Android * wake-up lock

WakeLock_WhitePaper.doc
BSD2.0.txt
WakeLock.zip

Christopher Bird

Android power management-wake up lock New Market

Most people may experience the embarrassment of a short battery period on their mobile phones. This is annoying. There is no difference between a cell phone with no power and a cement brick. In general, if a user's cell phone cannot last for a whole day, they will feel very dissatisfied. In addition, when the mobile phone is charged, the user cannot use the mobile phone, which also brings great inconvenience.

Traditionally, jobs that require laptop or PC processing can now be processed using smartphones even if they are not at the desk with all the new improved software. However, compared with a laptop, the small shape of the smartphone greatly limits the size of the battery that can be accommodated. It is difficult to provide a mobile phone with a laptop and a 24-hour battery life.

With excellent power management, Android and other mobile operating systems provide a durable battery life. Shortly after the phone is stopped, the monitor will be shut down, and the CPU will enter the deep energy-saving state. Therefore, when you do not use it, it will only consume a small amount of power. This is why the phone can be used for multiple days after it is fully charged. With the power manager of Android, the CPU is also disabled when the monitor is disabled.

However, Android Developers can (and have permissions) Prevent Android devices from entering sleep mode. They may want the CPU to be active-even if the monitor is off. Or they may want to prevent the monitor from automatically disabling when performing an activity. For this reason, Google * adds a wake-up lock in its PowerManager API. Applications that prevent devices from entering sleep mode can use wake-up locks. As long as there is an active wake-up lock on the system, the device cannot enter the suspend mode unless the wake-up lock is released. When using the wake-up lock, you must understand that when you do not need to wake up the lock, you must release it correctly, because the unreleased wake-up lock cannot enter the default status to save energy, this will soon exhaust the device's battery.

This article will introduce you to some Android apps (and usage modes) that use the wake-up lock by default in Android 4.0 to help you understand when to use this technology. Next, we will introduce a sample application "Wakelocks" in the SDPSamples set to show how to write the code for wakeup locks.

Waken Application Usage

With Android, there is a way to check which services enable the wake-up lock to prevent the system from entering the low power consumption status. The/proc/wakelocks file on the device lists the services and drivers that define the use of wake-up locks. By monitoring the content of the/sys/power/wake_lock file (root access is required), you can know when the wake-up lock is enabled for CPU resources and which service has wakelock2 enabled. I have captured several use cases where the wake-up lock is enabled on Galaxy Nexus running Android 4.0, as shown below:

Application used	Executed operation	Services with wake-up locks	Running status
Arbitrary	Press the UI Widget (for example, click a button or ListView)	PowerManagerService	Enabled and locked after 5 seconds
MAP/navigation	Enable map or enter navigation	Gps-lock	Enable locking and use GPS
YouTube	Watch Streaming Videos	PowerManagerService	Enable the wake-up lock throughout video playback.
Music	Listen to music	PowerManagerService	Enable the wake-up lock during music playing

Table: some default Android applications demonstrate the use of wake-up locks

The YouTube and Music apps can demonstrate different levels of wake-up locks. When you play a video, the YouTube app will enable the wake-up lock. During the video playing process, the display will remain on (ignore the system display settings ). However, if you press the power button during playback, the device will be suspended, which will cause the display to shut down and the audio/video to stop playing. The Music application uses different wake-up locks when playing audio. The display settings cannot be changed, so the screen of the device will be closed based on the user's display settings. When the monitor is off, the wake-up lock keeps the CPU active so that the audio can continue playing-even if the user presses the power button.

Select wake-up lock (before execution)

Before learning how to write a wake-up lock, you must understand the types of the wake-up lock to select the most appropriate wake-up lock for your application. The Android PowerManager API introduces multiple wake-up lock tags used to change the power status of a device:

Tag Value	CPU	Screen	Keyboard
PARTIAL_WAKE_LOCK	Enable	Close	Close
SCREEN_DIM_WAKE_LOCK	Enable	Dim)	Close
SCREEN_BRIGHT_WAKE_LOCK	Enable	Bright (Bright)	Close
FULL_WAKE_LOCK	Enable	Bright (Bright)	Bright (Bright)

Table: Derived from the Android PowerManager API.

