Question

The documentation for MFMessageComposeViewController says you should check [MFMessageComposeViewController canSendText] before trying to create a MFMessageComposeViewController.

It also says:

in iOS 5.0 or later, you should register as an observer of the MFMessageComposeViewControllerTextMessageAvailabilityDidChangeNotification notification to be notified of changes in the availability of sending text messages.

What possible reasons could there be for the availability of text messaging changing?
Is it just to make Linus Torvalds cry?

Background: I’m creating an iOS app that requires the ability to send SMS messages. I’ve added sms to UIRequiredDeviceCapabilities in Info.plist. It should only run on iPhones (according to Apple docs, I read somewhere this does not include iPod touches or iPads solely with iMessage)

If text messaging is not available, the app is useless, and should present some useful error message.

Update: I’ve tested with my iPhone 4S that simply having a locked SIM still returns YES from canSendText – still not a reason for message sending ability to change.

Update #2: I have created a simple test app to see what might cause it. I have not received the notification at all.

Things that don’t cause MFMessageComposeViewControllerTextMessageAvailabilityDidChangeNotification

• Turning iMessage on & off
• Losing WiFi connectivity
• Turning on airplane mode
• Locking iPhone SIM card

Things that can cause canSendText to change between YES/NO (but not send a notification)

• Turning iMessage on & off on an iPad (I assume iPod touch too)

I’m guessing the correct case is for iPads and iPod touches when iMessage is turned on or off, however, there must be a bug (either in my test code or Apple’s) that’s preventing this from working.

If you need this notification, I tried just checking [MFMessageComposeViewController canSendText] again on UIApplicationDidBecomeActiveNotification. This sometimes works, but if you switch quickly between Settings.app and your app, it may not have changed yet by the time you switch to your app, causing canSendText to still reflect the old value.

My solution for now is just going to be simply checking canSendText just before relevant decisions are made, and handle the (rare?) case where it may have changed without me knowing in some appropriate way.

Answer 1

Currently, there is no case where MFMessageComposeViewControllerTextMessageAvailabilityDidChangeNotification is sent.

The only time when the value of canSendText changes is when iMessage is turned on/off on an iPad or iPod Touch (not an iPhone, which has SMS messaging).

Even when canSendText changes, no notification is sent.
(Unless it’s just a bug in my test code?) This appears to be a bug or missing feature in Apple’s code.