【移動應用開發(fā)技術】Android怎么實現照片墻效果

【移動應用開發(fā)技術】Android怎么實現照片墻效果

這篇文章主要介紹了Android怎么實現照片墻效果，具有一定借鑒價值，感興趣的朋友可以參考下，希望大家閱讀完這篇文章之后大有收獲，下面讓在下帶著大家一起了解一下。照片墻這種功能現在應該算是挺常見了，在很多應用中你都可以經常看到照片墻的身影。它的設計思路其實也非常簡單，用一個GridView控件當作“墻”，然后隨著GridView的滾動將一張張照片貼在“墻”上，這些照片可以是手機本地中存儲的，也可以是從網上下載的。制作類似于這種的功能的應用，有一個非常重要的問題需要考慮，就是圖片資源何時應該釋放。因為隨著GridView的滾動，加載的圖片可能會越來越多，如果沒有一種合理的機制對圖片進行釋放，那么當圖片達到一定上限時，程序就必然會崩潰。今天我們照片墻應用的實現，重點也是放在了如何防止由于圖片過多導致程序崩潰上面。主要的核心算法使用了Android中提供的LruCache類，這個類是3.1版本中提供的，如果你是在更早的Android版本中開發(fā)，則需要導入android-support-v4的jar包。關于LruCache用法的詳細講解，可以參考Android高效加載大圖、多圖解決方案有效避免程序OOM。那我們開始動手吧，新建一個Android項目，起名叫PhotoWallDemo，這里我使用的是Android4.0的API。第一個要考慮的問題就是，我們從哪兒去收集這么多的圖片呢？這里我從谷歌官方提供的Demo里將圖片源取了出來，我們就從這些網址中下載圖片，代碼如下所示：public

class

Images

{

public

final

static

String[]

imageThumbUrls

=

new

String[]

{

"/-55osAWw3x0Q/URquUtcFr5I/AAAAAAAAAbs/rWlj1RUKrYI/s160-c/A%252520Photographer.jpg",

"/--dq8niRp7W4/URquVgmXvgI/AAAAAAAAAbs/-gnuLQfNnBA/s160-c/A%252520Song%252520of%252520Ice%252520and%252520Fire.jpg",

"/-7qZeDtRKFKc/URquWZT1gOI/AAAAAAAAAbs/hqWgteyNXsg/s160-c/Another%252520Rockaway%252520Sunset.jpg",

"/--L0Km39l5J8/URquXHGcdNI/AAAAAAAAAbs/3ZrSJNrSomQ/s160-c/Antelope%252520Butte.jpg",

"/-8HO-4vIFnlw/URquZnsFgtI/AAAAAAAAAbs/WT8jViTF7vw/s160-c/Antelope%252520Hallway.jpg",

"/-WIuWgVcU3Qw/URqubRVcj4I/AAAAAAAAAbs/YvbwgGjwdIQ/s160-c/Antelope%252520Walls.jpg",

"/-UBmLbPELvoQ/URqucCdv0kI/AAAAAAAAAbs/IdNhr2VQoQs/s160-c/Apre%2525CC%252580s%252520la%252520Pluie.jpg",

"/-s-AFpvgSeew/URquc6dF-JI/AAAAAAAAAbs/Mt3xNGRUd68/s160-c/Backlit%252520Cloud.jpg",

"/-bvmif9a9YOQ/URquea3heHI/AAAAAAAAAbs/rcr6wyeQtAo/s160-c/Bee%252520and%252520Flower.jpg",

"/-n7mdm7I7FGs/URqueT_BT-I/AAAAAAAAAbs/9MYmXlmpSAo/s160-c/Bonzai%252520Rock%252520Sunset.jpg",

"/-4CN4X4t0M1k/URqufPozWzI/AAAAAAAAAbs/8wK41lg1KPs/s160-c/Caterpillar.jpg",

"/-rrFnVC8xQEg/URqufdrLBaI/AAAAAAAAAbs/s69WYy_fl1E/s160-c/Chess.jpg",

"/-WVpRptWH8Yw/URqugh-QmDI/AAAAAAAAAbs/E-MgBgtlUWU/s160-c/Chihuly.jpg",

"/-0BDXkYmckbo/URquhKFW84I/AAAAAAAAAbs/ogQtHCTk2JQ/s160-c/Closed%252520Door.jpg",

