android中异步加载图片

浅谈Android ListView 异步图片获取

Android应用中，从远程获取图片算是经常要用的一个功能，而图片资源往往会消耗比较大的流量，而且如果加载在图片比较多的View中，如ListView GridView WebView中，加载速度会比较缓慢。Android中图片加载速度很慢的话，程序很容易就挂掉，等待的用户用户同样会等到崩溃。

我们一般的处理方法是：

异步下载

本地缓存

-----------------------------------------------------------
先说异步下载

如一个ListView的异步下载

Java代码
1.public class AsyncActivity extends Activity {
2. List<data> insList;
3. private ListView mListView;
4. private ArrayList<data> mListItems;
5. private ItemAdapter mAdapter;
6. private ImageView mLfteView;
7. private ImageView mRightView;
8.
9.
10.
11. private static final int REFRESH_LIST = 1;
12. Handler mHandler = new Handler() {
13. public void handleMessage(android.os.Message msg) {
14. int type = msg.what;
15. switch (type) {
16. case REFRESH_LIST: {
17.
18. if (mListItems.size() > 0) {
19. mListView.setVisibility(View.VISIBLE);
20. if (mAdapter== null) {
21. mAdapter = new Adapter(AsyncActivity.this,
22. mListItems);
23. mListView.setAdapter(mAdapter);
24. }
25. mAdapter.notifyDataSetChanged();
26.
27. }
28. mListView.requestFocus();
29.
30. unShowDialogLoading();
31. break;
32. }
33.
34.
35. default:
36. break;
37. }
38. };
39. };
40.
41.
42. public void onCreate(Bundle savedInstanceState) {
43. super.onCreate(savedInstanceState);
44. setContentView(R.layout.insurance_list);
45. initViews();
46. getList();
47. }
48.
49. /**
50. * 初始化view
51. */
52. private void initViews(){
53.
54. mListView = (ListView)findViewById(R.id.list);
55. mListView.setAdapter(mAdapter);
56. mListView.setOnItemClickListener(new OnItemClickListener(){
57. public void onItemClick(AdapterView<?> arg0, View v, int id,
58. long pos) {
59. // 获得被单击的项
60. //跳转
61.
62. }
63. });
64.
65.
66. mListItems = new ArrayList<data>();
67.
68. }
69.
70.
71.
72. private void getList() {
73. showDialogLoading();
74. //得到保险列表
75. insuranceData insurance = new insuranceData();
76. insList = insurance.getList();
77. mListItems.clear();
78. mListItems.addAll(insList);
79. mHandler.sendEmptyMessage(REFRESH_LIST);
80.
81.
82. }
83.
84.
85. private ProgressDialog mLoadingDialog;
86.
87. private void showDialogLoading() {
88. mLoadingDialog = new ProgressDialog(this);
89. mLoadingDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);
90. //spinner 自旋体像螺旋桨那样
91. mLoadingDialog.setMessage("载入中,请稍候...");
92. mLoadingDialog.setIndeterminate(false);//设置进度条是否为不明确
93. mLoadingDialog.setCancelable(true);//设置进度条是否可以按退回健取消
94. mLoadingDialog.show();
95. }
96.
97. private void unShowDialogLoading() {
98. if (mLoadingDialog == null)
99. return;
100. else
101. mLoadingDialog.dismiss();
102. }
103.
104.}
public class AsyncActivity extends Activity {
List<data> insList;
private ListView mListView;
private ArrayList<data> mListItems;
private ItemAdapter mAdapter;
private ImageView mLfteView;
private ImageView mRightView;

private static final int REFRESH_LIST = 1;
Handler mHandler = new Handler() {
public void handleMessage(android.os.Message msg) {
int type = msg.what;
switch (type) {
case REFRESH_LIST: {

if (mListItems.size() > 0) {
mListView.setVisibility(View.VISIBLE);
if (mAdapter== null) {
mAdapter = new Adapter(AsyncActivity.this,
mListItems);
mListView.setAdapter(mAdapter);
}
mAdapter.notifyDataSetChanged();

}
mListView.requestFocus();

unShowDialogLoading();
break;
}

default:
break;
}
};
};

public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.insurance_list);
initViews();
getList();
}

/**
* 初始化view
*/
private void initViews(){

mListView = (ListView)findViewById(R.id.list);
mListView.setAdapter(mAdapter);
mListView.setOnItemClickListener(new OnItemClickListener(){
public void onItemClick(AdapterView<?> arg0, View v, int id,
long pos) {
// 获得被单击的项
//跳转

}
});

mListItems = new ArrayList<data>();

}

private void getList() {
showDialogLoading();
//得到保险列表
insuranceData insurance = new insuranceData();
insList = insurance.getList();
mListItems.clear();
mListItems.addAll(insList);
mHandler.sendEmptyMessage(REFRESH_LIST);

