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andriod提供了(le) Handler 和 Looper 來(lai)滿(man)足線程(cheng)間的(de)通信。例如一個(ge)(ge)子線程(cheng)從網絡上下載了(le)一副圖片，當(dang)它(ta)下載完成后會發送消息給主線程(cheng)，這個(ge)(ge)消息是(shi)通過(guo)綁(bang)定(ding)在(zai)主線程(cheng)的(de)Handler來(lai)傳遞的(de)。

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在Android，這(zhe)(zhe)里(li)(li)的(de)(de)(de)(de)線(xian)(xian)(xian)(xian)(xian)程(cheng)(cheng)(cheng)分為有(you)(you)消(xiao)(xiao)息(xi)(xi)循(xun)環(huan)(huan)的(de)(de)(de)(de)線(xian)(xian)(xian)(xian)(xian)程(cheng)(cheng)(cheng)和沒有(you)(you)消(xiao)(xiao)息(xi)(xi)循(xun)環(huan)(huan)的(de)(de)(de)(de)線(xian)(xian)(xian)(xian)(xian)程(cheng)(cheng)(cheng)，有(you)(you)消(xiao)(xiao)息(xi)(xi)循(xun)環(huan)(huan)的(de)(de)(de)(de)線(xian)(xian)(xian)(xian)(xian)程(cheng)(cheng)(cheng)一(yi)(yi)(yi)(yi)般都會有(you)(you)一(yi)(yi)(yi)(yi)個(ge)Looper，這(zhe)(zhe)個(ge)是(shi)(shi)android的(de)(de)(de)(de)新(xin)概念。我(wo)(wo)(wo)們的(de)(de)(de)(de)主線(xian)(xian)(xian)(xian)(xian)程(cheng)(cheng)(cheng)(UI線(xian)(xian)(xian)(xian)(xian)程(cheng)(cheng)(cheng))就(jiu)(jiu)是(shi)(shi)一(yi)(yi)(yi)(yi)個(ge)消(xiao)(xiao)息(xi)(xi)循(xun)環(huan)(huan)的(de)(de)(de)(de)線(xian)(xian)(xian)(xian)(xian)程(cheng)(cheng)(cheng)。針(zhen)對這(zhe)(zhe)種消(xiao)(xiao)息(xi)(xi)循(xun)環(huan)(huan)的(de)(de)(de)(de)機制，我(wo)(wo)(wo)們引入(ru)一(yi)(yi)(yi)(yi)個(ge)新(xin)的(de)(de)(de)(de)機制Handler，我(wo)(wo)(wo)們有(you)(you)消(xiao)(xiao)息(xi)(xi)循(xun)環(huan)(huan)，就(jiu)(jiu)要(yao)(yao)往消(xiao)(xiao)息(xi)(xi)循(xun)環(huan)(huan)里(li)(li)面(mian)發送相(xiang)應(ying)(ying)的(de)(de)(de)(de)消(xiao)(xiao)息(xi)(xi)，自(zi)定義消(xiao)(xiao)息(xi)(xi)一(yi)(yi)(yi)(yi)般都會有(you)(you)自(zi)己對應(ying)(ying)的(de)(de)(de)(de)處(chu)理，消(xiao)(xiao)息(xi)(xi)的(de)(de)(de)(de)發送和清除，把這(zhe)(zhe)些(xie)都封(feng)裝在Handler里(li)(li)面(mian)，注意Handler只是(shi)(shi)針(zhen)對那 些(xie)有(you)(you)Looper的(de)(de)(de)(de)線(xian)(xian)(xian)(xian)(xian)程(cheng)(cheng)(cheng)，不(bu)管(guan)是(shi)(shi)UI線(xian)(xian)(xian)(xian)(xian)程(cheng)(cheng)(cheng)還是(shi)(shi)子線(xian)(xian)(xian)(xian)(xian)程(cheng)(cheng)(cheng)，只要(yao)(yao)你有(you)(you)Looper，我(wo)(wo)(wo)就(jiu)(jiu)可以往你的(de)(de)(de)(de)消(xiao)(xiao)息(xi)(xi)隊列(lie)里(li)(li)面(mian)添加東西，并做(zuo)相(xiang)應(ying)(ying)的(de)(de)(de)(de)處(chu)理。但是(shi)(shi)這(zhe)(zhe)里(li)(li)還有(you)(you)一(yi)(yi)(yi)(yi)點，就(jiu)(jiu)是(shi)(shi)只要(yao)(yao)是(shi)(shi)關于UI相(xiang)關的(de)(de)(de)(de)東西，就(jiu)(jiu)不(bu)能放在子線(xian)(xian)(xian)(xian)(xian)程(cheng)(cheng)(cheng)中，因(yin)為子線(xian)(xian)(xian)(xian)(xian)程(cheng)(cheng)(cheng)是(shi)(shi)不(bu)能操(cao)作UI的(de)(de)(de)(de)，只能進行(xing)數據、系(xi)統等其他非UI的(de)(de)(de)(de)操(cao)作。

一個Handler的(de)創(chuang)(chuang)建它就會被(bei)綁(bang)定到這個線(xian)(xian)程的(de)消息(xi)隊(dui)列中，如果是(shi)在主(zhu)線(xian)(xian)程創(chuang)(chuang)建的(de)，那就不(bu)需(xu)要寫代碼(ma)來創(chuang)(chuang)建消息(xi)隊(dui)列了，默認的(de)消息(xi)隊(dui)列會在主(zhu)線(xian)(xian)程被(bei)創(chuang)(chuang)建。但是(shi)如果是(shi)在子線(xian)(xian)程的(de)話，就必須在創(chuang)(chuang)建Handler之前先初始化線(xian)(xian)程的(de)消息(xi)隊(dui)列。請看如下代碼(ma)：

class ChildThread extends Thread {

public void run() {

/*

* 創(chuang)建 handler前先初始化Looper.

*/

Looper.prepare();

/*

* 在子線程創建handler，所以會(hui)綁定到(dao)子線程的消息隊列中

*

*/

mChildHandler = new Handler() {

public void handleMessage(Message msg) {

/*

* Do some expensive operations there.

*/

}

};

/*

* 啟動(dong)該線程的消(xiao)息(xi)隊(dui)列(lie)

*/

Looper.loop();

}

}

當(dang)Handler收到消息后，就會運行執行handlerMessage()方法，可以在(zai)里面(mian)獲取另外一個線(xian)程(cheng)發送過來的(de)Message對(dui)象，到達線(xian)程(cheng)之間通信的(de)過程(cheng)。

后完成(cheng)了操(cao)作要結束子(zi)線(xian)程時，記得調用(yong)quit()來結束消息(xi)循(xun)環隊列;

mChildHandler.getLooper().quit();