"/-PyggXXZRykM/URquh-kVvoI/AAAAAAAAAbs/hFtDwhtrHHQ/s160-c/Colorado%252520River%252520Sunset.jpg",

"/-ZAs4dNZtALc/URquikvOCWI/AAAAAAAAAbs/DXz4h4dll1Y/s160-c/Colors%252520of%252520Autumn.jpg",

"/-GztnWEIiMz8/URqukVCU7bI/AAAAAAAAAbs/jo2Hjv6MZ6M/s160-c/Countryside.jpg",

"/-bEg9EZ9QoiM/URquklz3FGI/AAAAAAAAAbs/UUuv8Ac2BaE/s160-c/Death%252520Valley%252520-%252520Dunes.jpg",

"/-ijQJ8W68tEE/URqulGkvFEI/AAAAAAAAAbs/zPXvIwi_rFw/s160-c/Delicate%252520Arch.jpg",

"/-Oh8mMy2ieng/URqullDwehI/AAAAAAAAAbs/TbdeEfsaIZY/s160-c/Despair.jpg",

"/-gl0y4UiAOlk/URqumC_KjBI/AAAAAAAAAbs/PM1eT7dn4oo/s160-c/Eagle%252520Fall%252520Sunrise.jpg",

"/-hYYHd2_vXPQ/URqumtJa9eI/AAAAAAAAAbs/wAalXVkbSh0/s160-c/Electric%252520Storm.jpg",

"/-PyY_yiyjPTo/URqunUOhHFI/AAAAAAAAAbs/azZoULNuJXc/s160-c/False%252520Kiva.jpg",

"/-PYvLVdvXywk/URqunwd8hfI/AAAAAAAAAbs/qiMwgkFvf6I/s160-c/Fitzgerald%252520Streaks.jpg",

"/-KIR_UobIIqY/URquoCZ9SlI/AAAAAAAAAbs/Y4d4q8sXu4c/s160-c/Foggy%252520Sunset.jpg",

"/-9lzOk_OWZH0/URquoo4xYoI/AAAAAAAAAbs/AwgzHtNVCwU/s160-c/Frantic.jpg",

"/-0X3JNaKaz48/URqupH78wpI/AAAAAAAAAbs/lHXxu_zbH8s/s160-c/Golden%252520Gate%252520Afternoon.jpg",

"/-95sb5ag7ABc/URqupl95RDI/AAAAAAAAAbs/g73R20iVTRA/s160-c/Golden%252520Gate%252520Fog.jpg",

"/-JB9v6rtgHhk/URqup21F-zI/AAAAAAAAAbs/64Fb8qMZWXk/s160-c/Golden%252520Grass.jpg",

"/-EIBGfnuLtII/URquqVHwaRI/AAAAAAAAAbs/FA4McV2u8VE/s160-c/Grand%252520Teton.jpg",

"/-WoMxZvmN9nY/URquq1v2AoI/AAAAAAAAAbs/grj5uMhL6NA/s160-c/Grass%252520Closeup.jpg",

"/-6hZiEHXx64Q/URqurxvNdqI/AAAAAAAAAbs/kWMXM3o5OVI/s160-c/Green%252520Grass.jpg",

"/-6LVb9OXtQ60/URquteBFuKI/AAAAAAAAAbs/4F4kRgecwFs/s160-c/Hanging%252520Leaf.jpg",

"/-zAvf__52ONk/URqutT_IuxI/AAAAAAAAAbs/D_bcuc0thoU/s160-c/Highway%2525201.jpg",

"/-H4SrUg615rA/URquuL27fXI/AAAAAAAAAbs/4aEqJfiMsOU/s160-c/Horseshoe%252520Bend%252520Sunset.jpg",

"/-JhFi4fb_Pqw/URquuX-QXbI/AAAAAAAAAbs/IXpYUxuweYM/s160-c/Horseshoe%252520Bend.jpg",

