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Phones with dual-role power capabilities like the Google Pixel 2 XL comes in handy for charging other compatible devices when there's just no access to AC power as well as being able to be charged itself. Since the Apple 10.5-inch iPad Pro supports USB Power Delivery based charging similar to the Google Pixel 2 XL, we'll see here how they work when connected over a Apple USB-C to Lightning Cable. 

Note that Apple iPhone and iPad Pros that support USB Power Delivery can only consume and not supply power. Thus, the Google Pixel 2 XL  will be acting as a power bank to the Apple 10.5-inch iPad Pro, thus consuming the Google Pixel 2 XL's own battery charge to support charging the iPad. For this reason, make sure there's sufficient battery charge left on the Google Pixel 2 XL, which we use here while 86% charged to supply power to a fully drained battery on the Apple 10.5-inch iPad Pro.

After connecting both the phone and iPad, under the phone's USB charging options, we select 'Charge connected device'. The Apple 10.5-inch iPad Pro when connected automatically just supports charging in.

As observed through the Granite River Labs GRL-USB-PD-A1 power analyzer, the Google Pixel 2 XL initiates USB Power Delivery negotiation over repeated display of 2.5 watts (5 volts @ .5 amps) and 4.5 watts (5 volts @ .9 amps) power capabilities.

After a long time, the Apple 10.5-inch iPad Pro only starts to request an initial 2.5 watts (5 volts @ .5 amps) which the Google Pixel 2 XL then accepts.

The Google Pixel 2 XL continues on to initiate multiple Vendor Defined Messages which include Discover Identity, SVID's and Discover Modes with the Apple 10.5-inch iPad Pro as seen below. 

At initial plugin from 0% battery, the Apple 10.5-inch iPad Pro during power off starts pulling current with frequent downward bands of around 0.5 amps at 4.9 volts over repeated power display by the Google phone. Halfway through this constant power declaration and over subsequent power request and Vendor Defined Messages, the current stabilizes to level out charging at only about 2.4 watts (4.9 volts @ 0.5 amps).

At not more than 3 watts, charging can only happen very slowly between the Apple 10.5-inch iPad Pro and the Google Pixel 2 XL. While the iPad can charge much faster using its own 12W based charger, you can still use certain Android phones like the Google Pixel 2 XL to charge up the iPad when there's no AC power source available.