*** 使用私有構(gòu)造方法***
讓構(gòu)造方法私有從而保證這是一個(gè)單例，這意味著你不需要在同步代碼塊(synchronized block)中額外訪問這個(gè)類：
public class PhotoManager {
...
* Constructs the work queues and thread pools used to download
* and decode images. Because the constructor is marked private,
* it's unavailable to other classes, even in the same package.
*/
private PhotoManager() {
...
}

通過調(diào)用線程池類里的方法開啟你的任務(wù)
在線程池類中定義一個(gè)能添加任務(wù)到線程池隊(duì)列的方法。例如：
public class PhotoManager {
...
// Called by the PhotoView to get a photo
static public PhotoTask startDownload(
PhotoView imageView,
boolean cacheFlag) {
...
// Adds a download task to the thread pool for execution
sInstance.
mDownloadThreadPool.
execute(downloadTask.getHTTPDownloadRunnable());
...
}

在構(gòu)造方法中實(shí)例化一個(gè)Handler，且將它附加到你APP的UI線程。
一個(gè)Handler允許你的APP安全地調(diào)用UI對象（例如 View對象）的方法。大多數(shù)UI對象只能從UI線程安全的代碼中被修改。這個(gè)方法將會在與UI線程進(jìn)行通信(Communicate with the UI Thread)這一課中進(jìn)行詳細(xì)的描述。例如：
private PhotoManager() {
...
// Defines a Handler object that's attached to the UI thread
mHandler = new Handler(Looper.getMainLooper()) {
/*
* handleMessage() defines the operations to perform when
* the Handler receives a new Message to process.
*/
@Override
public void handleMessage(Message inputMessage) {
...
}
...
}
}