Prototype implementation for template requirements

src/briefcase/commands/base.py

@@ -1,4 +1,5 @@
 import argparse
+import contextlib
 import importlib
 import inspect
 import os
@@ -105,6 +106,7 @@ class BaseCommand(ABC):
     cmd_line = "briefcase {command} {platform} {output_format}"
     supported_host_os = {"Darwin", "Linux", "Windows"}
     supported_host_os_reason = f"This command is not supported on {platform.system()}."
+    requires = []
     GLOBAL_CONFIG_CLASS = GlobalConfig
     APP_CONFIG_CLASS = AppConfig
     allows_passthrough = False
@@ -326,19 +328,33 @@ def binary_path(self, app):
         """
         ...
 
-    def _load_path_index(self, app: BaseConfig):
-        """Load the path index from the index file provided by the app template.
-
-        :param app: The config object for the app
-        :return: The contents of the application path index.
-        """
+    @contextlib.contextmanager
+    def _open_briefcase_toml(self, app: BaseConfig):
         try:
             with (self.bundle_path(app) / "briefcase.toml").open("rb") as f:
-                self._path_index[app] = tomllib.load(f)["paths"]
+                yield f
         except OSError as e:
             raise BriefcaseCommandError(
                 f"Unable to find '{self.bundle_path(app) / 'briefcase.toml'}'"
             ) from e
+
+    def build_requirements(self, app: BaseConfig) -> list[str]:
+        """The list of requirements to build an app from ``briefcase.toml``."""
+        with self._open_briefcase_toml(app) as f:
+            try:
+                return tomllib.load(f)["briefcase"]["requires"]
+            except KeyError:
+                return []
+
+    def _load_path_index(self, app: BaseConfig):
+        """Load the path index from the index file provided by the app template.
+
+        :param app: The config object for the app
+        :return: The contents of the application path index.
+        """
+        with self._open_briefcase_toml(app) as f:
+            self._path_index[app] = tomllib.load(f)["paths"]
+
         return self._path_index[app]
 
     def support_path(self, app: BaseConfig):

src/briefcase/commands/build.py

@@ -1,5 +1,8 @@
 from typing import Optional
 
+from packaging.requirements import Requirement
+from packaging.version import Version
+
 from briefcase.config import BaseConfig
 from briefcase.exceptions import BriefcaseCommandError
 
@@ -21,6 +24,40 @@ def build_app(self, app: BaseConfig, **options):
         """
         # Default implementation; nothing to build.
 
+    def verify_build_requirements(self, app: BaseConfig):
+        """Verify the template satisfies the Command's requirements."""
+        self.logger.info("Verifying template compatability...", prefix=app.app_name)
+        with self.input.wait_bar("Verifying build requirements..."):
+            template_requires = [
+                Requirement(r) for r in self.build_requirements(app=app)
+            ]
+            for req in [Requirement(r) for r in self.requires]:
+                template_reqs = [r for r in template_requires if r.name == req.name]
+                if len(template_reqs) > 1:
+                    raise BriefcaseCommandError(
+                        f"The requirement {req.name!r} can only be listed once in the template."
+                    )
+                if len(template_reqs) < 1:
+                    raise BriefcaseCommandError(
+                        f"The requirement {req.name!r} is not provided by the template."
+                    )
+
+                template_req = template_reqs[0]
+                if not str(template_req.specifier).startswith("=="):
+                    raise BriefcaseCommandError(
+                        f"The requirement {template_req!r} is not exact; for example, 'gradle==8.0.1'"
+                    )
+
+                if not req.specifier.contains(Version(str(template_req.specifier)[2:])):
+                    raise BriefcaseCommandError(
+                        f"""
+The template is not compatible with this version of Briefcase.
+
+Briefcase requires {req!r} from the template.
+The template is providing {template_req!r}.
+"""
+                    )
+
     def _build_app(
         self,
         app: BaseConfig,
@@ -68,6 +105,7 @@ def _build_app(
         else:
             state = None
 
+        self.verify_build_requirements(app)
         self.verify_app_tools(app)
 
         state = self.build_app(app, test_mode=test_mode, **full_options(state, options))

src/briefcase/platforms/android/gradle.py

@@ -62,6 +62,7 @@ def android_log_clean_filter(line):
 class GradleMixin:
     output_format = "gradle"
     platform = "android"
+    requires = ["gradle>=8.0", "android_gradle_plugin>=8.0.1"]
 
     @property
     def packaging_formats(self):