［通俗易懂］理解「委託」

委託的意義在於實現多態；在於讓對象可以在程序運行時知足外界對其的改變。

（1）一個對象屬性、動做，若是在編譯時就能肯定，能夠在這個對象的類裏面來實現。

（2）一個對象的屬性、動做，若是在運行時才能肯定，則只能經過這個對象的委託來實現。

換句話說：類，知足編譯時對對象的設置和要求。委託，用於知足運行時對對象的設置和要求。

舉個例子：UITableView 的一個實例對象 tableView。

當咱們在使用tableView時，咱們在編譯的時候就能肯定下來這個表格的分割線顏色（separatorColor ）、分割線風格（separatorStyle ）、背景圖片（backgroundView）等。還有一些，咱們在編譯的時候就能夠執行的方法。如：

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

固然，那些只有在運行時才能肯定的方面，咱們只能在類的委託中實現（由於在編譯的時候，咱們沒法肯定其狀態是怎樣的）。以下面這些：

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

以上，就是對委託的所有理解。若是看着還不過癮，咱們能夠經過以下篇幅，窺視一下蘋果公司是如何設計tableview的。

蘋果公司設計tableview在編譯時就要肯定的方面包括：＝＝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

 

蘋果公司設計tableview在運行時才能肯定的方面包括：＝＝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: