更順暢地裝載物品

如果你的 RecyclerView 中的專案從網路載入資料（通常是影象）或執行其他處理，則可能需要花費大量時間，並且你最終可能會在螢幕上顯示專案但未完全載入。為避免這種情況，你可以擴充套件現有的 LinearLayoutManager，以便在螢幕上顯示之前預先載入多個專案：

package com.example;

import android.content.Context;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.OrientationHelper;
import android.support.v7.widget.RecyclerView;

/**
 * A LinearLayoutManager that preloads items off-screen.
 * <p>
 * Preloading is useful in situations where items might take some time to load
 * fully, commonly because they have maps, images or other items that require
 * network requests to complete before they can be displayed.
 * <p>
 * By default, this layout will load a single additional page's worth of items,
 * a page being a pixel measure equivalent to the on-screen size of the
 * recycler view.  This can be altered using the relevant constructor, or
 * through the {@link #setPages(int)} method.
 */
public class PreLoadingLinearLayoutManager extends LinearLayoutManager {
  private int mPages = 1;
  private OrientationHelper mOrientationHelper;

  public PreLoadingLinearLayoutManager(final Context context) {
    super(context);
  }

  public PreLoadingLinearLayoutManager(final Context context, final int pages) {
    super(context);
    this.mPages = pages;
  }

  public PreLoadingLinearLayoutManager(final Context context, final int orientation, final boolean reverseLayout) {
    super(context, orientation, reverseLayout);
  }

  @Override
  public void setOrientation(final int orientation) {
    super.setOrientation(orientation);
    mOrientationHelper = null;
  }

  /**
   * Set the number of pages of layout that will be preloaded off-screen,
   * a page being a pixel measure equivalent to the on-screen size of the
   * recycler view.
   * @param pages the number of pages; can be {@code 0} to disable preloading
   */
  public void setPages(final int pages) {
    this.mPages = pages;
  }

  @Override
  protected int getExtraLayoutSpace(final RecyclerView.State state) {
    if (mOrientationHelper == null) {
      mOrientationHelper = OrientationHelper.createOrientationHelper(this, getOrientation());
    }
    return mOrientationHelper.getTotalSpace() * mPages;
  }
}