picasso是Square公司開源的一個Android圖形緩存庫,地址http://square.github.io/picasso/,可以實現圖片下載和緩存功能。僅僅只需要一行代碼就能完全實現圖片的異步加載:
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Api看起來非常獨特,是吧。
Picasso不僅實現了圖片異步加載的功能,還解決了android中加載圖片時需要解決的一些常見問題:
1.在adapter中需要取消已經不在視野範圍的ImageView圖片資源的加載,否則會導致圖片錯位,Picasso已經解決了這個問題。
2.使用複雜的圖片壓縮轉換來儘可能的減少內存消耗
3.自帶內存和硬盤二級緩存功能
特性以及示例代碼:
ADAPTER 中的下載:Adapter的重用會被自動檢測到,Picasso會取消上次的加載
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@Override
public void getView(int position, View convertView, ViewGroup parent) { SquaredImageView
view = (SquaredImageView) convertView; if
(view == null )
{ view
= new
SquaredImageView(context); } String
url = getItem(position); Picasso. with (context).load(url).into(view); } |
圖片轉換:轉換圖片以適應佈局大小並減少內存佔用
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Picasso. with (context) .load(url) .resize(50,
50) .centerCrop() .into(imageView); |
你還可以自定義轉換:
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public
class CropSquareTransformation implements Transformation { @Override
public Bitmap transform(Bitmap source) { int
size = Math.min(source.getWidth(), source.getHeight()); int
x = (source.getWidth() - size) / 2; int
y = (source.getHeight() - size) / 2; Bitmap
result = Bitmap.createBitmap(source, x, y, size, size); if
(result != source) { source.recycle(); } return
result; } @Override
public String key() { return
"square()" ;
} } |
將CropSquareTransformation 的對象傳遞給transform 方法即可。
Place holders-空白或者錯誤佔位圖片:picasso提供了兩種佔位圖片,未加載完成或者加載發生錯誤的時需要一張圖片作爲提示。
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Picasso. with (context) .load(url) .placeholder(R.drawable.user_placeholder) .error(R.drawable.user_placeholder_error) .into(imageView); |
如果加載發生錯誤會重複三次請求,三次都失敗纔會顯示erro Place holder
資源文件的加載:除了加載網絡圖片picasso還支持加載Resources, assets, files, content providers中的資源文件。
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Picasso. with (context).load(R.drawable.landing_screen).into(imageView1); Picasso. with (context).load( new
File(...)).into(imageView2); |
下面是picasso源碼的解析(不看不影響使用)
Cache,緩存類
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this .map
= new
LinkedHashMap<String, Bitmap>(0, 0.75f, true ); |
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private
void trimToSize(int maxSize) { while
( true )
{ String
key; Bitmap
value; synchronized
( this )
{ if
(size < 0 || (map.isEmpty() && size != 0)) { throw
new
IllegalStateException(getClass().getName() +
".sizeOf()
is reporting inconsistent results!" ); } if
(size <= maxSize || map.isEmpty()) { break ; } Map.Entry<String,
Bitmap> toEvict = map.entrySet().iterator() .next(); key
= toEvict.getKey(); value
= toEvict.getValue(); map.remove(key); size
-= Utils.getBitmapBytes(value); evictionCount++; } } } |
Request,操作封裝類
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public
interface Transformation { /** *
Transform the source bitmap into a new bitmap. If you create a new bitmap instance, you must *
call {@link android.graphics.Bitmap#recycle()} on {@code source}. You may return the original *
if no transformation is required. */ Bitmap
transform(Bitmap source); /** *
Returns a unique key for the transformation, used for caching purposes. If the transformation *
has parameters (e.g. size, scale factor, etc) then these should be part of the key. */ String
key(); } |
當操作封裝好以後,會將Request傳到另一個結構中Action。
Action
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@Override public
void complete(Bitmap result, Picasso.LoadedFrom from) { if
(result == null )
{ throw
new
AssertionError(String.format( "Attempted
to complete action with no result!\n%s" ,
this )); } ImageView
target = this .target.get(); if
(target == null )
{ return ; } Context
context = picasso.context; boolean
debugging = picasso.debugging; PicassoDrawable.setBitmap(target,
context, result, from, noFade, debugging); if
(callback != null )
{ callback.onSuccess(); } } |
有了加載任務,具體的圖片下載與解析是在哪裏呢?這些都是耗時的操作,應該放在異步線程中進行,就是下面的BitmapHunter。
BitmapHunter
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@Override public
void run() { try
{ Thread.currentThread() .setName(Utils.THREAD_PREFIX
+ data.getName()); result
= hunt(); if
(result == null )
{ dispatcher.dispatchFailed( this ); }
else
{ dispatcher.dispatchComplete( this ); } }
catch
(IOException e) { exception
= e; dispatcher.dispatchRetry( this ); }
catch
(Exception e) { exception
= e; dispatcher.dispatchFailed( this ); }
finally { Thread.currentThread().setName(Utils.THREAD_IDLE_NAME); } } abstract
Bitmap decode(Request data) throws IOException; Bitmap
hunt() throws IOException { Bitmap
bitmap; if
(!skipMemoryCache) { bitmap
= cache.get(key); if
(bitmap != null )
{ stats.dispatchCacheHit(); loadedFrom
= MEMORY; return
bitmap; } } bitmap
= decode(data); if
(bitmap != null )
{ stats.dispatchBitmapDecoded(bitmap); if
(data.needsTransformation() || exifRotation != 0) { synchronized
(DECODE_LOCK) { if
(data.needsMatrixTransform() || exifRotation != 0) { bitmap
= transformResult(data, bitmap, exifRotation); } if
(data.hasCustomTransformations()) { bitmap
= applyCustomTransformations( data.transformations,
bitmap); } } stats.dispatchBitmapTransformed(bitmap); } } return
bitmap; } |
Dispatcher任務調度器
外部調用的是dispatchXXX方法,然後通過handler將請求轉換到對應的performXXX方法。
例如生成Action以後就會調用dispather的dispatchSubmit()來請求執行,
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void
dispatchSubmit(Action action) { handler.sendMessage(handler.obtainMessage(REQUEST_SUBMIT,
action)); } |
handler接到消息後轉換到performSubmit方法
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void
performSubmit(Action action) { BitmapHunter
hunter = hunterMap.get(action.getKey()); if
(hunter != null )
{ hunter.attach(action); return ; } if
(service.isShutdown()) { return ; } hunter
= forRequest(context, action.getPicasso(), this ,
cache, stats, action,
downloader); hunter.future
= service.submit(hunter); hunterMap.put(action.getKey(),
hunter); } |
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public
static Picasso with (Context
context) { if
(singleton == null )
{ singleton
= new
Builder(context).build(); } return
singleton; } public
Picasso build() { Context
context = this .context; if
(downloader == null )
{ downloader
= Utils.createDefaultDownloader(context); } if
(cache == null )
{ cache
= new
LruCache(context); } if
(service == null )
{ service
= new
PicassoExecutorService(); } if
(transformer == null )
{ transformer
= RequestTransformer.IDENTITY; } Stats
stats = new
Stats(cache); Dispatcher
dispatcher = new
Dispatcher(context, service, HANDLER, downloader,
cache, stats); return
new
Picasso(context, dispatcher, cache, listener, transformer,
stats, debugging); } |
在Picasso.with()的時候會將執行所需的所有必備元素創建出來,如緩存cache、執行executorService、調度dispatch等,在load()時創建Request,在into()中創建action、bitmapHunter,並最終交給dispatcher執行。
參考:http://blog.csdn.net/xu_fu/article/details/17043231