WIFI电源管理

Android 对WiFi电源管理的代码主要在WifiService.java中。如果应用程序想在屏幕被关掉后继续使用WiFi则可以调用acquireWifiLock来锁住WiFi，该操作会阻止WiFi进入睡眠状态。当应用程序不再使用WiFi时需要调用releaseWifiLock来释放WiFi。之后WiFi可以进入睡眠状态以节省电源。
    默认情况下当屏幕被关掉以后，如果没有应用程序在使用WiFi，WiFi会在2分钟后进入睡眠状态。这主要是为防止频繁地改变WiFi的电源模式。

1. INTRODUCTION

    WifiLock Allows an application to keep the Wi-Fi radio awake. Normally the Wi-Fi radio may turn off when the user has not used the device in a while. Acquiring a WifiLock will keep the radio on until the lock is released. Multiple applications may hold WifiLocks, and the radio will only be allowed to turn off when no WifiLocks are held in any application.
     Before using a WifiLock, consider carefully if your application requires Wi-Fi access, or could function over a mobile network, if available. A program that needs to download large files should hold a WifiLock to ensure that the download will complete, but a program whose network usage is occasional or low-bandwidth should not hold a WifiLock to avoid adversely affecting battery life.
     Note that WifiLocks cannot override the user-level "Wi-Fi Enabled" setting, nor Airplane Mode. They simply keep the radio from turning off when Wi-Fi is already on but the device is idle.

2. API

    WifiLock APIs are implemented in android.net.wifi.WifiManager.
    2.1 acquire()
        Locks the Wi-Fi radio on until release is called.
        If this WifiLock is reference-counted, each call to acquire will increment the reference count, and the radio will remain locked as long as the reference count is above zero.
        If this WifiLock is not reference-counted, the first call to acquire will lock the radio, but subsequent calls will be ignored. Only one call to release will be required, regardless of the number of times that acquire is called.

    2.2 release()
        Unlocks the Wi-Fi radio, allowing it to turn off when the device is idle.
        If this WifiLock is reference-counted, each call to release will decrement the reference count, and the radio will be unlocked only when the reference count reaches zero. If the reference count goes below zero (that is, if release is called a greater number of times than acquire), an exception is thrown.
         If this WifiLock is not reference-counted, the first call to release (after the radio was locked using acquire) will unlock the radio, and subsequent calls will be ignored.

    2.3 setReferenceCounted(boolean refCounted)
        Controls whether this is a reference-counted or non-reference-counted WifiLock.
        Reference-counted WifiLocks keep track of the number of calls to acquire and release, and only allow the radio to sleep when every call to acquire has been balanced with a call to release. Non-reference-counted WifiLocks lock the radio whenever acquire is called and it is unlocked, and unlock the radio whenever release is called and it is locked.
        
         Param: refCounted true if this WifiLock should keep a reference count

    2.4 isHeld()
         Checks whether this WifiLock is currently held.
       
         return true if this WifiLock is held, false otherwise
        

3. EXAMPLE

    frameworks/base/services/java/com/android/server/LocationManagerService.java is a good example for your reference.

    Our simple example is as blew.

import android.os.PowerManager;
import android.net.wifi.WifiManager;

public class WifiLockExample{
    private final Context mContext;
    private PowerManager.WakeLock mWakeLock = null;
    private WifiManager.WifiLock mWifiLock = null;

    public WifiLockExample(Context context){
        mContext = context;
        // Create a wake lock
        PowerManager powerManager = (PowerManager) mContext.getSystemService(Context.POWER_SERVICE);
        mWakeLock = powerManager.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, WAKELOCK_KEY);
        mWakeLock.setReferenceCounted(true);
        // Create a wifi lock
        WifiManager wifiManager = (WifiManager) context.getSystemService(Context.WIFI_SERVICE);
        mWifiLock = wifiManager.createWifiLock(WIFILOCK_KEY);
        mWifiLock.setReferenceCounted(true);
    }

    private void acquire(){
        // Acquire wake lock
        mWakeLock.acquire();
        // Acquire wifi lock
        mWifiLock.acquire();
    }
    private void release(){
        // Release wifi lock
        mWifiLock.release();
        // Release wake lock
        mWakeLock.release();
    }
}

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