}

private ProgressDialog mLoadingDialog;

private void showDialogLoading() {
mLoadingDialog = new ProgressDialog(this);
mLoadingDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);
//spinner 自旋体像螺旋桨那样
mLoadingDialog.setMessage("载入中,请稍候...");
mLoadingDialog.setIndeterminate(false);//设置进度条是否为不明确
mLoadingDialog.setCancelable(true);//设置进度条是否可以按退回健取消
mLoadingDialog.show();
}

private void unShowDialogLoading() {
if (mLoadingDialog == null)
return;
else
mLoadingDialog.dismiss();
}

}

它的Adapter是

Java代码
1.public class InsuranceItemAdapter extends BaseAdapter{
2. private ArrayList<data> mList;
3. private Context mContext;
4. // 异步加载图片的线程
5. private AsyncImageLoader imageLoader = new AsyncImageLoader();
6. //当前的缓存
7. private Map<Integer, View> viewMap = new HashMap<Integer, View>();
8.
9. public InsuranceItemAdapter(Context context, ArrayList<data> ins) {
10. mContext = context;
11. mList= ins;
12. }
13.
14. public InsuranceItemAdapter(Context context, ArrayList<data> ins,
15. Handler handler) {
16. mContext = context;
17. mList= ins;
18. }
19.
20. public void setInsurance(ArrayList<data> ins) {
21. mList= ins;
22. }
23.
24.
25.
26. public int getCount() {
27. return mList.size();
28. }
29.
30. public Object getItem(int position) {
31. try {
32. return mList.get(position);
33. } catch (Exception ex) {
34. return null;
35. }
36. }
37.
38. public long getItemId(int position) {
39. return position;
40. }
41.
42.
43. private View newView() {
44. ItemView view = new ItemView(mContext);
45. return view;
46. }
47.
48.
49.
50. public View getView(int position, View convertView, ViewGroup parent) {
51. //先从缓存里面读取
52. ViewHolder holder = null;
53. View view;
54.
55. if (viewMap.get(position) == null) {
56. view = newView();
57. holder = new ViewHolder();
58. holder.mTitle = (TextView)view.findViewById(R.id.ins_title);
59. holder.mTime = (TextView)view.findViewById(R.id.ins_time);
60. holder.mType = (TextView)view.findViewById(R.id.ins_from);
61. holder.mPic = (ImageView)view.findViewById(R.id.ins_small_pic);
62. final int p = position;
63. viewMap.put(position, view);
64. view.setTag(holder);
65. }else{
66. Log.e("MainActivity","position2 = "+position);
67. view = viewMap.get(position);
68. holder = (ViewHolder)view.getTag();
69. }
70.
71. data ins = mList.get(position);
72. holder.mTitle.setText(ins.getTitle());
73.
74.
75. holder.mPic.setVisibility(View.VISIBLE);
76.
77. // 异步加载图片
78. Drawable cachedImage = imageLoader.loadDrawable(ins.getPic(), holder.mPic,
79. new ImageCallback() {
80. public void imageLoaded(Drawable imageDrawable,
81. ImageView imageView, String imageUrl) {
82. imageView.setImageDrawable(imageDrawable);
83. }
84. });
85. if (cachedImage != null) {
86. holder.mPic.setImageDrawable(cachedImage);
87. }
88.
89. return view;
90.
91. }
92.
93.
94. static class ViewHolder{
95. ImageView mPic;
96. TextView mTitle;
97. TextView mTime;
98. TextView mType;
99. }
100.}
public class InsuranceItemAdapter extends BaseAdapter{
private ArrayList<data> mList;
private Context mContext;
// 异步加载图片的线程
private AsyncImageLoader imageLoader = new AsyncImageLoader();
//当前的缓存
private Map<Integer, View> viewMap = new HashMap<Integer, View>();

public InsuranceItemAdapter(Context context, ArrayList<data> ins) {
mContext = context;
mList= ins;
}

public InsuranceItemAdapter(Context context, ArrayList<data> ins,
Handler handler) {
mContext = context;
mList= ins;
}

public void setInsurance(ArrayList<data> ins) {
mList= ins;
}

public int getCount() {
return mList.size();
}

public Object getItem(int position) {
try {
return mList.get(position);
} catch (Exception ex) {
return null;
}
}

public long getItemId(int position) {
return position;
}

private View newView() {
ItemView view = new ItemView(mContext);
return view;
}

public View getView(int position, View convertView, ViewGroup parent) {
//先从缓存里面读取
ViewHolder holder = null;
View view;

if (viewMap.get(position) == null) {
view = newView();
holder = new ViewHolder();
holder.mTitle = (TextView)view.findViewById(R.id.ins_title);
holder.mTime = (TextView)view.findViewById(R.id.ins_time);
holder.mType = (TextView)view.findViewById(R.id.ins_from);
holder.mPic = (ImageView)view.findViewById(R.id.ins_small_pic);
final int p = position;
viewMap.put(position, view);
view.setTag(holder);
}else{
Log.e("MainActivity","position2 = "+position);
view = viewMap.get(position);
holder = (ViewHolder)view.getTag();
}

data ins = mList.get(position);
holder.mTitle.setText(ins.getTitle());

holder.mPic.setVisibility(View.VISIBLE);

