Picasso框架源碼解析（一）。

相信做android開發的都會用到Picasso、Universal-ImageLoader等，和圖片相關的框架。這些開源的框架在很多情況下都可以使用，這個時候可能就有問題了，我該怎麼選擇框架呢？本人曾經或者現在也會遇到這樣的苦惱，（有時候就像剛學android一樣，當時的苦惱是如何實現這個功能）現在是用什麼方式實現來說“性價比最好”。

對於這個問題，只有唯一解。那就是對這幾個框架的源碼進行專研了。理解了其中的原理和邏輯，那麼這些問題就不在是爲什麼了。

先說說我在項目中關於圖片常用到的兩個框架Picasso和UIL（Universal-ImageLoader）
先分析源碼，之後在總結他們的適用場景。

Picasso

簡介

Images add much-needed context and visual flair to Android applications. Picasso allows for hassle-free image loading in your application—often in one line of code!
假如你想在你的安卓程序中添加你所需要的（帶上下文引用的視覺風格的）圖片，那麼使用Picasso框架僅僅一行代碼，就可以達到你所需要的效果，而且不需要考慮什麼oom之類的問題。

這是Picasso官網對其的說明，我的總結是簡單、粗暴，一行代碼。

Picasso.with(context).load("http://i.imgur.com/DvpvklR.png").into(imageView);

Picasso的特點

Handling ImageView recycling and download cancelation in an adapter.
對圖像自動進行回收以及在適配的時候getView()方法裏面可以取消加載圖片（可以用在listview快速滾動，不加載等場景）
Complex image transformations with minimal memory use.
對於複雜圖片用最小的內存去轉換。
Automatic memory and disk caching.
自動的對圖片進行磁盤緩存和內存。

看到這裏相信你對Picasso有了一些印象了吧，接下來分析源碼

Picasso的使用

Resources, assets, files, content providers are all supported as image sources.
一般我們的image可以是Resources, assets, files, content providers

Picasso.with(context).load(R.drawable.landing_screen).into(imageView1);
Picasso.with(context).load("file:///android_asset/DvpvklR.png").into(imageView2);
Picasso.with(context).load(new File(...)).into(imageView3);


     picasso_iv = (ImageView) findViewById(R.id.picasso_iv);
     Picasso.with(this).load("圖片").transform(new Transformation() {
            @Override
            public Bitmap transform(Bitmap source) {
                int size = Math.min(source.getWidth(), source.getHeight());
                int x = (source.getWidth() - size) / 4;
                int y = (source.getHeight() - size) / 4;
                Bitmap result = Bitmap.createBitmap(source, x, y, size, size);
                if (result != source) {
                    source.recycle();
                }
                return result;
            }
            @Override
            public String key() {
                return "square()";
            }
        }).placeholder(R.mipmap.ic_launcher).into(picasso_iv);

Picasso源碼

     我們看到 Picasso.with(this)，那麼Picasso是如我們所料的單例模式嗎？

public class Picasso {
....java
//volatile關鍵字修飾的對象每次都是都是從主存中獲取，而不是從各個子線程中獲取。也就是說它每次都是拿到最新的值，但是它不保證執行線程的有序性。
static volatile Picasso singleton = null;
...
 public static Picasso with(Context context) {
        if(singleton == null) {
            Class var1 = Picasso.class;
            synchronized(Picasso.class) {
                if(singleton == null) {
                    //這裏創建了一個Picasso實例
                    singleton = (new Picasso.Builder(context)).build();
                }
            }
        }

        return singleton;
    }
}
...

Volatile關鍵字

接下來看Picasso.Builder(context)).build()

  public Picasso build() {
         //...
         //這裏最後調用構造函數創建了一個Picasso對象
   return new Picasso(context, dispatcher, this.cache, this.listener, this.transformer,   this.requestHandlers, stats, this.defaultBitmapConfig, this.indicatorsEnabled,   this.loggingEnabled);
        }
    }

接着看
Picasso.with(this).load("圖片")，拿到Picasso對象後進行load()操作

  public RequestCreator load(Uri uri) {
        return new RequestCreator(this, uri, 0);
    }

    public RequestCreator load(String path) {
        if(path == null) {
            return new RequestCreator(this, (Uri)null, 0);
        } else if(path.trim().length() == 0) {
            throw new IllegalArgumentException("Path must not be empty.");
        } else {
            return this.load(Uri.parse(path));
        }
    }

    public RequestCreator load(File file) {
        return file == null?new RequestCreator(this, (Uri)null, 0):this.load(Uri.fromFile(file));
    }

    public RequestCreator load(int resourceId) {
        if(resourceId == 0) {
            throw new IllegalArgumentException("Resource ID must not be zero.");
        } else {
            return new RequestCreator(this, (Uri)null, resourceId);
        }
    }

    //獲取一個RequestCreator對象
   RequestCreator(Picasso picasso, Uri uri, int resourceId) {
        if(picasso.shutdown) {
          throw new IllegalStateException("Picasso instance already shut down. Cannot  submit new requests.");
        } else {
            this.picasso = picasso;
            this.data = new Builder(uri, resourceId, picasso.defaultBitmapConfig);
        }
    }

接着看，拿到RequestCreator我們在調用into()

 public void into(ImageView target) {
        this.into(target, (Callback)null);
    }



