iOS PRogramming UISplitViewController 

The ipad, on the other hand, has plenty of screen space to present both views using a built-in class called UISplitViewController.

iPad 有足够的屏幕空间来展现两个view用built-in 类称为UISplitViewController。

UISplitViewController is an iPad-only class that presents two view controllers in a master-detail relationship.

UISplitViewController 是iPad-only类，展现两个view controller 在master-detail 关系中。

The master view controller occupies a small strip on the lefthand side of the screen, and the detail view controller occupies the rest of the screen.

master view controller 占据了一个小的屏幕在左侧，detail view controller 占据了剩余的屏幕。

You will also make Nerdfeed a universal application and have it continue to use a UINavigationController when run on the iPhone.

1. Splitting Up Nerdfeed

Creating a UISplitViewController is simple since you have already learned about navigation controllers and tab bar controllers.

When you initialize a split view controller, you pass it an array of view controllers just like with a tab bar controller.

当你initialize  一个split view controller ,你就给他传递了一列 view controllers 就像一个tab bar controller. 

In application:didFinishLaunchingWithOptions:, check whether the device is an iPad before instantiating a UISplitViewController. The UISplitViewController class does not exist on the iPhone, and trying to create an instance of UISplitViewController will cause an exception to be thrown.

在application:didFinishLaunchingWithOptions:(NSDictionary*)launchOptions中检查设备是否是iPad应用在实例化UISplitViewController之前。

// Check to make sure we are running on the iPad

if ([UIDevice currentDevice].userInterfaceIdiom == UIUserInterfaceIdiomPad) {

// webViewController must be in navigation controller; you will see why

later
UINavigationController *detailNav =

[[UINavigationController alloc] initWithRootViewController:wvc];

UISplitViewController *svc = [[UISplitViewController alloc] init];

// Set the delegate of the split view controller to the detail VC

// You will need this later - ignore the warning for now

svc.delegate = wvc;

svc.viewControllers = @[masterNav, detailNav];

// Set the root view controller of the window to the split view controller

self.window.rootViewController = svc; }

else {

// On non-iPad devices, just use the navigation controller

self.window.rootViewController = masterNav;

}

A split view controller must have both the master and the detail view controller when it is created.

一个split view controller 一定即要有master 和detail view controller 当创建的时候。

This is how a UISplitViewController works: in landscape mode, there master view controller is shown in a small strip on the left hand side of the screen and the detail view controller takes over the rest of the screen.

UISplitViewController在landscape 模式，master view  controller 在左侧的一个小的地方展现，detail view controller 占据了屏幕的其他地方。

To address this problem, when a row is tapped, you need to check whether the BNRCoursesViewController is a member of a split view controller and, if it is, take a different action.

为了定位这个问题，当一行被选中后，你需要检查BNRCoursesViewController是否是split view controller 的一员，如果是，就要采取不同的动作。

You can send the message splitViewController to any UIViewController, and if that view controller is part of a split view controller it will return a pointer to the split view controller

你可以向splitViewController 发送消息给任何UIViewController ,如果view controller 是split view controller 的一部分，它将返回一个指针指向split view controller .

Otherwise, it returns nil.

否则就会返回nil。

View controllers are smart: a view controller will return this pointer if it is a member of the split view controller's array or if it belongs to another controller that is a member of a split view controller's array (as is the case with both BNRCoursesViewController and

BNRWebViewController).

View controllers 很聪明：view controller 将返回这个指针，如果他是split view controller 的一员或它属于另外一个controller ,它是split view controller 的成员。

In BNRCoursesViewController.m, locate the method tableView:didSelectRowAtIndexPath:. At the top of this method, check for a split view controller before pushing the BNRWebViewController onto the navigation stack.

在这个方法的开始，在把BNRWebViewController推到navigation stack 之前，先检查一下split view controller .

if (!self.splitViewController) {
[self.navigationController pushViewController:self.webViewController

animated:YES];

}

Now, if the BNRCoursesViewController is not in a split view controller, you assume the device is not an iPad and BNRCoursesViewController pushes the BNRWebViewController onto the navigation controller's stack. 

