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A fast and efficient image loading library for Android focused on smooth scrolling.
There is a newer version: 5.0.0-rc01
package com.bumptech.glide.request.target;

import android.graphics.drawable.Drawable;
import android.support.annotation.Nullable;
import com.bumptech.glide.manager.LifecycleListener;
import com.bumptech.glide.request.Request;
import com.bumptech.glide.request.transition.Transition;

/**
 * An interface that Glide can load a resource into and notify of relevant lifecycle events during a
 * load.
 *
 *  The lifecycle events in this class are as follows: 
 onLoadStarted
 * onResourceReady
 onLoadCleared
 onLoadFailed
 
 *
 * The typical lifecycle is onLoadStarted -> onResourceReady or onLoadFailed -> onLoadCleared.
 * However, there are no guarantees. onLoadStarted may not be called if the resource is in memory or
 * if the load will fail because of a null model object. onLoadCleared similarly may never be called
 * if the target is never cleared. See the docs for the individual methods for details. 
 *
 * @param  The type of resource the target can display.
 */
public interface Target extends LifecycleListener {
  /**
   * Indicates that we want the resource in its original unmodified width and/or height.
   */
  int SIZE_ORIGINAL = Integer.MIN_VALUE;

  /**
   * A lifecycle callback that is called when a load is started.
   *
   *  Note - This may not be called for every load, it is possible for example for loads to fail
   * before the load starts (when the model object is null).
   *
   * 
 Note - This method may be called multiple times before any other lifecycle method is
   * called. Loads can be paused and restarted due to lifecycle or connectivity events and each
   * restart may cause a call here.
   *
   * 
You must ensure that any current Drawable received in {@link #onResourceReady(Object,
   * Transition)} is no longer displayed before redrawing the container (usually a View) or
   * changing its visibility.
   *
   * @param placeholder The placeholder drawable to optionally show, or null.
   */
  void onLoadStarted(@Nullable Drawable placeholder);

  /**
   * A lifecycle callback that is called when a load fails.
   *
   * 
 Note - This may be called before {@link #onLoadStarted(android.graphics.drawable.Drawable)
   * } if the model object is null.
   *
   * 
You must ensure that any current Drawable received in {@link #onResourceReady(Object,
   * Transition)} is no longer displayed before redrawing the container (usually a View) or
   * changing its visibility.
   *
   * @param errorDrawable The error drawable to optionally show, or null.
   */
  void onLoadFailed(@Nullable Drawable errorDrawable);

  /**
   * The method that will be called when the resource load has finished.
   *
   * @param resource the loaded resource.
   */
  void onResourceReady(R resource, Transition transition);

  /**
   * A lifecycle callback that is called when a load is cancelled and its resources are freed.
   *
   * You must ensure that any current Drawable received in {@link #onResourceReady(Object,
   * Transition)} is no longer displayed before redrawing the container (usually a View) or
   * changing its visibility.
   *
   * @param placeholder The placeholder drawable to optionally show, or null.
   */
  void onLoadCleared(@Nullable Drawable placeholder);

  /**
   * A method to retrieve the size of this target.
   *
   * @param cb The callback that must be called when the size of the target has been determined
   */
  void getSize(SizeReadyCallback cb);

  /**
   * Removes the given callback from the pending set if it's still retained.
   *
   * @param cb The callback to remove.
   */
  void removeCallback(SizeReadyCallback cb);

  /**
   * Sets the current request for this target to retain, should not be called outside of Glide.
   */
  void setRequest(@Nullable Request request);

  /**
   * Retrieves the current request for this target, should not be called outside of Glide.
   */
  @Nullable
  Request getRequest();
}