不论是创建完全的 Flutter项目，还是 Native以 Moudle的方式集成 Flutter，亦或是 Native以 aar方式集成 Flutter，最终 Flutter在 Andorid端的 App 都是以 Native项目+ Flutter 的UI产物存在的。所以在这里拆开一个 Flutter在 release模式下编译后生成 aar包来做分析：
我们关注重点在 assets，jni，libs 这 3 个目录中，其他的文件都是 Nactive层壳工程的产物。
jni ：该目录下存在文件 libflutter.so，该文件为 Flutter Engine (引擎) 层的 C++实现，提供skia(绘制引擎)，Dart，Text(纹理绘制)等支持。
libs：该目录下存在文件为 flutter.jar，该文件为 Flutter embedding (嵌入) 层的 Java实现，该层提供给 Flutter 许多Native层平台系统功能的支持，比如创建线程。
assets:该目录下分为两部分：
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flutter_assets 目录：该目录下存放Flutter 我们应用层的资源，包括images,font等
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isolate_snapshot_data，isolate_snapshot_instr，vm_snapshot_data，vm_snapshot_instr 文件：这 4 个文件分别对应 isolate、VM 的数据段和指令段文件，这就是我们自己的 Flutter 代码的产物了。
Flutter 代码的热更新
代码探究
在我们的 Native 项目中，会在 FlutterMainActivity 中，通过调用 Flutter 这个类来创建 View：
flutterView = Flutter.createView(this, getLifecycle(), route);layoutParams = new FrameLayout.LayoutParams(FrameLayout.LayoutParams.MATCH_PARENT,FrameLayout.LayoutParams.MATCH_PARENT);addContentView(flutterView, layoutParams);
查看 Flutter 类代码，发现 Flutter 类主要做了几件事：
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使用 FlutterNative 加载 View，设置路由，使用 lifecycle 绑定生命周期
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使用 FlutterMain 初始化，重点关注这里。
public static FlutterView createView(@NonNull final Activity activity, @NonNull Lifecycle lifecycle, String initialRoute) {FlutterMain.startInitialization(activity.getApplicationContext());FlutterMain.ensureInitializationComplete(activity.getApplicationContext(), (String[])null);FlutterNativeView nativeView = new FlutterNativeView(activity);
所以，真正初始化的相关代码是在 FlutterMian 中：
public static void startInitialization(Context applicationContext, FlutterMain.Settings settings) { if (Looper.myLooper() != Looper.getMainLooper()) { throw new IllegalStateException("startInitialization must be called on the main thread"); } else if (sSettings == null) { sSettings = settings; long initStartTimestampMillis = SystemClock.uptimeMillis(); initConfig(applicationContext); initAot(applicationContext); initResources(applicationContext); System.loadLibrary("flutter"); long initTimeMillis = SystemClock.uptimeMillis() - initStartTimestampMillis; nativeRecordStartTimestamp(initTimeMillis); }}
在 startInitialization 中，主要执行了三个初始化方法 initConfig(applicationContext)，initAot(applicationContext)，initResources(applicationContext)，最后记录了执行时间。
在 initConfig 中：
private static void initConfig(Context applicationContext) { try { Bundle metadata = applicationContext.getPackageManager().getApplicationInfo(applicationContext.getPackageName(), 128).metaData; if (metadata != null) { sAotSharedLibraryPath = metadata.getString(PUBLIC_AOT_AOT_SHARED_LIBRARY_PATH, "app.so"); sAotVmSnapshotData = metadata.getString(PUBLIC_AOT_VM_SNAPSHOT_DATA_KEY, "vm_snapshot_data"); sAotVmSnapshotInstr = metadata.getString(PUBLIC_AOT_VM_SNAPSHOT_INSTR_KEY, "vm_snapshot_instr"); sAotIsolateSnapshotData = metadata.getString(PUBLIC_AOT_ISOLATE_SNAPSHOT_DATA_KEY, "isolate_snapshot_data"); sAotIsolateSnapshotInstr = metadata.getString(PUBLIC_AOT_ISOLATE_SNAPSHOT_INSTR_KEY, "isolate_snapshot_instr"); sFlx = metadata.getString(PUBLIC_FLX_KEY, "app.flx"); sFlutterAssetsDir = metadata.getString(PUBLIC_FLUTTER_ASSETS_DIR_KEY, "flutter_assets"); } } catch (NameNotFoundException var2) { throw new RuntimeException(var2); }}
在 initResources 中：
sResourceExtractor = new ResourceExtractor(applicationContext);sResourceExtractor.addResource(fromFlutterAssets(sFlx)).addResource(fromFlutterAssets(sAotVmSnapshotData)).addResource(fromFlutterAssets(sAotVmSnapshotInstr)).addResource(fromFlutterAssets(sAotIsolateSnapshotData)).addResource(fromFlutterAssets(sAotIsolateSnapshotInstr)).addResource(fromFlutterAssets("kernel_blob.bin"));if (sIsPrecompiledAsSharedLibrary) { sResourceExtractor.addResource(sAotSharedLibraryPath);} else { sResourceExtractor.addResource(sAotVmSnapshotData).addResource(sAotVmSnapshotInstr).addResource(sAotIsolateSnapshotData).addResource(sAotIsolateSnapshotInstr);} sResourceExtractor.start();
