What's the difference between enableReaderMode and enableForegroundDispatch?

Foreground dispatch system

The foreground dispatch system (NfcAdapter.enableForegroundDispatch()) exists since Android 2.3.3 (which is basically the beginning of Android NFC). Hence, this method is supported on all Android devices with NFC capabilities.

The foreground dispatch system is used to give an activity that is currently in the foreground precedence in handling NFC discovery events (i.e. discovered NFC tags and NDEF messages received from peer-to-peer devices). This means that even if another app is registered (by means of an intent filter in the AndroidManifest.xml) for a specific tag type or NDEF data, the NFC event will still be passed to the foreground activity instead of that other activity. Consequently, the method does not change the way Android listens for NFC devices (NFC tags, P2P devices), it only changes the priority for handling discovered devices.

Reader-mode API

The reader-mode API (NfcAdapter.enableReaderMode()) was introduced in Android 4.4. Consequently, not all Android devices with NFC capabilities support this method.

As opposed to the foreground dispatch system, the reader-mode API does change the way Android listens for NFC devices. The reader-mode API disables peer-to-peer mode. This, for instance, permits you to discover card emulation mode of other devices that have peer-to-peer mode and card emulation mode enabled simultaneously (as is the case with Android HCE). (Normally, such a device would be discovered as a peer-to-peer device and an Android app would not be able to access the card emulation functionality.)

Moreover, you can change specific parameters of the NFC reader mode, e.g. you can:

• define the tag technologies that the NFC reader polls for,
• define the interval in which Android tests if a tag is still present by sending a certain command sequence to the tag and checking if a response is still received,
• stop Android from automatically sending commands to the tag in order to test if the tag contains an NDEF message,
• stop Android from playing a sound upon tag discovery.

Note: Behavior may differ on Android 10

According to a comment by Adam Johns, the above may no longer be true on Android 10 (he tested on a Pixel 2). When using only enableReaderMode() (without an additional enableForegroundDispatch()), the devices seems to show a toast "No supported application for this NFC tag" eventhough tags are correctly dispatched to the registered reader-mode callback method (onTagDiscovered()).

enableReaderMode: Limit the NFC controller to reader mode while this Activity is in the foreground.

enableForegroundDispatch: This will give priority to the foreground activity when dispatching a discovered Tag to an application.

So basically you can use both for the same purpose, which is reading/writing a tag. enableReaderMode is used by Android phones in combination with a Broadcom NFC controller, because there is a bug on the presence check. As far as I know, only the enableReaderMode can dodge this by increasing EXTRA_READER_PRESENCE_CHECK_DELAY.