export function PropertyMixin(key, meta = {}) {
  // Param check.
  if (!meta || 'object' !== typeof meta) {
    throw new TypeError(
      `Expected meta for Property(${
        key
      }) to be object. ${
        JSON.stringify(meta)
      } given.`
    );
  }
  return (Superclass) => {
    // Sanity check.
    if (Superclass.prototype[key]) {
      throw new TypeError(`can't redefine Avocado property "${key}"`);
    }
    // Handle defaults.
    let metaDefault;
    if (null === meta.default) {
      metaDefault = null;
    }
    else if (undefined === meta.default) {
      metaDefault = undefined;
    }
    else {
      metaDefault = JSON.parse(JSON.stringify(meta.default));
    }
    // Transform key. TODO: dynamic? Comes at a price...
    let transformedProperty;
    if (meta.transformProperty) {
      transformedProperty = meta.transformProperty(key);
    }
    else {
      transformedProperty = `$$avocado_property_${key}`;
    }
    class Property extends Superclass {

      constructor(...args) {
        super(...args);
        // Set default.
        if (undefined !== metaDefault) {
          if (meta.set) {
            meta.set.call(this, metaDefault);
          }
          else {
            this[transformedProperty] = metaDefault;
          }
        }
      }

    }
    // Getter.
    const getter = meta.get ? meta.get : function() {
      return this[transformedProperty];
    };
    // Setter.
    let setter;
    // Helper to define assigner.
    function defineAssigner() {
      const assignMethod = `${transformedProperty}$assign`;
      Object.defineProperty(Property.prototype, assignMethod, {
        value: meta.set,
      });
      return assignMethod;
    }
    // Tracking?
    if (meta.track) {
      // Define emitter.
      const emitter = meta.emit ? meta.emit : function(...args) {
        if (this.emit) {
          this.emit(...args);
        }
      };
      const emitMethod = `${transformedProperty}$emit`;
      Object.defineProperty(Property.prototype, emitMethod, {
        value: emitter,
      });
      // Compare?
      if (meta.eq) {
        // Define comparator.
        const comparator = meta.eq ? meta.eq : function(l, r) {
          return l === r;
        };
        const compareMethod = `${transformedProperty}$compare`;
        Object.defineProperty(Property.prototype, compareMethod, {
          value: comparator,
        });
        // Assign?
        if (meta.set) {
          // Define assigner.
          const assignMethod = defineAssigner();
          // Set with assigner, comparator, and emitter.
          setter = function(value) {
            const old = this[key];
            this[assignMethod](value);
            if (!this[compareMethod](old, value)) {
              this[emitMethod](`${key}Changed`, old, value);
            }
          };
        }
        else {
          // Set with comparator and emitter.
          setter = function(value) {
            const old = this[key];
            this[transformedProperty] = value;
            if (!this[compareMethod](old, value)) {
              this[emitMethod](`${key}Changed`, old, value);
            }
          };
        }
      }
      // No compare.
      else {
        // Assign?
        if (meta.set) {
          // Define assigner.
          const assignMethod = defineAssigner();
          // Set with assigner and emitter.
          setter = function(value) {
            const old = this[key];
            this[assignMethod](value);
            if (old !== value) {
              this[emitMethod](`${key}Changed`, old, value);
            }
          };
        }
        else {
          // Set with emitter.
          setter = function(value) {
            const old = this[key];
            this[transformedProperty] = value;
            if (old !== value) {
              this[emitMethod](`${key}Changed`, old, value);
            }
          }
        }
      }
    }
    // No tracking?
    else {
      // Assign?
      if (meta.set) {
        // Define assigner.
        const assignMethod = defineAssigner();
        // Set with assigner.
        setter = function(value) {
          this[assignMethod](value);
        };
      }
      else {
        // Set raw.
        setter = function(value) {
          this[transformedProperty] = value;
        };
      }
    }
    Object.defineProperty(Property.prototype, key, {
      enumerable: true,
      get: getter,
      set: setter,
    });
    return Property;
  }
}