Detect when a tab bar item is pressed

问题:

I have a root view controller which isn’t set as the custom class for any of my view controllers on my storyboard. Instead, all of my view controllers are subclassing this class like so.

// RootViewController
class RootViewController: UIViewController, UITabBarDelegate { 

    // This is not getting executed on any of the view controllers
    func tabBar(tabBar: UITabBar, didSelectItem item: UITabBarItem) {
        print("ddd")
    }
}

// Subclassing it 
class TopStoriesViewController: RootViewController {

}

But I seem to be struggling with doing something when a tabbaritem is pressed on the view controller which is subclassing the rootviewcontroller, i.e, the message is not being printed.

回答1:

You don't want your view controller's base class to be a UITabBarDelegate. If you were to do that, all of your view controller subclasses would be tab bar delegates. What I think you want to do is to extend UITabBarController, something like this:

class MyTabBarController: UITabBarController, UITabBarControllerDelegate {

then, in that class, override viewDidLoad and in there set the delegate property to self:

self.delegate = self

Note: This is setting the tab bar controller delegate. The tab bar has it's own delegate (UITabBarDelegate), which the tab bar controller manages, and you are not allow to change.

So, now this class is both a UITabBarDelegate (because UITabBarController implements that protocol), and UITabBarControllerDelegate, and you can override/implement those delegate's methods as desired, such as:

// UITabBarDelegate
override func tabBar(tabBar: UITabBar, didSelectItem item: UITabBarItem) {
    print("Selected item")
}

// UITabBarControllerDelegate
func tabBarController(tabBarController: UITabBarController, didSelectViewController viewController: UIViewController) {
    print("Selected view controller")
}

I'm guessing you're probably more interested in the latter. Check out the documentation to see what each of these delegates provide.

Last thing, in your storyboard (assuming you are using storyboards), set your tab bar controller's class to MyTabBarController in the Identity Inspector, and you're good to go.

Swift 3/4

// UITabBarDelegate
override func tabBar(_ tabBar: UITabBar, didSelect item: UITabBarItem) {
    print("Selected item")
}

// UITabBarControllerDelegate
func tabBarController(_ tabBarController: UITabBarController, didSelect viewController: UIViewController) {
    print("Selected view controller")
}

回答2:

Doing it this way caused me an error

Changing the delegate of a tab bar managed by a tab bar controller is not allowed

This is what I did and it worked

1. In you ViewController you inherit UITabBarControllerDelegate
2. Set delegate in a viewDidLoad
3. Add a function

Example:

class MyClass: UIViewController, UITabBarControllerDelegate {

   func tabBarController(_ tabBarController: UITabBarController, didSelect viewController: UIViewController) {
        let tabBarIndex = tabBarController.selectedIndex
        if tabBarIndex == 0 {
            //do your stuff
        }
   }

   override func viewDidLoad() {
        super.viewDidLoad()
        self.tabBarController?.delegate = self
   }

}

回答3:

Here is a version of @mbeaty's answer with a little more context. It is adapted from my fuller answer here.

import UIKit

class MyTabBarController: UITabBarController, UITabBarControllerDelegate {

    override func viewDidLoad() {
        super.viewDidLoad()

        // tell our UITabBarController subclass to handle its own delegate methods
        self.delegate = self
    }

    // called whenever a tab button is tapped
    func tabBarController(_ tabBarController: UITabBarController, didSelect viewController: UIViewController) {

        if let firstVC = viewController as? FirstViewController {
            firstVC.doSomeAction()
        }

        if viewController is FirstViewController {
            print("First tab")
        } else if viewController is SecondViewController {
            print("Second tab")
        }
    }

    // alternate method if you need the tab bar item
    override func tabBar(_ tabBar: UITabBar, didSelect item: UITabBarItem) {
        // ...
    }
}

Set this as the Custom class of your Tab View Controller in IB.

Alternate solution

• Just do something in the viewDidLoad method of the tab's view controller. See this answer.