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作者微信號：christgreenlaw
重溫UINavigationBar的點點滴滴。
本文是閱讀UINavigationBar文檔的學習記錄。
文中的藍色鏈接，可以調用Xcode直接打開。
UINavigationBar是支持navigation層級結構的可視化控制組件，基本上多用于navigation controller。
Overview
主要的組件：
a left(back) button, a center title, and an optional right button.
You can use a navigation bar as a standalone object or in conjunction with a navigation controller object.
The NavigationController object creates, displays, and manages its associated navigation bar, and uses attributes of the view controllers you add to control the content displayed in the navigation bar.
使用navigation controller時，控制navigation bar 的步驟：
- Create a navigation controller in Interface Builder or in the code.
在IB或者代碼中創建navigation controller。
- Configure the appearance of the navigation bar using the navigationBar property on the UINavigationController object.
使用
UINavigationController對象的navigationBar屬性來配置navigation bar的外觀。
- Control the content of the navigation bar by setting the title and navigationItem properties on each UIViewController you push onto the navigation controller’s stack.
通過設置每個你push到navigation controller棧上的UIViewController的
title和navigationItem屬性來控制navigation bar的內容。
不使用navigation controller獨立使用navigation bar：
You can also use a standalone navigation bar, without using a navigation controller. To add a navigation bar to your interface, the following steps are required:
- Set up Auto Layout rules to govern the position of the navigation bar in your interface.
設置好Auto Layout約束來管理navigation bar在界面中的位置。
- Create a root navigation item to supply the initial title.
創建一個root navigation item來提供初始標題
- Configure a delegate object to handle user interactions with the navigation bar.
配置一個代理對象以處理用戶與navigation bar之間的交互
- Customize the appearance of the navigation bar.
配置bar的外觀
- Configure your app to push and pop relevant navigation items as the user navigates through the hierarchical screens.
配置應用以在相關的navigation item之間push和pop。
Using a Navigation Bar with a Navigation Controller
If you use a navigation controller to manage the navigation between different screens of content, the navigation controller creates a navigation bar automatically and pushes and pops navigation items when appropriate.
A navigation controller uses the navigationItem property on UIViewController to provide the model objects to its navigation bar when navigating a stack of view controllers. The default navigation item uses the view controller’s title, but you can override the navigationItem on a UIViewController subclass to gain complete control of the navigation bar’s content.
A navigation controller automatically assigns itself as the delegate of its navigation bar object. Therefore, when using a navigation controller, don’t assign a custom delegate object to the corresponding navigation bar.
navigation controller 會自動成為navigation bar 的代理對象。所以使用navigation controller時，不要自定義代理對象來處理navigation bar的行為。
To access the navigation bar associated with a navigation controller, use the navigationBar property on UINavigationController. See Customizing the Appearance of a Navigation Bar for details on how to customize the appearance of a navigation bar.
要訪問和 navigation controller相關聯的navigation bar，應該使用
UINavigationController的navigationBar屬性。
For information about navigation controllers, see UINavigationController.
Adding Content to a Standalone Navigation Bar
In the vast majority of scenarios you will use a navigation bar as part of a navigation controller. However, there are situations for which you might want to use the navigation bar UI and implement your own approach to content navigation. In these situations, you can use a standalone navigation bar.
想要實現自己的navigation跳轉邏輯，使用navigation bar的UI，此時可以使用standalone navigation bar.
When you use a navigation bar as a standalone object, you are responsible for providing its content. Unlike other types of views, you do not add subviews to a navigation bar directly. Instead, you use a navigation item (an instance of the UINavigationItem class) to specify what buttons or custom views you want displayed. A navigation item has properties for specifying views on the left, right, and center of the navigation bar and for specifying a custom prompt string. Figure 1 shows how the navigation item properties manifest themselves in a navigation bar.
