本文實例講述了android listview異步加載圖片方法。分享給大家供大家參考，具體如下：
先說說這篇文章的優點把，開啟線程異步加載圖片，然后刷新ui顯示圖片，而且通過弱引用緩存網絡加載的圖片，節省了再次連接網絡的開銷。
這樣做無疑是非常可取的方法，但是加載圖片時仍然會感覺到輕微的卡屏現象，特別是listview里的item在進行快速滑動的時候。
我找了一下原因，可能是在listview快速滑動屏幕的時候劃過的item太多 而且每次調用getview方法后就會異步的在過去某個時間內用handler刷新一下ui，
如果在同一時間調用handler刷新ui次數多了就會造成這樣的卡屏現象。
后來又一想，其實我們完全沒有必要在listview正在滑動的時候去后臺加載圖片（不管這是圖片是在緩存里還是在網絡上），這樣無疑造成了很大的資源浪費。
我們只需要在listview滑動停止之后再去加載listview里面顯示的幾個item里面的圖片就好了。
根據以上想法，我做了一些設計改造：
1.在adapter 的 getview方法里面啟動加載圖片的thread，如果listview在滑動則wait
2.監聽listview滑動停止事件，獲得listview顯示的item的最上面和最下面的序號，并喚醒所有加載圖片的thread，判斷加載圖片的序號是否是在范圍內，如果是則繼續加載，如果不是則結束thread
部分代碼如下：
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@overridepublic view getview(int position, view convertview, viewgroup parent){ if(convertview == null){ convertview = minflater.inflate(r.layout.book_item_adapter, null); } bookmodel model = mmodels.get(position); convertview.settag(position); imageview iv = (imageview) convertview.findviewbyid(r.id.sitemicon); textview sitemtitle = (textview) convertview.findviewbyid(r.id.sitemtitle); textview siteminfo = (textview) convertview.findviewbyid(r.id.siteminfo); sitemtitle.settext(model.book_name); siteminfo.settext(model.out_book_url); iv.setbackgroundresource(r.drawable.rc_item_bg); syncimageloader.loadimage(position,model.out_book_pic,imageloadlistener); return convertview;}syncimageloader.onimageloadlistener imageloadlistener = new syncimageloader.onimageloadlistener(){ @override public void onimageload(integer t, drawable drawable) { //bookmodel model = (bookmodel) getitem(t); view view = mlistview.findviewwithtag(t); if(view != null){ imageview iv = (imageview) view.findviewbyid(r.id.sitemicon); iv.setbackgrounddrawable(drawable); } } @override public void onerror(integer t) { bookmodel model = (bookmodel) getitem(t); view view = mlistview.findviewwithtag(model); if(view != null){ imageview iv = (imageview) view.findviewbyid(r.id.sitemicon); iv.setbackgroundresource(r.drawable.rc_item_bg); } }};public void loadimage(){ int start = mlistview.getfirstvisibleposition(); int end =mlistview.getlastvisibleposition(); if(end >= getcount()){ end = getcount() -1; } syncimageloader.setloadlimit(start, end); syncimageloader.unlock();}abslistview.onscrolllistener onscrolllistener = new abslistview.onscrolllistener() { @override public void onscrollstatechanged(abslistview view, int scrollstate) { switch (scrollstate) { case abslistview.onscrolllistener.scroll_state_fling: debugutil.debug("scroll_state_fling"); syncimageloader.lock(); break; case abslistview.onscrolllistener.scroll_state_idle: debugutil.debug("scroll_state_idle"); loadimage(); //loadimage(); break; case abslistview.onscrolllistener.scroll_state_touch_scroll: syncimageloader.lock(); break; default: break; } } @override public void onscroll(abslistview view, int firstvisibleitem, int visibleitemcount, int totalitemcount) { // todo auto-generated method stub }}; |
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package cindy.android.test.synclistview;import java.io.datainputstream;import java.io.file;import java.io.fileinputstream;import java.io.fileoutputstream;import java.io.ioexception;import java.io.inputstream;import java.lang.ref.softreference;import java.net.url;import java.util.hashmap;import android.graphics.drawable.drawable;import android.os.environment;import android.os.handler;public class syncimageloader { private object lock = new object(); private boolean mallowload = true; private boolean firstload = true; private int mstartloadlimit = 0; private int mstoploadlimit = 0; final handler handler = new handler(); private hashmap<string, softreference<drawable>> imagecache = new hashmap<string, softreference<drawable>>(); public interface onimageloadlistener { public void onimageload(integer t, drawable drawable); public void onerror(integer t); } public void setloadlimit(int startloadlimit,int stoploadlimit){ if(startloadlimit > stoploadlimit){ return; } mstartloadlimit = startloadlimit; mstoploadlimit = stoploadlimit; } public void restore(){ mallowload = true; firstload = true; } public void lock(){ mallowload = false; firstload = false; } public void unlock(){ mallowload = true; synchronized (lock) { lock.notifyall(); } } public void loadimage(integer t, string imageurl, onimageloadlistener listener) { final onimageloadlistener mlistener = listener; final string mimageurl = imageurl; final integer mt = t; new thread(new runnable() { @override public void run() { if(!mallowload){ debugutil.debug("prepare to load"); synchronized (lock) { try { lock.wait(); } catch (interruptedexception e) { // todo auto-generated catch block e.printstacktrace(); } } } if(mallowload && firstload){ loadimage(mimageurl, mt, mlistener); } if(mallowload && mt <= mstoploadlimit && mt >= mstartloadlimit){ loadimage(mimageurl, mt, mlistener); } } }).start(); } private void loadimage(final string mimageurl,final integer mt,final onimageloadlistener mlistener){ if (imagecache.containskey(mimageurl)) { softreference<drawable> softreference = imagecache.get(mimageurl); final drawable d = softreference.get(); if (d != null) { handler.post(new runnable() { @override public void run() { if(mallowload){ mlistener.onimageload(mt, d); } } }); return; } } try { final drawable d = loadimagefromurl(mimageurl); if(d != null){ imagecache.put(mimageurl, new softreference<drawable>(d)); } handler.post(new runnable() { @override public void run() { if(mallowload){ mlistener.onimageload(mt, d); } } }); } catch (ioexception e) { handler.post(new runnable() { @override public void run() { mlistener.onerror(mt); } }); e.printstacktrace(); } } public static drawable loadimagefromurl(string url) throws ioexception { debugutil.debug(url); if(environment.getexternalstoragestate().equals(environment.media_mounted)){ file f = new file(environment.getexternalstoragedirectory()+"/testsynclistview/"+md5.getmd5(url)); if(f.exists()){ fileinputstream fis = new fileinputstream(f); drawable d = drawable.createfromstream(fis, "src"); return d; } url m = new url(url); inputstream i = (inputstream) m.getcontent(); datainputstream in = new datainputstream(i); fileoutputstream out = new fileoutputstream(f); byte[] buffer = new byte[1024]; int byteread=0; while ((byteread = in.read(buffer)) != -1) { out.write(buffer, 0, byteread); } in.close(); out.close(); drawable d = drawable.createfromstream(i, "src"); return loadimagefromurl(url); }else{ url m = new url(url); inputstream i = (inputstream) m.getcontent(); drawable d = drawable.createfromstream(i, "src"); return d; } }} |
除了本身已有的弱引用緩存圖片，我還添加了本地sd卡緩存圖片（這兩種緩存方法各有好處，如果圖片經常變化建議內存緩存圖片，如果是不經常修改的圖片建議sd卡緩存）
希望本文所述對大家android程序設計有所幫助。
