PowerManagerService在updateDisplayPowerStateLocked中與DisplayPowerControler互動，詳細看程式碼：

 private boolean updateDisplayPowerStateLocked(int dirty) {
        final boolean oldDisplayReady = mDisplayReady;
        if ((dirty & (DIRTY_WAKE_LOCKS | DIRTY_USER_ACTIVITY | DIRTY_WAKEFULNESS
                | DIRTY_ACTUAL_DISPLAY_POWER_STATE_UPDATED | DIRTY_BOOT_COMPLETED
                | DIRTY_SETTINGS | DIRTY_SCREEN_BRIGHTNESS_BOOST)) != 0) {
            mDisplayPowerRequest.policy = getDesiredScreenPolicyLocked();

            // Determine appropriate screen brightness and auto-brightness adjustments.
            int screenBrightness = mScreenBrightnessSettingDefault;
            float screenAutoBrightnessAdjustment = 0.0f;
            boolean autoBrightness = (mScreenBrightnessModeSetting ==
                    Settings.System.SCREEN_BRIGHTNESS_MODE_AUTOMATIC);
            if (isValidBrightness(mScreenBrightnessOverrideFromWindowManager)) {
                screenBrightness = mScreenBrightnessOverrideFromWindowManager;
                autoBrightness = false;
            } else if (isValidBrightness(mTemporaryScreenBrightnessSettingOverride)) {
                screenBrightness = mTemporaryScreenBrightnessSettingOverride;
            } else if (isValidBrightness(mScreenBrightnessSetting)) {
                screenBrightness = mScreenBrightnessSetting;
            }
            if (autoBrightness) {
                screenBrightness = mScreenBrightnessSettingDefault;
                if (isValidAutoBrightnessAdjustment(
                        mTemporaryScreenAutoBrightnessAdjustmentSettingOverride)) {
                    screenAutoBrightnessAdjustment =
                            mTemporaryScreenAutoBrightnessAdjustmentSettingOverride;
                } else if (isValidAutoBrightnessAdjustment(
                        mScreenAutoBrightnessAdjustmentSetting)) {
                    screenAutoBrightnessAdjustment = mScreenAutoBrightnessAdjustmentSetting;
                }
            }
            screenBrightness = Math.max(Math.min(screenBrightness,
                    mScreenBrightnessSettingMaximum), mScreenBrightnessSettingMinimum);
            screenAutoBrightnessAdjustment = Math.max(Math.min(
                    screenAutoBrightnessAdjustment, 1.0f), -1.0f);

            // Update display power request.
            mDisplayPowerRequest.screenBrightness = screenBrightness;
            mDisplayPowerRequest.screenAutoBrightnessAdjustment =
                    screenAutoBrightnessAdjustment;
            mDisplayPowerRequest.useAutoBrightness = autoBrightness;
            mDisplayPowerRequest.useProximitySensor = shouldUseProximitySensorLocked();
            mDisplayPowerRequest.lowPowerMode = mLowPowerModeEnabled;
            mDisplayPowerRequest.boostScreenBrightness = mScreenBrightnessBoostInProgress;

            if (mDisplayPowerRequest.policy == DisplayPowerRequest.POLICY_DOZE) {
                mDisplayPowerRequest.dozeScreenState = mDozeScreenStateOverrideFromDreamManager;
                mDisplayPowerRequest.dozeScreenBrightness =
                        mDozeScreenBrightnessOverrideFromDreamManager;
            } else {
                mDisplayPowerRequest.dozeScreenState = Display.STATE_UNKNOWN;
                mDisplayPowerRequest.dozeScreenBrightness = PowerManager.BRIGHTNESS_DEFAULT;
            }

            mDisplayReady = mDisplayManagerInternal.requestPowerState(mDisplayPowerRequest,//呼叫DisplayPowerControler的requestPowerstate介面
                    mRequestWaitForNegativeProximity);
            mRequestWaitForNegativeProximity = false;

