pkg/compiler/lib/src/js_emitter/program_builder/field_visitor.dart

// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

part of dart2js.js_emitter.program_builder;

/**
 * [member] is a field (instance, static, or top level).
 *
 * [name] is the field name that the [Namer] has picked for this field's
 * storage, that is, the JavaScript property name.
 *
 * [accessorName] is the name of the accessor. For instance fields this is
 * mostly the same as [name] except when [member] is shadowing a field in its
 * superclass.  For other fields, they are rarely the same.
 *
 * [needsGetter] and [needsSetter] represent if a getter or a setter
 * respectively is needed.  There are many factors in this, for example, if the
 * accessor can be inlined.
 *
 * [needsCheckedSetter] indicates that a checked getter is needed, and in this
 * case, [needsSetter] is always false. [needsCheckedSetter] is only true when
 * type assertions are enabled (checked mode).
 */
typedef void AcceptField(
    VariableElement member,
    js.Name name,
    js.Name accessorName,
    bool needsGetter,
    bool needsSetter,
    bool needsCheckedSetter);

class FieldVisitor {
  final Compiler compiler;
  final Namer namer;

  JavaScriptBackend get backend => compiler.backend;

  FieldVisitor(this.compiler, this.namer);

  /**
   * Invokes [f] for each of the fields of [element].
   *
   * [element] must be a [ClassElement] or a [LibraryElement].
   *
   * If [element] is a [ClassElement], the static fields of the class are
   * visited if [visitStatics] is true and the instance fields are visited if
   * [visitStatics] is false.
   *
   * If [element] is a [LibraryElement], [visitStatics] must be true.
   *
   * When visiting the instance fields of a class, the fields of its superclass
   * are also visited if the class is instantiated.
   *
   * Invariant: [element] must be a declaration element.
   */
  void visitFields(Element element, bool visitStatics, AcceptField f) {
    assert(invariant(element, element.isDeclaration));

    bool isClass = false;
    bool isLibrary = false;
    if (element.isClass) {
      isClass = true;
    } else if (element.isLibrary) {
      isLibrary = true;
      assert(invariant(element, visitStatics));
    } else {
      throw new SpannableAssertionFailure(
          element, 'Expected a ClassElement or a LibraryElement.');
    }

    // If the class is never instantiated we still need to set it up for
    // inheritance purposes, but we can simplify its JavaScript constructor.
    bool isInstantiated =
        compiler.codegenWorld.directlyInstantiatedClasses.contains(element);

    void visitField(Element holder, FieldElement field) {
      assert(invariant(element, field.isDeclaration));

      // Keep track of whether or not we're dealing with a field mixin
      // into a native class.
      bool isMixinNativeField =
          isClass && backend.isNative(element) && holder.isMixinApplication;

      // See if we can dynamically create getters and setters.
      // We can only generate getters and setters for [element] since
      // the fields of super classes could be overwritten with getters or
      // setters.
      bool needsGetter = false;
      bool needsSetter = false;
      if (isLibrary || isMixinNativeField || holder == element) {
        needsGetter = fieldNeedsGetter(field);
        needsSetter = fieldNeedsSetter(field);
      }

      if ((isInstantiated && !backend.isNative(holder)) ||
          needsGetter ||
          needsSetter) {
        js.Name accessorName = namer.fieldAccessorName(field);
        js.Name fieldName = namer.fieldPropertyName(field);
        bool needsCheckedSetter = false;
        if (compiler.options.enableTypeAssertions &&
            needsSetter &&
            !canAvoidGeneratedCheckedSetter(field)) {
          needsCheckedSetter = true;
          needsSetter = false;
        }
        // Getters and setters with suffixes will be generated dynamically.
        f(field, fieldName, accessorName, needsGetter, needsSetter,
            needsCheckedSetter);
      }
    }

    if (isLibrary) {
      LibraryElement library = element;
      library.implementation.forEachLocalMember((Element member) {
        if (member.isField) visitField(library, member);
      });
    } else if (visitStatics) {
      ClassElement cls = element;
      cls.implementation.forEachStaticField(visitField);
    } else {
      ClassElement cls = element;
      // TODO(kasperl): We should make sure to only emit one version of
      // overridden fields. Right now, we rely on the ordering so the
      // fields pulled in from mixins are replaced with the fields from
      // the class definition.

      // If a class is not instantiated then we add the field just so we can
      // generate the field getter/setter dynamically. Since this is only
      // allowed on fields that are in [element] we don't need to visit
      // superclasses for non-instantiated classes.
      cls.implementation.forEachInstanceField(visitField,
          includeSuperAndInjectedMembers: isInstantiated);
    }
  }

  bool fieldNeedsGetter(VariableElement field) {
    assert(field.isField);
    if (fieldAccessNeverThrows(field)) return false;
    if (backend.shouldRetainGetter(field)) return true;
    return field.isClassMember &&
        compiler.codegenWorld.hasInvokedGetter(field, compiler.closedWorld);
  }

  bool fieldNeedsSetter(VariableElement field) {
    assert(field.isField);
    if (fieldAccessNeverThrows(field)) return false;
    if (field.isFinal || field.isConst) return false;
    if (backend.shouldRetainSetter(field)) return true;
    return field.isClassMember &&
        compiler.codegenWorld.hasInvokedSetter(field, compiler.closedWorld);
  }

  static bool fieldAccessNeverThrows(VariableElement field) {
    return
        // We never access a field in a closure (a captured variable) without
        // knowing that it is there.  Therefore we don't need to use a getter
        // (that will throw if the getter method is missing), but can always
        // access the field directly.
        field is ClosureFieldElement;
  }

  bool canAvoidGeneratedCheckedSetter(VariableElement member) {
    // We never generate accessors for top-level/static fields.
    if (!member.isInstanceMember) return true;
    DartType type = member.type;
    return type.treatAsDynamic || type.isObject;
  }
}