General Dart

Question 1

What is this operator called and what does it do?

??=

E.g.
a ??= 3;

Answer 1

Null Assignment operator

If the value to the left is null then it will assign it the value to the right. E.g.

int? a; // = null
a ??= 3;
print(a); // Prints 3

Question 2

What is this operator called and what does it do?

??

E.g.
print(1 ?? 3);

Answer 2

Null Aware operator

It will return the value on the left, however if that value is null then it will return the value on the right.

E.g.
print(1 ?? 3); // Prints ‘1’
print(null ?? 12); // Prints ‘12’

Question 3

What is this operator called and what does this operator do?

?.

E.g.
obj?.method()

Answer 3

Null Conditional Member Access operator

If the object on the left is null then null will be returned, however if it is not null then the result of the method (or property getter) will be returned.

Use ?. when you want to call a method/getter on an object IFF that object is not null (otherwise, return null).

Question 4

What is this operator called and what does it do?

…?

Answer 4

Null Spread operator

Placing … before an expression inside a collection literal unpacks the result of the expression and inserts its elements directly inside the new collection. If the expression is null then it will be ignored.

List lowerNumbers = [1, 2, 3, 4, 5];
List? upperNumbers = null;
List numbers = […lowerNumbers, …?upperNumbers];
numbers.forEach(print); // Prints ‘1 2 3 4 5’

Question 5

4 reasons to use getters and setters…

Answer 5

1. Read-only access
2. Write only access
3. Verify on set
4. Compute a value before set

Question 6

What is the format of the single-line ternary operator?

Answer 6

aBoolean ? isTrue : ifFalse;

Question 7

What is this operator and what does it do?

.. (2 dots)

Also is there a null-safety version?

Answer 7

Cascade operator

It perform a sequence of operations on the same object.

E.g.

BigObject fillBigObject(BigObject obj) {
  return obj
    ..anInt = 1
    ..aString = 'String!'
    ..aList = [3.0]
    ..allDone();
}

Note that the return value of each operation is ignored and after all of the operations have completed the cascade will return the original object that is being operated on. In this case… obj.

The null-safety version is ?.. (null-shorting) and it will only perform the cascading operations if the object is not null. You only use it on the first operator and the rest are all normal ones. E.g.

result1
?..doStuff()
..doMoreStuff();

Question 8

What are these two operators:

1. .. (2 dots)
2. … (3 dots)

Answer 8

1. .. (2 dots) The cascade operator. It’s for chaining multiple operations on the same object. It returns the object itself.
2. … (3 dots) The Spread operator. It’s for unpacking the result of an expression into a collection literal definition.

Question 9

When do we need to use initialiser lists?

Answer 9

All final final fields must be initialised in the initialiser list section of a class’ constructor. They can’t even be used in the constructors body until they have been initialised. The initialiser list runs before the constructor’s body.

Question 10

What is the purpose of assert() calls?

Answer 10

To find errors during debugging.

Asserts will not be run during production.

They can also be used to run specific logic during debug builds only. See https://stackoverflow.com/questions/56537718/what-assert-do-in-dart

Question 11

How do you lazily initialise a field?

Answer 11

Use the late modifier and provide a function call as the return value.

E.g.

class Weather {
  late int _temperature = _readThermometer();
}

Instead of running it as soon as the instance is constructed, it is deferred and run lazily the first time the field is accessed.

The function (in this case _readThermometer) is called only on the first time. This appears to make a lazy initialised field final by nature.

A constructor call can also be used to late initialise objects. e.g.
class Weather {
  late TempSensor _tempSensor = TempSensor();
}

Question 12

Why would you use lazy initialisation?

Answer 12

When the initialisation expression is costly and may not be needed.

Question 13

How can you access ‘this’ when initialising fields?

Answer 13

Usually instance field initialisers cannot access ‘this’ because you don’t have access to the new object until all field initialisers have completed. But with a late field, that’s no longer true, so you can access ‘this’, call methods, or access fields on the instance.

Question 14

When is it okay to use ‘var’ instead of explicitly declaring the type?

Answer 14

When they are local variables in small functions with very little scope. If the function is large enough that the reader must scroll then the full type should be declared instead.

