Teach flutter to build app.a (#4141)

On iOS, we use Xcode to build a static library that contains the precompiled
code. This code is currently unused, but it will be used by the new Xcode
harness to factor out as much complexity as possible into the flutter tool.

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

@@ -38,6 +38,7 @@ class BuildAotCommand extends FlutterCommand {
   Future<int> runInProject() async {
     String outputPath = buildAotSnapshot(
       findMainDartFile(argResults['target']),
+      TargetPlatform.android_arm,
       getBuildMode(),
       outputPath: argResults['output-dir']
     );
@@ -56,6 +57,7 @@ String _getSdkExtensionPath(String packagesPath, String package) {
 
 String buildAotSnapshot(
   String mainPath,
+  TargetPlatform platform,
   BuildMode buildMode, {
   String outputPath: _kDefaultAotOutputDir
 }) {
@@ -69,14 +71,15 @@ String buildAotSnapshot(
   String engineSrc = tools.engineSrcPath;
   if (engineSrc != null) {
     entryPointsDir  = path.join(engineSrc, 'sky', 'engine', 'bindings');
-    String engineOut = tools.getEngineArtifactsDirectory(
-        TargetPlatform.android_arm, buildMode).path;
-
+    String engineOut = tools.getEngineArtifactsDirectory(platform, buildMode).path;
+    if (platform == TargetPlatform.ios) {
+      genSnapshot = path.join(engineOut, 'clang_x64', 'gen_snapshot');
+    } else {
       String host32BitToolchain = getCurrentHostPlatform() == HostPlatform.darwin_x64 ? 'clang_i386' : 'clang_x86';
       genSnapshot = path.join(engineOut, host32BitToolchain, 'gen_snapshot');
+    }
   } else {
-    String artifactsDir = tools.getEngineArtifactsDirectory(
-        TargetPlatform.android_arm, buildMode).path;
+    String artifactsDir = tools.getEngineArtifactsDirectory(platform, buildMode).path;
     entryPointsDir = artifactsDir;
     String hostToolsDir = path.join(artifactsDir, getNameForHostPlatform(getCurrentHostPlatform()));
     genSnapshot = path.join(hostToolsDir, 'gen_snapshot');
@@ -90,7 +93,6 @@ String buildAotSnapshot(
   String rodataBlob = path.join(outputDir.path, 'snapshot_aot_rodata');
 
   String vmEntryPoints = path.join(entryPointsDir, 'dart_vm_entry_points.txt');
-  String vmEntryPointsAndroid = path.join(entryPointsDir, 'dart_vm_entry_points_android.txt');
 
   String packagesPath = path.absolute(Directory.current.path, 'packages');
   if (!FileSystemEntity.isDirectorySync(packagesPath)) {
@@ -107,11 +109,46 @@ String buildAotSnapshot(
   String skyEngineSdkExt = _getSdkExtensionPath(packagesPath, 'sky_engine');
   String uiPath = path.join(skyEngineSdkExt, 'dart_ui.dart');
   String vmServicePath = path.join(skyEngineSdkExt, 'dart', 'runtime', 'bin', 'vmservice', 'vmservice_io.dart');
-  String jniPath = path.join(skyEngineSdkExt, 'dart_jni', 'jni.dart');
 
   List<String> filePaths = <String>[
-    genSnapshot, vmEntryPoints, vmEntryPointsAndroid, mojoInternalPath, uiPath, vmServicePath, jniPath
+    genSnapshot,
+    vmEntryPoints,
+    mojoInternalPath,
+    uiPath,
+    vmServicePath,
   ];
+
+  // These paths are used only on Android.
+  String vmEntryPointsAndroid;
+  String jniPathAndroid;
+
+  // These paths are used only on iOS.
+  String snapshotDartIOS;
+  String assembly;
+
+  switch (platform) {
+    case TargetPlatform.android_arm:
+    case TargetPlatform.android_x64:
+    case TargetPlatform.android_x86:
+      vmEntryPointsAndroid = path.join(entryPointsDir, 'dart_vm_entry_points_android.txt');
+      jniPathAndroid = path.join(skyEngineSdkExt, 'dart_jni', 'jni.dart');
+      filePaths.addAll(<String>[
+        vmEntryPointsAndroid,
+        jniPathAndroid,
+      ]);
+      break;
+    case TargetPlatform.ios:
+      snapshotDartIOS = path.join(entryPointsDir, 'snapshot.dart');
+      assembly = path.join(outputDir.path, 'snapshot_assembly.S');
+      filePaths.addAll(<String>[
+        snapshotDartIOS,
+      ]);
+      break;
+    case TargetPlatform.darwin_x64:
+    case TargetPlatform.linux_x64:
+      assert(false);
+  }
+
   List<String> missingFiles = filePaths.where((String p) => !FileSystemEntity.isFileSync(p)).toList();
   if (missingFiles.isNotEmpty) {
     printError('Missing files: $missingFiles');
@@ -122,18 +159,35 @@ String buildAotSnapshot(
     genSnapshot,
     '--vm_isolate_snapshot=$vmIsolateSnapshot',
     '--isolate_snapshot=$isolateSnapshot',
-    '--instructions_blob=$instructionsBlob',
-    '--rodata_blob=$rodataBlob',
     '--embedder_entry_points_manifest=$vmEntryPoints',
-    '--embedder_entry_points_manifest=$vmEntryPointsAndroid',
     '--package_root=$packagesPath',
     '--url_mapping=dart:mojo.internal,$mojoInternalPath',
     '--url_mapping=dart:ui,$uiPath',
     '--url_mapping=dart:vmservice_sky,$vmServicePath',
-    '--url_mapping=dart:jni,$jniPath',
-    '--no-sim-use-hardfp',
   ];
 
