Android性能優化之電量篇

Google近期在Udacity上發佈了Android性能優化的在線課程，分別從渲染，運算與內存，電量幾個方面介紹瞭如何去優化性能，這些課程是Google以前在Youtube上發佈的Android性能優化典範專題課程的細化與補充。

 

下面是電量篇章的學習筆記，部份內容與前面的性能優化典範有重合，歡迎你們一塊兒學習交流！

(1). Understanding Battery Drain

手機各個硬件模塊的耗電量是不同的，有些模塊很是耗電，而有些模塊則相對顯得耗電量小不少。

                                        

電量消耗的計算與統計是一件麻煩並且矛盾的事情，記錄電量消耗自己也是一個費電量的事情。惟一可行的方案是使用第三方監測電量的設備，這樣纔可以獲取到真實的電量消耗。

當設備處於待機狀態時消耗的電量是極少的，以N5爲例，打開飛行模式，能夠待機接近1個月。但是點亮屏幕，硬件各個模塊就須要開始工做，這會須要消耗不少電量。

使用WakeLock或者JobScheduler喚醒設備處理定時的任務以後，必定要及時讓設備回到初始狀態。每次喚醒蜂窩信號進行數據傳遞，都會消耗不少電量，它比WiFi等操做更加的耗電。

                  

(2). Battery Historian

Battery Historian是Android 5.0開始引入的新API。經過下面的指令，能夠獲得設備上的電量消耗信息：

$ adb shell dumpsys batterystats > xxx.txt //獲得整個設備的電量消耗信息 $ adb shell dumpsys batterystats > com.package.name > xxx.txt //獲得指定app相關的電量消耗信息

獲得了原始的電量消耗數據以後，咱們須要經過Google編寫的一個python腳本把數據信息轉換成可讀性更好的html文件：

$ python historian.py xxx.txt > xxx.html

打開這個轉換事後的html文件，能夠看到相似TraceView生成的列表數據，這裏的數據信息量很大，這裏就不展開了。

             

(3). Track Battery Status & Battery Manager

咱們能夠經過下面的代碼來獲取手機的當前充電狀態：

// It is very easy to subscribe to changes to the battery state, but you can get the current // state by simply passing null in as your receiver. Nifty, isn't that? IntentFilter filter = new IntentFilter(Intent.ACTION_BATTERY_CHANGED); Intent batteryStatus = this.registerReceiver(null, filter); int chargePlug = batteryStatus.getIntExtra(BatteryManager.EXTRA_PLUGGED, -1); boolean acCharge = (chargePlug == BatteryManager.BATTERY_PLUGGED_AC); if (acCharge) { Log.v(LOG_TAG,「The phone is charging!」); }

在上面的例子演示瞭如何當即獲取到手機的充電狀態，獲得充電狀態信息以後，咱們能夠有針對性的對部分代碼作優化。好比咱們能夠判斷只有當前手機爲AC充電狀態時 纔去執行一些很是耗電的操做。

/** * This method checks for power by comparing the current battery state against all possible * plugged in states. In this case, a device may be considered plugged in either by USB, AC, or * wireless charge. (Wireless charge was introduced in API Level 17.) */ private boolean checkForPower() { // It is very easy to subscribe to changes to the battery state, but you can get the current // state by simply passing null in as your receiver. Nifty, isn't that? IntentFilter filter = new IntentFilter(Intent.ACTION_BATTERY_CHANGED); Intent batteryStatus = this.registerReceiver(null, filter); // There are currently three ways a device can be plugged in. We should check them all. int chargePlug = batteryStatus.getIntExtra(BatteryManager.EXTRA_PLUGGED, -1); boolean usbCharge = (chargePlug == BatteryManager.BATTERY_PLUGGED_USB); boolean acCharge = (chargePlug == BatteryManager.BATTERY_PLUGGED_AC); boolean wirelessCharge = false; if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) { wirelessCharge = (chargePlug == BatteryManager.BATTERY_PLUGGED_WIRELESS); } return (usbCharge || acCharge || wirelessCharge); }