Due to the small scope size, the reader does not have to scroll to where the variable is initialised to see the declared type. The initialisation code will always be visible.

Omitting the type focuses the reader’s attention on the more important name of the variable and its initialised value.

This will be most beneficial when you have long Types cluttering the code. E.g. List>

Question 15

What does the asterisk mean in async* and sync*?

Answer 15

It makes that function a ‘Generator Function’.

Question 16

What does a ‘Generator Function’ do?

Answer 16

It returns a sequence of values.

Question 17

What does a synchronous generator (sync*) function return?

Answer 17

An Iterable.

Question 18

What does an asynchronous generator (async*) function return?

Answer 18

A Stream.

Question 19

What’s the key difference between sync* and async* generator functions?

Answer 19

The calling code has to wait for a sync* generator to finish and return the iterable. An async* generator does not halt the calling code and the returned Stream can be listened to asynchronously.

Question 20

When does a generator function, generate its results?

Answer 20

Only when then are requested. Either when the Iterable is iterated over or when the Stream is listened to.

Question 21

What does yield do?

Answer 21

It is used to emit values from a generator without returning from the function.

Question 22

What does yield* do?

Answer 22

It delegates the call to another generator and after that generator stops producing the values, it resumes generating its own values.

Example…

Stream stream1() async* {
  yield 100;
  yield* stream2(3);
  yield 200;
  yield* stream2(2);
  yield 300;
}

Stream stream2(int input) async* {
  for (int i = input; i >0; i--) {
    yield i;
  }
}
void main() {
  Future fut = stream1().forEach(print);
}

This will print: 100, 3, 2, 1, 200, 2, 1, 300.

Note: You can yourself recursively using this.

Question 23

Two way to create a stream?

Answer 23

1. Create a function marked with async*

2. Using a StreamController.

Question 24

The ‘is’ operator checks if an object is of a certain type. What is the ‘is not’ operator?

Answer 24

is!

Note: The ! is after the is and not before.

Question 25

What does this operator do…

?[]

Answer 25

This the conditional subscript access operator.

Much like the subscript access operator… [ i ] it is used to access an item in a list. However, this null-safety version will only do so if the list object itself is not null. If it is then null is returned.

Question 26

Error vs Exception

Answer 26

Error: An Error object represents a program failure that the programmer should have avoided.

An Exception is intended to convey information to the user about a failure, so that the error can be addressed programmatically. It is intended to be caught, and it should contain useful data fields.

Question 27

In a try/catch block what does the ‘on’ keyword do?

Answer 27

Use ‘on’ when you need to specify the exception type.

Question 28

In a try/catch block what does the ‘catch’ keyword do?

Answer 28

Use ‘catch’ when your exception handler needs the exception object.

Question 29

In a try/catch block how do you catch everything?

Answer 29

You don’t specify any ‘on’.

E.g.

} catch (e) {
  // No specified type, handles all
  print('Something really unknown: $e');
}

Question 30

The catch call can take 2 parameters. The first is the Exception itself but what is the 2nd optional argument?

Answer 30

The second is the stack trace (a StackTrace object).

Question 31

What is a Dart package?

What is a plugin?

Answer 31

Packages have only dart code.

Plugins have dart + native code. E.g. platform plugins for getting device data.

Question 32

When would you use the following parameterized type names?
E
R
K
V
T
S
U

Answer 32

E = Element in a collection.
R = Return type.
K = Key in a map.
V = Value in a map.
T, S, U = Any other types, start with T and use S and U to avoid shadowing.

Question 33

Dart generic types are reified, which means that they…

Answer 33

...carry their type information around at runtime.
E.g.
var names = [];
names.addAll(['Seth', 'Kathy', 'Lars']);
print(names is List); // true

Question 34

What does ‘show’ and ‘hide’ do to import statements?

Answer 34

hide = Import all except the class/function/variable that you name.
show = Import only the class/function/variable that you name.

Note; You can show/hide multiple things by separating them with a comma.

Question 35

What’s an easy way to wait for multiple Futures to complete before running some other code?

Answer 35

await Future.wait(List);