If BNRCoursesViewController is in a split view controller, then it is left to the UISplitViewController to place the BNRWebViewController on the screen.

2 Displaying the Master View Controller in Portrait Mode

在portrait mode 下显示master view controller 

UISplitViewController lets you do just that by supplying its delegate with a UIBarButtonItem.

UISplitViewController 让你做这些通过提供它的delegate用UIBarButtonItem。

Tapping this button shows the master view controller in a specialized UIPopoverController

点击这个button 将会在specialized UIPopoverController 中展现master view controller .

In your code, whenever a detail view controller was given to the split view controller, that detail

view controller was set as the split view controller's delegate.

在你的代码中，当一个detail

 view controller 被给定了split view controller ,detail view  controller 被设置为split view controller 的delegate.

As the delegate, the detail view controller will get a pointer to the UIBarButtonItem when rotating to portrait mode.

作为一个delegate,detail view controller 将会有一个指向UIBarButtonItem的指针当在portrait mode .

In BNRWebViewController.h, add this declaration:

@interface BNRWebViewController : UIViewController

<UISplitViewControllerDelegate>

In BNRWebViewController.m, implement the following delegate method to place the bar button item in  the BNRWebViewController's navigation item.

BNRWebViewController.m方法中，实现下面的委托方法来放置bar button item 到BNRWebViewController的navigation item中。

- (void)splitViewController:(UISplitViewController *)svc willHideViewController:(UIViewController *)aViewController

withBarButtonItem:(UIBarButtonItem *)barButtonItem forPopoverController:(UIPopoverController *)pc

{

// If this bar button item does not have a title, it will not appear at all

barButtonItem.title = @"Courses";

// Take this bar button item and put it on the left side of the nav item

self.navigationItem.leftBarButtonItem = barButtonItem;

}

Notice that you explicitly set the title of the button. If the button does not have a title, it will not appear at all.

注意到你明确的设置了button的标题。如果button 没有title，你将完全不会显示。

If the master view controller's navigationItem has a title, then the button will be automatically set to that title.

如果master view controller 的navigationItem 有一个title ,那么button 将会自动设为那个title。

Rotate to portrait mode, and you will see the bar button item appear on the left of the navigation bar. Tap that button, and the master view controller's view will appear in a UIPopoverController.

在navigation bar 的左侧，你就会看到bar button item.点击这个button，master view controller 的view就会显示在UIPopoverController中。

This bar button item is why we always had you put the detail view controller inside a navigation controller.

这个bar button item 是为什么我们总是把detail view controller 放在一个navigation controller 里面的原因。

You do not have to use a navigation controller to put a view controller in a split view controller, but it makes using the bar button item much easier.

你不需要必须使用navigation controller 来放置在split view controller中的一个view controller ，但是那样会使用bar button item 更容易。

There are two small issues left to address with your Courses button. First, when the device is rotated back to landscape mode, the button is still there.

现在有两个小问题，首先，当你的设备旋转到landscape mode ,button仍然在哪儿。

To remove it, the delegate needs to respond to another message from the UISplitViewController.

为了移除这些，delegate 需要响应另一个消息从UISplitViewController .

Implement this delegate method in BNRWebViewController.m.

- (void)splitViewController:(UISplitViewController *)svc willShowViewController:(UIViewController *)aViewController

invalidatingBarButtonItem:(UIBarButtonItem *)barButtonItem

{
// Remove the bar button item from the navigation item
// Double check that it is the correct button, even though we know it is if (barButtonItem == self.navigationItem.leftBarButtonItem) {

self.navigationItem.leftBarButtonItem = nil; }

}

The Courses button will now appear and disappear as you rotate between portrait and landscape modes.

3  Universalizing Nerdfeed 通用的Nerdfeed

（1）As you built Nerdfeed, you were careful about the device differences in the classes used.

（2）Nerdfeed is still a relatively simple application. It is always easier to universalize an application early in development. As an application grows, its details get buried in the massive pile of code. Finding and fixing issues as you are writing code is much easier than coming back later. Details are harder to find, and there is the risk of breaking what already works.