Helper functions to read values

[g-ortuno]

In Android we have helper functions to more easily read and write values to and from characteristics:

[getFloatValue()](https://developer.android.com/reference/android/bluetooth/BluetoothGattCharacteristic.html#getFloatValue%28int, int%29)
[getIntValue()](https://developer.android.com/reference/android/bluetooth/BluetoothGattCharacteristic.html#getIntValue%28int, int%29)
getStringValue()

They are really useful since developers don't have to worry about bytes if all they are doing is reading/writing integers. Could we add similar helper functions?

[jyasskin]

I think these features should be prototyped in a helper library before we put them in the spec, so they have a chance to evolve faster than they would in Blink. Shall we start a sibling repository to hold that helper library?

[g-ortuno]

Eventually we should, for now let's focus on the spec and implementation.

[beaufortfrancois]

I'd be happy to start this helper library.
Which other helper functions would you add?

[beaufortfrancois]

@g-ortuno @jyasskin Before going further, is it what you were thinking of?
https://github.com/beaufortfrancois/sandbox/blob/99fd1453a12a1513f2731c9ffa561199294a9cc5/web-bluetooth/utils/utils.js

[jyasskin]

Yes, that's great. I think I'd make 2 changes: I'd either default the byteOffset to 0 or omit it entirely, since if you're reading multiple items from a single characteristic, you probably want to only call readValue once. And I'd default littleEndian to true, since nearly all Bluetooth values are little-endian, contrary to DataView's default.

[beaufortfrancois]

Will do that tomorrow then. Thanks for the feedback.

On Thu, Jan 7, 2016, 7:15 PM Jeffrey Yasskin notifications@github.com
wrote:

Yes, that's great. I think I'd make 2 changes: I'd either default the
byteOffset to 0 or omit it entirely, since if you're reading multiple
items from a single characteristic, you probably want to only call
readValue once. And I'd default littleEndian to true, since nearly all
Bluetooth values are little-endian, contrary to DataView's default.

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[beaufortfrancois]

I've changed default littleEndian to true as advised. Thank you!

Regarding the default/removal of byteOffset, that made me realized we might want to implement BluetoothGattCharacteristic.prototype.getFloat32Value instead of BluetoothGattCharacteristic.prototype.readFloat32Value so that it returns a Float32 value of the stored cached read value.
What do you think?

[beaufortfrancois]

Here's what it would look like: https://github.com/beaufortfrancois/sandbox/blob/gh-pages/web-bluetooth/utils/utils.js

It will work when https://code.google.com/p/chromium/issues/detail?id=575577 is fixed

[g-ortuno]

I wonder how useful these function will be when characteristic returns a
DataView instead of an ArrayBuffer.

On Fri, Jan 8, 2016 at 1:48 AM François Beaufort notifications@github.com
wrote:

Here's what it would look like:
https://github.com/beaufortfrancois/sandbox/blob/gh-pages/web-bluetooth/utils/utils.js

[image: screenshot 2016-01-08 at 10 47 07 am]
https://cloud.githubusercontent.com/assets/634478/12195118/3896530a-b5f5-11e5-97e1-98e0a4da98f8.png

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[beaufortfrancois]

Hopefully it won't be needed anymore.

[beaufortfrancois]

Here's what it looks like at https://github.com/beaufortfrancois/sandbox/blob/gh-pages/web-bluetooth/utils/utils.js below. What do you think?

BluetoothGATTCharacteristic.prototype.getFloat32Value = function(byteOffset, littleEndian = true) {
  return this.value.getFloat32(byteOffset, littleEndian);
};

BluetoothGATTCharacteristic.prototype.getFloat64Value = function(byteOffset, littleEndian = true) {
  return this.value.getFloat64(byteOffset, littleEndian);
};

BluetoothGATTCharacteristic.prototype.getInt16Value = function(byteOffset, littleEndian = true) {
  return this.value.getInt16(byteOffset, littleEndian);
};

BluetoothGATTCharacteristic.prototype.getInt32Value = function(byteOffset, littleEndian = true) {
  return this.value.getInt32(byteOffset, littleEndian);
};

BluetoothGATTCharacteristic.prototype.getInt8Value = function(byteOffset) {
  return this.value.getInt8(byteOffset);
};

BluetoothGATTCharacteristic.prototype.getStringValue = function(utfLabel = 'utf8') {
  var decoder = new TextDecoder(utfLabel);
  return decoder.decode(this.value);
};

BluetoothGATTCharacteristic.prototype.getUint16Value = function(byteOffset, littleEndian = true) {
  return this.value.getUint16(byteOffset, littleEndian);
};

BluetoothGATTCharacteristic.prototype.getUint32Value = function(byteOffset, littleEndian = true) {
  return this.value.getUint32(byteOffset, littleEndian);
};

BluetoothGATTCharacteristic.prototype.getUint8Value = function(byteOffset) {
  return this.value.getUint8(byteOffset);
};

[jyasskin]

I'd be happy to have this as a simple npm module, but I'd like to wait to change the spec until we see it being widely used. I'll close this issue in this repository, but we can reopen later if it turns out people want this.