(4). Wakelock and Battery Drain

高效的保留更多的電量與不斷促使用戶使用你的App會消耗電量，這是矛盾的選擇題。不過咱們可使用一些更好的辦法來平衡二者。

假設你的手機裏面裝了大量的社交類應用，即便手機處於待機狀態，也會常常被這些應用喚醒用來檢查同步新的數據信息。Android會不斷關閉各類硬件來延長手機的待機時間，首先屏幕會逐漸變暗直相當閉，而後CPU進入睡眠，這一切操做都是爲了節約寶貴的電量資源。可是即便在這種睡眠狀態下，大多數應用仍是會嘗試進行工做，他們將不斷的喚醒手機。一個最簡單的喚醒手機的方法是使用PowerManager.WakeLock的API來保持CPU工做並防止屏幕變暗關閉。這使得手機能夠被喚醒，執行工做，而後回到睡眠狀態。知道如何獲取WakeLock是簡單的，但是及時釋放WakeLock也是很是重要的，不恰當的使用WakeLock會致使嚴重錯誤。例如網絡請求的數據返回時間不肯定，致使原本只須要10s的事情一直等待了1個小時，這樣會使得電量白白浪費了。這也是爲什麼使用帶超時參數的wakelock.acquice()方法是很關鍵的。

可是僅僅設置超時並不足夠解決問題，例如設置多長的超時比較合適？何時進行重試等等？解決上面的問題，正確的方式多是使用非精準定時器。一般狀況下，咱們會設定一個時間進行某個操做，可是動態修改這個時間也許會更好。例如，若是有另一個程序須要比你設定的時間晚5分鐘喚醒，最好可以等到那個時候，兩個任務捆綁一塊兒同時進行，這就是非精肯定時器的核心工做原理。咱們能夠定製計劃的任務，但是系統若是檢測到一個更好的時間，它能夠推遲你的任務，以節省電量消耗。

                                 

這正是JobScheduler API所作的事情。它會根據當前的狀況與任務，組合出理想的喚醒時間，例如等到正在充電或者鏈接到WiFi的時候，或者集中任務一塊兒執行。咱們能夠經過這個API實現不少免費的調度算法。

(5). Network and Battery Drain

下面內容來自官方Training文檔中高效下載章節關於手機(Radio)蜂窩信號對電量消耗的介紹。

一般狀況下，使用3G移動網絡傳輸數據，電量的消耗有三種狀態：

• Full power: 能量最高的狀態，移動網絡鏈接被激活，容許設備以最大的傳輸速率進行操做。
• Low power: 一種中間狀態，對電量的消耗差很少是Full power狀態下的50%。
• Standby: 最低的狀態，沒有數據鏈接須要傳輸，電量消耗最少。

下圖是一個典型的3G Radio State Machine的圖示(來自AT&T，詳情請點擊這裏):

             

總之，爲了減小電量的消耗，在蜂窩移動網絡下，最好作到批量執行網絡請求，儘可能避免頻繁的間隔網絡請求。

經過前面學習到的Battery Historian咱們能夠獲得設備的電量消耗數據，若是數據中的移動蜂窩網絡(Mobile Radio)電量消耗呈現下面的狀況，間隔很小，又頻繁斷斷續續的出現，說明電量消耗性能很很差：

             

通過優化以後，若是呈現下面的圖示，說明電量消耗的性能是良好的：

              

另外WiFi鏈接下，網絡傳輸的電量消耗要比移動網絡少不少，應該儘可能減小移動網絡下的數據傳輸，多在WiFi環境下傳輸數據。

             

那麼如何纔可以把任務緩存起來，作到批量化執行呢？下面就輪到Job Scheduler出場了。

(6). Using Job Scheduler

使用Job Scheduler，應用須要作的事情就是判斷哪些任務是不緊急的，能夠交給Job Scheduler來處理，Job Scheduler集中處理收到的任務，選擇合適的時間，合適的網絡，再一塊兒進行執行。

