prepare method

@override void prepare(String deviceId, String filepath, bool isAssets)

准备就绪后会回调{@link OADListener#onPrepared(String)}

• deviceId
• filepath 要加载的OAD文件（.bin）路径
• isAssets 是否是assets目录文件

Implementation

@override
void prepare(String deviceId, String filepath, bool isAssets) async {
  if (state == OADState.programming || state == OADState.waitingImgInfo) {
    throw ("Can't prepare() in $state state.");
  }

  bool initOk = await _initCharacteristics(deviceId);
  int readLen = await _loadFile(filepath, isAssets);
  bool prepared = initOk && (readLen != -1);
  if (prepared) {
    _startNotifyTimer(deviceId);
  }
}