        }
        return mDisplayReady && !oldDisplayReady;
    }
 

接下來我們再看看mDisplayManagerInternal這個成員變數：在systemready中

mDisplayManagerInternal = getLocalService(DisplayManagerInternal.class);
。。。。
mDisplayManagerInternal.initPowerManagement(
                    mDisplayPowerCallbacks, mHandler, sensorManager);

而DisplayPowerControler是在DisplayManagerService中定義，並且放在localservice中，publish出來。

    private final class LocalService extends DisplayManagerInternal {
        @Override
        public void initPowerManagement(final DisplayPowerCallbacks callbacks, Handler handler,
                SensorManager sensorManager) {
            synchronized (mSyncRoot) {
                DisplayBlanker blanker = new DisplayBlanker() {
                    @Override
                    public void requestDisplayState(int state) {
                        // The order of operations is important for legacy reasons.
                        if (state == Display.STATE_OFF) {
                            requestGlobalDisplayStateInternal(state);
                        }

                        callbacks.onDisplayStateChange(state);//注意這個介面後續會介紹

                        if (state != Display.STATE_OFF) {
                            requestGlobalDisplayStateInternal(state);
                        }
                    }
                };
                mDisplayPowerController = new DisplayPowerController(//new一個DisplayPowerController
                        mContext, callbacks, handler, sensorManager, blanker);
            }
        }

        @Override
        public boolean requestPowerState(DisplayPowerRequest request,
                boolean waitForNegativeProximity) {
            return mDisplayPowerController.requestPowerState(request,
                    waitForNegativeProximity);
        }
。。。。。
 

好接下來我們先分析DisplayPowerControler中的requestPowerState函式：

    public boolean requestPowerState(DisplayPowerRequest request,
            boolean waitForNegativeProximity) {
        synchronized (mLock) {
            boolean changed = false;

            if (waitForNegativeProximity
                    && !mPendingWaitForNegativeProximityLocked) {
                mPendingWaitForNegativeProximityLocked = true;
                changed = true;
            }

            if (mPendingRequestLocked == null) {
                mPendingRequestLocked = new DisplayPowerRequest(request);
                changed = true;
            } else if (!mPendingRequestLocked.equals(request)) {
                mPendingRequestLocked.copyFrom(request);
                changed = true;
            }

            if (changed) {
                mDisplayReadyLocked = false;//注意有變化，返回的是false
            }

            if (changed && !mPendingRequestChangedLocked) {
                mPendingRequestChangedLocked = true;
                sendUpdatePowerStateLocked();//傳送訊號，到updatePowerState
            }

            return mDisplayReadyLocked;
        }
    }

 private void updatePowerState() {
        // Update the power state request.
        final boolean mustNotify;
        boolean mustInitialize = false;
        boolean autoBrightnessAdjustmentChanged = false;

        synchronized (mLock) {
            mPendingUpdatePowerStateLocked = false;
            if (mPendingRequestLocked == null) {
                return; // wait until first actual power request
            }

            if (mPowerRequest == null) {
                mPowerRequest = new DisplayPowerRequest(mPendingRequestLocked);
                mWaitingForNegativeProximity = mPendingWaitForNegativeProximityLocked;
                mPendingWaitForNegativeProximityLocked = false;
                mPendingRequestChangedLocked = false;
                mustInitialize = true;
            } else if (mPendingRequestChangedLocked) {
                autoBrightnessAdjustmentChanged = (mPowerRequest.screenAutoBrightnessAdjustment
                        != mPendingRequestLocked.screenAutoBrightnessAdjustment);
                mPowerRequest.copyFrom(mPendingRequestLocked);
                mWaitingForNegativeProximity |= mPendingWaitForNegativeProximityLocked;
                mPendingWaitForNegativeProximityLocked = false;
                mPendingRequestChangedLocked = false;
                mDisplayReadyLocked = false;//這時候mDisplayReadyLocked還是false，這是requestPowerState的返回值
            }

            mustNotify = !mDisplayReadyLocked;
        }

        // Initialize things the first time the power state is changed.
        if (mustInitialize) {
            initialize();
        }

