ACursor abstract

abstract class ACursor

Annotations: @immutable

Abstract cursor over a Json tree, providing navigation and decoding.

An ACursor is either an HCursor (succeeded) or a FailedCursor (failed). Navigation methods return a new cursor; failed cursors propagate silently until decode is called, at which point the error is surfaced.

Implementers

• FailedCursor
• HCursor

Constructors

ACursor() const

const ACursor(HCursor? lastCursor, CursorOp? lastOp)

Implementation

const ACursor(this.lastCursor, this.lastOp);

Properties

failed no setter

bool get failed

Returns true if the cursor is in a failed state.

Implementation

bool get failed => !succeeded;

hashCode no setter inherited

int get hashCode

The hash code for this object.

A hash code is a single integer which represents the state of the object that affects operator == comparisons.

All objects have hash codes. The default hash code implemented by Object represents only the identity of the object, the same way as the default operator == implementation only considers objects equal if they are identical (see identityHashCode).

If operator == is overridden to use the object state instead, the hash code must also be changed to represent that state, otherwise the object cannot be used in hash based data structures like the default Set and Map implementations.

Hash codes must be the same for objects that are equal to each other according to operator ==. The hash code of an object should only change if the object changes in a way that affects equality. There are no further requirements for the hash codes. They need not be consistent between executions of the same program and there are no distribution guarantees.

Objects that are not equal are allowed to have the same hash code. It is even technically allowed that all instances have the same hash code, but if clashes happen too often, it may reduce the efficiency of hash-based data structures like HashSet or HashMap.

If a subclass overrides hashCode, it should override the operator == operator as well to maintain consistency.

Inherited from Object.

Implementation

external int get hashCode;

index no setter

Option<int> get index

If the cursor is inside a JArray, returns Some with the current index.

Implementation

Option<int> get index => none();

key no setter

Option<String> get key

If the cursor is inside a JObject, returns Some with the current key.

Implementation

Option<String> get key => none();

keys no setter

Option<IList<String>> get keys

If the focus is a JObject, returns Some with its keys.

Implementation

Option<IList<String>> get keys;

lastCursor final

final HCursor? lastCursor

The cursor that was active before the last operation, or null at the root.

Implementation

final HCursor? lastCursor;

lastOp final

final CursorOp? lastOp

The operation that produced this cursor from lastCursor, or null at the root.

Implementation

final CursorOp? lastOp;

pathString no setter

String get pathString

A dot/bracket string representing the current cursor position (e.g. .user.address[0]).

Implementation

String get pathString => PathToRoot.toPathString(pathToRoot());

runtimeType no setter inherited

Type get runtimeType

A representation of the runtime type of the object.

Inherited from Object.

Implementation

external Type get runtimeType;

succeeded no setter

bool get succeeded

Returns true if the cursor is in a successful state.

Implementation

bool get succeeded;

values no setter

Option<IList<Json>> get values

If the focus is a JArray, returns Some with its elements.

Implementation

Option<IList<Json>> get values;

Methods

decode()

DecodeResult<DecodingFailure, A> decode<A>(Decoder<A> decoder)

Decodes the focused value using decoder.

Implementation

DecodeResult<A> decode<A>(Decoder<A> decoder) => decoder.tryDecodeC(this);

delete()

ACursor delete()

Delete the focus and move to the parent.

Implementation

ACursor delete();

downArray()

ACursor downArray()

If the focus is a JSON array, move to the first element.

Implementation

ACursor downArray();

downField()

ACursor downField(String key)

If the focus is a JSON object, move to the value at the given key.

Implementation

ACursor downField(String key);

downN()

ACursor downN(int n)

If the focus is a JSON array, move to element at the given index.

Implementation

ACursor downN(int n);

field()

ACursor field(String key)

If the focus is a JSON object, move to the sibling at the given key.

Implementation

ACursor field(String key);

focus()

Option<Json> focus()

Returns Some with the focused Json if the cursor succeeded, or None if it failed.

