Hi Emil, On Thu, Jan 31, 2019 at 6:19 PM Emil Lenngren <emil.lenngren@xxxxxxxxx> wrote: > > Hi, > > I was looking through the quite lengthy discussion at > https://github.com/WebBluetoothCG/web-bluetooth/issues/238 on the > issue that in Web-Bluetooth, only a single "write value" API is > available, causing Web-Bluetooth to decide on its own if Write With > Response or Write Without Response should be used, in case both are > supported by the characteristic. > > But in the Bluetooth spec about Write Without Response: > > "This sub-procedure is used to write a Characteristic Value to a > server when the client knows the Characteristic Value Handle and the > client does not need an acknowledgement that the write was > successfully performed." > > Basically, it says it's up to the client/application to decide if an > acknowledgement is needed or not, and hence it's the app that should > decide if Write With or Without Response should be used. The "client" > can't mean a bluetooth stack here since it can of course not know if > an acknowledgement is needed or not. There is a property indicating if write without response is supported though, but you are right regarding that not excluding regular write so at that point the client would have a choice whether to use it or not. > I noticed that according to gatt-api.txt, BlueZ has the same > limitation in the WriteValue method, in that the stack chooses the > write type "arbitrarily" if both write types are supported (or really > the Write With Response is chosen, which might cause unwanted > latency). Therefore I suggest that an option should be added to the > WriteValue method, for example "write-without-response" (bool) to > force Write Without Response. It gets a bit trickier if the attribute is in fact a control point in which case perhaps only write-without-response really works, anyway control points are better off using AcquireWrite. > Note how iOS has a write type parameter to the write method, and > Android has a write type property you set before you execute the > write. > > I see that it might be possible to achieve the same result with > AcquireWrite -> write to socket -> release but that wouldn't be a good > solution for bluetooth stacks built on top of BlueZ that would like to > differentiate between the two write types (such as Web-Bluetooth) > since AcquireWrite can fail, for example if two apps write the value > at the same time (I guess the lock is exclusive?). It also seems like > unnecessary overhead to open and close sockets. AcquireWrite is to be used when the app needs exclusive access, like control points such as those commonly used for things like DFU, I don't think that is your intent here (or is it?) so I guess adding an option for WriteValue is probably better. Note though that obviously one cannot use such a flag with things like e.g. offset as that is not supported which makes the API a little trickier to use but I guess that ok given that setting flags is optional. -- Luiz Augusto von Dentz
