【移动应用开发技术】Android 实现异步加载图片

【移动应用开发技术】Android实现异步加载图片

麦洛开通博客以来,有一段时间没有更新博文了.主要是麦洛这段时间因项目开发实在太忙了.今天周六还在公司加班,苦逼程序猿都是这样生活的.今天在做项目的时候,有一个实现异步加载图片的功能,虽然比较简单但还是记录一下吧.因为麦洛之前实现异步加载图片都是使用了AsynTask这个API,继续这个类,实现起来非常简单也很方便.在doInBackground()方法里实现下载逻辑.具体实现如下实现逻辑是:先从内存中读取,如果内存中有这张图片,则直接使用;如果内存没有再到sdcard上读取,如果有则显示;如果sdcard上还没有则到网络上读取.内存中开启缓存是参考了网上的实现.麦洛在这里非常感谢喜欢分享的程序猿们.public

class

ImageDownloader

extends

AsyncTask<String,

Integer,

Object>

{

private

static

final

String

TAG

=

"ImageDownloader";

//

为了加快速度，在内存中开启缓存（主要应用于重复图片较多时，或者同一个图片要多次被访问，比如在ListView时来回滚动）

private

Map<String,

SoftReference<Drawable>>

p_w_picpathCache

=

new

HashMap<String,

SoftReference<Drawable>>();

/**

*

显示图片的控件

*/

private

ImageView

mImageView;

public

ImageDownloader(ImageView

p_w_picpath)

{

mImageView

=

p_w_picpath;

}

@Override

protected

void

onPreExecute()

{

super.onPreExecute();

}

@Override

protected

Object

doInBackground(String...

params)

{

//

Log.i("ImageDownloader",

"loading

p_w_picpath...");

String

url

=

params[0];

Drawable

drawable

=

null;

try

{

if

(!"".equals(url)

&&

url

!=

null)

{

String

fileName

=

url.hashCode()+".jpg";

//

如果缓存过就从缓存中取出数据

if

(p_w_picpathCache.containsKey(fileName))

{

SoftReference<Drawable>

softReference

=

p_w_picpathCache.get(fileName);

drawable

=

softReference.get();

if

(drawable

!=

null)

{

return

drawable;

}

}

File

dir

=

new

File(FileConstant.IMAGE_FILE_PATH);

if

(!dir.exists())

{

boolean

m

=

dir.mkdirs();

}

File

file

=

new

File(dir,

fileName);

if

(file.exists()

&&

file.length()

>

0)

{

Log.i(TAG,

"load

p_w_picpath

from

sd

card");

//

如果文件存在则直接读取sdcard

drawable

=

readFromSdcard(file);

}

else

{

//file.createNewFile();

Log.i(TAG,

"load

p_w_picpath

from

network");

URL

p_w_picpathUrl

=

new

URL(url);

//

写入sdcard

if

(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED))

{

saveImageFile(p_w_picpathUrl,

file);

drawable

=

Drawable.createFromStream(new

FileInputStream(file),

fileName);

}else{

//直接从流读取

drawable

=

Drawable.createFromStream(p_w_picpathUrl.openStream(),

fileName);

}

}

if(drawable!=null){

//保存在缓存中

p_w_picpathCache.put(fileName,

new

SoftReference<Drawable>(drawable));

}

}

}

catch

(Exception

e)

{

e.printStackTrace();

}

return

drawable;

}

/**

*

save

p_w_picpath*/

private

void

saveImageFile(URL

url,

File

file)

{

FileOutputStream

out

=

null;

InputStream

in

=

null;

try

{

file.deleteOnExit();

out

=

new

FileOutputStream(file);

in

=

url.openStream();

byte[]

buf

=

new

byte[1024];

int

len

=

-1;

while((len

=

in.read(buf))!=-1){

out.write(buf,

0,

len);

out.flush();

}

}

catch

(Exception

e)

{

e.printStackTrace();

}

finally

{

if(out!=null){

try

{

out.close();

}

catch

(IOException

e)

{

e.printStackTrace();

}

}

if(in!=null){

try

{

in.close();

}

catch

(IOException

e)

{

e.printStackTrace();

}

}

}

}

/**

*

从sdcard中获取图片*/

private

Drawable

readFromSdcard(File

file)

throws

Exception

{

FileInputStream

in

=

new

FileInputStream(file);

return

Drawable.createFromStream(in,

file.getName());

}

@Override

protected

void

onPostExecute(Object

result)

{

super.onPostExecute(result);

Drawable

drawable

=

(Drawable)

result;

if

(mImageView

!=

null

&&

drawable

!=

null)

{

mImageView.setBackgroundDrawable(drawable);

}

}

@Override

protected

void

onProgressUpdate(Integer...

values)

{

super.onProgressUpdate(values);

}

@Override

protected

void

onCancelled()

{

super.onCancelled();

}

}使用时:ImageDownloader

loader

=

new

ImageDownloader(p_w_picpathView);

loader.execute(url);其实这样的话,还有一些隐患的,就是说这个类实现还是有些问题的.比如每次都在p_w_picpathView中设置网络上的图片时,其实是没有使用到这个类里面的内存缓存的,就是p_w_picpathCacheMap<String,

SoftReference<Drawable>>

p_w_picpathCache

=

new

HashMap<String,

SoftReference<Drawable>>();因为每次设置p_w_picpathView的时候,都是new了一个ImageDownloader的对象.所以每个ImageDownloader对象里面都是独立的一个p_w_picpathCache.

