如何使UITabBarController懒惰地加载视图控制器？

Question

我有一个UITabBarController创建programaticaly管理UIViewController的4个子类.就像是:

//Create Controller 1
    self.terminal = [[[TerminalController alloc] initWithNibName:@"TerminalView" bundle:nil] autorelease];
    UINavigationController* navTerminal = [[[UINavigationController alloc] initWithRootViewController:terminal] autorelease];
    navTerminal.title = __(@"Terminal");
    navTerminal.navigationBar.barStyle = UIBarStyleBlackOpaque;
    navTerminal.tabBarItem.image = [UIImage imageNamed:@"tab_terminal.png"];        

    //Create Controller 2
    self.history = [[[HistoryController alloc] initWithNibName:@"HistoryView" bundle:nil] autorelease];
    UINavigationController* navHistory =  [[[UINavigationController alloc] initWithRootViewController:history] autorelease];
    navHistory.title = __(@"History");
    navHistory.navigationBar.barStyle = UIBarStyleBlackOpaque;
    navHistory.tabBarItem.image = [UIImage imageNamed:@"tab_history.png"];

    //Create Controller 3
    self.settings = [[[SettingsController alloc] initWithNibName:@"SettingsView" bundle:nil] autorelease];
    UINavigationController* navSettings =  [[[UINavigationController alloc] initWithRootViewController:settings] autorelease];
    navSettings.title = __(@"Settings");
    navSettings.navigationBar.barStyle = UIBarStyleBlackOpaque;
    navSettings.tabBarItem.image = [UIImage imageNamed:@"tab_settings.png"];

    //Create Controller 4
    HelpController* help = [[[HelpController alloc] initWithNibName:@"HelpView" bundle:nil] autorelease];
    UINavigationController* navHelp =  [[[UINavigationController alloc] initWithRootViewController:help] autorelease];
    navHelp.title = __(@"Help");
    navHelp.navigationBar.barStyle = UIBarStyleBlackOpaque;
    navHelp.tabBarItem.image = [UIImage imageNamed:@"tab_help.png"];

    //Create Tab Bar an add it's view to window.
    self.tabBar = [[[UITabBarController alloc] initWithNibName:nil bundle:nil] autorelease];
    tabBar.viewControllers = [[[NSArray alloc] initWithObjects:navTerminal, navHistory, navSettings, navHelp, nil] autorelease];
    tabBar.delegate = self;     

    [window addSubview:tabBar.view];

有没有办法告诉UITabBarController懒惰地加载视图控制器？ej,当用户点击其中一个标签栏项目或者调用tabBarController setSelectedIndex时？

Answer 1

我在我的一个应用程序中做了这件事.诀窍是让你的视图控制器不是UITabBarController的子类,而是UIViewController,并实现UITabBarDelegate.我在IB中为此创建了视图,布置了标签栏(带有正确的按钮,图像,标签等)和占位符UIView,用于正确放置交换的子视图.视图切换发生在tabBar:didSelectItem:它看起来像这样:

// MyTabBarController.h
@class MyFirstViewController;
@class MySecondViewController;
@class MyThirdViewController;

@interface MyTabBarController : UIViewController <UITabBarDelegate> {
    IBOutlet UIView *placeholderView;
    IBOutlet UITabBar *tabBar;
    MyFirstViewController *firstViewController;
    MySecondViewController *secondViewController;
    MyThirdViewController *thirdViewController;
    UIViewController *currentViewController;
}
@property (nonatomic, retain) MyFirstViewController *firstViewController;
@property (nonatomic, retain) MySecondViewController *secondViewController;
@property (nonatomic, retain) MyThirdViewController *thirdViewController;

- (void) switchToView:(UIViewController*)aViewController;
@end


//  MyTabBarController.m
#import "MyTabBarController.h"
#import "MyFirstViewController.h"
#import "MySecondViewController.h"
#import "MyThirdViewController.h"

enum {
    kView_First = 1,
    kView_Second,
    kView_Third
};

@implementation MyTabBarController

@synthesize firstViewController, secondViewController, thirdViewController;

- (void) viewDidLoad {
    // Default to first view.
    tabBar.selectedItem = [tabBar.items objectAtIndex:0];
    MyFirstViewController *viewController = [[MyFirstViewController alloc] initWithNibName:@"FirstView" bundle:nil];
    self.firstViewController = viewController;
    [viewController release];
    [self switchToView:firstViewController];
}

- (void)viewWillAppear:(BOOL)animated {
    // Tell our subview.
    if( currentViewController != nil ) {
        [currentViewController viewWillAppear:animated];
    }
}

- (void)tabBar:(UITabBar *)tabBar didSelectItem:(UITabBarItem *)item {
    switch (item.tag) {
        case kView_First: {
            if (firstViewController == nil) {
                MyFirstViewController *viewController = [[MyFirstViewController alloc]
                    initWithNibName:@"FirstView" bundle:nil];
                self.firstViewController = viewController;
                [viewController release];
            }

            [self switchToView:firstViewController];
        }
        break;

        case kView_Second:
            if (secondViewController == nil) {
                MySecondViewController *viewController = [[MySecondViewController alloc]
                initWithNibName:@"SecondView" bundle:nil];
                self.secondViewController = viewController;
                [viewController release];
            }

            [self switchToView:secondViewController];
            break;

        case kView_Third: {
            if (timesViewController == nil) {
                MyThirdViewController *viewController = [[MyThirdViewController alloc]
                initWithNibName:@"ThirdView" bundle:nil];
                self.thirdViewController = viewController;
                [viewController release];
            }

            [self switchToView:thirdViewController];
        }
        break;              
    }
}

- (void) switchToView:(UIViewController*)aViewController {
    if( aViewController == currentViewController ) return;

    UIView *aView= aViewController.view;                
    [aViewController viewWillAppear:NO];
    if( currentViewController != nil ) {
        [currentViewController viewWillDisappear:NO];
        [currentViewController.view removeFromSuperview];       
    }
    aView.frame = placeholderView.frame;
    [self.view insertSubview:aView aboveSubview:placeholderView];
    if( currentViewController != nil ) {
        [currentViewController viewDidDisappear:NO];
    }
    [aViewController viewDidAppear:NO];
    currentViewController = aViewController;
}
@end

代码肯定会变得更干,但你明白了.

Answer 2

您要延迟加载什么？

这是一个非常标准的UITabBarController实现。在我正在编写的应用程序中，我有一个与此非常相似的东西。在我的代码中，在触摸选项卡之前，不会调用viewDidLoad（在控制器将其关联的视图加载到内存后调用的方法）。

我确实相信（除了所有自动发布的对象之外）您编码的方式是创建此类UI的首选方法。