Improve Android builds. (#9801)

Eagerly generate local.properties, and always update the flutter.sdk
setting in it, in case FLUTTER_ROOT has changed.

Fixes #8365.
Fixes #9716 - at least the specific issue reported. My Android Studio
still complains about Gradle versions - it ships with v3.2, but requires
v3.3...

Add a 'generate dependencies' task to the Gradle build, which checks if
the snapshot dependencies file exists, and runs an extra build before
the actual FlutterTask if it doesn't. This makes the first build slower,
but sub-sequent builds (without source changes) much faster.

Fixes #9717.

packages/flutter_tools/gradle/flutter.gradle

@@ -194,7 +194,20 @@ class FlutterPlugin implements Plugin<Project> {
                 throw new GradleException("Build variant must be one of \"debug\", \"profile\", or \"release\" but was \"${variant.name}\"")
             }
 
+            GenerateDependencies dependenciesTask = project.tasks.create("flutterDependencies${variant.name.capitalize()}", GenerateDependencies) {
+                flutterRoot this.flutterRoot
+                flutterExecutable this.flutterExecutable
+                buildMode variant.name
+                localEngine this.localEngine
+                localEngineSrcPath this.localEngineSrcPath
+                targetPath target
+                kernelFile kernel
+                sourceDir project.file(project.flutter.source)
+                intermediateDir project.file("${project.buildDir}/${AndroidProject.FD_INTERMEDIATES}/flutter/${variant.name}")
+            }
+
             FlutterTask flutterTask = project.tasks.create("flutterBuild${variant.name.capitalize()}", FlutterTask) {
+                dependsOn dependenciesTask
                 flutterRoot this.flutterRoot
                 flutterExecutable this.flutterExecutable
                 buildMode variant.name
@@ -222,7 +235,7 @@ class FlutterExtension {
     String target
 }
 
-class FlutterTask extends DefaultTask {
+abstract class BaseFlutterTask extends DefaultTask {
     File flutterRoot
     File flutterExecutable
     String buildMode
@@ -232,11 +245,80 @@ class FlutterTask extends DefaultTask {
     String targetPath
     @Optional @InputFile
     File kernelFile
-
     File sourceDir
+    File intermediateDir
+
+    @OutputFile
+    File getDependenciesFile() {
+        if (buildMode != 'debug') {
+            return project.file("${intermediateDir}/snapshot.d")
+        }
+        return  project.file("${intermediateDir}/snapshot_blob.bin.d")
+    }
+
+    void buildFlx() {
+        if (!sourceDir.isDirectory()) {
+            throw new GradleException("Invalid Flutter source directory: ${sourceDir}")
+        }
+
+        intermediateDir.mkdirs()
+
+        if (buildMode != "debug") {
+            project.exec {
+                executable flutterExecutable.absolutePath
+                workingDir sourceDir
+                if (localEngine != null) {
+                    args "--local-engine", localEngine
+                    args "--local-engine-src-path", localEngineSrcPath
+                }
+                args "build", "aot"
+                args "--quiet"
+                args "--target", targetPath
+                args "--target-platform", "android-arm"
+                args "--output-dir", "${intermediateDir}"
+                args "--${buildMode}"
+            }
+        }
 
+        project.exec {
+            executable flutterExecutable.absolutePath
+            workingDir sourceDir
+            if (localEngine != null) {
+                args "--local-engine", localEngine
+                args "--local-engine-src-path", localEngineSrcPath
+            }
+            args "build", "flx"
+            args "--target", targetPath
+            if (kernelFile != null) {
+                args "--kernel", kernelFile.absolutePath
+            }
+            args "--output-file", "${intermediateDir}/app.flx"
+            if (buildMode != "debug") {
+                args "--precompiled"
+            } else {
+                args "--snapshot", "${intermediateDir}/snapshot_blob.bin"
+                args "--depfile", "${intermediateDir}/snapshot_blob.bin.d"
+            }
+            args "--working-dir", "${intermediateDir}/flx"
+        }
+    }
+}
+
+class GenerateDependencies extends BaseFlutterTask {
+    @TaskAction
+    void build() {
+        File dependenciesFile = getDependenciesFile();
+        if (!dependenciesFile.exists()) {
+            buildFlx()
+        }
+    }
+}
+
+class FlutterTask extends BaseFlutterTask {
     @OutputDirectory
-    File intermediateDir
+    File getOutputDirectory() {
+        return intermediateDir
+    }
 
     CopySpec getAssets() {
         return project.copySpec {
@@ -266,12 +348,7 @@ class FlutterTask extends DefaultTask {
 
     @InputFiles
     FileCollection getSourceFiles() {
-        File dependenciesFile
-        if (buildMode != 'debug') {
-            dependenciesFile = project.file("${intermediateDir}/snapshot.d")
-        } else {
-            dependenciesFile = project.file("${intermediateDir}/snapshot_blob.bin.d")
-        }
+        File dependenciesFile = getDependenciesFile()
         FileCollection sources = readDependencies(dependenciesFile)
         if (sources != null) {
             // We have a dependencies file. Add a dependency on gen_snapshot as well, since the
@@ -293,49 +370,6 @@ class FlutterTask extends DefaultTask {
 
