在做Service簡單練習時,在Service中的OnCreate、OnStart、OnDestroy三個方法中都像在Activity中同樣的方法調用了Toast.makeText,並在Acitivy中通過兩個按鈕來調用該服務的onStart和onDestroy方法:
DemoService代碼如下:
- <span style="font-size:16px;">@Override
- public void onCreate()
- {
- super.onCreate();
- Toast.makeText(getApplicationContext(), "Service is created!", Toast.LENGTH_LONG).show();
- }
- @Override
- public void onStart(Intent intent,int startId)
- {
- super.onStart(intent, startId);
- Toast.makeText(getApplicationContext(), "Service is on!", Toast.LENGTH_LONG).show();
- }
- @Override
- public void onDestroy(){
- super.onDestroy();
- Toast.makeText(getApplicationContext(), "Service is off!", Toast.LENGTH_LONG).show();
- }
- </span>
運行之後,DemoService中的信息都沒有顯示出來。
剛開始以爲所得到的Context不正確,在Service直接調用getApplicationContext()得到的是Service的Context,但是細究來看,Toast應該得到主UI的Context才能顯示,所以找了一下,Google對Toast的說明中,有一句:
“A toast can be created and displayed from an Activity or Service. If you create a toast notification from a Service,it appears in front of the Activity currently in focus.”
(http://developer.android.com/guide/topics/ui/notifiers/toasts.html)
那麼按照這句來看,service中創建的toast會在Acivity的UI前面聚焦顯示。但爲什麼運行沒有效果呢?再來查看一下makeText方法。
果然還是Context的問題,所以想要toast能夠正常工作,需要在Activity的主線程上運行纔行,那麼如何得到主線程UI的Context呢?可以通過Handler將一個自定義的線程運行於主線程之上。
再來看一下Toast.show方法的src:
- <span style="font-size:16px;">public void show() {
- ...
- service.enqueueToast(pkg, tn, mDuration); //將該toast插入到一個消息隊列中
- ...
- }
- </span>
原理上看,Android中大致上是消息隊列和消息循環,主線程從消息隊列中取得消息並處理。而Handler看作是一個工具類,用來向消息隊列中插入消息。所以我們重構原來的代碼:
- <span style="font-size:16px;">@Override
- public void onCreate()
- {
- super.onCreate();
- handler=new Handler(Looper.getMainLooper());
- handler.post(new Runnable(){
- public void run(){
- Toast.makeText(getApplicationContext(), "Service is created!", Toast.LENGTH_LONG).show();
- }
- });
- }
- @Override
- public void onStart(Intent intent,int startId)
- {
- super.onStart(intent, startId);
- handler=new Handler(Looper.getMainLooper());
- handler.post(new Runnable(){
- public void run(){
- Toast.makeText(getApplicationContext(), "Service is on!", Toast.LENGTH_LONG).show();
- }
- });
- }
- @Override
- public void onDestroy(){
- super.onDestroy();
- handler=new Handler(Looper.getMainLooper());
- handler.post(new Runnable(){
- public void run(){
- Toast.makeText(getApplicationContext(), "Service is off!", Toast.LENGTH_LONG).show();
- }
- });
- </span>
運行之後的效果如下:
總結:在Android的Framework中使用Toast,要將Toast添加到主線程裏才能正常工作。