Universal-Image-Loader源碼

public void displayImage(String uri, ImageAware imageAware, DisplayImageOptions options,
			ImageSize targetSize, ImageLoadingListener listener, ImageLoadingProgressListener progressListener) {
		checkConfiguration();
		if (imageAware == null) {
			throw new IllegalArgumentException(ERROR_WRONG_ARGUMENTS);
		}
		if (listener == null) {
			listener = defaultListener;
		}
		if (options == null) {
			options = configuration.defaultDisplayImageOptions;
		}

		if (TextUtils.isEmpty(uri)) {
			engine.cancelDisplayTaskFor(imageAware);
			listener.onLoadingStarted(uri, imageAware.getWrappedView());
			if (options.shouldShowImageForEmptyUri()) {
				imageAware.setImageDrawable(options.getImageForEmptyUri(configuration.resources));
			} else {
				imageAware.setImageDrawable(null);
			}
			listener.onLoadingComplete(uri, imageAware.getWrappedView(), null);
			return;
		}

		if (targetSize == null) {
			targetSize = ImageSizeUtils.defineTargetSizeForView(imageAware, configuration.getMaxImageSize());
		}
		String memoryCacheKey = MemoryCacheUtils.generateKey(uri, targetSize);
		engine.prepareDisplayTaskFor(imageAware, memoryCacheKey);

		listener.onLoadingStarted(uri, imageAware.getWrappedView());

		Bitmap bmp = configuration.memoryCache.get(memoryCacheKey);<span style="white-space:pre">	</span>//判斷是不是在記憶體中
		if (bmp != null && !bmp.isRecycled()) {
			L.d(LOG_LOAD_IMAGE_FROM_MEMORY_CACHE, memoryCacheKey);

			if (options.shouldPostProcess()) {
				ImageLoadingInfo imageLoadingInfo = new ImageLoadingInfo(uri, imageAware, targetSize, memoryCacheKey,
						options, listener, progressListener, engine.getLockForUri(uri));
				ProcessAndDisplayImageTask displayTask = new ProcessAndDisplayImageTask(engine, bmp, imageLoadingInfo,
						defineHandler(options));
				if (options.isSyncLoading()) {
					displayTask.run();
				} else {
					engine.submit(displayTask);
				}
			} else {
				options.getDisplayer().display(bmp, imageAware, LoadedFrom.MEMORY_CACHE);
				listener.onLoadingComplete(uri, imageAware.getWrappedView(), bmp);
			}
		} else {
			if (options.shouldShowImageOnLoading()) {
				imageAware.setImageDrawable(options.getImageOnLoading(configuration.resources));
			} else if (options.isResetViewBeforeLoading()) {
				imageAware.setImageDrawable(null);
			}

			ImageLoadingInfo imageLoadingInfo = new ImageLoadingInfo(uri, imageAware, targetSize, memoryCacheKey,
					options, listener, progressListener, engine.getLockForUri(uri));
			LoadAndDisplayImageTask displayTask = new LoadAndDisplayImageTask(engine, imageLoadingInfo,
					defineHandler(options));<span style="white-space:pre">	</span>//先判斷是不是在外存中，再從網絡上下載下傳
			if (options.isSyncLoading()) {
				displayTask.run();
			} else {
				engine.submit(displayTask);
			}
		}
	}
           

public void run() {
		if (waitIfPaused()) return;
		if (delayIfNeed()) return;

		ReentrantLock loadFromUriLock = imageLoadingInfo.loadFromUriLock;
		L.d(LOG_START_DISPLAY_IMAGE_TASK, memoryCacheKey);
		if (loadFromUriLock.isLocked()) {
			L.d(LOG_WAITING_FOR_IMAGE_LOADED, memoryCacheKey);
		}

		loadFromUriLock.lock();
		Bitmap bmp;
		try {
			checkTaskNotActual();

			bmp = configuration.memoryCache.get(memoryCacheKey);
			if (bmp == null || bmp.isRecycled()) {
				bmp = tryLoadBitmap();<span style="white-space:pre">	</span>//在這個函數中先從外存緩存中找圖檔，再從網上加載
				if (bmp == null) return; // listener callback already was fired

