Android代碼實現(xiàn)AdapterViews和RecyclerView無限滾動
應(yīng)用的一個共同的特點就是當用戶歡動時自動加載更多的內(nèi)容,這是通過用戶滑動觸發(fā)一定的閾值時發(fā)送數(shù)據(jù)請求實現(xiàn)的。
相同的是:信息實現(xiàn)滑動的效果需要定義在列表中最后一個可見項,和某些類型的閾值以便于開始在最后一項到達之前開始抓取數(shù)據(jù),實現(xiàn)無限的滾動。
實現(xiàn)無限滾動的現(xiàn)象的重要之處就在于在用戶滑動到最低端之前就行數(shù)據(jù)的獲取,所以需要加上一個閾值來幫助實現(xiàn)獲取數(shù)據(jù)的預(yù)期。
使用ListView和GridView實現(xiàn)
每個AdapterView 例如ListView 和GridView 當用戶開始進行滾動操作時候都會觸發(fā)OnScrollListener .使用這個系統(tǒng)我們就可以定義一個基本的EndlessScrollListener ,通過創(chuàng)造繼承OnScrollListener 的類來支持大多數(shù)情況下的使用。
package com.codepath.customadapter; import android.widget.AbsListView; /** * Created by Administrator on 2016/7/11. */ public abstract class EndlessScrollListener implements AbsListView.OnScrollListener { //在你滑動項下最少為多少時開始加載數(shù)據(jù) private int visibleThreshold = 5; //已經(jīng)加載數(shù)據(jù)的當前頁碼 private int currentPage = 0; //上一次加載數(shù)據(jù)后數(shù)據(jù)庫的數(shù)據(jù)量 private int previousTotalItemCount = 0; //我們是否在等待最后一組數(shù)據(jù)的加載 private boolean loading = true; //設(shè)置開始頁的下標 private int startingPageIndex = 0; public EndlessScrollListener() { } public EndlessScrollListener(int visibleThreshold) { this.visibleThreshold = visibleThreshold; } public EndlessScrollListener(int visibleThreshold, int startingPageIndex) { this.visibleThreshold = visibleThreshold; this.startingPageIndex = startingPageIndex; } //這個方法可能會在滑動調(diào)用很多次,所以在設(shè)計時要保持謹慎 //我們需要一些有用的參數(shù)來幫助我們,當我們需要加載更多數(shù)據(jù)的時候 //但是我們首先要檢查是否我們在等待先前的加載結(jié)束 //onScroll()當列表或網(wǎng)格視圖被滾動后將會調(diào)用,參數(shù)一:報告狀態(tài)的視圖參數(shù)二:第一個可以看見的項的下標,參數(shù)三:可見項的數(shù)量參數(shù)四:listAdapter中所有的項數(shù) @Override public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) { //如果總項數(shù)為0,而且先前沒有項,那么這個列表是無效的應(yīng)該被設(shè)定為初始狀態(tài) if (totalItemCount < previousTotalItemCount) { this.currentPage = this.startingPageIndex; this.previousTotalItemCount = totalItemCount; if (totalItemCount == 0) {this.loading = true;} } //如果仍在加載中我們可以檢查一下數(shù)據(jù)集合是否改變了,如果改變的話那就是已經(jīng)完成了loading需要更新當前 //頁數(shù)和數(shù)據(jù)總量 if (loading && (totalItemCount > previousTotalItemCount)) { loading = false; previousTotalItemCount = totalItemCount; currentPage++; } //如果當前沒有加載,我們需要檢查當前是否達到了閾值,如果是的話我們需要 //加載更多的數(shù)據(jù),執(zhí)行onLoadMore if (!loading && (firstVisibleItem + visibleItemCount + visibleThreshold) >= totalItemCount) { loading = onLoadMore(currentPage + 1, totalItemCount); } } //定義實際加載數(shù)據(jù)的過程,如果數(shù)據(jù)加載完成返回false,如果正在加載返回true; public abstract boolean onLoadMore(int page, int totalItemCount); @Override public void onScrollStateChanged(AbsListView view, int scrollState) { //不采取動作 } }
要注意的是這是一個抽象的類,為了要使用這些,必須繼承這個基本的類并且實現(xiàn)onLoadMore()方法實際的獲取數(shù)據(jù), 我們在一個活動中定義一個匿名的類來繼承EndlessScrollListener然后將其連接AdapterView上
