Android ListView异步加载图片方法详解

时间:2021-09-14 07:15:17

本文实例讲述了android listview异步加载图片方法。分享给大家供大家参考,具体如下:

先说说这篇文章的优点把,开启线程异步加载图片,然后刷新ui显示图片,而且通过弱引用缓存网络加载的图片,节省了再次连接网络的开销。

这样做无疑是非常可取的方法,但是加载图片时仍然会感觉到轻微的卡屏现象,特别是listview里的item在进行快速滑动的时候。

我找了一下原因,可能是在listview快速滑动屏幕的时候划过的item太多 而且每次调用getview方法后就会异步的在过去某个时间内用handler刷新一下ui,

如果在同一时间调用handler刷新ui次数多了就会造成这样的卡屏现象。

后来又一想,其实我们完全没有必要在listview正在滑动的时候去后台加载图片(不管这是图片是在缓存里还是在网络上),这样无疑造成了很大的资源浪费。

我们只需要在listview滑动停止之后再去加载listview里面显示的几个item里面的图片就好了。

根据以上想法,我做了一些设计改造:

1.在adapter 的 getview方法里面启动加载图片的thread,如果listview在滑动则wait

2.监听listview滑动停止事件,获得listview显示的item的最上面和最下面的序号,并唤醒所有加载图片的thread,判断加载图片的序号是否是在范围内,如果是则继续加载,如果不是则结束thread

部分代码如下:

?
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
@override
public 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
  }
};
?
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
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缓存图片,我还添加了本地sd卡缓存图片(这两种缓存方法各有好处,如果图片经常变化建议内存缓存图片,如果是不经常修改的图片建议sd卡缓存)

希望本文所述对大家android程序设计有所帮助。