Add comments and be sure to segment action cache by architecture.

.github/workflows/build_apk.yml

@@ -52,10 +52,14 @@ jobs:
         python-version: 3.9
     - uses: actions/cache@v2
       with:
+        # This is where python for android puts its intermediary build
+        # files - we cache this to improve build performance, but be
+        # aggressive in clearing the cache whenever any file changes
+        # in the repository.
         path: ~/.local
-        key: ${{ runner.os }}-local-${{ hashFiles('*') }}
+        key: ${{ runner.os }}-local-${{ github.event.inputs.arch || inputs.arch }}-${{ hashFiles('*') }}
         restore-keys: |
-          ${{ runner.os }}-local-
+          ${{ runner.os }}-local-${{ github.event.inputs.arch || inputs.arch }}-
     - uses: actions/cache@v2
       with:
         path: ~/.cache/pip
@@ -76,6 +80,9 @@ jobs:
     - name: Build the app
       env:
         ARCH: ${{ github.event.inputs.arch || inputs.arch }}
+        # No need to set the ANDROID_HOME environment variable here that is used for
+        # setting up the ANDROID SDK and ANDROID NDK, as the github actions images
+        # have these SDKs and NDKs already installed.
         # https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu2004-Readme.md#environment-variables-3
         ANDROIDSDK: ${{ env.ANDROID_SDK_ROOT }}
         ANDROIDNDK: ${{ env.ANDROID_NDK_ROOT }}

Makefile

@@ -21,6 +21,11 @@ ANDROIDNDKVER := 21.4.7075529
 
 SDK := ${ANDROID_HOME}/android-sdk-$(PLATFORM)
 
+# This checks if an environment variable with a specific name
+# exists. If it doesn't, it prints an error message and exits.
+# For example to check for the presence of the ANDROIDSDK environment
+# variable, you could use:
+# make guard-ANDROIDSDK
 guard-%:
 	@ if [ "${${*}}" = "" ]; then \
 		echo "Environment variable $* not set"; \
@@ -72,6 +77,7 @@ needs-version:
 
 .PHONY: kolibri.apk
 # Build the signed version of the apk
+# For some reason, p4a defauls to adding a final '-' to the filename, so we remove it in the final step.
 kolibri.apk: p4a_android_distro src/kolibri needs-version
 	$(MAKE) guard-P4A_RELEASE_KEYSTORE
 	$(MAKE) guard-P4A_RELEASE_KEYALIAS
@@ -84,6 +90,7 @@ kolibri.apk: p4a_android_distro src/kolibri needs-version
 
 .PHONY: kolibri.apk.unsigned
 # Build the unsigned debug version of the apk
+# For some reason, p4a defauls to adding a final '-' to the filename, so we remove it in the final step.
 kolibri.apk.unsigned: p4a_android_distro src/kolibri needs-version
 	@echo "--- :android: Build APK (unsigned)"
 	p4a apk --arch=$(P4A_ARCH) --version=$(APK_VERSION) --numeric-version=$(BUILD_NUMBER)
@@ -125,6 +132,9 @@ sdk:
 	@echo "Accepting all licenses"
 	yes | $(SDK)/cmdline-tools/bin/sdkmanager --licenses --sdk_root=$(SDK)
 
+# All of these commands are non-destructive, so if the cmdline-tools are already installed, make will skip
+# based on the directory existing.
+# The SDK installations will take a little time, but will not attempt to redownload if already installed.
 setup:
 	$(MAKE) guard-ANDROID_HOME
 	mkdir -p $(SDK)