How to detect Light\Dark mode change in iOS 13?

问题

Some of the UI setups not working automatically with the Dark/Light mode change as the UIColor. For example shadow in layer. As I need to remove and drop shadow in dark and light mode, I need somewhere to put updateShadowIfNeeded() function. I know how to detect what is the mode currently:

func dropShadowIfNeeded() {
    switch traitCollection.userInterfaceStyle {
    case .dark: removeShadow()
    case .light: dropShadowIfNotDroppedYet()
    default: assertionFailure("Unknown userInterfaceStyle")
    }
}

Now I put the function inside the layoutSubviews, since it gets called every time appearance change:

override func layoutSubviews() {
    super.layoutSubviews()
    dropShadowIfNeeded()
}

But this function is getting called A LOT. What is the proper function to trigger only if userInterfaceStyle changed?

回答1:

I found the answer on WWDC 2019 - Session 214 around 23:30.

As I expected, this function is getting called a lot including when colors changing. Along side with many other functions for ViewController and presentationController. But there is some especial function designed for that has a similar signature in all View representers.

Take a look at this image from that session:

Gray: Calling but not good for my issue, Green: Designed for this

So I should call it and check it inside this function:

override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
    super.traitCollectionDidChange(previousTraitCollection)

    if traitCollection.hasDifferentColorAppearance(comparedTo: previousTraitCollection) {
        dropShadowIfNeeded()
    }
}

This will guarantee to be called just once per change.

回答2:

I think this should get called significantly less often, plus the guard makes sure you only react to user interface style changes:

override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
    super.traitCollectionDidChange(previousTraitCollection)

    guard previousTraitCollection?.userInterfaceStyle != traitCollection.userInterfaceStyle else {
        return
    }
    dropShadowIfNeeded()
}

回答3:

With RxSwift and ObjectiveC runtime, you can achieve it without inheritance

here is the encapsulated version:

import UIKit
import RxSwift
import RxCocoa

enum SystemTheme {
    static func get(on view: UIView) -> UIUserInterfaceStyle {
        view.traitCollection.userInterfaceStyle
    }

    static func observe(on view: UIView) -> Observable<UIUserInterfaceStyle> {
        view.rx.methodInvoked(#selector(UIView.traitCollectionDidChange(_:)))
            .map { _ in SystemTheme.get(on: view) }
            .distinctUntilChanged()
    }
}

来源：https://stackoverflow.com/questions/58016866/how-to-detect-light-dark-mode-change-in-ios-13

标签

swiftui

uitraitcollection

ios-darkmode