"/-UGgssvFRJ7g/URquueyJzGI/AAAAAAAAAbs/yYIBlLT0toM/s160-c/Into%252520the%252520Blue.jpg",

"/-CH7KoupI7uI/URquu0FF__I/AAAAAAAAAbs/R7GDmI7v_G0/s160-c/Jelly%252520Fish%2525202.jpg",

"/-pwuuw6yhg8U/URquvPxR3FI/AAAAAAAAAbs/VNGk6f-tsGE/s160-c/Jelly%252520Fish%2525203.jpg",

"/-GoUQVw1fnFw/URquv6xbC0I/AAAAAAAAAbs/zEUVTQQ43Zc/s160-c/Kauai.jpg",

"/-8QdYYQEpYjw/URquwvdh88I/AAAAAAAAAbs/cktDy-ysfHo/s160-c/Kyoto%252520Sunset.jpg",

"/-vPeekyDjOE0/URquwzJ28qI/AAAAAAAAAbs/qxcyXULsZrg/s160-c/Lake%252520Tahoe%252520Colors.jpg",

"/-xBPxWpD4yxU/URquxWHk8AI/AAAAAAAAAbs/ARDPeDYPiMY/s160-c/Lava%252520from%252520the%252520Sky.jpg",

"/-897VXrJB6RE/URquxxxd-5I/AAAAAAAAAbs/j-Cz4T4YvIw/s160-c/Leica%25252050mm%252520Summilux.jpg",

"/-qSJ4D4iXzGo/URquyDWiJ1I/AAAAAAAAAbs/k2pBXeWehOA/s160-c/Leica%25252050mm%252520Summilux.jpg",

"/-dwlPg83vzLg/URquylTVuFI/AAAAAAAAAbs/G6SyQ8b4YsI/s160-c/Leica%252520M8%252520%252528Front%252529.jpg",

"/-R3_EYAyJvfk/URquzQBv8eI/AAAAAAAAAbs/b9xhpUM3pEI/s160-c/Light%252520to%252520Sand.jpg",

"/-fHY5h77QPi0/URqu0Cp4J1I/AAAAAAAAAbs/0lG6m94Z6vM/s160-c/Little%252520Bit%252520of%252520Paradise.jpg",

"/-TzF_LwrCnRM/URqu0RddPOI/AAAAAAAAAbs/gaj2dLiuX0s/s160-c/Lone%252520Pine%252520Sunset.jpg",

"/-4HdpJ4_DXU4/URqu046dJ9I/AAAAAAAAAbs/eBOodtk2_uk/s160-c/Lonely%252520Rock.jpg",

"/-erbF--z-W4s/URqu1ajSLkI/AAAAAAAAAbs/xjDCDO1INzM/s160-c/Longue%252520Vue.jpg",

"/-0CXJRdJaqvc/URqu1opNZNI/AAAAAAAAAbs/PFB2oPUU7Lk/s160-c/Look%252520Me%252520in%252520the%252520Eye.jpg",

"/-D_5lNxnDN6g/URqu2Tk7HVI/AAAAAAAAAbs/p0ddca9W__Y/s160-c/Lost%252520in%252520a%252520Field.jpg",

"/-flsqwMrIk2Q/URqu24PcmjI/AAAAAAAAAbs/5ocIH85XofM/s160-c/Marshall%252520Beach%252520Sunset.jpg",

"/-Y4lgryEVTmU/URqu28kG3gI/AAAAAAAAAbs/OjXpekqtbJ4/s160-c/Mono%252520Lake%252520Blue.jpg",

"/-AaHAJPmcGYA/URqu3PIldHI/AAAAAAAAAbs/lcTqk1SIcRs/s160-c/Monument%252520Valley%252520Overlook.jpg",

"/-vKxfdQ83dQA/URqu31Yq_BI/AAAAAAAAAbs/OUoGk_2AyfM/s160-c/Moving%252520Rock.jpg",

"/-CG62QiPpWXg/URqu4ia4vRI/AAAAAAAAAbs/0YOdqLAlcAc/s160-c/Napali%252520Coast.jpg",

