Google近期在Udacity上发布了Android性能优化的在线课程，分别从渲染，运算与内存，电量几个方面介绍了如何去优化性能，这些课程是Google之前在Youtube上发布的Android性能优化典范专题课程的细化与补充。

下面是电量篇章的学习笔记，部分内容与前面的性能优化典范有重合，欢迎大家一起学习交流！

1)Understanding Battery Drain

手机各个硬件模块的耗电量是不一样的，有些模块非常耗电，而有些模块则相对显得耗电量小很多。

电量消耗的计算与统计是一件麻烦而且矛盾的事情，记录电量消耗本身也是一个费电量的事情。唯一可行的方案是使用第三方监测电量的设备，这样才能够获取到真实的电量消耗。

当设备处于待机状态时消耗的电量是极少的，以N5为例，打开飞行模式，可以待机接近1个月。可是点亮屏幕，硬件各个模块就需要开始工作，这会需要消耗很多电量。

使用WakeLock或者JobScheduler唤醒设备处理定时的任务之后，一定要及时让设备回到初始状态。每次唤醒蜂窝信号进行数据传递，都会消耗很多电量，它比WiFi等操作更加的耗电。

2)Battery Historian

Battery Historian是Android 5.0开始引入的新API。通过下面的指令，可以得到设备上的电量消耗信息：

$ adb shell dumpsys batterystats > xxx.txt  //得到整个设备的电量消耗信息
$ adb shell dumpsys batterystats > com.package.name > xxx.txt //得到指定app相关的电量消耗信息

得到了原始的电量消耗数据之后，我们需要通过Google编写的一个python脚本把数据信息转换成可读性更好的html文件：

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

打开这个转换过后的html文件，可以看到类似TraceView生成的列表数据，这里的数据信息量很大，这里就不展开了。

3)Track Battery Status & Battery Manager

我们可以通过下面的代码来获取手机的当前充电状态：

// 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获取到手机的充电状态，得到充电状态信息之后，我们可以有针对性的对部分代码做优化。比如我们可以判断只有当前手机为AC充电状态时 才去执行一些非常耗电的操作。

/** 
 * 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

高效的保留更多的电量与不断促使用户使用你的App会消耗电量，这是矛盾的选择题。不过我们可以使用一些更好的办法来平衡两者。

假设你的手机里面装了大量的社交类应用，即使手机处于待机状态，也会经常被这些应用唤醒用来检查同步新的数据信息。Android会不断关闭各种硬件来延长手机的待机时间，首先屏幕会逐渐变暗直至关闭，然后CPU进入睡眠，这一切操作都是为了节约宝贵的电量资源。但是即使在这种睡眠状态下，大多数应用还是会尝试进行工作，他们将不断的唤醒手机。一个最简单的唤醒手机的方法是使用PowerManager.WakeLock的API来保持CPU工作并防止屏幕变暗关闭。这使得手机可以被唤醒，执行工作，然后回到睡眠状态。知道如何获取WakeLock是简单的，可是及时释放WakeLock也是非常重要的，不恰当的使用WakeLock会导致严重错误。例如网络请求的数据返回时间不确定，导致本来只需要10s的事情一直等待了1个小时，这样会使得电量白白浪费了。这也是为何使用带超时参数的wakelock.acquice()方法是很关键的。

但是仅仅设置超时并不足够解决问题，例如设置多长的超时比较合适？什么时候进行重试等等？解决上面的问题，正确的方式可能是使用非精准定时器。通常情况下，我们会设定一个时间进行某个操作，但是动态修改这个时间也许会更好。例如，如果有另外一个程序需要比你设定的时间晚5分钟唤醒，最好能够等到那个时候，两个任务捆绑一起同时进行，这就是非精确定时器的核心工作原理。我们可以定制计划的任务，可是系统如果检测到一个更好的时间，它可以推迟你的任务，以节省电量消耗。

这正是JobScheduler API所做的事情。它会根据当前的情况与任务，组合出理想的唤醒时间，例如等到正在充电或者连接到WiFi的时候，或者集中任务一起执行。我们可以通过这个API实现很多免费的调度算法。 5)Network and Battery Drain

下面内容来自官方Training文档中高效下载章节关于手机(Radio)蜂窝信号对电量消耗的介绍。

通常情况下，使用3G移动网络传输数据，电量的消耗有三种状态：

Full power: 能量最高的状态，移动网络连接被激活，允许设备以最大的传输速率进行操作。 Low power: 一种中间状态，对电量的消耗差不多是Full power状态下的50%。 Standby: 最低的状态，没有数据连接需要传输，电量消耗最少。

下图是一个典型的3G Radio State Machine的图示

总之，为了减少电量的消耗，在蜂窝移动网络下，最好做到批量执行网络请求，尽量避免频繁的间隔网络请求。

通过前面学习到的Battery Historian我们可以得到设备的电量消耗数据，如果数据中的移动蜂窝网络(Mobile Radio)电量消耗呈现下面的情况，间隔很小，又频繁断断续续的出现，说明电量消耗性能很不好：

经过优化之后，如果呈现下面的图示，说明电量消耗的性能是良好的：

另外WiFi连接下，网络传输的电量消耗要比移动网络少很多，应该尽量减少移动网络下的数据传输，多在WiFi环境下传输数据。

那么如何才能够把任务缓存起来，做到批量化执行呢？下面就轮到Job Scheduler出场了。

6)Using Job Scheduler

使用Job Scheduler，应用需要做的事情就是判断哪些任务是不紧急的，可以交给Job Scheduler来处理，Job Scheduler集中处理收到的任务，选择合适的时间，合适的网络，再一起进行执行。

下面是使用Job Scheduler的一段简要示例，需要先有一个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);  
        }  
    }  
}

然后模拟通过点击Button触发N个任务，交给JobService来处

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);  
        }  
    }  
}