How to tell if UIViewController's view is visi

问题:

I have a tab bar application, with many views. Is there a way to know if a particular UIViewController is currently visible from within the UIViewController? (looking for a property)

回答1:

The view\'s window property is non-nil if a view is currently visible, so check the main view in the view controller:

[EDIT] Invoking the view method causes the view to load (if it is not loaded) which is unnecessary and may be undesirable. It would be better to check first to see if it is already loaded. I\'ve added the call to isViewLoaded to avoid this problem.

if (viewController.isViewLoaded && viewController.view.window) {
    // viewController is visible
}

Or if you have a UINavigationController managing the view controllers, you could check its visibleViewController property instead.

Also, in Swift on iOS 9 (or later):

if viewController.viewIfLoaded?.window != nil {
    // viewController is visible
}

回答2:

Here\'s @progrmr\'s solution as a UIViewController category:

// UIViewController+Additions.h

@interface UIViewController (Additions)

- (BOOL)isVisible;

@end


// UIViewController+Additions.m

#import \"UIViewController+Additions.h\"

@implementation UIViewController (Additions)

- (BOOL)isVisible {
    return [self isViewLoaded] && self.view.window;
}

@end

回答3:

There are a couple of issues with the above solutions. If you are using, for example, a UISplitViewController, the master view will always return true for

if(viewController.isViewLoaded && viewController.view.window) {
    //Always true for master view in split view controller
}

Instead, take this simple approach which seems to work well in most, if not all cases:

- (void)viewDidDisappear:(BOOL)animated {
    [super viewDidDisappear:animated];

    //We are now invisible
    self.visible = false;
}

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];

    //We are now visible
    self.visible = true;
}

回答4:

For those of you looking for a Swift 2.2 version of the answer:

if self.isViewLoaded() && (self.view.window != nil) {
     // viewController is visible
}

and Swift 3:

if self.isViewLoaded && (self.view.window != nil) {
         // viewController is visible
}

回答5:

You want to use the UITabBarController\'s selectedViewController property. All view controllers attached to a tab bar controller have a tabBarController property set, so you can, from within any of the view controllers\' code:

if([[[self tabBarController] selectedViewController] isEqual:self]){
     //we\'re in the active controller
}else{
     //we are not
}

回答6:

For over-full-screen or over-context modal presentation, \"is visible\" could mean it is on top of the view controller stack or just visible but covered by another view controller.

To check if the view controller \"is the top view controller\" is quite different from \"is visible\", you should check the view controller\'s navigation controller\'s view controller stack.

I wrote a piece of code to solve this problem:

extension UIViewController {
    public var isVisible: Bool {
        if isViewLoaded {
            return view.window != nil
        }
        return false
    }

    public var isTopViewController: Bool {
        if self.navigationController != nil {
            return self.navigationController?.visibleViewController === self
        } else if self.tabBarController != nil {
            return self.tabBarController?.selectedViewController == self && self.presentedViewController == nil
        } else {
            return self.presentedViewController == nil && self.isVisible
        }
    }
}

回答7:

I made a swift extension based on @progrmr\'s answer.

It allows you to easily check if a UIViewController is on screen like so:

if someViewController.isOnScreen {
    // Do stuff here
}

The extension:

//
//  UIViewControllerExtension.swift
//

import UIKit

extension UIViewController{
    var isOnScreen: Bool{
        return self.isViewLoaded() && view.window != nil
    }
}

回答8:

For my purposes, in the context of a container view controller, I\'ve found that

- (BOOL)isVisible {
    return (self.isViewLoaded && self.view.window && self.parentViewController != nil);
}

works well.

回答9:

if you\'re utilizing a UINavigationController and also want to handle modal views, the following is what i use:

#import <objc/runtime.h>

UIViewController* topMostController = self.navigationController.visibleViewController;
if([[NSString stringWithFormat:@\"%s\", class_getName([topMostController class])] isEqualToString:@\"NAME_OF_CONTROLLER_YOURE_CHECKING_IN\"]) {
    //is topmost visible view controller
}

回答10:

The approach that I used for a modal presented view controller was to check the class of the presented controller. If the presented view controller was ViewController2 then I would execute some code.

UIViewController *vc = [self presentedViewController];

if ([vc isKindOfClass:[ViewController2 class]]) {
    NSLog(@\"this is VC2\");
}

回答11:

I found those function in UIViewController.h.

/*
  These four methods can be used in a view controller\'s appearance callbacks to determine if it is being
  presented, dismissed, or added or removed as a child view controller. For example, a view controller can
  check if it is disappearing because it was dismissed or popped by asking itself in its viewWillDisappear:
  method by checking the expression ([self isBeingDismissed] || [self isMovingFromParentViewController]).
*/

- (BOOL)isBeingPresented NS_AVAILABLE_IOS(5_0);
- (BOOL)isBeingDismissed NS_AVAILABLE_IOS(5_0);

- (BOOL)isMovingToParentViewController NS_AVAILABLE_IOS(5_0);
- (BOOL)isMovingFromParentViewController NS_AVAILABLE_IOS(5_0);

Maybe the above functions can detect the ViewController is appeared or not.

回答12:

XCode 6.4, for iOS 8.4, ARC enabled

Obviously lots of ways of doing it. The one that has worked for me is the following...

@property(nonatomic, readonly, getter=isKeyWindow) BOOL keyWindow

This can be used in any view controller in the following way,

[self.view.window isKeyWindow]

If you call this property in -(void)viewDidLoad you get 0, then if you call this after -(void)viewDidAppear:(BOOL)animated you get 1.

Hope this helps someone. Thanks! Cheers.

回答13:

If you are using a navigation controller and just want to know if you are in the active and topmost controller, then use:

if navigationController?.topViewController == self {
    // Do something
}

This answer is based on @mattdipasquale\'s comment.

If you have a more complicated scenario, see the other answers above.

回答14:

you can check it by window property

if(viewController.view.window){

// view visible

}else{

// no visible

}

回答15:

I needed this to check if the view controller is the current viewed controller, I did it via checking if there\'s any presented view controller or pushed through the navigator, I\'m posting it in case anyone needed such a solution:

if presentedViewController != nil || navigationController?.topViewController != self {
      //Viewcontroller isn\'t viewed
}else{
     // Now your viewcontroller is being viewed 
}