使用standalone navigation bar的時候，你就需要為其提供內容了。navigation bar不能直接添加子視圖。你需要使用UINavigationItem的實例，來指明你要用什么按鈕、什么自定義view。navigation item 有左右中三個內容。
A navigation bar manages a stack of UINavigationItem objects. Although the stack is there mostly to support navigation controllers, you can use it to implement your own custom navigation interface. The topmost item in the stack represents the navigation item whose contents are currently displayed by the navigation bar. You push new navigation items onto the stack using the pushNavigationItem:animated: method and pop items off the stack using the popNavigationItemAnimated: method. Both of these changes can be animated for the benefit of the user.
navigation bar管理一個棧，棧中有很多個UINavigationItem對象。雖然這個棧主要是用來支持navigation controller的，你也可以用它來實現你自己的自定義navigation 界面。棧中最頂部的item是當前navigation bar所展示的內容對應的item。用pushNavigationItem:animated:方法向棧頂部推一個navigation item，用popNavigationItemAnimated:將item彈出。這兩個行為是可以帶動畫的。
In addition to pushing and popping items, you can also set the contents of the stack directly using either the items property or the setItems:animated: method. You might use this method at launch time to restore your interface to its previous state or to push or pop more than one navigation item at a time.Figure 2 shows the part of the UINavigationBar API responsible for managing the stack of navigation items.
除了push和pop item，你也可以通過
items屬性或者setItems:animated:方法來直接設置stack中的內容。你可能需要用這個方法在啟動時恢復到上次的界面狀態，或者一次性push和pop多個navigation item。下圖展示了控制navigation item棧的 UINavigationBar API。
If you are using a navigation bar as a standalone object, assign a custom delegate object to the delegate property and use that object to intercept messages coming from the navigation bar. Delegate objects must conform to the UINavigationBarDelegateprotocol. The delegate notifications let you track when navigation items are pushed or popped from the stack. You use these notifications to update the rest of your app’s user interface.
使用standalone的navigation bar就需要為其
delegate屬性賦值一個代理對象，用這個代理對象截獲navigation bar 的消息。代理對象服從UINavigationBarDelegate協議，代理通知可以讓你知道棧中的push 和pop活動。用這個活動通知來完成你的其他界面功能。
For more information about creating navigation items, see UINavigationItem. For more information about implementing a delegate object, see UINavigationBarDelegate.
Customizing the Appearance of a Navigation Bar
Navigation bars have two standard appearance styles: white with dark text or black with light text. Use the barStyle property to select the style. Any changes you make to other navigation bar appearance properties override those inferred from the bar style.
兩種標準樣式：黑底白字，白底黑字。barStyle可以選擇樣式。對navigation bar外觀進行的改動將會覆蓋從barStyle推斷出的樣式。（就是說你可以修改默認的樣式）
Navigation bars are translucent by default; that is, their background color is semitransparent. You can make the navigation bar opaque by setting the translucent property to NO.
navigation bar默認是半透明的。（背景色是半透明的）你可以將 translucent設置為NO使navigation bar變為不透明。
You can specify a custom tint color for a navigation bar background using the barTintColor property. Setting this property overrides the default color inferred from the bar style. As with all UIView subclasses, you can control the color of the interactive elements within navigation bars, including button images and titles, using the tintColorproperty.
barTintColor屬性可以自定義navigation bar的背景色。修改這個屬性將會覆蓋barStyle推斷出的默認顏色。對于所有的UIView子類來說，只需要使用tintColor屬性，你就可以控制navigation bar上所有可交互元素的顏色，包括按鈕圖片和title。
The titleTextAttributes property specifies the attributes for displaying the bar’s title text. You can specify the font, text color, text shadow color, and text shadow offset for the title in the text attributes dictionary using the NSFontAttributeName, NSForegroundColorAttributeName, and NSShadowAttributeName keys, respectively. For more information about string-formatting attributes, see Character Attributes.
titleTextAttributes屬性表明了bar上title text的attributes。對應的使用 NSFontAttributeName, NSForegroundColorAttributeName, and NSShadowAttributeName三個key可以在text attributes dictionary中指明字體，字色，字陰影，字陰影偏移量。
Use the setTitleVerticalPositionAdjustment:forBarMetrics: method to adjust the vertical position of the title. This method allows you to specify the adjustment dependent on the bar height, which is represented by the UIBarMetrics enum.Figure 3 shows a navigation bar with custom tint color, title text attributes and bar tint color.
用setTitleVerticalPositionAdjustment:forBarMetrics:方法可以調整title的豎直位置。該方法允許你根據bar的高度（由 UIBarMetrics枚舉量表示）來調整位置。下圖展示了一個自定義的bar。
To allow complete customization over the appearance of navigation bars, you can additionally provide custom background and shadow images. To provide a custom background image, use the setBackgroundImage:forBarPosition:barMetrics: method, providing a UIImage object for the appropriate bar position and metrics values. Use a UIBarPosition value for the bar position argument to specify whether to use the supplied image at the bottom or the top of the window, and if it appears at the top, whether to extend it upward under the status bar. Similarly, you can specify that the image should be used for either compact or default bar metrics, with or without a prompt, by providing a UIBarMetrics value to the bar metrics argument.