另外,AsynTask也是一个线程.而每次使用都开一个线程来load图片,对线程个数没有进行显示,毕竟线程数目还是有限制的.所以麦洛今天发现了这个问题,于是参考了别人的实现,使用了线程池,实现逻辑也上面的代码一样,先从内存读取,如果没有到sdcard读取,如果还是没有,则是网络读取;实现没有使用AsynTask,具体代码如下:/**

*

异步加载图片，并将图片设置到ImageView控件中*/public

class

ImageDownloader

extends

AsyncTask<String,

Integer,

Object>

{

private

static

final

String

TAG

=

"ImageDownloader";

//

为了加快速度，在内存中开启缓存（主要应用于重复图片较多时，或者同一个图片要多次被访问，比如在ListView时来回滚动）

private

Map<String,

SoftReference<Drawable>>

p_w_picpathCache

=

new

HashMap<String,

SoftReference<Drawable>>();

/**

*

显示图片的控件

*/

private

ImageView

mImageView;

public

ImageDownloader(ImageView

p_w_picpath)

{

mImageView

=

p_w_picpath;

}

@Override

protected

void

onPreExecute()

{

super.onPreExecute();

}

@Override

protected

Object

doInBackground(String...

params)

{

//

Log.i("ImageDownloader",

"loading

p_w_picpath...");

String

url

=

params[0];

Drawable

drawable

=

null;

try

{

if

(!"".equals(url)

&&

url

!=

null)

{

String

fileName

=

url.hashCode()+".jpg";

//

如果缓存过就从缓存中取出数据

if

(p_w_picpathCache.containsKey(fileName))

{

SoftReference<Drawable>

softReference

=

p_w_picpathCache.get(fileName);

drawable

=

softReference.get();

if

(drawable

!=

null)

{

return

drawable;

}

}

File

dir

=

new

File(FileConstant.IMAGE_FILE_PATH);

if

(!dir.exists())

{

boolean

m

=

dir.mkdirs();

}

File

file

=

new

File(dir,

fileName);

if

(file.exists()

&&

file.length()

>

0)

{

Log.i(TAG,

"load

p_w_picpath

from

sd

card");

//

如果文件存在则直接读取sdcard

drawable

=

readFromSdcard(file);

}

else

{

//file.createNewFile();

Log.i(TAG,

"load

p_w_picpath

from

network");

URL

p_w_picpathUrl

=

new

URL(url);

//

写入sdcard

if

(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED))

{

saveImageFile(p_w_picpathUrl,

file);

drawable

=

Drawable.createFromStream(new

FileInputStream(file),

fileName);

}else{

//直接从流读取

drawable

=

Drawable.createFromStream(p_w_picpathUrl.openStream(),

fileName);

}

}

if(drawable!=null){

//保存在缓存中

p_w_picpathCache.put(fileName,

new

SoftReference<Drawable>(drawable));

}

}

}

catch

(Exception

e)

{

e.printStackTrace();

}

return

drawable;

}

/**

*

save

p_w_picpath*/

private

void

saveImageFile(URL

url,

File

file)

{

FileOutputStream

out

=

null;

InputStream

in

=

null;

try

{

file.deleteOnExit();

out

=

new

FileOutputStream(file);

in

=

url.openStream();

byte[]

buf

=

new

byte[1024];

int

len

=

-1;

while((len

=

in.read(buf))!=-1){

out.write(buf,

0,

len);

out.flush();

}

}

catch

(Exception

e)

{

e.printStackTrace();

}

finally

{

if(out!=null){

try

{

out.close();

}

catch

(IOException

e)

{

e.printStackTrace();

}

}

if(in!=null){

try

{

in.close();

}

catch

(IOException

e)

{

e.printStackTrace();

}

}

}

}

/**

*

从sdcard中获取图片

*/

private

Drawable

readFromSdcard(File

file)

throws

Exception

{

FileInputStream

in

=

new

FileInputStream(file);

return

Drawable.createFromStream(in,

file.getName());

}

@Override

protected

void

onPostExecute(Object

result)

{

super.onPostExecute(result);

Drawable

drawable

=

(Drawable)

result;

if

(mImageView

!=

null

&&

drawable

!=

null)

{