[main]: Add mdp to lecture notes
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# Power Management
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#### Power Saving
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- Knowledge about the power consumption of each component and app
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- Screen / Network / CPU off or release other device resources as soon as not needed
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- Limit resources for less frequently used apps
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- Set device to sleep as soon as there are no user interactions
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- Global management of running jobs across all apps
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- Perform syncs, uploads & downloads together in a fixed time window
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#### Battery Consumption Statistics
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- Framework tracks the time that devices spend in different states
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- e.g. WiFi chip-set: on/off
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- Display: low/high brightness
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- Controlling service **pushes** state changes to `BatteryStats` service
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- Framework **pulls** the data at these transition points
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- App power consumption is calculated based on CPU run time at specific speeds
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#### Android Power Management Concept
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- **G0** - working
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- **G1** - sleeping
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- **S1** - CPU stops executing instructions, power to CPU and RAM maintained
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- **S2** - CPU powered off, cache is flushed
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- **S3** - Standby / sleep / suspend power to RAM
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- **S4** - Hibernate, suspend disk, RAM powered off
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- **G2** (S5) - soft off
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- **G3** - mechanical off
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#### Power Management Design
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- A wrapper to Linux Power Management
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- Added to the Kernel
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- Wake lock mechanism
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- Apps need to request the CPU & screen to be on with `WakeLocks`; otherwise, Android will shut down the CPU
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- Wake locks and timeouts constantly switch the state of the system’s power
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- Overall system power consumption decreases
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- *Better* use of battery capacity
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### WAKE_LOCK
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Firstly, request the WAKE_LOCK permission in the manifest
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```xml
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<uses-permission android:name="android.permission.WAKE_LOCK"/>
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```
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Use `newWakeLock()` to create a `PowerManager.WakeLock` object
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```java
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PowerManager pm = (PowerManager) getSystemService(Context.POWER_SERVICE);
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PowerManager.WakeLock wl = pm.newWakeLock(PowerManager.SCREEN_DIM_WAKE_LOCK, "My Tag");
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wl.acquire();
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..screen will stay on during this section..
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wl.release();
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```
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| Flag Value | CPU | Screen | Keyboard |
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| :------------------------ | :------------------------------------------------- | ------ | -------- |
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| `PARTIAL_WAKE_LOCK` | On (Don’t sleep even when power button is pressed) | Off | Off |
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| `SCREEN_DIM_WAKE_LOCK` | On | Dim | Off |
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| `SCREEN_BRIGHT_WAKE_LOCK` | On | Bright | Off |
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| `FULL_WAKE_LOCK` | On | Bright | Bright |
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> Creating and holding wake locks can have a **dramatic impact** on the host device's battery life. Thus you should use wake locks only when **strictly necessary** and hold them for as **short a time as possible**.
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>
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> - If app is performing long-running HTTP downloads, consider `DownloadManager`
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> - If app is synchronising data, consider sync adapter
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> - If your app relies on background services, consider `JobScheduler` to trigger services at specific intervals
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> - One use case for wake lock might be a background service that needs to grab a wake lock to keep the CPU running to do work while the screen is off.
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#### Battery Life Enhancement in Android 6.0+
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##### App Standby
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Defer background activity for apps with no recent user interaction
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###### Start Conditions:
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An app that is not actively used for a certain time will be placed in an idle mode
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- Not doing foreground work (activities, services, pending intents, notifications)
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- Hasn’t been explicitly launched for a certain time (can be days)
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###### Actions
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- No network access
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- No background jobs
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- Can set alarms
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- Can use wake locks
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- Is allowed network access once per day
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###### Exit conditions
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- When the device is plugged in
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- Notice when your phone is plugged in, the screen will stay on for longer
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- When a foreground task is performed by the user
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##### Doze
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A deep sleep if the user has not actively used the device for extended periods of time
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###### Starts Doze
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- When device is idle, screen is off, on battery & stationary
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###### In doze
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- No network access
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- No CPU-intensive work
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- Wake locks ignored
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- No Wi-Fi scan
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- Deferred alarm manager alarms
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- Only high-priority notifications received
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- No job scheduler
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- No sync adaptors
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###### Exits Doze
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- User interaction
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- Device motion
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- Screen on
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- Alarm clock alarm
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- Notifications **do not** cause Doze exit
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###### Maintenance windows
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Maintenance windows complete pending activities (syncs, jobs, etc.)
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- SMS & Telephony services are **excluded** from Doze.
