Add 'maestro.platform' for javascript to determine platform

[Fishbowler]

Proposed Changes

Add a platform property to the maestro object in JavaScript, so as to determine iOS or Android at runtime.

output.welcomeMessage = "Your app is running on " + maestro.platform

Why though?

Previously, an app that requested a few permissions would have lots of this:

- tapOn: "Enable Location"
- runFlow:
    when:
      platform: 'Android'
    commands:
      - assertVisible: "Allow access to this device's location?"
      - tapOn: "While using the app"
- runFlow:
    when:
        platform: 'iOS'
    commands:
        - assertVisible: "This app uses your location to show you information about your local environment."
        - tapOn: "Allow While Using App"
- assertVisible:
    id: "location_permission_true"

I wanted to set the strings in JavaScript, but this would require prepending the flow with something like:

- runFlow:
    when:
      platform: 'Android'
    commands:
      - runScript: 
          file: setPermissionsVars.js
          env:
            MOBILE_OS: 'Android'
- runFlow:
    when:
      platform: 'iOS'
    commands:
      - runScript: 
          file: setPermissionsVars.js
          env:
            MOBILE_OS: 'iOS'

After this change:

- runScript: setPermissionsVars.js
- tapOn: "Enable Location"
- assertVisible: ${output.locationPermissionAlert}
- tapOn: ${output.locationPermissionButton}
- assertVisible:
    id: "location_permission_true"

Testing

I've tested this with a more trivial example:

flow.yml:

appId: org.wikimedia.wikipedia #iOS
#appId: org.wikipedia #Android
jsEngine: graaljs
---
- launchApp
- runScript: 'setSearchTerm.js'
- tapOn: "Search Wikipedia"
- inputText: ${output.searchTerm}

- runFlow:
    when:
      platform: 'Android'
    commands:
      - assertVisible: Robot
- runFlow:
    when:
      platform: 'iOS'
    commands:
      - assertVisible: Fruit.*

setSearchTerm.js:

output.searchTerm = 'blackberries'
if (maestro.platform == 'android'){
    output.searchTerm = 'robots'
}
if (maestro.platform == 'ios'){
    output.searchTerm = 'apples'
}

Tested on iOS and Android, in Rhino & Graal.

[axelniklasson]

Thanks for raising this @Fishbowler, I'll review it next week. Please do also raise a PR to https://github.com/mobile-dev-inc/maestro-docs updating this page with information about the platform object and a code example. Thanks!

[Fishbowler]

Done. Potentially a bit of overkill on the docs, but we can deal with that in the review of that PR :D

[axelniklasson]

Thanks for this @Fishbowler, we really appreciate the contribution 🎉