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[react-native] Specify more detailed type of hardwareBackPress handler #42618

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[react-native] Specify more detailed type of hardwareBackPress handler #42618

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11 changes: 8 additions & 3 deletions types/react-native/index.d.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -7000,12 +7000,17 @@ export type BackPressEventName = 'hardwareBackPress';
* Detect hardware back button presses, and programmatically invoke the
* default back button functionality to exit the app if there are no
* listeners or if none of the listeners return true.
* Methods don't have more detailed documentation as of 0.25.
* The event subscriptions are called in reverse order
* (i.e. last registered subscription first), and if one subscription
* returns true then subscriptions registered earlier
* will not be called.
*
* @see https://reactnative.dev/docs/backhandler.html
*/
export interface BackHandlerStatic {
exitApp(): void;
addEventListener(eventName: BackPressEventName, handler: () => void): NativeEventSubscription;
removeEventListener(eventName: BackPressEventName, handler: () => void): void;
addEventListener(eventName: BackPressEventName, handler: () => true | void): NativeEventSubscription;
removeEventListener(eventName: BackPressEventName, handler: () => true | void): void;

@sheetalkamat sheetalkamat Feb 26, 2020

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This change seems incorrect.. Even without this change, you should be able to pass handle that returns true

@ghost ghost Feb 26, 2020

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Yes, indeed, one is already able to pass a handler that returns anything at all, as no return value is strictly needed. However the handler must return true, as per documentation (or actually any truthy value, as per implementation), to cause specific behaviour - stopping all the subsequent listeners, including the default one, from executing. And it seems appropriate to document that the handler "should" return true or nothing (even though, in the implementation, the return value is typecasted to boolean, and any value is possible).

@alloy alloy Feb 26, 2020

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The Flow version doesn’t really say anything about return type. Looking at the docs and the implementation, it seems to me like this should just be () => boolean, but perhaps () => boolean | null | undefined is acceptable too.

In any case, it would be good to send a similar change upstream, unless I’m overlooking a reason for this to not be typed as such already. /cc @TheSavior

@elicwhite elicwhite Feb 26, 2020

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I’m not familiar with this. If the flow code doesn’t include boolean then it is likely that we don’t have any callsites doing that, which makes me not trust the documentation. Can you verify the runtime behavior of the listeners and behavior of returning true?

@ghost ghost Feb 26, 2020

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Yes, the links are attached

@elicwhite elicwhite Feb 26, 2020 •
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Ah, okay, I understand better now. The flow code for those functions is untyped. Function is the same as any. I think having the type be boolean | void is reasonable. Please upstream a PR making it typed as returning ?boolean.

@alloy alloy Feb 27, 2020 •
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I think having the type be boolean | void is reasonable. Please upstream a PR making it typed as returning ?boolean.

Just to be sure we’re all on the same page; I assume you meant the Flow equivalent that I posted: () => boolean | null | undefined. Using void would I think send the wrong message, i.e. that the return value would not lead to effects.

@elicwhite elicwhite Feb 27, 2020

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Correct. I was agreeing with what you posted :-) Thanks for checking

}

export interface ButtonProps {
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1 change: 1 addition & 0 deletions types/react-native/test/index.tsx
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Expand Up @@ -126,6 +126,7 @@ function TextUseWindowDimensions() {
}

BackHandler.addEventListener("hardwareBackPress", () => {}).remove();
BackHandler.addEventListener("hardwareBackPress", () => true).remove();

interface LocalStyles {
container: ViewStyle;
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