"/-wdGrP5PMmJQ/URqu5PZvn7I/AAAAAAAAAbs/m0abEcdPXe4/s160-c/One%252520Wheel.jpg",

"/-6WS5DoCGuOA/URqu5qx1UgI/AAAAAAAAAbs/giMw2ixPvrY/s160-c/Open%252520Sky.jpg",

"/-u8EHKj8G8GQ/URqu55sM6yI/AAAAAAAAAbs/lIXX_GlTdmI/s160-c/Orange%252520Sunset.jpg",

"/-74Z5qj4bTDE/URqu6LSrJrI/AAAAAAAAAbs/XzmVkw90szQ/s160-c/Orchid.jpg",

"/-lEQE4h7TePE/URqu6t_lSkI/AAAAAAAAAbs/zvGYKOea_qY/s160-c/Over%252520there.jpg",

"/-cauH-53JH2M/URqu66v_USI/AAAAAAAAAbs/EucwwqclfKQ/s160-c/Plumes.jpg",

"/-eDLT2jHDoy4/URqu7axzkAI/AAAAAAAAAbs/iVZE-xJ7lZs/s160-c/Rainbokeh.jpg",

"/-j1NLqEFIyco/URqu8L1CGcI/AAAAAAAAAbs/aqZkgX66zlI/s160-c/Rainbow.jpg",

"/-DRnqmK0t4VU/URqu8XYN9yI/AAAAAAAAAbs/LgvF_592WLU/s160-c/Rice%252520Fields.jpg",

"/-hwh2v3EOGcQ/URqu8qOaKwI/AAAAAAAAAbs/IljRJRnbJGw/s160-c/Rockaway%252520Fire%252520Sky.jpg",

"/-wjV6FQk7tlk/URqu9jCQ8sI/AAAAAAAAAbs/RyYUpdo-c9o/s160-c/Rockaway%252520Flow.jpg",

"/-6cAXNfo7D20/URqu-BdzgPI/AAAAAAAAAbs/OmsYllzJqwo/s160-c/Rockaway%252520Sunset%252520Sky.jpg",

"/-sl8fpGPS-RE/URqu_BOkfgI/AAAAAAAAAbs/Dg2Fv-JxOeg/s160-c/Russian%252520Ridge%252520Sunset.jpg",

"/-gVtY36mMBIg/URqu_q91lkI/AAAAAAAAAbs/3CiFMBcy5MA/s160-c/Rust%252520Knot.jpg",

"/-GHeImuHqJBE/URqu_FKfVLI/AAAAAAAAAbs/axuEJeqam7Q/s160-c/Sailing%252520Stones.jpg",

"/-hBbYZjTOwGc/URqu_ycpIrI/AAAAAAAAAbs/nAdJUXnGJYE/s160-c/Seahorse.jpg",

"/-Iwi6-i6IexY/URqvAYZHsVI/AAAAAAAAAbs/5ETWl4qXsFE/s160-c/Shinjuku%252520Street.jpg",

"/-amhnySTM_MY/URqvAlb5KoI/AAAAAAAAAbs/pFCFgzlKsn0/s160-c/Sierra%252520Heavens.jpg",

"/-dJgjepFrYSo/URqvBVJZrAI/AAAAAAAAAbs/v-F5QWpYO6s/s160-c/Sierra%252520Sunset.jpg",

"/-Z4zGiC5nWdc/URqvBdEwivI/AAAAAAAAAbs/ZRZR1VJ84QA/s160-c/Sin%252520Lights.jpg",

"/-_0cYiWW8ccY/URqvBz3iM4I/AAAAAAAAAbs/9N_Wq8MhLTY/s160-c/Starry%252520Lake.jpg",

"/-A9LMoRyuQUA/URqvCYx_JoI/AAAAAAAAAbs/s7sde1Bz9cI/s160-c/Starry%252520Night.jpg",

"/-KtLJ3k858eY/URqvC_2h_bI/AAAAAAAAAbs/zzEBImwDA_g/s160-c/Stream.jpg",

"/-dFB7Lad6RcA/URqvDUftwWI/AAAAAAAAAbs/BrhoUtXTN7o/s160-c/Strip%252520Sunset.jpg",

