Summary:
In preparation for Blob support (wherein binary XHR and WebSocket responses can be retained as native data blobs on the native side and JS receives a web-like opaque Blob object), this change makes RCTNetworking aware of the responseType that JS requests. A `xhr.responseType` of `''` or `'text'` translates to a native response type of `'text'`. A `xhr.responseType` of `arraybuffer` translates to a native response type of `base64`, as we currently lack an API to transmit TypedArrays directly to JS. This is analogous to how the WebSocket module already works, and it's a lot more versatile and much less brittle than converting a JS *string* back to a TypedArray, which is what's currently going on.
Now that we don't always send text down to JS, JS consumers might still want to get progress updates about a binary download. This is what the `'progress'` event is designed for, so this change also implements that. This change also follows the XHR spec with regards to `xhr.response` and `xhr.responseText`:
- if the response type is `'text'`, `xhr.responseText` can be peeked at by the JS consumer. It will be updated periodically as the download progresses, so long as there's either an `onreadystatechange` or `onprogress` handler on the XHR.
- if the response type is not `'text'`, `xhr.responseText` can't be accessed and `xhr.response` remains `null` until the response is fully received. `'progress'` events containing response details (total bytes, downloaded so far) are dispatched if there's an `onprogress` handler.
Once Blobs are landed, `xhr.responseType` of `'blob'` will correspond to the same native response type, which will cause RCTNetworking to only send a blob ID down to JS, which can then create a `Blob` object from that for consumers.
Closes https://github.com/facebook/react-native/pull/8324
Reviewed By: javache
Differential Revision: D3508822
Pulled By: davidaurelio
fbshipit-source-id: 441b2d4d40265b6036559c3ccb9fa962999fa5df
08c375f828