// 异步加载图片
Drawable cachedImage = imageLoader.loadDrawable(ins.getPic(), holder.mPic,
new ImageCallback() {
public void imageLoaded(Drawable imageDrawable,
ImageView imageView, String imageUrl) {
imageView.setImageDrawable(imageDrawable);
}
});
if (cachedImage != null) {
holder.mPic.setImageDrawable(cachedImage);
}

return view;

}

static class ViewHolder{
ImageView mPic;
TextView mTitle;
TextView mTime;
TextView mType;
}
}

原理简单，不罗嗦了

本地缓存

就是先读取本地的数据，如果本地没有再从网络上获取

WebView中很简单，

Java代码
1.//优先缓存
2. mWebView.getSettings().setCacheMode(WebSettings.LOAD_CACHE_ELSE_NETWORK);
//优先缓存
mWebView.getSettings().setCacheMode(WebSettings.LOAD_CACHE_ELSE_NETWORK); 就能实现了。

其他地方我们就得自己写一个异步图片下载了，

Java代码
1.public class AsyncImageLoader {
2.
3. //SoftReference是软引用，是为了更好的为了系统回收变量
4. private static HashMap<String, SoftReference<Drawable>> imageCache;
5.
6. static {
7. imageCache = new HashMap<String, SoftReference<Drawable>>();
8. }
9.
10.
11. public AsyncImageLoader() {
12.
13. }
14. public Drawable loadDrawable(final String imageUrl,final ImageView imageView, final ImageCallback imageCallback){
15. if (imageCache.containsKey(imageUrl)) {
16. //从缓存中获取
17. SoftReference<Drawable> softReference = imageCache.get(imageUrl);
18. Drawable drawable = softReference.get();
19. if (drawable != null) {
20. return drawable;
21. }
22. }
23. final Handler handler = new Handler() {
24. public void handleMessage(Message message) {
25. imageCallback.imageLoaded((Drawable) message.obj, imageView,imageUrl);
26. }
27. };
28. //建立新一个新的线程下载图片
29. new Thread() {
30. @Override
31. public void run() {
32. Drawable drawable = null;
33. try {
34. drawable = ImageUtil.geRoundDrawableFromUrl(imageUrl, 20);
35. } catch (Exception e) {
36. e.printStackTrace();
37. }
38. imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));
39. Message message = handler.obtainMessage(0, drawable);
40. handler.sendMessage(message);
41. }
42. }.start();
43. return null;
44. }
45. //回调接口
46. public interface ImageCallback {
47. public void imageLoaded(Drawable imageDrawable,ImageView imageView, String imageUrl);
48. }
49.}
public class AsyncImageLoader {

//SoftReference是软引用，是为了更好的为了系统回收变量
private static HashMap<String, SoftReference<Drawable>> imageCache;

static {
imageCache = new HashMap<String, SoftReference<Drawable>>();
}

public AsyncImageLoader() {

}
public Drawable loadDrawable(final String imageUrl,final ImageView imageView, final ImageCallback imageCallback){
if (imageCache.containsKey(imageUrl)) {
//从缓存中获取
SoftReference<Drawable> softReference = imageCache.get(imageUrl);
Drawable drawable = softReference.get();
if (drawable != null) {
return drawable;
}
}
final Handler handler = new Handler() {
public void handleMessage(Message message) {
imageCallback.imageLoaded((Drawable) message.obj, imageView,imageUrl);
}
};
//建立新一个新的线程下载图片
new Thread() {
@Override
public void run() {
Drawable drawable = null;
try {
drawable = ImageUtil.geRoundDrawableFromUrl(imageUrl, 20);
} catch (Exception e) {
e.printStackTrace();
}
imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));
Message message = handler.obtainMessage(0, drawable);
handler.sendMessage(message);
}
}.start();
return null;
}
//回调接口
public interface ImageCallback {
public void imageLoaded(Drawable imageDrawable,ImageView imageView, String imageUrl);
}
}

当然我们也可以存数据库中，从数据库中优先获取，当没有网络的时候达到离线阅读的功能。

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