Since we use states manage the input flow in McBopomofo, implementing this function becomes easy. What I did is to create a new state, Associated Phrases state, and let the key handler to emit such a state just after emitting a Committing state.
When the input method controller is under Associated Phrase state, it shows the candidate window with a tooltip, and only accept candidate keys with the shift key. The key handler uses the characters without modifiers in an NSEvent object to find if there is any matching candidate label, so I added a new member "inputTextIgnoringModifiers" to KeyHandlerInput.
I use KeyValueBlobReader to read the associated phrases. I use the cin file from OpenVanilla project but removed the head and tail of the file to pass KeyValueBlobReader's validation.
zonble