Codable

Codable is the protocol a class implements to be archived by a Coder. It is the xc form of Foundation’s NSCoding.

class Point { ... }        // every class is already Codable through Object

Overview

Object adopts Codable with two empty methods, so every class conforms without saying so. A class with state to keep overrides both methods, calls super first, and hands each field to the coder under a string key:

class Point {
    i32    x;
    i32    y;
    Point* next;

    void encodeWithCoder(Coder* coder) {
        super.encodeWithCoder(coder);
        coder.encodeI32(x, "x");
        coder.encodeI32(y, "y");
        coder.encodeObject(next, "next");
    }

    void initWithCoder(Coder* coder) {
        super.initWithCoder(coder);
        x    = coder.decodeI32("x");
        y    = coder.decodeI32("y");
        next = (Point* ?)coder.decodeObject("next");
    }
}

Calling super in both methods lets each class in a hierarchy code its own fields, as in Objective-C.

initWithCoder is not a constructor. The unarchiver creates each instance by class name, which runs the class’s init(void) if it has one, and then calls initWithCoder on that instance. A class that is unarchived therefore needs an init(void) or no init at all. Both methods are ordinary virtual methods.

An object referred to from several places is archived once and comes back as one object. A cycle is allowed: each instance is registered before its initWithCoder runs, so a reference back to it resolves to the same instance, even though that instance has not finished decoding.

String, Number, Data, Array, Map and Set do not use these methods. The coder writes them in a native JSON form; see Coder.

Required methods

encodeWithCoder

void encodeWithCoder(Coder* coder);

Write this object’s state into coder with its encode… methods. Called while Coder.archive writes the graph. The default on Object writes nothing.

initWithCoder

void initWithCoder(Coder* coder);

Read the state back with the coder’s decode… methods. Called on an instance the unarchiver has just created with init(void). The default on Object reads nothing.

Cost

Codable.xc names Coder* without importing Coder.xc: the language treats a pointer to a class as opaque until something needs the class’s layout. A program that never archives compiles and links none of Coder. What every program does carry is the two empty methods on Object and their two vtable entries in each class.