下面是使用Job Scheduler的一段簡要示例，須要先有一個JobService：

public class MyJobService extends JobService { private static final String LOG_TAG = "MyJobService"; @Override public void onCreate() { super.onCreate(); Log.i(LOG_TAG, "MyJobService created"); } @Override public void onDestroy() { super.onDestroy(); Log.i(LOG_TAG, "MyJobService destroyed"); } @Override public boolean onStartJob(JobParameters params) { // This is where you would implement all of the logic for your job. Note that this runs // on the main thread, so you will want to use a separate thread for asynchronous work // (as we demonstrate below to establish a network connection). // If you use a separate thread, return true to indicate that you need a "reschedule" to // return to the job at some point in the future to finish processing the work. Otherwise, // return false when finished. Log.i(LOG_TAG, "Totally and completely working on job " + params.getJobId()); // First, check the network, and then attempt to connect. if (isNetworkConnected()) { new SimpleDownloadTask() .execute(params); return true; } else { Log.i(LOG_TAG, "No connection on job " + params.getJobId() + "; sad face"); } return false; } @Override public boolean onStopJob(JobParameters params) { // Called if the job must be stopped before jobFinished() has been called. This may // happen if the requirements are no longer being met, such as the user no longer // connecting to WiFi, or the device no longer being idle. Use this callback to resolve // anything that may cause your application to misbehave from the job being halted. // Return true if the job should be rescheduled based on the retry criteria specified // when the job was created or return false to drop the job. Regardless of the value // returned, your job must stop executing. Log.i(LOG_TAG, "Whelp, something changed, so I'm calling it on job " + params.getJobId()); return false; } /** * Determines if the device is currently online. */ private boolean isNetworkConnected() { ConnectivityManager connectivityManager = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE); NetworkInfo networkInfo = connectivityManager.getActiveNetworkInfo(); return (networkInfo != null && networkInfo.isConnected()); } /** * Uses AsyncTask to create a task away from the main UI thread. This task creates a * HTTPUrlConnection, and then downloads the contents of the webpage as an InputStream. * The InputStream is then converted to a String, which is logged by the * onPostExecute() method. */ private class SimpleDownloadTask extends AsyncTask<JobParameters, Void, String> { protected JobParameters mJobParam; @Override protected String doInBackground(JobParameters... params) { // cache system provided job requirements mJobParam = params[0]; try { InputStream is = null; // Only display the first 50 characters of the retrieved web page content. int len = 50; URL url = new URL("https://www.google.com"); HttpURLConnection conn = (HttpURLConnection) url.openConnection(); conn.setReadTimeout(10000); //10sec conn.setConnectTimeout(15000); //15sec conn.setRequestMethod("GET"); //Starts the query conn.connect(); int response = conn.getResponseCode(); Log.d(LOG_TAG, "The response is: " + response); is = conn.getInputStream(); // Convert the input stream to a string Reader reader = null; reader = new InputStreamReader(is, "UTF-8"); char[] buffer = new char[len]; reader.read(buffer); return new String(buffer); } catch (IOException e) { return "Unable to retrieve web page."; } } @Override protected void onPostExecute(String result) { jobFinished(mJobParam, false); Log.i(LOG_TAG, result); } } }

而後模擬經過點擊Button觸發N個任務，交給JobService來處理

public class FreeTheWakelockActivity extends ActionBarActivity { public static final String LOG_TAG = "FreeTheWakelockActivity"; TextView mWakeLockMsg; ComponentName mServiceComponent; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_wakelock); mWakeLockMsg = (TextView) findViewById(R.id.wakelock_txt); mServiceComponent = new ComponentName(this, MyJobService.class); Intent startServiceIntent = new Intent(this, MyJobService.class); startService(startServiceIntent); Button theButtonThatWakelocks = (Button) findViewById(R.id.wakelock_poll); theButtonThatWakelocks.setText(R.string.poll_server_button); theButtonThatWakelocks.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { pollServer(); } }); } /** * This method polls the server via the JobScheduler API. By scheduling the job with this API, * your app can be confident it will execute, but without the need for a wake lock. Rather, the * API will take your network jobs and execute them in batch to best take advantage of the * initial network connection cost. * * The JobScheduler API works through a background service. In this sample, we have * a simple service in MyJobService to get you started. The job is scheduled here in * the activity, but the job itself is executed in MyJobService in the startJob() method. For * example, to poll your server, you would create the network connection, send your GET * request, and then process the response all in MyJobService. This allows the JobScheduler API * to invoke your logic without needed to restart your activity. * * For brevity in the sample, we are scheduling the same job several times in quick succession, * but again, try to consider similar tasks occurring over time in your application that can * afford to wait and may benefit from batching. */ public void pollServer() { JobScheduler scheduler = (JobScheduler) getSystemService(Context.JOB_SCHEDULER_SERVICE); for (int i=0; i<10; i++) { JobInfo jobInfo = new JobInfo.Builder(i, mServiceComponent) .setMinimumLatency(5000) // 5 seconds .setOverrideDeadline(60000) // 60 seconds (for brevity in the sample) .setRequiredNetworkType(JobInfo.NETWORK_TYPE_ANY) // WiFi or data connections .build(); mWakeLockMsg.append("Scheduling job " + i + "!\n"); scheduler.schedule(jobInfo); } } }