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##### Exemptions
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System apps and cloud messaging services pre-loaded on the phone are exempt from App Standby and Doze
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Uses whitelist for apps to be partially exempt from Doze & App standby
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- `isIgnoringBatterOptimizations()` to check if it's in the whitelist
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- `ACTION_IGNORE_BATTERY_OPTIMIZATION_SETTINGS` intent to redirect user to battery optimisation options
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- `REQUEST_IGNORE_BATTERY_OPTIMIZATIONS` allow user to add app to white-list directly
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- White-listed apps can use network and hold partial wake locks during Doze and App standby, but jobs and syncs are still deferred
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- The app should not be on the whitelist unless it cannot use Firebase Cloud Messaging, high-priority messages or the app’s core functionality is affected.
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#### App Stand-By Buckets
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Prioritise apps based on how recently and how frequently the apps are used
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- Active: currently or recently used
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- Working set: regular use
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- Frequent: often used but not every day
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- Rare: not frequently
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- Never: never
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#### Firebase Cloud Messaging
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Replaced Google Cloud Messaging
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- A cross-platform reliable battery-efficient messaging solution that allows messages to be delivered between the server and client.
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- Provides a single and persistent connection to the server.
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- Can notify a client app that a new email or other data is available to sync
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- Can deliver to single device, groups of devices or devices subscribed to topics.
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- Send messages from client apps to the server (e.g. chats, messages, etc.)
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#### Sustained Performance Mode
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- Thermal throttling prevents long-running apps from maintaining their performance (e.g. games, cameras, VR, etc.)
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- App can request to enter SPM, and keep a consistent level of performance
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- Set using `Window.setSustainedPerformanceMode()`
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- System automatically disables the mode when no longer in focus
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# Power 2
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**Background processes**
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- Threads
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- Services
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- `AsyncTasks`
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- `AlarmManager`
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- Based on time
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- `JobScheduler`
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- Based on conditions
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- `WorkManager`
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- Requires Google Play service
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- Automatic use of `alarmManager` and `jobScheduler`
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The bottom three will guarantee completion when we are not worried about when it will happen
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#### Alarm Manager
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- Schedule a task to run at a specific time point or time interval
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- `AlarmManager` holds a CPU wake lock, when the `onReceive()` method is executing
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- Alarm manager operates outside of your application, so it can be used to trigger events when the app is not running or the device is asleep.
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- Alarm delivery is inexact
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- Use `setWindow` and `setExact` for exact delivery
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##### Usage
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- Specify an intent to be broadcast at some time
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- `setRepeating(int type, long triggerAtMillis, long intervalMillis, PendingIntent operation)`
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- `setExact(int type, long triggerAtMillis, PendingIntent operation)`
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- Extend Broadcast receiver
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- `onReceive` method is called when the alarm goes off
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- Start a service to actually do the work
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#### Job Scheduler
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- Many apps perform tasks asynchronously outside the main activity (downloading files)
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- Job Scheduler collects pending jobs across all apps and schedules them to run at about the same time, so that the phone can sleep for longer and save more power
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- Specify requirements for network and timing for each job, then the JS robustly optimises the execution time.
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- May defer jobs that comply with Doze and App StandBy
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- `JobService` **will run on the main thread**; you need to manage any asynchronous tasks yourself (using threads)
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##### Usage
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- Construct `JobInfo` objects using `JobInfo.Builder` using schedule conditions
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- Network type (metered/unmetered)
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- Charging and Idle
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- Content Provider update
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- Back-off (retry) criteria
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- Minimum latency and override deadline
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- Periodic
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- Persistent
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- Extras (customised information from app)
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- Pass it to `JobScheduler` using *schedule* (`JobInfo`)
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- Jobs implemented in the app’s `JobService`
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- Jobs will be executed (batch & defer jobs smartly) at a certain time, but not an exact time (minimum period length is 15 minutes for periodic jobs)
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#### Work Manager
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- Guaranteed but deferrable
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- Supports one-off or periodic tasks
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- Queryable - check if work is successful
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- Chainable: Job 2 is dependent on the execution of job 1
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- Opportunistic: do the background work as soon as it can
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##### When to use work manager
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✓ Upload media to a server
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✓ Parse data and store it in a database
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✓ Periodically sync local data with the network
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✗ Extract paint colour and update image views (use thread pools)
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✗ Parse data and update contents of a View (use thread pools)
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✗ Process payment transactions (use a foreground service)
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##### Usage
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- Add dependencies
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```groovy
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dependencies {
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implementation "androidx.work:work-runtime:$versions.work"
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}
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```
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- Work Request
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```java
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mWorkManager = WorkManager.getInstance(application);
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mWorkManager.enqueue(OneTimeWorkRequest.from(Worker.class));
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```
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- Chain workers
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```java
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WorkContinuation continuation = mWorkManager.beginWith(workA);
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continuation.then(workB).then(workC).enqueue();
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```
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- Set constraint
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```java
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Constraints constraints = new Constraints.Builder()
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.setReuiresCharging(false)
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.build();
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```
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