From: https://developer.apple.com/library/content/qa/qa1754/_index.html
Q: Can you explain the difference between calling the AVAudioSession method overrideOutputAudioPort: with the value AVAudioSessionPortOverrideSpeaker and using the category option AVAudioSessionCategoryOptionDefaultToSpeaker with setCategory:withOptions:error:.
A: The difference is that setting the AVAudioSessionPortOverride by calling overrideOutputAudioPort: is more transient than using the category option AVAudioSessionCategoryOptionDefaultToSpeaker.
Important: The use of both AVAudioSessionPortOverrideSpeaker and AVAudioSessionCategoryOptionDefaultToSpeaker are only applicable to the AVAudioSessionCategoryPlayAndRecord category.
Calling overrideOutputAudioPort: and setting the AVAudioSessionPortOverride to AVAudioSessionPortOverrideSpeaker is a way of temporarily overriding the output to play to the speaker. Any route change or interruption will cause the audio to be routed back to its normal route, following the last-in wins rule. Think of using overrideOutputAudioPort: in terms of what you might use to implement a Speakerphone button where you want to be able to toggle between the speaker (AVAudioSessionPortOverrideSpeaker) and the normal output route (AVAudioSessionPortOverrideNone).
Note: This property is intended to allow 3rd party applications to mimic the behavior of a Speakerphone button and therefore may change the input route as well as output route. For example, setting the AVAudioSessionPortOverride to AVAudioSessionPortOverrideSpeaker while a headset is plugged in will cause the route to change to built-in mic / built-in speaker.
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TODO
NEED
-[ ] icon in route picker
-[ ] commit cleanup
NICE
-[ ] present action sheet automatically when making outgoing bluetooth call
-[ ] left align icons
-[ ] audio is paused when switching between video mode (maybe existing behavior, not sure)
-[ ] Copy: iPhone/iPad/iPod instead of "iPhone Microphone"
DONE
-[x] remove "receiver" from options while in video mode
-[x] show available audio routes
-[x] select available audio routes
-[x] notification if availabe inputs change so we can update call screen
mid call with available BT route
-[x] include speakerphone in choices
-[x] Enabled button shows active speakerphone. Should still show
bluetooth picker.
-[x] toggle back and forth between audio devices
-[x] hide audio route button in video mode if no BT available
-[x] Fixed: When on speakerphone - switching to video mode goes back to bluetooth.
-[x] Fixed: When switching to video w/ bluetooth device connected there is no
audio picker.
-[x] respect speakerphone/BT selection when in or toggling to/from video
-[x] do not hide audio route button when in video mode and bluetooth
connected
-[x] Show which is currently selected audio route
-[x] switching to speakerphone no longer works
-[x] switching *back* to bluetooth no longer works
-[x] add proper bluetooth button for audio calls
-[x] add proper bluetooth button for video calls
This ensures message ordering for clients that can't handle out of order
call messages (legacy iOS and Android).
Even when we revert the previous commit, to send ICE Updates sooner,
we'll want to keep this (until we're confident all clients can receive
out of order call messages)
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That change makes it more likely that the remote client will receive an
ICE update before their peer connection client is set.
We want to ship the robustness fix (waitForPeerConnection in
handleRemoteAddedIceCandidate) before we have the caller start sending
ICE updates immediately, in a future release.
At that time, we can simply revert this commit.
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