main/src/com/google/android/setupdesign/items/ReflectionInflater.java

/*
 * Copyright (C) 2017 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.android.setupdesign.items;

import android.content.Context;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import android.util.AttributeSet;
import android.view.InflateException;
import java.lang.reflect.Constructor;
import java.util.HashMap;

/**
 * An XML inflater that creates items by reading the tag as a class name, and constructs said class
 * by invoking the 2-argument constructor {@code Constructor(Context, AttributeSet)} via reflection.
 *
 * <p>Optionally a "default package" can be specified so that for unqualified tag names (i.e. names
 * that do not contain "."), the default package will be prefixed onto the tag.
 *
 * @param <T> The class where all instances (including child elements) belong to. If parent and
 *     child elements belong to different class hierarchies, it's OK to set this to {@link Object}.
 */
public abstract class ReflectionInflater<T> extends SimpleInflater<T> {

  /* static section */

  private static final Class<?>[] CONSTRUCTOR_SIGNATURE =
      new Class<?>[] {Context.class, AttributeSet.class};

  private static final HashMap<String, Constructor<?>> constructorMap = new HashMap<>();

  /* non-static section */

  // Array used to contain the constructor arguments (Context, AttributeSet), to avoid allocating
  // a new array for creation of every item.
  private final Object[] tempConstructorArgs = new Object[2];

  @Nullable private String defaultPackage;

  @NonNull private final Context context;

  /**
   * Create a new inflater instance associated with a particular Context.
   *
   * @param context The context used to resolve resource IDs. This context is also passed to the
   *     constructor of the items created as the first argument.
   */
  protected ReflectionInflater(@NonNull Context context) {
    super(context.getResources());
    this.context = context;
  }

  @NonNull
  public Context getContext() {
    return context;
  }

  /**
   * Instantiate the class by name. This attempts to instantiate class of the given {@code name}
   * found in this inflater's ClassLoader.
   *
   * @param tagName The full name of the class to be instantiated.
   * @param attrs The XML attributes supplied for this instance.
   * @return The newly instantiated item.
   */
  @NonNull
  public final T createItem(String tagName, String prefix, AttributeSet attrs) {
    String qualifiedName = tagName;
    if (prefix != null && qualifiedName.indexOf('.') == -1) {
      qualifiedName = prefix.concat(qualifiedName);
    }
    @SuppressWarnings("unchecked") // qualifiedName should correspond to a subclass of T
    Constructor<? extends T> constructor =
        (Constructor<? extends T>) constructorMap.get(qualifiedName);

    try {
      if (constructor == null) {
        // Class not found in the cache, see if it's real, and try to add it
        @SuppressWarnings("unchecked") // qualifiedName should correspond to a subclass of T
        Class<? extends T> clazz =
            (Class<? extends T>) context.getClassLoader().loadClass(qualifiedName);
        constructor = clazz.getConstructor(CONSTRUCTOR_SIGNATURE);
        constructor.setAccessible(true);
        constructorMap.put(tagName, constructor);
      }

      tempConstructorArgs[0] = context;
      tempConstructorArgs[1] = attrs;
      final T item = constructor.newInstance(tempConstructorArgs);
      tempConstructorArgs[0] = null;
      tempConstructorArgs[1] = null;
      return item;
    } catch (Exception e) {
      throw new InflateException(
          attrs.getPositionDescription() + ": Error inflating class " + qualifiedName, e);
    }
  }

  @Override
  protected T onCreateItem(String tagName, AttributeSet attrs) {
    return createItem(tagName, defaultPackage, attrs);
  }

  /**
   * Sets the default package that will be searched for classes to construct for tag names that have
   * no explicit package.
   *
   * @param defaultPackage The default package. This will be prepended to the tag name, so it should
   *     end with a period.
   */
  public void setDefaultPackage(@Nullable String defaultPackage) {
    this.defaultPackage = defaultPackage;
  }

  /**
   * Returns the default package, or null if it is not set.
   *
   * @see #setDefaultPackage(String)
   * @return The default package.
   */
  @Nullable
  public String getDefaultPackage() {
    return defaultPackage;
  }
}