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Lazy load background tabs at app startup (#553)
Task/Issue URL: https://app.asana.com/0/1177771139624306/1201065943414949/f Description: I added a mechanism that records selected tabs in chronological order and reloads them in that order at app startup. The logic is as follows: * app launches * if there are no URL tabs or only 1 URL tab and it's currently selected, do nothing, don't even initialize * if current tab is a URL tab, wait until it finishes loading (or fails to load), otherwise proceed immediately * pick up to 3 most recently visited URL tab and reload them in background * as background tabs finish (or fail) loading, repeat previous step (keeping at most 3 concurrent loads) until 20 background tabs are loaded or there are no more URL tabs to load * as the user switches through tabs during lazy loading, record visited tabs and remove them from the lazy loading queue (they were visited manually which triggered a reload) * if 20 tabs were reloaded in background or there are no more not activated URL tabs, report finished work Additional details: * TabLazyLoader is owned by TabCollectionViewModel which serves as its data source * When lazy loader reports completion, it gets deallocated * Lazy loader does not differentiate between successful and failed loads, it only records website load attempts and completion (regardless of the outcome) - it's therefore not affected by "no internet" scenario * the implementation is based on generic types to allow for more thorough unit testing * the mechanism is inspired by how Chromium-based browsers work, but magic numbers 3 and 20 mentioned above were picked by Gabriel - you'll find more information in the Asana task. * OSLog.tabLazyLoading was added to help debugging lazy loading (disabled by default) * If there are more than 20 tabs at app startup, lazy loading starts with 10 tabs adjacent to the current one, before proceeding to 10 most recently selected. * Lazy loading is paused every time the user interacts with the current tab (triggers a navigation).
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DuckDuckGo/Common/Utilities/AdjacentItemEnumerator.swift
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| // | ||
| // AdjacentItemEnumerator.swift | ||
| // | ||
| // Copyright © 2022 DuckDuckGo. All rights reserved. | ||
| // | ||
| // Licensed under the Apache License, Version 2.0 (the "License"); | ||
| // you may not use this file except in compliance with the License. | ||
| // You may obtain a copy of the License at | ||
| // | ||
| // http://www.apache.org/licenses/LICENSE-2.0 | ||
| // | ||
| // Unless required by applicable law or agreed to in writing, software | ||
| // distributed under the License is distributed on an "AS IS" BASIS, | ||
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| // See the License for the specific language governing permissions and | ||
| // limitations under the License. | ||
| // | ||
|
|
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| import Foundation | ||
|
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| /** | ||
| * This struct generates array indices adjacent to a given index. | ||
| * | ||
| * The _adjacent_ indices are generated as diffs against given index | ||
| * with the following pattern: | ||
| * | ||
| * 1, -1, 2, -2, 3, -3, 4, -4, ... | ||
| * | ||
| */ | ||
| struct AdjacentItemEnumerator { | ||
|
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| /** | ||
| * The index of an item for which adjacent items indices are computed. | ||
| */ | ||
| var itemIndex: Int | ||
|
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| /** | ||
| * Last valid adjacent item index returned by `nextIndex(arraySize:)`. | ||
| */ | ||
| var currentAdjacentIndex: Int { | ||
| itemIndex + currentDiff | ||
| } | ||
|
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| /** | ||
| * Computes next adjacent index, constrained by `arraySize`. | ||
| * | ||
| * Returns the next available and valid index, between by `0` and `arraySize-1`, | ||
| * or `nil` if the index falls outside array bounds. | ||
| */ | ||
| mutating func nextIndex(arraySize: Int) -> Int? { | ||
| if currentDiff != 0 { | ||
| operation = operation.next | ||
| } | ||
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| previousDiff = currentDiff | ||
| currentDiff = operation.perform(with: currentDiff) | ||
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| let newIndex = itemIndex + currentDiff | ||
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| if newIndex < 0 || newIndex >= arraySize { | ||
| let previousIndex = itemIndex + previousDiff | ||
| if previousIndex <= 0 || previousIndex >= arraySize - 1 { | ||
| return nil | ||
| } | ||
| return nextIndex(arraySize: arraySize) | ||
| } | ||
| return newIndex | ||
| } | ||
|
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| /** | ||
| * Resets the enumerator internal state | ||
| * | ||
| * Calling this function is equivalent to reinstantiating the enumerator. | ||
| */ | ||
| mutating func reset() { | ||
| currentDiff = 0 | ||
| previousDiff = 0 | ||
| operation = .toggleSignAndAdvance | ||
| } | ||
|
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| init(itemIndex: Int = 0) { | ||
| self.itemIndex = itemIndex | ||
| } | ||
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| // MARK: - Private | ||
|
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| private var currentDiff = 0 | ||
| private var previousDiff = 0 | ||
| private var operation: Operation = .toggleSignAndAdvance | ||
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| private enum Operation { | ||
| case toggleSign, toggleSignAndAdvance | ||
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| func perform(with value: Int) -> Int { | ||
| switch self { | ||
| case .toggleSign: | ||
| return value * -1 | ||
| case .toggleSignAndAdvance: | ||
| return (value * -1) + 1 | ||
| } | ||
| } | ||
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| var next: Operation { | ||
| switch self { | ||
| case .toggleSign: | ||
| return .toggleSignAndAdvance | ||
| case .toggleSignAndAdvance: | ||
| return .toggleSign | ||
| } | ||
| } | ||
| } | ||
| } |
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