Implementation

Option<Json> focus();

get()

DecodeResult<DecodingFailure, A> get<A>(String key, Decoder<A> decoder)

Navigates to key and decodes the value using decoder.

Implementation

DecodeResult<A> get<A>(String key, Decoder<A> decoder) => downField(key).decode(decoder);

getOrElse()

DecodeResult<DecodingFailure, A> getOrElse<A>(
  String key,
  Decoder<A> decoder,
  A Function() fallback,
)

Navigates to key and decodes the value; returns fallback if the field is absent or decoding yields None.

Implementation

DecodeResult<A> getOrElse<A>(String key, Decoder<A> decoder, Function0<A> fallback) =>
    get(key, decoder.optional()).fold(
      (err) => fallback().asRight(),
      (aOpt) => aOpt.fold(() => fallback().asRight(), (a) => a.asRight()),
    );

history()

IList<CursorOp> history()

Returns the list of CursorOps that led to this cursor, most-recent first. Used to reconstruct a path for DecodingFailure messages.

Implementation

IList<CursorOp> history() {
  final ops = <CursorOp>[];
  var current = this as ACursor?;

  while (current != null && current.lastOp != null) {
    ops.add(current.lastOp!);
    current = current.lastCursor;
  }

  return ilist(ops);
}

left()

ACursor left()

If the focus is an element in a JSON array, move to the left.

Implementation

ACursor left();

noSuchMethod() inherited

dynamic noSuchMethod(Invocation invocation)

Invoked when a nonexistent method or property is accessed.

A dynamic member invocation can attempt to call a member which doesn't exist on the receiving object. Example:

dynamic object = 1;
object.add(42); // Statically allowed, run-time error

This invalid code will invoke the noSuchMethod method of the integer 1 with an Invocation representing the .add(42) call and arguments (which then throws).

Classes can override noSuchMethod to provide custom behavior for such invalid dynamic invocations.

A class with a non-default noSuchMethod invocation can also omit implementations for members of its interface. Example:

class MockList<T> implements List<T> {
  noSuchMethod(Invocation invocation) {
    log(invocation);
    super.noSuchMethod(invocation); // Will throw.
  }
}
void main() {
  MockList().add(42);
}

This code has no compile-time warnings or errors even though the MockList class has no concrete implementation of any of the List interface methods. Calls to List methods are forwarded to noSuchMethod, so this code will log an invocation similar to Invocation.method(#add, [42]) and then throw.

If a value is returned from noSuchMethod, it becomes the result of the original invocation. If the value is not of a type that can be returned by the original invocation, a type error occurs at the invocation.

The default behavior is to throw a NoSuchMethodError.

Inherited from Object.

Implementation

@pragma("vm:entry-point")
@pragma("wasm:entry-point")
external dynamic noSuchMethod(Invocation invocation);

pathToRoot()

PathToRoot pathToRoot()

Reconstructs the full PathToRoot from the cursor chain.

Implementation

PathToRoot pathToRoot() {
  // TODO: Revisit lastCursorParentOrLastCursor
  var currentCursor = this as ACursor?;
  var acc = PathToRoot.empty;

  while (currentCursor != null) {
    if (currentCursor.failed) {
      // If the cursor is in a failed state, we lose context on what the
      // attempted last position was. Since we usually want to know this
      // for error reporting, we use the lastOp to attempt to recover that
      // state. We only care about operations which imply a path to the
      // root, such as a field selection.