"/-at6apgFiN20/URqvDyffUZI/AAAAAAAAAbs/clABCx171bE/s160-c/Sunset%252520Hills.jpg",

"/-7-EHhtQthII/URqvEYTk4vI/AAAAAAAAAbs/QSJZoB3YjVg/s160-c/Tenaya%252520Lake%2525202.jpg",

"/-8MrjV_a-Pok/URqvFC5repI/AAAAAAAAAbs/9inKTg9fbCE/s160-c/Tenaya%252520Lake.jpg",

"/-B1HW-z4zwao/URqvFWYRwUI/AAAAAAAAAbs/8Peli53Bs8I/s160-c/The%252520Cave%252520BW.jpg",

"/-PO4E-xZKAnQ/URqvGRqjYkI/AAAAAAAAAbs/42nyADFsXag/s160-c/The%252520Fisherman.jpg",

"/-iLyZlzfdy7s/URqvG0YScdI/AAAAAAAAAbs/1J9eDKmkXtk/s160-c/The%252520Night%252520is%252520Coming.jpg",

"/-G-k7YkkUco0/URqvHhah7fI/AAAAAAAAAbs/_taQQG7t0vo/s160-c/The%252520Road.jpg",

"/-h-ALJt7kSus/URqvIThqYfI/AAAAAAAAAbs/ejiv35olWS8/s160-c/Tokyo%252520Heights.jpg",

"/-Hy9k-TbS7xg/URqvIjQMOxI/AAAAAAAAAbs/RSpmmOATSkg/s160-c/Tokyo%252520Highway.jpg",

"/-83oOvMb4OZs/URqvJL0T7lI/AAAAAAAAAbs/c5TECZ6RONM/s160-c/Tokyo%252520Smog.jpg",

"/-FB-jfgREEfI/URqvJI3EXAI/AAAAAAAAAbs/XfyweiRF4v8/s160-c/Tufa%252520at%252520Night.jpg",

"/-vngKD5Z1U8w/URqvJUCEgPI/AAAAAAAAAbs/ulxCMVcU6EU/s160-c/Valley%252520Sunset.jpg",

"/-DOz5I2E2oMQ/URqvKMND1kI/AAAAAAAAAbs/Iqf0IsInleo/s160-c/Windmill%252520Sunrise.jpg",

"/-biyiyWcJ9MU/URqvKculiAI/AAAAAAAAAbs/jyPsCplJOpE/s160-c/Windmill.jpg",

"/-PDT167_xRdA/URqvK36mLcI/AAAAAAAAAbs/oi2ik9QseMI/s160-c/Windmills.jpg",

"/-kI_QdYx7VlU/URqvLXCB6gI/AAAAAAAAAbs/N31vlZ6u89o/s160-c/Yet%252520Another%252520Rockaway%252520Sunset.jpg",

"/-e9NHZ5k5MSs/URqvMIBZjtI/AAAAAAAAAbs/1fV810rDNfQ/s160-c/Yosemite%252520Tree.jpg",

};

}圖片源已經有了，現在我們就該考慮在哪里放置這些圖片了。新建或打開activity_main.xml作為程序的主布局，加入如下代碼：<LinearLayout

xmlns:android="/apk/res/android"

xmlns:tools="/tools"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

>

<GridView

android:id="@+id/photo_wall"

android:layout_width="match_parent"

android:layout_height="wrap_content"

android:columnWidth="90dip"

android:stretchMode="columnWidth"

android:numColumns="auto_fit"

android:verticalSpacing="10dip"

android:gravity="center"

></GridView>

</LinearLayout>可以看到，我們在這個布局文件中僅加入了一個GridView，這也就是我們程序中的“墻”，所有的圖片都將貼在這個“墻”上。接著我們定義GridView中每一個子View的布局，新建一個photo_layout.xml布局，加入如下代碼：<RelativeLayout

xmlns:android="/apk/res/android"

xmlns:tools="/tools"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

>

<ImageView

android:id="@+id/photo"

android:layout_width="90dip"

android:layout_height="90dip"

android:src="@drawable/empty_photo"

android:layout_centerInParent="true"

