问题
Tapping the tab bar icon for the current navigation controller already returns the user to the root view, but if they are scrolled way down, if they tap it again I want it to scroll to the top (same effect as tapping the status bar). How would I do this?
A good example is Instagram's feed, scroll down then tap the home icon in the tab bar to scroll back to top.
The scrolling back to the top is easy, but connecting it to the tab bar controller is what I'm stuck on.
回答1:
Implement the UITabBarControllerDelegate method tabBarController:didSelectViewController: to be notified when the user selects a tab. This method is also called when the same tab button is tapped again, even if that tab is already selected.
A good place to implement this delegate would probably be your AppDelegate. Or the object that logically "owns" the tab bar controller.
I would declare and implement a method that can be called on your view controllers to scroll the UICollectionView.
- (void)tabBarController:(UITabBarController *)tabBarController
didSelectViewController:(UIViewController *)viewController
{
static UIViewController *previousController = nil;
if (previousController == viewController) {
// the same tab was tapped a second time
if ([viewController respondsToSelector:@selector(scrollToTop)]) {
[viewController scrollToTop];
}
}
previousController = viewController;
}
回答2:
SWIFT 3
Here goes..
First implement the UITabBarControllerDelegate in the class and make sure the delegate is set in viewDidLoad
class DesignStoryStreamVC: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource, UITabBarControllerDelegate {
@IBOutlet weak var collectionView: UICollectionView!
override func viewDidLoad() {
super.viewDidLoad()
self.tabBarController?.delegate = self
collectionView.delegate = self
collectionView.dataSource = self
}
}
Next, put this delegate function somewhere in your class.
func tabBarController(_ tabBarController: UITabBarController, didSelect viewController: UIViewController) {
let tabBarIndex = tabBarController.selectedIndex
print(tabBarIndex)
if tabBarIndex == 0 {
self.collectionView.setContentOffset(CGPoint.zero, animated: true)
}
}
Make sure to select the correct index in the "if" statement. I included the print function so you can double check.
回答3:
You can use shouldSelect rather than didSelect, which would omit the need for an external variable to keep track of the previous view controller.
- (BOOL)tabBarController:(UITabBarController *)tabBarController shouldSelectViewController:(UIViewController *)viewController
{
if ([viewController isEqual:self] && [tabBarController.selectedViewController isEqual:viewController]) {
// Do custom stuff here
}
return YES;
}
回答4:
extension UIViewController {
func scrollToTop() {
func scrollToTop(view: UIView?) {
guard let view = view else { return }
switch view {
case let scrollView as UIScrollView:
if scrollView.scrollsToTop == true {
scrollView.setContentOffset(CGPoint(x: 0.0, y: -scrollView.contentInset.top), animated: true)
return
}
default:
break
}
for subView in view.subviews {
scrollToTop(view: subView)
}
}
scrollToTop(view: self.view)
}
}
This is my answer in Swift 3. It uses a helper function for recursive calls and it automatically scrolls to top on call. Tested on a UICollectionViewController embedded into a UINavigationController embedded in a UITabBarController
回答5:
I was using this View hierarchy.
UITabBarController > UINavigationController > UIViewController
I got a reference to the UITabBarController in the UIViewController
tabBarControllerRef = self.tabBarController as! CustomTabBarClass
tabBarControllerRef!.navigationControllerRef = self.navigationController as! CustomNavigationBarClass
tabBarControllerRef!.viewControllerRef = self
Then I created a Bool that was called at the correct times, and a method that allows scrolling to top smoothly
var canScrollToTop:Bool = true
// Called when the view becomes available
override func viewWillAppear(animated: Bool) {
super.viewWillAppear(animated)
canScrollToTop = true
}
// Called when the view becomes unavailable
override func viewWillDisappear(animated: Bool) {
super.viewWillDisappear(animated)
canScrollToTop = false
}
// Scrolls to top nicely
func scrollToTop() {
self.collectionView.setContentOffset(CGPoint(x: 0, y: 0), animated: true)
}
Then in my UITabBarController Custom Class I called this
func tabBarController(tabBarController: UITabBarController, didSelectViewController viewController: UIViewController) {
// Allows scrolling to top on second tab bar click
if (viewController.isKindOfClass(CustomNavigationBarClass) && tabBarController.selectedIndex == 0) {
if (viewControllerRef!.canScrollToTop) {
viewControllerRef!.scrollToTop()
}
}
}
The Result is identical to Instagram and Twitter's feed :)
回答6:
Swift 3 approach::
//MARK: Properties
var previousController: UIViewController?
func tabBarController(_ tabBarController: UITabBarController, shouldSelect viewController: UIViewController) -> Bool {
if self.previousController == viewController || self.previousController == nil {
// the same tab was tapped a second time
let nav = viewController as! UINavigationController
// if in first level of navigation (table view) then and only then scroll to top
if nav.viewControllers.count < 2 {
let tableCont = nav.topViewController as! UITableViewController
tableCont.tableView.setContentOffset(CGPoint(x: 0.0, y: -tableCont.tableView.contentInset.top), animated: true)
}
}
self.previousController = viewController;
return true
}
A few notes here:: "shouldSelect" instead of "didSelect" because the latter is taking place after transition, meaning viewController local var already changed. 2. We need to handle the event before changing controller, in order to have the information of navigation's view controllers regarding scrolling (or not) action.
Explanation:: We want to scroll to top, if current view is actually a List/Table view controller. If navigation has advanced and we tap same tab bar, desired action would be to just pop one step (default functionality) and not scroll to top. If navigation hasn't advanced meaning we are still in table/list controller then and only then we want to scroll to top when tapping again. (Same thing Facebook does when tapping "Feed" from a user's profile. It only goes back to feed without scrolling to top.