				checkTaskNotActual();
				checkTaskInterrupted();

				if (options.shouldPreProcess()) {
					L.d(LOG_PREPROCESS_IMAGE, memoryCacheKey);
					bmp = options.getPreProcessor().process(bmp);
					if (bmp == null) {
						L.e(ERROR_PRE_PROCESSOR_NULL, memoryCacheKey);
					}
				}

				if (bmp != null && options.isCacheInMemory()) {
					L.d(LOG_CACHE_IMAGE_IN_MEMORY, memoryCacheKey);
					configuration.memoryCache.put(memoryCacheKey, bmp);<span style="white-space:pre">	</span>//存入記憶體緩存
				}
			} else {
				loadedFrom = LoadedFrom.MEMORY_CACHE;
				L.d(LOG_GET_IMAGE_FROM_MEMORY_CACHE_AFTER_WAITING, memoryCacheKey);
			}

			if (bmp != null && options.shouldPostProcess()) {
				L.d(LOG_POSTPROCESS_IMAGE, memoryCacheKey);
				bmp = options.getPostProcessor().process(bmp);
				if (bmp == null) {
					L.e(ERROR_POST_PROCESSOR_NULL, memoryCacheKey);
				}
			}
			checkTaskNotActual();
			checkTaskInterrupted();
		} catch (TaskCancelledException e) {
			fireCancelEvent();
			return;
		} finally {
			loadFromUriLock.unlock();
		}

		DisplayBitmapTask displayBitmapTask = new DisplayBitmapTask(bmp, imageLoadingInfo, engine, loadedFrom);<span style="white-space:pre">	</span>//顯示圖檔在另一個線程中
		runTask(displayBitmapTask, syncLoading, handler, engine);
	}
           

private Bitmap tryLoadBitmap() throws TaskCancelledException {
		Bitmap bitmap = null;
		try {
			File imageFile = configuration.diskCache.get(uri);	//從外存緩存中加載圖檔
			if (imageFile != null && imageFile.exists() && imageFile.length() > 0) {
				L.d(LOG_LOAD_IMAGE_FROM_DISK_CACHE, memoryCacheKey);
				loadedFrom = LoadedFrom.DISC_CACHE;


				checkTaskNotActual();
				bitmap = decodeImage(Scheme.FILE.wrap(imageFile.getAbsolutePath()));
			}
			if (bitmap == null || bitmap.getWidth() <= 0 || bitmap.getHeight() <= 0) {
				L.d(LOG_LOAD_IMAGE_FROM_NETWORK, memoryCacheKey);
				loadedFrom = LoadedFrom.NETWORK;


				String imageUriForDecoding = uri;
				if (options.isCacheOnDisk() && tryCacheImageOnDisk()) {	//tryCacheImageOnDisk()就會從嘗試從網上下載下傳圖檔
					imageFile = configuration.diskCache.get(uri);
					if (imageFile != null) {
						imageUriForDecoding = Scheme.FILE.wrap(imageFile.getAbsolutePath());
					}
				}


				checkTaskNotActual();
				bitmap = decodeImage(imageUriForDecoding);	//如果option中不在外存中緩沖的話，這裡會調用BaseImageDecoder decoder的decode()方法，也會嘗試從網上下載下傳；否則，圖檔已經嘗試從網上下載下傳緩存在外存中了。


				if (bitmap == null || bitmap.getWidth() <= 0 || bitmap.getHeight() <= 0) {
					fireFailEvent(FailType.DECODING_ERROR, null);
				}
			}
		} catch (IllegalStateException e) {
			fireFailEvent(FailType.NETWORK_DENIED, null);
		} catch (TaskCancelledException e) {
			throw e;
		} catch (IOException e) {
			L.e(e);
			fireFailEvent(FailType.IO_ERROR, e);
		} catch (OutOfMemoryError e) {
			L.e(e);
			fireFailEvent(FailType.OUT_OF_MEMORY, e);
		} catch (Throwable e) {
			L.e(e);
			fireFailEvent(FailType.UNKNOWN, e);
		}
		return bitmap;
	}