[SOLVED] How to work Dispatch_Sempahore when first call signal and

How to work Dispatch_Sempahore when first call signal and

During the DispatchSemaphore function, Is it OK to call Signal() first and Wait() later? If call Signal() N continuously, does the value internally become N, Or I wonder if invoking Signal() several times does not increase the value when Wait() is not called.

let sempahore = DispatchSemaphore(value: 0)
semaphore.signal()
semaphore.signal()
semaphore.signal()
// in time, what is sempahore value? 1 or 3?

sempahore.wait()
// in time, what is semaphore value? 0 or 2?
// wait for more signal? or not?

Solution

Every .signal is '+1', and every .wait is '-1' or block as documented, and code that demo is

let semaphore = DispatchSemaphore(value: 0)
semaphore.signal() // = 1
semaphore.signal() // = 2
semaphore.signal() // = 3

semaphore.wait() // = 2  - pass
semaphore.wait() // = 1  - pass
semaphore.wait() // = 0  - pass
semaphore.wait() // = -1 - hang - waiting for new signal()

Here is from Apple Documentation

You increment a semaphore count by calling the signal() method, and decrement a semaphore count by calling wait() or one of its variants that specifies a timeout.
@discardableResult func signal() -> Int
Discussion
Increment the counting semaphore. If the previous value was less than zero, 
  this function wakes a thread currently waiting

func wait()
Discussion
Decrement the counting semaphore. If the resulting value is less than zero, 
  this function waits for a signal to occur before returning.