要完全自定義navigation bar的外觀，可以額外提供自定義的背景和陰影圖。要提供自定義的背景圖片，使用setBackgroundImage:forBarPosition:barMetrics:方法，在適當的位置提供一個UIImage對象。在bar position參數用UIBarPosition值以確定將提供的圖片放在底部還是頂部，如果它出現在頂部，要不要將其延伸到status bar底部。相似地，你可以指明圖片應該是compact還是默認的bar那么大，通過提供一個UIBarMetrics值。
To add a shadow, provide a resizable UIImage to the shadowImage property. To use the custom shadow image, you need to have specified a custom background image.Figure 4shows a navigation bar with a custom background image, supplied using setBackgroundImage:forBarPosition:barMetrics: with a bar position value of UIBarPositionTopAttached and a bar metrics value of UIBarMetricsDefault. A custom image has also been provided to the shadowImage property.
給 shadowImage 屬性賦值一個resizable的UIImage以添加陰影。要使用自定義的陰影圖片，你需要先指明一個自定義的背景圖片。下圖展示了有自定義背景圖片的navigation bar，supplied using setBackgroundImage:forBarPosition:barMetrics: with a bar position value of UIBarPositionTopAttached and a bar metrics value of UIBarMetricsDefault.自定義的陰影圖片由shadowImage 屬性提供。
Interface Builder Attributes
Table 1 lists the core attributes that you configure for navigations bars in the Attributes Inspector within Interface Builder.
可在IB的Attributes Inspector中配置的主要屬性。
| Attribute | Description |
|---|---|
| Style | Specifies the UI bar style to apply to the navigation bar. The bar style controls the title color and the bar tint color, but you can override it by providing values for those attributes. Select Translucent to make the navigation bar semitransparent. Access these values at runtime with the barStyle and translucent properties. |
| Bar Tint | Controls the tint color of the navigation bar. This overrides the value implied by the Style attribute. If the Translucent attribute is selected, the Bar Tint color is automatically made semitransparent. Access this value at runtime with the barTintColor property. |
| Shadow Image | Represents the image used as a shadow beneath the navigation bar. This image is stretched horizontally to match the width of the bar. Access this value at runtime with the shadowImage property. |
| Back Image | Specifies the image that appears at the leading edge of the back button. This attribute must be used in combination with the Back Mask attribute. Access this value at runtime with the backIndicatorImage property. |
| Back Mask | Specifies the mask associated with the Back Image attribute. This is used to control the appearance of the Back button during animated transitions, and therefore must be used in conjunction with the Back Image attribute. Access this value at runtime with the backIndicatorTransitionMaskImage property. |
Table 2 lists the Interface Builder attributes that affect the appearance of the navigation bar’s title.
| Attribute | Description |
|---|---|
| Title Font | The font used to render the title in the center of the navigation bar. Access this value at runtime with the value stored against the NSFontAttributeName key in the dictionary in the titleTextAttributes property. |
| Title Color | The color used to render the navigation bar title. Access this value at runtime using the NSForegroundColorAttributeName key in the dictionary in the titleTextAttributes property. |
| Title Shadow | Specifies the color and offset of the shadow used when rendering the navigation bar’s title. Access these values at runtime with the dictionary in the [titleTextAttributes] property, using the NSShadowAttributeName key. |
Internationalization
To internationalize navigation bars, specify a localized string for each of the displayed string properties of the navigation item model objects.
For more information about internationalizing your interface, see Internationalization and Localization Guide.
Accessibility
Navigation bars are accessible by default. The default accessibility trait for a navigation bar is User Interaction Enabled.
With VoiceOver enabled on an iOS device, after the user navigates to a new view in the hierarchy, VoiceOver reads the navigation bar’s title, followed by the name of the left bar button item. When the user taps an element in a navigation bar, VoiceOver reads the name and the type of the element; for example, "General back button," "Keyboard heading," and "Edit button."
For general information about making your interface accessible, see Accessibility Programming Guide for iOS.