     @TaskAction
     void build() {
-        if (!sourceDir.isDirectory()) {
-            throw new GradleException("Invalid Flutter source directory: ${sourceDir}")
-        }
-
-        intermediateDir.mkdirs()
-
-        if (buildMode != "debug") {
-            project.exec {
-                executable flutterExecutable.absolutePath
-                workingDir sourceDir
-                if (localEngine != null) {
-                    args "--local-engine", localEngine
-                    args "--local-engine-src-path", localEngineSrcPath
-                }
-                args "build", "aot"
-                args "--quiet"
-                args "--target", targetPath
-                args "--target-platform", "android-arm"
-                args "--output-dir", "${intermediateDir}"
-                args "--${buildMode}"
-            }
-        }
-
-        project.exec {
-            executable flutterExecutable.absolutePath
-            workingDir sourceDir
-            if (localEngine != null) {
-                args "--local-engine", localEngine
-                args "--local-engine-src-path", localEngineSrcPath
-            }
-            args "build", "flx"
-            args "--target", targetPath
-            if (kernelFile != null) {
-                args "--kernel", kernelFile.absolutePath
-            }
-            args "--output-file", "${intermediateDir}/app.flx"
-            if (buildMode != "debug") {
-                args "--precompiled"
-            } else {
-                args "--snapshot", "${intermediateDir}/snapshot_blob.bin"
-                args "--depfile", "${intermediateDir}/snapshot_blob.bin.d"
-            }
-            args "--working-dir", "${intermediateDir}/flx"
-        }
+        buildFlx()
     }
 }

packages/flutter_tools/lib/src/android/gradle.dart

@@ -78,7 +78,7 @@ String getGradleAppOutDirV2() {
 // Note: this call takes about a second to complete.
 String _calculateGradleAppOutDirV2() {
   final String gradle = ensureGradle();
-  ensureLocalProperties();
+  updateLocalProperties();
   try {
     final String properties = runCheckedSync(
       <String>[gradle, 'app:properties'],
@@ -141,28 +141,42 @@ String ensureGradle() {
   return gradle;
 }
 
-/// Create android/local.properties if needed.
-File ensureLocalProperties() {
-  final File localProperties = fs.file('android/local.properties');
-  if (!localProperties.existsSync()) {
-    localProperties.writeAsStringSync(
-        'sdk.dir=${escapePath(androidSdk.directory)}\n'
-        'flutter.sdk=${escapePath(Cache.flutterRoot)}\n'
-    );
+/// Create android/local.properties if needed, and update Flutter settings.
+void updateLocalProperties({String projectPath, String buildMode}) {
+  final File localProperties = (projectPath == null)
+      ? fs.file(fs.path.join('android', 'local.properties'))
+      : fs.file(fs.path.join(projectPath, 'android', 'local.properties'));
+  bool changed = false;
+
+  SettingsFile settings;
+  if (localProperties.existsSync()) {
+    settings = new SettingsFile.parseFromFile(localProperties);
+  } else {
+    settings = new SettingsFile();
+    settings.values['sdk.dir'] = escapePath(androidSdk.directory);
+    changed = true;
+  }
+  final String escapedRoot = escapePath(Cache.flutterRoot);
+  if (changed || settings.values['flutter.sdk'] != escapedRoot) {
+    settings.values['flutter.sdk'] = escapedRoot;
+    changed = true;
   }
-  return localProperties;
+  if (buildMode != null && settings.values['flutter.buildMode'] != buildMode) {
+    settings.values['flutter.buildMode']  = buildMode;
+    changed = true;
+  }
+
+  if (changed)
+    settings.writeContents(localProperties);
 }
 
 Future<Null> buildGradleProject(BuildMode buildMode, String target, String kernelPath) async {
-  final File localProperties = ensureLocalProperties();
   // Update the local.properties file with the build mode.
   // FlutterPlugin v1 reads local.properties to determine build mode. Plugin v2
   // uses the standard Android way to determine what to build, but we still
   // update local.properties, in case we want to use it in the future.
   final String buildModeName = getModeName(buildMode);
-  final SettingsFile settings = new SettingsFile.parseFromFile(localProperties);
-  settings.values['flutter.buildMode'] = buildModeName;
-  settings.writeContents(localProperties);
+  updateLocalProperties(buildMode: buildModeName);
 
   injectPlugins();
 

packages/flutter_tools/lib/src/base/utils.dart

@@ -151,6 +151,8 @@ class ItemListNotifier<T> {
 }
 
 class SettingsFile {
+  SettingsFile();
+
   SettingsFile.parse(String contents) {
     for (String line in contents.split('\n')) {
       line = line.trim();

packages/flutter_tools/lib/src/commands/create.dart

@@ -6,6 +6,7 @@ import 'dart:async';
 
 import '../android/android.dart' as android;
 import '../android/android_sdk.dart' as android_sdk;
+import '../android/gradle.dart' as gradle;
 import '../base/common.dart';
 import '../base/file_system.dart';
 import '../base/utils.dart';
@@ -121,6 +122,9 @@ class CreateCommand extends FlutterCommand {
       if (argResults['pub'])
         await pubGet(directory: dirPath);
 
+      if (android_sdk.androidSdk != null)
+        gradle.updateLocalProperties(projectPath: dirPath);
+
       appPath = fs.path.join(dirPath, 'example');
       final String androidPluginIdentifier = templateContext['androidIdentifier'];
       final String exampleProjectName = projectName + '_example';
@@ -153,6 +157,9 @@ class CreateCommand extends FlutterCommand {
       injectPlugins(directory: appPath);
     }
 
+    if (android_sdk.androidSdk != null)
+      gradle.updateLocalProperties(projectPath: appPath);
+
     printStatus('');
 
     // Run doctor; tell the user the next steps.