> 原文出處:http://hukai.me/android-performance-battery/
> 作者: 胡凱
Google近期在Udacity上發(fā)布了[Android性能優(yōu)化的在線課程](https://www.udacity.com/course/ud825)，分別從渲染，運(yùn)算與內(nèi)存，電量幾個(gè)方面介紹了如何去優(yōu)化性能，這些課程是Google之前在Youtube上發(fā)布的[Android性能優(yōu)化典范](http://hukai.me/android-performance-patterns/)專題課程的細(xì)化與補(bǔ)充。
下面是電量篇章的學(xué)習(xí)筆記，部分內(nèi)容與前面的性能優(yōu)化典范有重合，歡迎大家一起學(xué)習(xí)交流！
## 1)Understanding Battery Drain
手機(jī)各個(gè)硬件模塊的耗電量是不一樣的，有些模塊非常耗電，而有些模塊則相對(duì)顯得耗電量小很多。
電量消耗的計(jì)算與統(tǒng)計(jì)是一件麻煩而且矛盾的事情，記錄電量消耗本身也是一個(gè)費(fèi)電量的事情。唯一可行的方案是使用第三方監(jiān)測(cè)電量的設(shè)備，這樣才能夠獲取到真實(shí)的電量消耗。
當(dāng)設(shè)備處于待機(jī)狀態(tài)時(shí)消耗的電量是極少的，以N5為例，打開飛行模式，可以待機(jī)接近1個(gè)月?？墒屈c(diǎn)亮屏幕，硬件各個(gè)模塊就需要開始工作，這會(huì)需要消耗很多電量。
使用WakeLock或者JobScheduler喚醒設(shè)備處理定時(shí)的任務(wù)之后，一定要及時(shí)讓設(shè)備回到初始狀態(tài)。每次喚醒蜂窩信號(hào)進(jìn)行數(shù)據(jù)傳遞，都會(huì)消耗很多電量，它比WiFi等操作更加的耗電。
## 2)Battery Historian
[Battery Historian](https://developer.android.com/about/versions/android-5.0.html#Power)是Android 5.0開始引入的新API。通過下面的指令，可以得到設(shè)備上的電量消耗信息：
~~~
$ adb shell dumpsys batterystats > xxx.txt //得到整個(gè)設(shè)備的電量消耗信息
$ adb shell dumpsys batterystats > com.package.name > xxx.txt //得到指定app相關(guān)的電量消耗信息
~~~
得到了原始的電量消耗數(shù)據(jù)之后，我們需要通過Google編寫的一個(gè)[python腳本](https://github.com/google/battery-historian)把數(shù)據(jù)信息轉(zhuǎn)換成可讀性更好的html文件：
~~~
$ python historian.py xxx.txt > xxx.html
~~~
打開這個(gè)轉(zhuǎn)換過后的html文件，可以看到類似TraceView生成的列表數(shù)據(jù)，這里的數(shù)據(jù)信息量很大，這里就不展開了。
## 3)Track Battery Status & Battery Manager
我們可以通過下面的代碼來獲取手機(jī)的當(dāng)前充電狀態(tài)：
~~~
// 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!”);
}
~~~
在上面的例子演示了如何立即獲取到手機(jī)的充電狀態(tài)，得到充電狀態(tài)信息之后，我們可以有針對(duì)性的對(duì)部分代碼做優(yōu)化。比如我們可以判斷只有當(dāng)前手機(jī)為AC充電狀態(tài)時(shí) 才去執(zhí)行一些非常耗電的操作。
~~~
/**
* 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(huì)消耗電量，這是矛盾的選擇題。不過我們可以使用一些更好的辦法來平衡兩者。
假設(shè)你的手機(jī)里面裝了大量的社交類應(yīng)用，即使手機(jī)處于待機(jī)狀態(tài)，也會(huì)經(jīng)常被這些應(yīng)用喚醒用來檢查同步新的數(shù)據(jù)信息。Android會(huì)不斷關(guān)閉各種硬件來延長(zhǎng)手機(jī)的待機(jī)時(shí)間，首先屏幕會(huì)逐漸變暗直至關(guān)閉，然后CPU進(jìn)入睡眠，這一切操作都是為了節(jié)約寶貴的電量資源。但是即使在這種睡眠狀態(tài)下，大多數(shù)應(yīng)用還是會(huì)嘗試進(jìn)行工作，他們將不斷的喚醒手機(jī)。一個(gè)最簡(jiǎn)單的喚醒手機(jī)的方法是使用PowerManager.WakeLock的API來保持CPU工作并防止屏幕變暗關(guān)閉。這使得手機(jī)可以被喚醒，執(zhí)行工作，然后回到睡眠狀態(tài)。知道如何獲取WakeLock是簡(jiǎn)單的，可是及時(shí)釋放WakeLock也是非常重要的，不恰當(dāng)?shù)氖褂肳akeLock會(huì)導(dǎo)致嚴(yán)重錯(cuò)誤。例如網(wǎng)絡(luò)請(qǐng)求的數(shù)據(jù)返回時(shí)間不確定，導(dǎo)致本來只需要10s的事情一直等待了1個(gè)小時(shí)，這樣會(huì)使得電量白白浪費(fèi)了。這也是為何使用帶超時(shí)參數(shù)的wakelock.acquice()方法是很關(guān)鍵的。
