，我们在编译的时候就能确定下来这个表格的分割线颜色（separatorColor ）、分割线风格（separatorStyle ）、背景图片（backgroundView）等。还有一些，我们在编译的时候就可以执行的方法。如：

• – cellForRowAtIndexPath:
• – indexPathForCell:
• – indexPathForRowAtPoint:
• – indexPathsForRowsInRect:
• – visibleCells
• – indexPathsForVisibleRows

当然，那些只有在运行时才能确定的方面，我们只能在类的委托中实现（因为在编译的时候，我们无法确定其状态是怎样的）。如下面这些：

• – tableView:heightForRowAtIndexPath:
• – tableView:estimatedHeightForRowAtIndexPath:
• – tableView:indentationLevelForRowAtIndexPath:
• – tableView:willDisplayCell:forRowAtIndexPath:

以上，就是对委托的全部理解。如果看着还不过瘾，我们可以通过如下篇幅，窥视一下苹果公司是如何设计tableview的。

苹果公司设计tableview在编译时就要确定的方面包括：＝＝UITableView Class Reference

Tasks

Initializing a UITableView Object

• – initWithFrame:style:

Configuring a Table View

•    style  property
• – numberOfRowsInSection:
• – numberOfSections
•    rowHeight  property
•    separatorStyle  property
•    separatorColor  property
•    backgroundView  property
•    separatorInset  property

Creating Table View Cells

• – registerNib:forCellReuseIdentifier:
• – registerClass:forCellReuseIdentifier:
• – dequeueReusableCellWithIdentifier:forIndexPath:
• – dequeueReusableCellWithIdentifier:

Accessing Header and Footer Views

• – registerNib:forHeaderFooterViewReuseIdentifier:
• – registerClass:forHeaderFooterViewReuseIdentifier:
• – dequeueReusableHeaderFooterViewWithIdentifier:
•    tableHeaderView  property
•    tableFooterView  property
•    sectionHeaderHeight  property
•    sectionFooterHeight  property
• – headerViewForSection:
• – footerViewForSection:

Accessing Cells and Sections

• – cellForRowAtIndexPath:
• – indexPathForCell:
• – indexPathForRowAtPoint:
• – indexPathsForRowsInRect:
• – visibleCells
• – indexPathsForVisibleRows

Estimating Element Heights

•    estimatedRowHeight  property
•    estimatedSectionHeaderHeight  property
•    estimatedSectionFooterHeight  property

Scrolling the Table View

• – scrollToRowAtIndexPath:atScrollPosition:animated:
• – scrollToNearestSelectedRowAtScrollPosition:animated:

Managing Selections

• – indexPathForSelectedRow
• – indexPathsForSelectedRows
• – selectRowAtIndexPath:animated:scrollPosition:
• – deselectRowAtIndexPath:animated:
•    allowsSelection  property
•    allowsMultipleSelection  property
•    allowsSelectionDuringEditing  property
•    allowsMultipleSelectionDuringEditing  property

Inserting, Deleting, and Moving Rows and Sections

• – beginUpdates
• – endUpdates
• – insertRowsAtIndexPaths:withRowAnimation:
• – deleteRowsAtIndexPaths:withRowAnimation:
• – moveRowAtIndexPath:toIndexPath:
• – insertSections:withRowAnimation:
• – deleteSections:withRowAnimation:
• – moveSection:toSection:

Managing the Editing of Table Cells

•    editing  property
• – setEditing:animated:

Reloading the Table View

• – reloadData
• – reloadRowsAtIndexPaths:withRowAnimation:
• – reloadSections:withRowAnimation:
• – reloadSectionIndexTitles

Accessing Drawing Areas of the Table View

• – rectForSection:
• – rectForRowAtIndexPath:
• – rectForFooterInSection:
• – rectForHeaderInSection:

Managing the Delegate and the Data Source

•    dataSource  property
•    delegate  property

Configuring the Table Index

•    sectionIndexMinimumDisplayRowCount  property
•    sectionIndexColor  property
•    sectionIndexBackgroundColor  property
•    sectionIndexTrackingBackgroundColor  property

苹果公司设计tableview在运行时才能确定的方面包括：＝＝UITableViewDelegate Protocol Reference

Tasks

Configuring Rows for the Table View

• – tableView:heightForRowAtIndexPath:
• – tableView:estimatedHeightForRowAtIndexPath:
• – tableView:indentationLevelForRowAtIndexPath:
• – tableView:willDisplayCell:forRowAtIndexPath:

Managing Accessory Views

• – tableView:accessoryButtonTappedForRowWithIndexPath:
• – tableView:accessoryTypeForRowWithIndexPath: Deprecated in iOS 3.0

Managing Selections

• – tableView:willSelectRowAtIndexPath:
• – tableView:didSelectRowAtIndexPath:
• – tableView:willDeselectRowAtIndexPath:
• – tableView:didDeselectRowAtIndexPath:

Modifying the Header and Footer of Sections

• – tableView:viewForHeaderInSection:
• – tableView:viewForFooterInSection:
• – tableView:heightForHeaderInSection:
• – tableView:estimatedHeightForHeaderInSection:
• – tableView:heightForFooterInSection:
• – tableView:estimatedHeightForFooterInSection:
• – tableView:willDisplayHeaderView:forSection:
• – tableView:willDisplayFooterView:forSection:

Editing Table Rows

• – tableView:willBeginEditingRowAtIndexPath:
• – tableView:didEndEditingRowAtIndexPath:
• – tableView:editingStyleForRowAtIndexPath:
• – tableView:titleForDeleteConfirmationButtonForRowAtIndexPath:
• – tableView:shouldIndentWhileEditingRowAtIndexPath:

Reordering Table Rows

• – tableView:targetIndexPathForMoveFromRowAtIndexPath:toProposedIndexPath:

Tracking the Removal of Views

• – tableView:didEndDisplayingCell:forRowAtIndexPath:
• – tableView:didEndDisplayingHeaderView:forSection:
• – tableView:didEndDisplayingFooterView:forSection:

Copying and Pasting Row Content

• – tableView:shouldShowMenuForRowAtIndexPath:
• – tableView:canPerformAction:forRowAtIndexPath:withSender:
• – tableView:performAction:forRowAtIndexPath:withSender:

Managing Table View Highlighting

• – tableView:shouldHighlightRowAtIndexPath:
• – tableView:didHighlightRowAtIndexPath:
• – tableView:didUnhighlightRowAtIndexPath: