DrawableCrossFadeTransition.java

package com.bumptech.glide.request.transition;

import android.graphics.drawable.Drawable;
import android.graphics.drawable.TransitionDrawable;

/**
 * A cross fade {@link Transition} for {@link android.graphics.drawable.Drawable}s that uses an
 * {@link android.graphics.drawable.TransitionDrawable} to transition from an existing drawable
 * already visible on the target to a new drawable. If no existing drawable exists, this class can
 * instead fall back to a default animation that doesn't rely on {@link
 * android.graphics.drawable.TransitionDrawable}.
 */
public class DrawableCrossFadeTransition implements Transition<Drawable> {
  private final Transition<Drawable> defaultAnimation;
  private final int duration;
  private final boolean isCrossFadeEnabled;

  /**
   * Constructor that takes a default animation and a duration in milliseconds that the cross fade
   * animation should last.
   *
   * @param defaultAnimation The {@link Transition} to use if there is no previous
   *                         {@link Drawable} (either a placeholder or previous resource) to
   *                         transition from.
   * @param duration The duration that the cross fade animation should run if there is something to
   *                 cross fade from when a new {@link android.graphics.drawable.Drawable} is put.
   * @param isCrossFadeEnabled If {@code true}, animates the previous resource's alpha to 0 while
   *                         animating the new resource's alpha to 100. Otherwise, only animates
   *                         the new resource's alpha to 100 while leaving the previous resource's
   *                         alpha at 100. See
   *                         {@link TransitionDrawable#setCrossFadeEnabled(boolean)}.
   */
  public DrawableCrossFadeTransition(Transition<Drawable> defaultAnimation, int duration,
      boolean isCrossFadeEnabled) {
    this.defaultAnimation = defaultAnimation;
    this.duration = duration;
    this.isCrossFadeEnabled = isCrossFadeEnabled;
  }

  /**
   * Animates from the previous drawable to the current drawable in one of two ways.
   *
   * <ol> <li>Using the default animation provided in the constructor if the previous drawable is
   * null</li> <li>Using the cross fade animation with the duration provided in the constructor if
   * the previous drawable is non null</li> </ol>
   *
   * @param current {@inheritDoc}
   * @param adapter {@inheritDoc}
   * @return {@inheritDoc}
   */
  @Override
  public boolean transition(Drawable current, ViewAdapter adapter) {
    Drawable previous = adapter.getCurrentDrawable();
    if (previous != null) {
      TransitionDrawable transitionDrawable =
          new TransitionDrawable(new Drawable[] { previous, current });
      transitionDrawable.setCrossFadeEnabled(isCrossFadeEnabled);
      transitionDrawable.startTransition(duration);
      adapter.setDrawable(transitionDrawable);
      return true;
    } else {
      defaultAnimation.transition(current, adapter);
      return false;
    }
  }
}