但是僅僅設(shè)置超時(shí)并不足夠解決問題，例如設(shè)置多長(zhǎng)的超時(shí)比較合適？什么時(shí)候進(jìn)行重試等等？解決上面的問題，正確的方式可能是使用非精準(zhǔn)定時(shí)器。通常情況下，我們會(huì)設(shè)定一個(gè)時(shí)間進(jìn)行某個(gè)操作，但是動(dòng)態(tài)修改這個(gè)時(shí)間也許會(huì)更好。例如，如果有另外一個(gè)程序需要比你設(shè)定的時(shí)間晚5分鐘喚醒，最好能夠等到那個(gè)時(shí)候，兩個(gè)任務(wù)捆綁一起同時(shí)進(jìn)行，這就是非精確定時(shí)器的核心工作原理。我們可以定制計(jì)劃的任務(wù)，可是系統(tǒng)如果檢測(cè)到一個(gè)更好的時(shí)間，它可以推遲你的任務(wù)，以節(jié)省電量消耗。
這正是JobScheduler API所做的事情。它會(huì)根據(jù)當(dāng)前的情況與任務(wù)，組合出理想的喚醒時(shí)間，例如等到正在充電或者連接到WiFi的時(shí)候，或者集中任務(wù)一起執(zhí)行。我們可以通過這個(gè)API實(shí)現(xiàn)很多免費(fèi)的調(diào)度算法。
## 5)Network and Battery Drain
下面內(nèi)容來自官方Training文檔中[高效下載](http://hukai.me/android-training-course-in-chinese/connectivity/efficient-downloads/efficient-network-access.html)章節(jié)關(guān)于手機(jī)(Radio)蜂窩信號(hào)對(duì)電量消耗的介紹。
通常情況下，使用3G移動(dòng)網(wǎng)絡(luò)傳輸數(shù)據(jù)，電量的消耗有三種狀態(tài)：
* **Full power**: 能量最高的狀態(tài)，移動(dòng)網(wǎng)絡(luò)連接被激活，允許設(shè)備以最大的傳輸速率進(jìn)行操作。
* **Low power**: 一種中間狀態(tài)，對(duì)電量的消耗差不多是Full power狀態(tài)下的50%。
* **Standby**: 最低的狀態(tài)，沒有數(shù)據(jù)連接需要傳輸，電量消耗最少。
下圖是一個(gè)典型的3G Radio State Machine的圖示(來自AT&T，詳情請(qǐng)點(diǎn)擊[這里](http://www.research.att.com/articles/featured_stories/2011_03/201102_Energy_efficient?fbid=SYuI20FzBum)):
**總之，為了減少電量的消耗，在蜂窩移動(dòng)網(wǎng)絡(luò)下，最好做到批量執(zhí)行網(wǎng)絡(luò)請(qǐng)求，盡量避免頻繁的間隔網(wǎng)絡(luò)請(qǐng)求。**
通過前面學(xué)習(xí)到的Battery Historian我們可以得到設(shè)備的電量消耗數(shù)據(jù)，如果數(shù)據(jù)中的移動(dòng)蜂窩網(wǎng)絡(luò)(Mobile Radio)電量消耗呈現(xiàn)下面的情況，間隔很小，又頻繁斷斷續(xù)續(xù)的出現(xiàn)，說明電量消耗性能很不好：
經(jīng)過優(yōu)化之后，如果呈現(xiàn)下面的圖示，說明電量消耗的性能是良好的：
另外WiFi連接下，網(wǎng)絡(luò)傳輸?shù)碾娏肯囊纫苿?dòng)網(wǎng)絡(luò)少很多，應(yīng)該盡量減少移動(dòng)網(wǎng)絡(luò)下的數(shù)據(jù)傳輸，多在WiFi環(huán)境下傳輸數(shù)據(jù)。
那么如何才能夠把任務(wù)緩存起來，做到批量化執(zhí)行呢？下面就輪到Job Scheduler出場(chǎng)了。
## 6)Using Job Scheduler
使用[Job Scheduler](https://developer.android.com/reference/android/app/job/JobScheduler.html)，應(yīng)用需要做的事情就是判斷哪些任務(wù)是不緊急的，可以交給Job Scheduler來處理，Job Scheduler集中處理收到的任務(wù)，選擇合適的時(shí)間，合適的網(wǎng)絡(luò)，再一起進(jìn)行執(zhí)行。
下面是使用Job Scheduler的一段簡(jiǎn)要示例，需要先有一個(gè)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);
}
}
}
~~~
然后模擬通過點(diǎn)擊Button觸發(fā)N個(gè)任務(wù)，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);
}
}
}
~~~
**Notes:**關(guān)于更多電量?jī)?yōu)化，還有一篇文章，請(qǐng)點(diǎn)擊[這里](http://developer.android.com/training/efficient-downloads/index.html)
