Bug in response handling semantics

[sadfsdfsdaf]

When acknowledge enabled some modules may send only response and no acknowledge.
For example "MH2024K-24SS". Library behaves badly in this and similar situations.
The problem code is here:

    case 0x4F:
      _isAvailable = true;
// _handleType=TimeOut-1; // placeholder
      break;

Here _handleType will have previous (uninitialized) value, in case the previous value was TimeOut then the timeout will be received. Problem is generated here:

bool DFRobotDFPlayerMini::waitAvailable(){
  _isSending = true;
  while (!available()){
    delay(0);
  }
  return _handleType != TimeOut;
}

By forcing the standard example to run myDFPlayer.readFileCountsInFolder(1); the code here will return TimeOut two times:

  if (myDFPlayer.available()) {
    printDetail(myDFPlayer.readType(), myDFPlayer.read()); //Print the detail message from DFPlayer to handle different errors and states.
  }

Result of myDFPlayer.readFileCountsInFolder(1); will be -1.

Correct answer should be number and one TimeOut value.

Here goes the command denoted as "<" and response from MH2024K-24SS denoted as ">" and response from YX5200-24SS denoted as "="

Query number of files in folder 1 (with acknowledge):
< 0x7E 0xFF 0x06 0x4E 0x01 0x00 0x01 0xFE 0xAB 0xEF
> 0x7E 0xFF 0x06 0x4E 0x00 0x00 0x05 0xFE 0xA8 0xEF
= 0x7E 0xFF 0x06 0x41 0x00 0x00 0x00 0xFE 0xBA 0xEF 0x7E 0xFF 0x06 0x4E 0x00 0x00 0x05 0xFE 0xA8 0xEF

The code inside void DFRobotDFPlayerMini::parseStack() should be reanalyzed and rewritten to avoid usage of uninitialized variables. My _handleType=TimeOut-1; is just a placeholder.

[jimaobian]

Thank you so much for your detailed analysis.
I've done some fix in 1.0.3. Try it here:
https://github.com/DFRobot/DFRobotDFPlayerMini/archive/1.0.3.zip