Implementation:Webdriverio Webdriverio MSPO Service

Knowledge Sources	Webdriverio_Webdriverio
Domains	Mobile_Testing, Performance_Optimization
Last Updated	2026-02-12 00:00 GMT

Overview

SelectorPerformanceService intercepts mobile selector commands during test execution, measures their performance, and stores timing metrics for later aggregation.

Description

The SelectorPerformanceService class implements Services.ServiceInstance and hooks into the WebdriverIO command lifecycle via beforeCommand and afterCommand. It tracks XPath-based element-finding commands ($, $$, custom$, custom$$) and their underlying findElement/findElements protocol calls. The service records timing data including original XPath duration and test context (file, suite, test name, line number). It automatically registers the MobileSelectorPerformanceReporter and applies command overwrites for optimization. On session end, it writes per-worker JSON data files for later aggregation.

Usage

This service is enabled by configuring the trackSelectorPerformance option on the Appium service. It runs automatically within each worker process during test execution.

Code Reference

Source Location

Repository: Webdriverio_Webdriverio
File: packages/wdio-appium-service/src/mobileSelectorPerformanceOptimizer/mspo-service.ts
Lines: 28-302

Signature

export default class SelectorPerformanceService implements Services.ServiceInstance {
    constructor(
        private _options: AppiumServiceConfig,
        private _config?: Options.Testrunner
    )

    async beforeSession(config: Options.Testrunner, _capabilities: never, _specs: never): Promise<void>
    async before(_capabilities: never, _specs: never, browser: WebdriverIO.Browser | WebdriverIO.MultiRemoteBrowser): Promise<void>
    async beforeCommand(commandName: string, args: unknown[]): Promise<void>
    async afterCommand(commandName: string, args: unknown[], _result: unknown, _error?: Error): Promise<void>
    async afterSession(): Promise<void>
}

Import

import SelectorPerformanceService from './mobileSelectorPerformanceOptimizer/mspo-service.js'

I/O Contract

Inputs

Name	Type	Required	Description
_options	`AppiumServiceConfig`	Yes	Service config containing trackSelectorPerformance with pageObjectPaths, enableCliReport, enableMarkdownReport, reportPath, and maxLineLength
_config	`Options.Testrunner`	No	WebdriverIO test runner config used for outputDir fallback and reporter registration

Outputs

Name	Type	Description
Worker JSON file	`JSON file`	Per-worker performance data written to `{reportDirectory}/selector-performance-worker-data/worker-data-{pid}.json`

Usage Examples

Enabling the Service via Configuration

// wdio.conf.ts
export const config: WebdriverIO.Config = {
    services: [
        ['appium', {
            trackSelectorPerformance: {
                pageObjectPaths: ['./tests/pageobjects'],
                enableCliReport: true,
                enableMarkdownReport: false,
                reportPath: './reports',
                maxLineLength: 120
            }
        }]
    ]
}

How Command Tracking Works Internally

// beforeCommand tracks the start time when a user command like $ is called
// with an XPath selector:
//   $('//XCUIElementTypeButton[@name="Login"]')
//
// afterCommand captures the end time when the underlying findElement completes
// and stores the performance data with test context

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Principle

Implementation

Heuristic

Environment