iOS: Made it clear that the "Audio Background Capability" setting must be enabled for MidiInput/Output::createNewDevice() to succeed

extras/Projucer/Source/ProjectSaving/jucer_ProjectExport_Xcode.h

@@ -334,7 +334,8 @@ public:
         if (iOS)
         {
             props.add (new ChoicePropertyComponent (iosBackgroundAudioValue, "Audio Background Capability"),
-                       "Enable this to grant your app the capability to access audio when in background mode.");
+                       "Enable this to grant your app the capability to access audio when in background mode. "
+                       "This permission is required if your app creates a MIDI input or output device.");
 
             props.add (new ChoicePropertyComponent (iosBackgroundBleValue, "Bluetooth MIDI Background Capability"),
                        "Enable this to grant your app the capability to connect to Bluetooth LE devices when in background mode.");

modules/juce_audio_devices/midi_io/juce_MidiDevices.h

@@ -112,6 +112,9 @@ public:
         This will attempt to create a new midi input device with the specified name for other
         apps to connect to.
 
+        NB - if you are calling this method on iOS you must have enabled the "Audio Background Capability"
+        setting in the iOS exporter otherwise this method will fail.
+
         Returns nullptr if a device can't be created.
 
         @param deviceName  the name of the device to create
@@ -265,6 +268,9 @@ public:
         This will attempt to create a new midi output device with the specified name that other
         apps can connect to and use as their midi input.
 
+        NB - if you are calling this method on iOS you must have enabled the "Audio Background Capability"
+        setting in the iOS exporter otherwise this method will fail.
+
         Returns nullptr if a device can't be created.
 
         @param deviceName  the name of the device to create

modules/juce_audio_devices/native/juce_mac_CoreMidi.cpp

@@ -459,7 +459,19 @@ MidiInput* MidiInput::createNewDevice (const String& deviceName, MidiInputCallba
         MIDIEndpointRef endpoint;
         ScopedCFString name (deviceName);
 
-        if (CHECK_ERROR (MIDIDestinationCreate (client, name.cfString, midiInputProc, mpc.get(), &endpoint)))
+        auto err = MIDIDestinationCreate (client, name.cfString, midiInputProc, mpc.get(), &endpoint);
+
+       #if JUCE_IOS
+        if (err == kMIDINotPermitted)
+        {
+            // If you've hit this assertion then you probably haven't enabled the "Audio Background Capability"
+            // setting in the iOS exporter for your app - this is required if you want to create a MIDI device!
+            jassertfalse;
+            return nullptr;
+        }
+       #endif
+
+        if (CHECK_ERROR (err))
         {
             auto deviceIdentifier = createUniqueIDForMidiPort (deviceName, true);
 
@@ -582,7 +594,19 @@ MidiOutput* MidiOutput::createNewDevice (const String& deviceName)
 
         ScopedCFString name (deviceName);
 
-        if (CHECK_ERROR (MIDISourceCreate (client, name.cfString, &endpoint)))
+        auto err = MIDISourceCreate (client, name.cfString, &endpoint);
+
+       #if JUCE_IOS
+        if (err == kMIDINotPermitted)
+        {
+            // If you've hit this assertion then you probably haven't enabled the "Audio Background Capability"
+            // setting in the iOS exporter for your app - this is required if you want to create a MIDI device!
+            jassertfalse;
+            return nullptr;
+        }
+       #endif
+
+        if (CHECK_ERROR (err))
         {
             auto deviceIdentifier = createUniqueIDForMidiPort (deviceName, true);