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41 lines
1.7 KiB
Kotlin
41 lines
1.7 KiB
Kotlin
/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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package org.mozilla.fenix.components
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import mozilla.components.lib.state.Action
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import mozilla.components.lib.state.Middleware
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import mozilla.components.lib.state.MiddlewareContext
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import mozilla.components.lib.state.State
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/**
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* A Middleware for detecting changes to a state property, and offering a callback that captures the action that changed
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* the property, the property before the change, and the property after the change.
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*
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* For example, this can be useful for debugging:
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* ```
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* val selectedTabChangedMiddleware: Middleware<BrowserState, BrowserAction> = ChangeDetectionMiddleware(
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* val selector = { it.selectedTabId }
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* val onChange = { actionThatCausedResult, preResult, postResult ->
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* logger.debug("$actionThatCausedResult changed selectedTabId from $preResult to $postResult")
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* }
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* ```
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*
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* @param selector A function to map from the State to the properties that are being inspected.
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* @param onChange A callback to react to changes to the properties defined by [selector].
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*/
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class ChangeDetectionMiddleware<S : State, A : Action, T>(
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private val selector: (S) -> T,
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private val onChange: (A, pre: T, post: T) -> Unit,
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) : Middleware<S, A> {
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override fun invoke(context: MiddlewareContext<S, A>, next: (A) -> Unit, action: A) {
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val pre = selector(context.store.state)
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next(action)
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val post = selector(context.store.state)
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if (pre != post) {
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onChange(action, pre, post)
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}
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}
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}
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