/>

</RelativeLayout>在每一個子View中我們就簡單使用了一個ImageView來顯示一張圖片。這樣所有的布局就已經定義好了。接下來新建PhotoWallAdapter做為GridView的適配器，代碼如下所示：public

class

PhotoWallAdapter

extends

ArrayAdapter<String>

implements

OnScrollListener

{

/**

*

記錄所有正在下載或等待下載的任務。

*/

private

Set<BitmapWorkerTask>

taskCollection;

/**

*

圖片緩存技術的核心類，用于緩存所有下載好的圖片，在程序內存達到設定值時會將最少最近使用的圖片移除掉。

*/

private

LruCache<String,

Bitmap>

mMemoryCache;

/**

*

GridView的實例

*/

private

GridView

mPhotoWall;

/**

*

第一張可見圖片的下標

*/

private

int

mFirstVisibleItem;

/**

*

一屏有多少張圖片可見

*/

private

int

mVisibleItemCount;

/**

*

記錄是否剛打開程序，用于解決進入程序不滾動屏幕，不會下載圖片的問題。

*/

private

boolean

isFirstEnter

=

true;

public

PhotoWallAdapter(Context

context,

int

textViewResourceId,

String[]

objects,

GridView

photoWall)

{

super(context,

textViewResourceId,

objects);

mPhotoWall

=

photoWall;

taskCollection

=

new

HashSet<BitmapWorkerTask>();

//

獲取應用程序最大可用內存

int

maxMemory

=

(int)

Runtime.getRuntime().maxMemory();

int

cacheSize

=

maxMemory

/

8;

//

設置圖片緩存大小為程序最大可用內存的1/8

mMemoryCache

=

new

LruCache<String,

Bitmap>(cacheSize)

{

@Override

protected

int

sizeOf(String

key,

Bitmap

bitmap)

{

return

bitmap.getByteCount();

}

};

mPhotoWall.setOnScrollListener(this);

}

@Override

public

View

getView(int

position,

View

convertView,

ViewGroup

parent)

{

final

String

url

=

getItem(position);

View

view;

if

(convertView

==

null)

{

view

=

LayoutInflater.from(getContext()).inflate(R.layout.photo_layout,

null);

}

else

{

view

=

convertView;

}

final

ImageView

photo

=

(ImageView)

view.findViewById(R.id.photo);

//

給ImageView設置一個Tag，保證異步加載圖片時不會亂序

photo.setTag(url);

setImageView(url,

photo);

return

view;

}

/**

*

給ImageView設置圖片。首先從LruCache中取出圖片的緩存，設置到ImageView上。如果LruCache中沒有該圖片的緩存，

*

就給ImageView設置一張默認圖片。

*

*

@param

imageUrl

*

圖片的URL地址，用于作為LruCache的鍵。

*

@param

imageView

*

用于顯示圖片的控件。

*/

private

void

setImageView(String

imageUrl,

ImageView

imageView)

{

Bitmap

bitmap

=

getBitmapFromMemoryCache(imageUrl);

if

(bitmap

!=

null)

{

imageView.setImageBitmap(bitmap);

}

else

{

imageView.setImageResource(R.drawable.empty_photo);

}

}

/**

*

將一張圖片存儲到LruCache中。

*

*

@param

key

*

LruCache的鍵，這里傳入圖片的URL地址。

*

@param

bitmap

*

LruCache的鍵，這里傳入從網絡上下載的Bitmap對象。

*/

public

void

addBitmapToMemoryCache(String

key,

Bitmap

bitmap)

{

if

(getBitmapFromMemoryCache(key)

==

null)

{

mMemoryCache.put(key,

bitmap);

}

}

/**

*

從LruCache中獲取一張圖片，如果不存在就返回null。

*

*

@param

key

*

LruCache的鍵，這里傳入圖片的URL地址。

*

@return

對應傳入鍵的Bitmap對象，或者null。

*/

public

Bitmap

getBitmapFromMemoryCache(String

key)

{

return

mMemoryCache.get(key);

}

@Override

public

void

onScrollStateChanged(AbsListView

view,

int

scrollState)

{

//

僅當GridView靜止時才去下載圖片，GridView滑動時取消所有正在下載的任務

if

(scrollState

==

SCROLL_STATE_IDLE)

