Avoid Custom button subscription in case HMI incompatibility (smartdevicelink#613)

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Author: GetmanetsIrina

proposals/NNNN-Avoid_custom_button_subscription_when_HMI_does_not_support.md

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+# Avoid Custom button subscription in case HMI incompatibility
+
+* Proposal: [SDL-NNNN](NNNN-Avoid_custom_button_subscription_when_HMI_does_not_support.md)
+* Author: [Getmanets Irina](https://github.com/GetmanetsIrina)
+* Status: **Awaiting review**
+* Impacted Platforms: [Core]
+
+## Introduction
+
+In case HMI does not support `CUSTOM_BUTTON`, SDL should not try to subscribe an application to it.
+
+## Motivation
+
+SDL does not check hmi_capabilities before subscription to `CUSTOM_BUTTON`, so it tries to subscribe each registered application to `CUSTOM_BUTTON` even if it is unsupported by HMI.
+
+SDL should check hmi_capabilities and subscribe application to `CUSTOM_BUTTON` if it is **supported by HMI only**.
+
+## Proposed solution
+
+SDL should check hmi_capabilities for `CUSTOM_BUTTON` by the start and save support state for `CUSTOM_BUTTON` internally.
+On the app registration, SDL should check the saved support state for `CUSTOM_BUTTON`.
+
+In case `CUSTOM_BUTTON` is supported by hmi_capabilities:
+SDL should send `Buttons.SubscribeButtons(CUSTOM_BUTTON)` to HMI and wait for a response.
+
+
+In case `CUSTOM_BUTTON` is not supported by hmi_capabilities:
+SDL should **not** send `Buttons.SubscribeButtons(CUSTOM_BUTTON)` to HMI.
+
+If HMI supports `CUSTOM_BUTTON`, it should be in the `Buttons[capabilities]` section of `hmi_capabilities.json`:
+
+```
+"Buttons": {
+       "capabilities": [
+        ...,
+        {
+               "name": "CUSTOM_BUTTON",
+               "shortPressAvailable": true,
+               "longPressAvailable": true,
+               "upDownAvailable": true
+           }
+        ]
+}
+```
+
+
+If HMI supports `CUSTOM_BUTTON`, response to `Buttons.GetCapabilities` should contain following structure:
+
+```
+"capabilities": [
+        ...,
+        {
+               "name": "CUSTOM_BUTTON",
+               "shortPressAvailable": true,
+               "longPressAvailable": true,
+               "upDownAvailable": true
+           }
+        ]
+```
+
+
+`Buttons.GetCapabilities` request has higher priority than `hmi_capabilities.json`.
+
+## Potential downsides
+
+N/A
+
+## Impact on existing code
+
+SDL core will need to be updated.
+
+## Alternatives considered
+
+N/A