//////////

 public void into(ImageView target, Callback callback) {
       //...
            //前面對請求參數進行校驗，這裏
            Request request1 = this.createRequest(started);
            String requestKey1 = Utils.createKey(request1);
            //查看內存或磁盤中是否有這個圖片
            if(MemoryPolicy.shouldReadFromMemoryCache(this.memoryPolicy)) {
                Bitmap action = this.picasso.quickMemoryCacheCheck(requestKey1);
                //當有緩存的時候
                if(action != null) {
                    this.picasso.cancelRequest(target);
                    PicassoDrawable.setBitmap(target, this.picasso.context, action, LoadedFrom.MEMORY, this.noFade, this.picasso.indicatorsEnabled);
                    if(this.picasso.loggingEnabled) {
                        Utils.log("Main", "completed", request1.plainId(), "from " + LoadedFrom.MEMORY);
                    }

                    if(callback != null) {
                        callback.onSuccess();
                    }

                    return;
                }
            }
            //沒緩存
            ImageViewAction action1 = new ImageViewAction(this.picasso, target, request1, this.memoryPolicy, this.networkPolicy, this.errorResId, this.errorDrawable, requestKey1, this.tag, callback, this.noFade);
            //前面所做的工作就是爲了創建一個ImageViewAction對象
            this.picasso.enqueueAndSubmit(action1);
        }
    }

接着看this.picasso.enqueueAndSubmit(action1);

void enqueueAndSubmit(Action action) {
        Object target = action.getTarget();
        if(target != null && this.targetToAction.get(target) != action) {
            //當將其做個標記
            this.cancelExistingRequest(target);
            this.targetToAction.put(target, action);
        }

        this.submit(action);
    }


void submit(Action action) {
        this.dispatcher.dispatchSubmit(action);
    }
    ///跟蹤到這裏，是不是有點欣慰啊。。哈哈，在構造函數裏面已經對handler有賦值
void dispatchSubmit(Action action) {
        //<font color="red">msg.what=1</font>
        this.handler.sendMessage(this.handler.obtainMessage(1, action));
    }

///如下Dispatcher
    Dispatcher(Context context, ExecutorService service, Handler mainThreadHandler, Downloader downloader, Cache cache, Stats stats) {
        this.dispatcherThread.start();
        Utils.flushStackLocalLeaks(this.dispatcherThread.getLooper());
        this.context = context;
        this.service = service;
        this.hunterMap = new LinkedHashMap();
        this.failedActions = new WeakHashMap();
        this.pausedActions = new WeakHashMap();
        this.pausedTags = new HashSet();
        ////......拿到當前線程的looper，而looper是唯一的，則可以將數據加載到image上
        this.handler = new Dispatcher.DispatcherHandler(this.dispatcherThread.getLooper(), this);
        ////......
        this.downloader = downloader;
        this.mainThreadHandler = mainThreadHandler;
        this.cache = cache;
        this.stats = stats;
        this.batch = new ArrayList(4);
        this.airplaneMode = Utils.isAirplaneModeOn(this.context);
        this.scansNetworkChanges = Utils.hasPermission(context, "android.permission.ACCESS_NETWORK_STATE");
        this.receiver = new Dispatcher.NetworkBroadcastReceiver(this);
        this.receiver.register();
    }

接着往下走,拿到了hanlder之後 msg.what=1; 繼續跟蹤代碼

private static class DispatcherHandler extends Handler {
        private final Dispatcher dispatcher;

        public DispatcherHandler(Looper looper, Dispatcher dispatcher) {
            super(looper);
            this.dispatcher = dispatcher;
        }

        public void handleMessage(final Message msg) {
            Object info;
            BitmapHunter info2;
            Action info3;
            switch(msg.what) {
            case 1:
                info3 = (Action)msg.obj;
                ////.......看這裏..
                this.dispatcher.performSubmit(info3);
                /////.........
                break;
            case 2:
                info3 = (Action)msg.obj;
                this.dispatcher.performCancel(info3);
                break;
            //...
            case 12:
                info = msg.obj;
                this.dispatcher.performResumeTag(info);
            }

        }
    }

繼續go~~!!!!!

void performSubmit(Action action, boolean dismissFailed) {
        if(this.pausedTags.contains(action.getTag())) {
            this.pausedActions.put(action.getTarget(), action);
            if(action.getPicasso().loggingEnabled) {
                Utils.log("Dispatcher", "paused", action.request.logId(), "because tag \'" + action.getTag() + "\' is paused");
            }

        } else {
            ///看這裏，看這裏，，當己錄表有這個的action key
            BitmapHunter hunter = (BitmapHunter)this.hunterMap.get(action.getKey());
            if(hunter != null) {
                hunter.attach(action);
            } else if(this.service.isShutdown()) {
                if(action.getPicasso().loggingEnabled) {
                    Utils.log("Dispatcher", "ignored", action.request.logId(), "because shut down");
                }

            } else {
                //完全第一次加載進來，××× 這裏也是最終的高地
                hunter = BitmapHunter.forRequest(action.getPicasso(), this, this.cache, this.stats, action);
                hunter.future = this.service.submit(hunter);
                this.hunterMap.put(action.getKey(), hunter);
                if(dismissFailed) {
                    this.failedActions.remove(action.getTarget());
                }

                if(action.getPicasso().loggingEnabled) {
                    Utils.log("Dispatcher", "enqueued", action.request.logId());
                }

            }
        }
    }

//////上面函數中最主要的  BitmapHunter  其實就是一個runnable  負責網絡加載圖片
hunter = BitmapHunter.forRequest(action.getPicasso(), this, this.cache, this.stats, action);

///

總結

雖然我們在用Picasso的時候十分簡單，就一行代碼。但是源碼的實現的過程，做爲一名開發者還是十分有必要的仔細閱讀的。

Picasso框架源碼解析（一）。

Picasso

Picasso源碼

總結

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