      final lastCursor = currentCursor.lastCursor;
      final lastOp = currentCursor.lastOp;

      switch (lastOp) {
        case Field _:
          currentCursor = currentCursor.lastCursor;
          acc = acc.prependElem(PathElem.objectKey(lastOp.key));
        case DownField _:
          // We tried to move down, and then that failed, so the field was missing.
          currentCursor = currentCursor.lastCursor;
          acc = acc.prependElem(PathElem.objectKey(lastOp.key));
        case DownArray _:
          // We tried to move into an array, but it must have been empty.
          currentCursor = currentCursor.lastCursor;
          acc = acc.prependElem(PathElem.arrayIndex(0));
        case DownN _:
          // We tried to move into an array at index N, but there was no element there.
          currentCursor = currentCursor.lastCursor;
          acc = acc.prependElem(PathElem.arrayIndex(lastOp.n));
        case MoveLeft _:
          // We tried to move to before the start of the array.
          currentCursor = currentCursor.lastCursor;
          acc = acc.prependElem(PathElem.arrayIndex(-1));
        case MoveRight _:
          if (lastCursor is ArrayCursor) {
            // We tried to move to past the end of the array.
            currentCursor = lastCursor.parent;
            acc = acc.prependElem(PathElem.arrayIndex(lastCursor.indexValue + 1));
          } else {
            // Invalid state, skip for now.
            currentCursor = currentCursor.lastCursor;
          }
        default:
          // CursorOp.MoveUp or CursorOp.DeleteGoParent, both are move up
          // events.
          //
          // Recalling we are in a failed branch here, this should only
          // fail if we are already at the top of the tree or if the
          // cursor state is broken, in either
          // case this is the only valid action to take.
          currentCursor = currentCursor.lastCursor;
      }
    } else {
      switch (currentCursor) {
        case ArrayCursor _:
          final cursor = currentCursor;
          currentCursor = cursor.parent;
          acc = acc.prependElem(PathElem.arrayIndex(cursor.indexValue));
        case ObjectCursor _:
          final cursor = currentCursor;
          currentCursor = cursor.parent;
          acc = acc.prependElem(PathElem.objectKey(cursor.keyValue));
        case TopCursor _:
          currentCursor = null; // Exit loop
        default:
          currentCursor = currentCursor.lastCursor;
      }
    }
  }

  return acc;
}

right()

ACursor right()

If the focus is an element in a JSON array, move to the right.

Implementation

ACursor right();

root()

HCursor? root()

Returns the root HCursor of the cursor chain, or null if unavailable.

Implementation

HCursor? root() => null;

set()

ACursor set(Json j)

Returns a new cursor with the focus replaced by j.

Implementation

ACursor set(Json j) => withFocus((_) => j);

success()

Option<HCursor> success()

Returns Some with this cursor if it succeeded, or None if it failed.

Implementation

Option<HCursor> success();

top()

Option<Json> top()

Returns Some with the root Json value if the cursor succeeded.

Implementation

Option<Json> top();

toString() override

String toString()

A string representation of this object.

Some classes have a default textual representation, often paired with a static parse function (like int.parse). These classes will provide the textual representation as their string representation.

Other classes have no meaningful textual representation that a program will care about. Such classes will typically override toString to provide useful information when inspecting the object, mainly for debugging or logging.

Implementation

@override
String toString() => 'ACursor($lastCursor, $lastOp)';

up()

ACursor up()

Move focus to the parent.

Implementation

ACursor up();

withFocus()

ACursor withFocus(Json Function(Json) f)

Returns a new cursor with the focused Json replaced by f(focused).

Implementation

ACursor withFocus(Function1<Json, Json> f);

Operators

operator ==() inherited

bool operator ==(Object other)

The equality operator.

The default behavior for all Objects is to return true if and only if this object and other are the same object.

Override this method to specify a different equality relation on a class. The overriding method must still be an equivalence relation. That is, it must be:

• Total: It must return a boolean for all arguments. It should never throw.

• Reflexive: For all objects o, o == o must be true.

• Symmetric: For all objects o1 and o2, o1 == o2 and o2 == o1 must either both be true, or both be false.

• Transitive: For all objects o1, o2, and o3, if o1 == o2 and o2 == o3 are true, then o1 == o3 must be true.

The method should also be consistent over time, so whether two objects are equal should only change if at least one of the objects was modified.

If a subclass overrides the equality operator, it should override the hashCode method as well to maintain consistency.

Inherited from Object.

Implementation

external bool operator ==(Object other);