Codecs: should -openReturningError: be idempotent?

[jlama]

I noticed that the -openReturningError: method in decoders and encoders does not check if it's already open. This can lead to weird behavior as this method does not properly reset all the state for some codecs. For example _framePosition in the FLAC & MPEG decoders is not reset to 0.

[sbooth]

In the original C++ version of SFBAudioEngine I used the NVI pattern to handle this scenario.

SFBAudioEngine/Decoders/AudioDecoder.cpp

Line 229 in 0041637

bool SFB::Audio::Decoder::Open(CFErrorRef *error)

Even though there is no access control in Objective-C for methods there might be a benefit to emulating the pattern. Subclasses could implement -_openReturningError: and the superclass -openReturningError: would handle the logistics of checking to see if it's already open, opening the input source, etc.

[jlama]

Seems like a good idea. However all the state still needs to be reset if a decoder is reused (multiple open/close).
Here's a potential fix:
jlama@e578285

[sbooth]

#578 contains a partial initial fix.

[sbooth]

I'm investigating an alternative approach in #599

[sbooth]

The fix for decoders is committed but the same probably applies to the encoders.