This API highlights that wake-up locks can significantly shorten the battery life of Android devices. Therefore, you should avoid using them as much as possible. If used, release it as soon as possible.

The application that uses the wake-up lock must apply for a special license before it can be executed. This can be achieved through the android. permission. WAKE_LOCK license in the Application List file. That is to say, when a user installs an application that uses the wake-up lock through Google Play, the system will remind them that the features contained in the application may be "Prevent phone from sleeping (preventing the phone from going to sleep )". If developers want to prevent the display of an application from being dimmed under certain conditions, they can use another method provided by Google, which requires no special permission. WindowManager includes a FLAG_KEEP_SCREEN_ON variable. You can set the FLAG_KEEP_SCREEN_ON variable when the application view is disabled. We recommend that you use this method in display control because its impact is independent of each other in applications. After the user task is switched to another application, WindowManager immediately releases the wake-up lock.

Keep the monitor on (derived from SDPSamples)

The wake-up lock application of the SDPSamples set demonstrates (including code) how an application uses Window Manager instead of enabling the display by writing the wake-up lock code. After the wake-up lock application is enabled, select the "Win Man Screen On" list item.

As long as the button status shows "Screen is LOCKED", the interface will remain in the enabled status. After the button status is changed to "Screen is UNLOCKED", if it is not operated for more than 5 seconds, the display will be closed.

In the Code, each time you press the button and the status changes, you can use the screenLockUpdateState () function in WakeLockActivity. java to set and release the FLAG_KEEP_SCREEN_ON variable of the current window.

 
 

  
  
 
   
 
    
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public void screenLockUpdateState() {
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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    if (mIsDisplayLocked)
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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     {
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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        ...
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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        // update display state
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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      getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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    } else
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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{
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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        ...
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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        // update display state
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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          getWindow().clearFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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    }
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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} 
 
   
 
  
 
 

Write a wake-up lock (from SDPSamples)

The wake-up lock application in the SDPSamples set also includes the code for executing different wake-up locks. After the Wake-up Lock application is started, choose from the following four types of Wake-up locks: Power Wake Lock Full, Power Wake Lock Bright, Power Wake Lock Dim, and Power Wake Lock Partial. The four list items correspond to the four wake-up lock tags described in the PowerManager API. Each item demonstrates how the device responds when it tries to shut down the screen within five seconds.

By monitoring the content of the/sys/power/wake_lock file (root access is required), you can see that after the power button is pressed, PARTIAL_WAKE_LOCK is the only and still valid wake-up lock. Other wake-up locks prevent the display from being closed with multiple levels of brightness.

The first step in writing a wake-up lock is to apply for a license before you can use the wake-up lock in the list AndroidManifest. xml:

After completing this step, you can create a wake-up Lock Object, including the functions for controlling the wake-up lock acquisition () and release. The encoding in the WakeLockActivity. java file shows the situation well:

 
 

  
  
 
   
 
    
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public void onCreate(Bundle savedInstanceState) {
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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    ...
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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    mPowerManager = (PowerManager) getSystemService(Context.POWER_SERVICE);
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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    ...
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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    mWakeLock = mPowerManager.newWakeLock(mWakeLockState,
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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                        "UMSE PowerTest");
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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    if (mWakeLock != null) {
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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        mWakeLock.acquire();
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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    ... 
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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    }
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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}
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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protected void onDestroy() {
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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    if (mWakeLock != null) {
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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        mWakeLock.release();
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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        mWakeLock = null;
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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    }
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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    ... 
 
   
 
  
 
 
 
 

  
  
 
   
 
    
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}
 
   
 
  
 
 

Conclusion

Wake-up lock is a powerful concept in Android. It allows developers to modify the default Power status of a device. The risk of using a wake-up lock in an application: it reduces the battery life of the device. The obvious advantages of the wake-up lock can be seen in a variety of default applications provided by Google, such as road navigation and music/video playback. Therefore, developers should determine whether their application models can benefit from the use of wake-up locks.

About the author

Christopher Bird entered Intel's SSG in 2007 and is committed to building an ecosystem that supports lingdong devices (mobile phones and tablets ).

Reference

1 Android reference: http://developer.android.com/reference/android/ OS /PowerManager.html

2 LWN-"wakeup lock and embedded problems": http://lwn.net/Articles/318611/

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