public class MainActivity extends Activity { @Override protected void onCreate(Bundle savedInstance) { //向平常一樣 ListView lvItems = (ListView) findViewById(R.id.lvItens); //將監(jiān)聽器綁定到上面 lvItems.setOnScrollListener(new EndlessScrollListener() { @Override public boolean onLoadMore(int page, int totalItemsCount) { // 當新數(shù)據(jù)需要綁定到列表上的時候觸發(fā) // 加載數(shù)據(jù)需要的代碼Add whatever code is needed to append new items to your AdapterView customLoadMoreDataFromApi(page); // or customLoadMoreDataFromApi(totalItemsCount); return true; //數(shù)據(jù)加載中為true,不然為false; ONLY if more data is actually being loaded; false otherwise. } }); } //加載更多的數(shù)據(jù) public void customLoadMoreDataFromApi(int offset) { //這個方法通常會發(fā)起一些網(wǎng)絡(luò)請求,然后向適配器添加更多的數(shù)據(jù) //將偏移量數(shù)據(jù)作為參數(shù)附在請求里來獲得一個數(shù)據(jù)的分頁 //解析API返回的值并且獲得新的對象構(gòu)建適配器 } }
現(xiàn)在當用戶滑動并且觸發(fā)閾值時會自動觸發(fā)onloadMore() 方法,而且監(jiān)聽器給予了對于頁數(shù)和數(shù)據(jù)總量的訪問權(quán)限。
實現(xiàn)RecyclerView的無限滑動
我們可以使用相同的方法來定義一個接口EndlessRecyclerViewScrollListener 然后定義一個onLoadMore() 的方法來進行實現(xiàn)。由于LayoutManager負責項的生成和滑動的管理,我們需要一個LayoutManage的實例來收集必要的信息。
實現(xiàn)必要的分頁需要這樣的步驟:
1).直接復制EndlessRecyclerViewScrollListener.java
2).調(diào)用addOnScrollListener(...) 在RecyclerView 中來實現(xiàn)無限的分頁,傳遞EndlessRecyclerViewScrollListener的實例來實現(xiàn)onLoadMore 方法來決定什么時候來加載新的數(shù)據(jù)
3).在onLoadMore 方法中通過發(fā)送網(wǎng)絡(luò)請求或者從源數(shù)據(jù)加載來獲得更多item。
protected void onCreate(Bundle savedInstanceState) { // Configure the RecyclerView獲得RecylerView的實例 RecyclerView rvItems = (RecyclerView) findViewById(R.id.rvContacts); LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this); recyclerView.setLayoutManager(linearLayoutManager); // Add the scroll listener rvItems.addOnScrollListener(new EndlessRecyclerViewScrollListener(linearLayoutManager) { @Override public void onLoadMore(int page, int totalItemsCount) { // Triggered only when new data needs to be appended to the list // Add whatever code is needed to append new items to the bottom of the list customLoadMoreDataFromApi(page); } }); } // Append more data into the adapter // This method probably sends out a network request and appends new data items to your adapter. public void customLoadMoreDataFromApi(int page) { // Send an API request to retrieve appropriate data using the offset value as a parameter. // --> Deserialize API response and then construct new objects to append to the adapter // --> Notify the adapter of the changes } }
EndlessRecyclerView
public abstract class EndlessRecyclerViewScrollListener extends RecyclerView.OnScrollListener { // The minimum amount of items to have below your current scroll position // before loading more. private int visibleThreshold = 5; // The current offset index of data you have loaded private int currentPage = 0; // The total number of items in the dataset after the last load private int previousTotalItemCount = 0; // True if we are still waiting for the last set of data to load. private boolean loading = true; // Sets the starting page index private int startingPageIndex = 0; RecyclerView.LayoutManager mLayoutManager; public EndlessRecyclerViewScrollListener(LinearLayoutManager layoutManager) { this.mLayoutManager = layoutManager; } public EndlessRecyclerViewScrollListener(GridLayoutManager layoutManager) { this.mLayoutManager = layoutManager; visibleThreshold = visibleThreshold * layoutManager.getSpanCount(); } public EndlessRecyclerViewScrollListener(StaggeredGridLayoutManager layoutManager) { this.mLayoutManager = layoutManager; visibleThreshold = visibleThreshold * layoutManager.getSpanCount(); } public int getLastVisibleItem(int[] lastVisibleItemPositions) { int maxSize = 0; for (int i = 0; i < lastVisibleItemPositions.length; i++) { if (i == 0) { maxSize = lastVisibleItemPositions[i]; } else if (lastVisibleItemPositions[i] > maxSize) { maxSize = lastVisibleItemPositions[i]; } } return maxSize; } // This happens many times a second during a scroll, so be wary of the code you place here. // We are given a few useful parameters to help us work out if we need to load some more data, // but first we check if we are waiting for the previous load to finish. @Override public void onScrolled(RecyclerView view, int dx, int dy) { int lastVisibleItemPosition = 0; int totalItemCount = mLayoutManager.getItemCount(); if (mLayoutManager instanceof StaggeredGridLayoutManager) { int[] lastVisibleItemPositions = ((StaggeredGridLayoutManager) mLayoutManager).findLastVisibleItemPositions(null); // get maximum element within the list lastVisibleItemPosition = getLastVisibleItem(lastVisibleItemPositions); } else if (mLayoutManager instanceof LinearLayoutManager) { lastVisibleItemPosition = ((LinearLayoutManager) mLayoutManager).findLastVisibleItemPosition(); } else if (mLayoutManager instanceof GridLayoutManager) { lastVisibleItemPosition = ((GridLayoutManager) mLayoutManager).findLastVisibleItemPosition(); } // If the total item count is zero and the previous isn't, assume the // list is invalidated and should be reset back to initial state if (totalItemCount < previousTotalItemCount) { this.currentPage = this.startingPageIndex; this.previousTotalItemCount = totalItemCount; if (totalItemCount == 0) { this.loading = true; } } // If it's still loading, we check to see if the dataset count has // changed, if so we conclude it has finished loading and update the current page // number and total item count. if (loading && (totalItemCount > previousTotalItemCount)) { loading = false; previousTotalItemCount = totalItemCount; } // If it isn't currently loading, we check to see if we have breached // the visibleThreshold and need to reload more data. // If we do need to reload some more data, we execute onLoadMore to fetch the data. // threshold should reflect how many total columns there are too if (!loading && (lastVisibleItemPosition + visibleThreshold) > totalItemCount) { currentPage++; onLoadMore(currentPage, totalItemCount); loading = true; } } // Defines the process for actually loading more data based on page public abstract void onLoadMore(int page, int totalItemsCount); }
注意問題:
1.對于ListView,確定將綁定監(jiān)聽器的步驟放在onCreate()的方法中
2.為可加可靠的進行分頁,需要確定在向列表中添加新數(shù)據(jù)的時候先清理適配器中的數(shù)據(jù)
對于RecyclerView來說在通知適配器時推薦更細致的更新。
3.對于RecyclerView來說確保在清除列表中的數(shù)據(jù)的時候迅速的通知適配器內(nèi)容更新了,以便于可以觸發(fā)新的onScroll事件,重置自己
展示進度條
為了在底部展示進度條證明ListView正在加載。我們可以在Adapter中進行設(shè)置,我們可以定義兩類,可以是進度條類型或者是文本表明達到了最底行,參考:http://guides.codepath.com/android/Endless-Scrolling-with-AdapterViews-and-RecyclerView
以上就是本文的全部內(nèi)容,希望對大家的學習有所幫助,也希望大家多多支持腳本之家。
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