MvpNullObjectBasePresenter.java

package com.hannesdorfmann.mosby3.mvp;

import android.support.annotation.NonNull;
import android.support.annotation.UiThread;
import java.lang.ref.WeakReference;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;

/**
 * A {@link MvpPresenter} implmenetation that implements the <a href="https://en.wikipedia.org/wiki/Null_Object_pattern">null
 * object pattern</a> for the attached mvp view. So whenever the view gets detached from this
 * presenter by calling{@link #detachView(boolean)}, a new "null object" view gets dynamically
 * instantiated by using reflections and set as the current
 * view (instead of null). The new "null object" view simply does nothing. This avoids
 * NullPointerExceptions and checks like {@code if (getView() != null)}
 *
 * <p>
 * Please note that when creating the "null object" the first generic parameter (left depth-first
 * search) that will be found that is subtype of {@link MvpView} will be used as the type of the
 * view. So avoid having multiple generic parameters for "View" like this {@code
 * MyPresenter<FooMvpView, OtherMvpView>} because we can't know wheter FooMvpView or OtherMvpView is
 * the
 * real type of this presenter's view. In that case (left depth-first search) FooMvpView will be
 * used (may cause ClassCastException if OtherMvpView was the desired one)
 * </p>
 *
 * @param <V> The type of the {@link MvpView}
 * @author Jens Dirller , Hannes Dorfmann
 * @since 1.2.0
 */
public abstract class MvpNullObjectBasePresenter<V extends MvpView> implements MvpPresenter<V> {

  private WeakReference<V> view;
  private final V nullView;
  private boolean viewAttachedAtLeastOnce = false;

  public MvpNullObjectBasePresenter() {
    try {

      // Scan the inheritance hierarchy until we reached MvpNullObjectBasePresenter
      Class<V> viewClass = null;
      Class<?> currentClass = getClass();

      while (viewClass == null) {

        Type genericSuperType = currentClass.getGenericSuperclass();

        while (!(genericSuperType instanceof ParameterizedType)) {
          // Scan inheritance tree until we find ParameterizedType which is probably a MvpSubclass
          currentClass = currentClass.getSuperclass();
          genericSuperType = currentClass.getGenericSuperclass();
        }

        Type[] types = ((ParameterizedType) genericSuperType).getActualTypeArguments();

        for (int i = 0; i < types.length; i++) {
          Class<?> genericType = (Class<?>) types[i];
          if (genericType.isInterface() && isSubTypeOfMvpView(genericType)) {
            viewClass = (Class<V>) genericType;
            break;
          }
        }

        // Continue with next class in inheritance hierachy (see genericSuperType assignment at start of while loop)
        currentClass = currentClass.getSuperclass();
      }

      nullView = NoOp.of(viewClass);
    } catch (Throwable t) {
      throw new IllegalArgumentException(
          "The generic type <V extends MvpView> must be the first generic type argument of class "
              + getClass().getSimpleName()
              + " (per convention). Otherwise we can't determine which type of View this"
              + " Presenter coordinates.", t);
    }
  }

  /**
   * Scans the interface inheritnace hierarchy and checks if on the root is MvpView.class
   *
   * @param klass The leaf interface where to begin to scan
   * @return true if subtype of MvpView, otherwise false
   */
  private boolean isSubTypeOfMvpView(Class<?> klass) {
    if (klass.equals(MvpView.class)) {
      return true;
    }
    Class[] superInterfaces = klass.getInterfaces();
    for (int i = 0; i < superInterfaces.length; i++) {
      if (isSubTypeOfMvpView(superInterfaces[i])) {
        return true;
      }
    }
    return false;
  }

  @UiThread @Override public void attachView(@NonNull V view) {
    this.view = new WeakReference<V>(view);
    viewAttachedAtLeastOnce = true;
  }

  @UiThread @NonNull protected V getView() {
    if (!viewAttachedAtLeastOnce){
      throw new IllegalStateException("No view has ever been attached to this presenter!");
    }
    if (view != null) {
      V realView = view.get();
      if (realView != null) {
        return realView;
      }
    }

    return nullView;
  }

  @UiThread @Override public void detachView(boolean retainInstance) {
    if (view != null) {
      view.clear();
      view = null;
    }
  }
}