{

loadBitmaps(mFirstVisibleItem,

mVisibleItemCount);

}

else

{

cancelAllTasks();

}

}

@Override

public

void

onScroll(AbsListView

view,

int

firstVisibleItem,

int

visibleItemCount,

int

totalItemCount)

{

mFirstVisibleItem

=

firstVisibleItem;

mVisibleItemCount

=

visibleItemCount;

//

下載的任務應該由onScrollStateChanged里調用，但首次進入程序時onScrollStateChanged并不會調用，

//

因此在這里為首次進入程序開啟下載任務。

if

(isFirstEnter

&&

visibleItemCount

>

0)

{

loadBitmaps(firstVisibleItem,

visibleItemCount);

isFirstEnter

=

false;

}

}

/**

*

加載Bitmap對象。此方法會在LruCache中檢查所有屏幕中可見的ImageView的Bitmap對象，

*

如果發(fā)現任何一個ImageView的Bitmap對象不在緩存中，就會開啟異步線程去下載圖片。

*

*

@param

firstVisibleItem

*

第一個可見的ImageView的下標

*

@param

visibleItemCount

*

屏幕中總共可見的元素數

*/

private

void

loadBitmaps(int

firstVisibleItem,

int

visibleItemCount)

{

try

{

for

(int

i

=

firstVisibleItem;

i

<

firstVisibleItem

+

visibleItemCount;

i++)

{

String

imageUrl

=

Images.imageThumbUrls[i];

Bitmap

bitmap

=

getBitmapFromMemoryCache(imageUrl);

if

(bitmap

==

null)

{

BitmapWorkerTask

task

=

new

BitmapWorkerTask();

taskCollection.add(task);

task.execute(imageUrl);

}

else

{

ImageView

imageView

=

(ImageView)

mPhotoWall.findViewWithTag(imageUrl);

if

(imageView

!=

null

&&

bitmap

!=

null)

{

imageView.setImageBitmap(bitmap);

}

}

}

}

catch

(Exception

e)

{

e.printStackTrace();

}

}

/**

*

取消所有正在下載或等待下載的任務。

*/

public

void

cancelAllTasks()

{

if

(taskCollection

!=

null)

{

for

(BitmapWorkerTask

task

:

taskCollection)

{

task.cancel(false);

}

}

}

/**

*

異步下載圖片的任務。

*

*

@author

guolin

*/

class

BitmapWorkerTask

extends

AsyncTask<String,

Void,

Bitmap>

{

/**

*

圖片的URL地址

*/

private

String

imageUrl;

@Override

protected

Bitmap

doInBackground(String...

params)

{

imageUrl

=

params[0];

//

在后臺開始下載圖片

Bitmap

bitmap

=

downloadBitmap(params[0]);

if

(bitmap

!=

null)

{

//

圖片下載完成后緩存到LrcCache中

addBitmapToMemoryCache(params[0],

bitmap);

}

return

bitmap;

}

@Override

protected

void

onPostExecute(Bitmap

bitmap)

{

super.onPostExecute(bitmap);

//

根據Tag找到相應的ImageView控件，將下載好的圖片顯示出來。

ImageView

imageView

=

(ImageView)

mPhotoWall.findViewWithTag(imageUrl);

if

(imageView

!=

null

&&

bitmap

!=

null)

{

imageView.setImageBitmap(bitmap);

}

taskCollection.remove(this);

}

/**

*

建立HTTP請求，并獲取Bitmap對象。

*

*

@param

imageUrl

*

圖片的URL地址

*

@return

解析后的Bitmap對象

*/

private

Bitmap

downloadBitmap(String

imageUrl)

{

Bitmap

bitmap

=

null;

HttpURLConnection

con

=

null;

try

{

URL

url

=

new

URL(imageUrl);

con

=

(HttpURLConnection)

url.openConnection();

con.setConnectTimeout(5

*

1000);

con.setReadTimeout(10

*

1000);

bitmap

=

BitmapFactory.decodeStream(con.getInputStream());

}

catch

(Exception

e)

{

e.printStackTrace();

}

finally

{

if

(