在 ResourceExtractor 类中，通过名字就能知道这个类是做资源提取的。把 add 的 Flutter 相关文件从 assets 目录中取出来，该类中 ExtractTask 的 doInBackground 方法中：
File dataDir = new File(PathUtils.getDataDirectory(ResourceExtractor.this.mContext))
这句话指定了资源提取的目的地，即 data/data/包名/app_flutter，如下：
如图，可以看到该目录是的访问权限是可读可写，所以理论上，我们只要把自己的 Flutter 产物下载后，从内存 copy 到这里，便能够实现代码的动态更新。
代码实现
public class FlutterUtils { private static String TAG = "FlutterUtils.class"; private static String flutterZipName = "flutter-code.zip"; private static String fileSuffix = ".zip"; private static String zipPath = Environment.getExternalStorageDirectory().getPath() + "/k12/" + flutterZipName; private static String targetDirPath = zipPath.replace(fileSuffix, ""); private static String targetDirDataPath = zipPath.replace(fileSuffix, "/data"); /** * Flutter 代码热更新第一步： 解压 Flutter 的压缩文件 / public static void unZipFlutterFile() { Log.i(TAG, "unZipFile: Start"); try { unZipFile(zipPath, targetDirPath); Log.i(TAG, "unZipFile: Finish"); } catch (Exception e) { e.printStackTrace(); } } /* * Flutter 代码热更新第二步： 将 Flutter 的相关文件移动到 AppData 的相关目录,APP启动时调用 * * @param mContext 获取 AppData 目录需要 / public static void copyDataToFlutterAssets(Context mContext) { String appDataDirPath = PathUtils.getDataDirectory(mContext.getApplicationContext()) + File.separator; Log.d(TAG, "copyDataToFlutterAssets-filesDirPath:" + targetDirDataPath); Log.d(TAG, "copyDataToFlutterAssets-appDataDirPath:" + appDataDirPath); File appDataDirFile = new File(appDataDirPath); File filesDirFile = new File(targetDirDataPath); File[] files = filesDirFile.listFiles(); for (File srcFile : files) { if (srcFile.getPath().contains("isolate_snapshot_data") || srcFile.getPath().contains("isolate_snapshot_instr") || srcFile.getPath().contains("vm_snapshot_data") || srcFile.getPath().contains("vm_snapshot_instr")) { File targetFile = new File(appDataDirFile + "/" + srcFile.getName()); FileUtil.copyFileByFileChannels(srcFile, targetFile); Log.i(TAG, "copyDataToFlutterAssets-copyFile:" + srcFile.getPath()); } } Log.i(TAG, "copyDataToFlutterAssets: Finish"); } /* * 解压缩文件到指定目录 * * @param zipFileString 压缩文件路径 * @param outPathString 目标路径 * @throws Exception / private static void unZipFile(String zipFileString, String outPathString) { try { ZipInputStream inZip = new ZipInputStream(new FileInputStream(zipFileString)); ZipEntry zipEntry; String szName = ""; while ((zipEntry = inZip.getNextEntry()) != null) { szName = zipEntry.getName(); if (zipEntry.isDirectory()) { szName = szName.substring(0, szName.length() - 1); File folder = new File(outPathString + File.separator + szName); folder.mkdirs(); } else { File file = new File(outPathString + File.separator + szName); if (!file.exists()) { Log.d(TAG, "Create the file:" + outPathString + File.separator + szName); file.getParentFile().mkdirs(); file.createNewFile(); } FileOutputStream out = new FileOutputStream(file); int len; byte[] buffer = new byte[1024]; while ((len = inZip.read(buffer)) != -1) { out.write(buffer, 0, len); out.flush(); } out.close(); } } inZip.close(); } catch (Exception e) { Log.i(TAG,e.getMessage()); e.printStackTrace(); } } /* * 使用FileChannels复制文件。 * * @param source 原路径 * @param dest 目标路径 / public static void copyFileByFileChannels(File source, File dest) { FileChannel inputChannel = null; FileChannel outputChannel = null; try { inputChannel = new FileInputStream(source).getChannel(); outputChannel = new FileOutputStream(dest).getChannel(); outputChannel.transferFrom(inputChannel, 0, inputChannel.size()); refreshMedia(BaseApplication.getBaseApplication(), dest); } catch (Exception e) { e.printStackTrace(); } finally { try { inputChannel.close(); outputChannel.close(); } catch (IOException e) { e.printStackTrace(); } } } /* * 更新媒体库 * * @param cxt * @param files */ public static void refreshMedia(Context cxt, File... files) { for (File file : files) { String filePath = file.getAbsolutePath(); refreshMedia(cxt, filePath); } } public static void refreshMedia(Context cxt, String... filePaths) { MediaScannerConnection.scanFile(cxt.getApplicationContext(), filePaths, null, null); }}