+  switch (platform) {
+    case TargetPlatform.android_arm:
+    case TargetPlatform.android_x64:
+    case TargetPlatform.android_x86:
+      genSnapshotCmd.addAll(<String>[
+        '--rodata_blob=$rodataBlob',
+        '--instructions_blob=$instructionsBlob',
+        '--embedder_entry_points_manifest=$vmEntryPointsAndroid',
+        '--url_mapping=dart:jni,$jniPathAndroid',
+        '--no-sim-use-hardfp',
+      ]);
+      break;
+    case TargetPlatform.ios:
+      genSnapshotCmd.addAll(<String>[
+        '--assembly=$assembly'
+      ]);
+      break;
+    case TargetPlatform.darwin_x64:
+    case TargetPlatform.linux_x64:
+      assert(false);
+  }
+
   if (buildMode != BuildMode.release) {
     genSnapshotCmd.addAll(<String>[
       '--no-checked',
@@ -146,5 +200,44 @@ String buildAotSnapshot(
   printStatus('Building snapshot...');
   runCheckedSync(genSnapshotCmd);
 
+  // On iOS, we use Xcode to compile the snapshot into a static library that the
+  // end-developer can link into their app.
+  if (platform == TargetPlatform.ios) {
+    printStatus('Building app.a...');
+
+    // These names are known to from the engine.
+    const String kDartVmIsolateSnapshotBuffer = 'kDartVmIsolateSnapshotBuffer';
+    const String kDartIsolateSnapshotBuffer = 'kDartIsolateSnapshotBuffer';
+
+    runCheckedSync(<String>['mv', vmIsolateSnapshot, path.join(outputDir.path, kDartVmIsolateSnapshotBuffer)]);
+    runCheckedSync(<String>['mv', isolateSnapshot, path.join(outputDir.path, kDartIsolateSnapshotBuffer)]);
+
+    String kDartVmIsolateSnapshotBufferC = path.join(outputDir.path, '$kDartVmIsolateSnapshotBuffer.c');
+    String kDartIsolateSnapshotBufferC = path.join(outputDir.path, '$kDartIsolateSnapshotBuffer.c');
+
+    runCheckedSync(<String>[
+      'xxd', '--include', kDartVmIsolateSnapshotBuffer, path.basename(kDartVmIsolateSnapshotBufferC)
+    ], workingDirectory: outputDir.path);
+    runCheckedSync(<String>[
+      'xxd', '--include', kDartIsolateSnapshotBuffer, path.basename(kDartIsolateSnapshotBufferC)
+    ], workingDirectory: outputDir.path);
+
+    String assemblyO = path.join(outputDir.path, 'snapshot_assembly.o');
+    String kDartVmIsolateSnapshotBufferO = path.join(outputDir.path, '$kDartVmIsolateSnapshotBuffer.o');
+    String kDartIsolateSnapshotBufferO = path.join(outputDir.path, '$kDartIsolateSnapshotBuffer.o');
+
+    runCheckedSync(<String>['xcrun', 'cc', '-c', assembly, '-o', assemblyO]);
+    runCheckedSync(<String>['xcrun', 'cc', '-c', kDartVmIsolateSnapshotBufferC, '-o', kDartVmIsolateSnapshotBufferO]);
+    runCheckedSync(<String>['xcrun', 'cc', '-c', kDartIsolateSnapshotBufferC, '-o', kDartIsolateSnapshotBufferO]);
+
+    String appLib = path.join(outputDir.path, 'app.a');
+
+    runCheckedSync(<String>['rm', '-f', appLib]);
+    runCheckedSync(<String>[
+      'xcrun', 'ar', 'rcs', appLib,
+      assemblyO, kDartVmIsolateSnapshotBufferO, kDartIsolateSnapshotBufferO,
+    ]);
+  }
+
   return outputPath;
 }

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

@@ -513,7 +513,7 @@ Future<int> buildAndroid(
 
   // Build an AOT snapshot if needed.
   if (isAotBuildMode(buildMode) && aotPath == null) {
-    aotPath = buildAotSnapshot(findMainDartFile(target), buildMode);
+    aotPath = buildAotSnapshot(findMainDartFile(target), platform, buildMode);
     if (aotPath == null) {
       printError('Failed to build AOT snapshot');
       return 1;