What is the Dart null checking idiom or best practice?

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As of Dart 1.12 null-aware operators are available for this type of situation:

bool isConnected(a, b) {
  bool outConn = outgoing[a]?.contains(b) ?? false;
  bool inConn = incoming[a]?.contains(b) ?? false;
  return outConn || inConn;
}

The ?. operator short-circuits to null if the left-hand side is null, and the ?? operator returns the left-hand side if it is not null, and the right-hand side otherwise.

The statement

outgoing[a]?.contains(b)

will thus either evaluate to null if outgoing[a] is null, or the boolean result of contains(b) if it is not.

That means the resulting statement will be one of the following: 

bool outConn = null ?? false; // false
bool outConn = false ?? false; // false
bool outConn = true ?? false; // true

The same applies to the inConn boolean, which means both inConn and outConn are guaranteed to be non-null, allowing us to return the result of ||ing the two.

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