Transition.java

package com.bumptech.glide.request.transition;

import android.graphics.drawable.Drawable;
import android.support.annotation.Nullable;
import android.view.View;

/**
 * An interface that allows a transition to be applied to {@link android.view.View}s in {@link
 * com.bumptech.glide.request.target.Target}s in across resource types. Targets that wrap views will
 * be able to provide all of the necessary arguments and start the transition. Those that do not
 * will be unable to provide the necessary arguments and will therefore be forced to ignore the
 * transition. This interface is a compromise that allows view specific transition in Glide's
 * complex world of arbitrary resource types and arbitrary target types.
 *
 * @param <R> The type of the resource whose entrance will be transitioned.
 */
public interface Transition<R> {

  /**
   * An interface wrapping a view that exposes the necessary methods to run the various types of
   * android animations as transitions: ({@link ViewTransition}, {@link ViewPropertyTransition} and
   * animated {@link android.graphics.drawable.Drawable}s).
   */
  interface ViewAdapter {
    /**
     * Returns the wrapped {@link android.view.View}.
     */
    View getView();

    /**
     * Returns the current drawable being displayed in the view, or null if no such drawable exists
     * (or one cannot be retrieved).
     */
    @Nullable
    Drawable getCurrentDrawable();

    /**
     * Sets the current drawable (usually an animated drawable) to display in the wrapped view.
     *
     * @param drawable The drawable to display in the wrapped view.
     */
    void setDrawable(Drawable drawable);
  }

  /**
   * Animates from the previous {@link android.graphics.drawable.Drawable} that is currently being
   * displayed in the given view, if not null, to the new resource that should be displayed in the
   * view.
   *
   * @param current The new resource that will be displayed in the view.
   * @param adapter The {@link Transition.ViewAdapter} wrapping a view that can at least return an
   *                {@link android.view.View} from {@link Transition.ViewAdapter#getView()}.
   * @return True if in the process of running the transition, the new resource was put on the view,
   * false if the caller needs to manually put the current resource on the view.
   */
  boolean transition(R current, ViewAdapter adapter);
}