Flutter动态化框架Kraken中C同步调用Dart实现原理

Flutter中使用dart:ffi Dart同步调用C是官方已经支持的。可是C调用Dart官方并无说怎么实现，网上也找不到相关的实现。 听说阿里已经实现了C调用Dart的方法，而且已经申请了专利，在Kraken项目中开源了。火烧眉毛的看了下源码。下面进行总结下。

首先看注册方法（Dart层），整体来讲就是把要暴露的Dart方法地址经过ffi传给C Pointer是本机C内存的指针， _dartNativeMethods是一个数组，用于存放dart 方法的地址

void registerDartMethodsToCpp() {
  Pointer<Uint64> bytes = allocate<Uint64>(count: _dartNativeMethods.length);
  Uint64List nativeMethodList = bytes.asTypedList(_dartNativeMethods.length);
  nativeMethodList.setAll(0, _dartNativeMethods);
  _registerDartMethods(bytes, _dartNativeMethods.length);
}
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那么如何获取Dart方法地址的呢？使用Pointer.fromFunction能够将Dart方法转为C方法指针，而后使用_nativeInvokeModule.address就能够获取到Dart方法地址了。

final Pointer<NativeFunction<Native_InvokeModule>> _nativeInvokeModule = Pointer.fromFunction(_invokeModule);
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而后看下Dart层的注册方法，lookup是dart:ffi暴露的方法用于调用C层代码，能够看到调用了C层的registerDartMethods(Pointer methodBytes, Int32 length);

final Dart_RegisterDartMethods _registerDartMethods =
    nativeDynamicLibrary.lookup<NativeFunction<Native_RegisterDartMethods>>('registerDartMethods').asFunction();
    
typedef Native_RegisterDartMethods = Void Function(Pointer<Uint64> methodBytes, Int32 length);
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下面是C层的代码，使用reinterpret_cast将Dart层的函数地址转为函数指针

void registerDartMethods(uint64_t *methodBytes, int32_t length) {
  size_t i = 0;

  methodPointer->invokeModule = reinterpret_cast<InvokeModule>(methodBytes[i++]);
  ......
}

typedef NativeString *(*InvokeModule)(void *callbackContext, int32_t contextId, NativeString *moduleName, NativeString *method, NativeString *params, AsyncModuleCallback callback);
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最后就能够在C层调用Dart方法了~

NativeString *response = getDartMethod()->invokeModule(bridgeContext, contextId, moduleName, method, params,
                                                               handleInvokeModuleTransientCallback);
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这里有一点要注意的是，Dart方法必须在Flutter UI线程调用，不然会Crash

还有一些要补充的 String类型须要转换下Dart是UTF-16，C是UTF-8，Dart String转C String转换方法 Utf8.toUtf8 Dart String和JS String都是UTF-16 ,转换方法以下，官方宣称编码格式都是UTF-16没有转换成本

class NativeString extends Struct {
  Pointer<Uint16> string;

  @Int32()
  int length;
}

String uint16ToString(Pointer<Uint16> pointer, int length) {
  return String.fromCharCodes(pointer.asTypedList(length));
}

Pointer<Uint16> _stringToUint16(String string) {
  final units = string.codeUnits;
  final Pointer<Uint16> result = allocate<Uint16>(count: units.length);
  final Uint16List nativeString = result.asTypedList(units.length);
  nativeString.setAll(0, units);
  return result;
}

Pointer<NativeString> stringToNativeString(String string) {
  assert(string != null);
  Pointer<NativeString> nativeString = allocate<NativeString>();
  nativeString.ref.string = _stringToUint16(string);
  nativeString.ref.length = string.length;
  return nativeString;
}

String nativeStringToString(Pointer<NativeString> pointer) {
  return uint16ToString(pointer.ref.string, pointer.ref.length);
}
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