回答7:
In this implementation you no need static variable and previous view controller state
If your UITableViewController in UINavigationController you can implement protocol and function:
protocol ScrollableToTop {
func scrollToTop()
}
extension UIScrollView {
func scrollToTop(_ animated: Bool) {
var topContentOffset: CGPoint
if #available(iOS 11.0, *) {
topContentOffset = CGPoint(x: -safeAreaInsets.left, y: -safeAreaInsets.top)
} else {
topContentOffset = CGPoint(x: -contentInset.left, y: -contentInset.top)
}
setContentOffset(topContentOffset, animated: animated)
}
}
Then in your UITableViewController:
class MyTableViewController: UITableViewController: ScrollableToTop {
func scrollToTop() {
if isViewLoaded {
tableView.scrollToTop(true)
}
}
}
Then in UITabBarControllerDelegate:
extension MyTabBarController: UITabBarControllerDelegate {
func tabBarController(_ tabBarController: UITabBarController, shouldSelect viewController: UIViewController) -> Bool {
guard tabBarController.selectedViewController === viewController else { return true }
guard let navigationController = viewController as? UINavigationController else {
assertionFailure()
return true
}
guard
navigationController.viewControllers.count <= 1,
let destinationViewController = navigationController.viewControllers.first as? ScrollableToTop
else {
return true
}
destinationViewController.scrollToTop()
return false
}
}
回答8:
Swift 5: no need for stored properties in the UITabBarController.
In MyTabBarController.swift, implement tabBarController(_:shouldSelect) to detect when the user re-selects the tab bar item:
protocol TabBarReselectHandling {
func handleReselect()
}
class MyTabBarController: UITabBarController, UITabBarControllerDelegate {
override func viewDidLoad() {
super.viewDidLoad()
delegate = self
}
func tabBarController(
_ tabBarController: UITabBarController,
shouldSelect viewController: UIViewController
) -> Bool {
if tabBarController.selectedViewController === viewController,
let handler = viewController as? TabBarReselectHandling {
// NOTE: viewController in line above might be a UINavigationController,
// in which case you need to access its contents
handler.handleReselect()
}
return true
}
}
In MyTableViewController.swift, handle the re-selection by scrolling the table view to the top:
class MyTableViewController: UITableViewController, TabBarReselectHandling {
func handleReselect() {
tableView?.setContentOffset(.zero, animated: true)
}
}
Now you can easily extend this to other tabs by just implementing TabBarReselectHandling.
回答9:
I have a collection view embedded in a navigation controller, in Swift this works.
var previousController: UIViewController?
func tabBarController(tabBarController: UITabBarController, didSelectViewController viewController: UIViewController) {
if previousController == viewController {
if let navVC = viewController as? UINavigationController, vc = navVC.viewControllers.first as? UICollectionViewController {
vc.collectionView?.setContentOffset(CGPointZero, animated: true)
}
}
previousController = viewController;
}
回答10:
I've implemented a plug & play UITabBarController that you can freely re-use in your projects. To enable the scroll-to-top functionality, you should just have to use the subclass, nothing else.
Should work out of the box with Storyboards also.
Code:
/// A UITabBarController subclass that allows "scroll-to-top" gestures via tapping
/// tab bar items. You enable the functionality by simply subclassing.
class ScrollToTopTabBarController: UITabBarController, UITabBarControllerDelegate {
/// Determines whether the scrolling capability's enabled.
var scrollEnabled: Bool = true
private var previousIndex = 0
override func viewDidLoad() {
super.viewDidLoad()
delegate = self
}
/*
Always call "super" if you're overriding this method in your subclass.
*/
func tabBarController(tabBarController: UITabBarController, didSelectViewController viewController: UIViewController) {
guard scrollEnabled else {
return
}
guard let index = viewControllers?.indexOf(viewController) else {
return
}
if index == previousIndex {
dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0), { [weak self] () in
guard let scrollView = self?.iterateThroughSubviews(self?.view) else {
return
}
dispatch_async(dispatch_get_main_queue(), {
scrollView.setContentOffset(CGPointZero, animated: true)
})
})
}
previousIndex = index
}
/*
Iterates through the view hierarchy in an attempt to locate a UIScrollView with "scrollsToTop" enabled.
Since the functionality relies on "scrollsToTop", it plugs easily into existing architectures - you can
control the behaviour by modifying "scrollsToTop" on your UIScrollViews.
*/
private func iterateThroughSubviews(parentView: UIView?) -> UIScrollView? {
guard let view = parentView else {
return nil
}
for subview in view.subviews {
if let scrollView = subview as? UIScrollView where scrollView.scrollsToTop == true {
return scrollView
}
if let scrollView = iterateThroughSubviews(subview) {
return scrollView
}
}
return nil
}
}
Edit (09.08.2016):
After attempting to compile with the default Release configuration (archiving) the compiler would not allow the possibility of creating a large number of closures that were captured in a recursive function, thus it would not compile. Changed out the code to return the first found UIScrollView with "scrollsToTop" set to true without using closures.
回答11:
I found the scrollRectToVisible method works better than the setContentOffset.
Swift:
After you catch the click on the tab bar from the delegate, something like below:
func tabBarController(tabBarController: UITabBarController, didSelectViewController viewController: UIViewController) {
if (viewController.isKindOfClass(SomeControllerClass) && tabBarController.selectedIndex == 0)
{
viewController.scrollToTop()
}
}
Now for the scrollToTop function inside the controller:
func scrollToTop()
{
self.tableView.scrollRectToVisible(CGRectMake(0,0,CGRectGetWidth(self.tableView.frame), CGRectGetHeight(self.tableView.frame)), animated: true)
}
来源：https://stackoverflow.com/questions/22392151/tap-tab-bar-to-scroll-to-top-of-uitableviewcontroller