        // Compute the basic display state using the policy.
        // We might override this below based on other factors.
        int state;
        int brightness = PowerManager.BRIGHTNESS_DEFAULT;
        boolean performScreenOffTransition = false;
        switch (mPowerRequest.policy) {//根據policy，得到顯示的state
            case DisplayPowerRequest.POLICY_OFF:
                state = Display.STATE_OFF;
                performScreenOffTransition = true;
                break;
            case DisplayPowerRequest.POLICY_DOZE:
                if (mPowerRequest.dozeScreenState != Display.STATE_UNKNOWN) {
                    state = mPowerRequest.dozeScreenState;
                } else {
                    state = Display.STATE_DOZE;
                }
                if (!mAllowAutoBrightnessWhileDozingConfig) {
                    brightness = mPowerRequest.dozeScreenBrightness;
                }
                break;
            case DisplayPowerRequest.POLICY_DIM:
            case DisplayPowerRequest.POLICY_BRIGHT:
            default:
                state = Display.STATE_ON;
                break;
        }
        assert(state != Display.STATE_UNKNOWN);

        // Apply the proximity sensor.這塊是距離感測器
        if (mProximitySensor != null) {
            if (mPowerRequest.useProximitySensor && state != Display.STATE_OFF) {
                setProximitySensorEnabled(true);
                if (!mScreenOffBecauseOfProximity
                        && mProximity == PROXIMITY_POSITIVE) {
                    mScreenOffBecauseOfProximity = true;
                    sendOnProximityPositiveWithWakelock();
                }
            } else if (mWaitingForNegativeProximity
                    && mScreenOffBecauseOfProximity
                    && mProximity == PROXIMITY_POSITIVE
                    && state != Display.STATE_OFF) {
                setProximitySensorEnabled(true);
            } else {
                setProximitySensorEnabled(false);
                mWaitingForNegativeProximity = false;
            }
            if (mScreenOffBecauseOfProximity
                    && mProximity != PROXIMITY_POSITIVE) {
                mScreenOffBecauseOfProximity = false;
                sendOnProximityNegativeWithWakelock();
            }
        } else {
            mWaitingForNegativeProximity = false;
        }
        if (mScreenOffBecauseOfProximity) {
            state = Display.STATE_OFF;
        }

        // Animate the screen state change unless already animating.
        // The transition may be deferred, so after this point we will use the
        // actual state instead of the desired one.
        //後面詳細分析下這函式，主要設定顯示的state，和displayPowerState互動。
        animateScreenStateChange(state, performScreenOffTransition);
        state = mPowerState.getScreenState();

        // Use zero brightness when screen is off.
        if (state == Display.STATE_OFF) {
            brightness = PowerManager.BRIGHTNESS_OFF;
        }

        // Configure auto-brightness.
        boolean autoBrightnessEnabled = false;
        if (mAutomaticBrightnessController != null) {
            final boolean autoBrightnessEnabledInDoze = mAllowAutoBrightnessWhileDozingConfig
                    && (state == Display.STATE_DOZE || state == Display.STATE_DOZE_SUSPEND);
            autoBrightnessEnabled = mPowerRequest.useAutoBrightness
                    && (state == Display.STATE_ON || autoBrightnessEnabledInDoze)
                    && brightness < 0;
            mAutomaticBrightnessController.configure(autoBrightnessEnabled,
                    mPowerRequest.screenAutoBrightnessAdjustment, state != Display.STATE_ON);
        }

        // Apply brightness boost.
        // We do this here after configuring auto-brightness so that we don't
        // disable the light sensor during this temporary state.  That way when
        // boost ends we will be able to resume normal auto-brightness behavior
        // without any delay.
        if (mPowerRequest.boostScreenBrightness
                && brightness != PowerManager.BRIGHTNESS_OFF) {
            brightness = PowerManager.BRIGHTNESS_ON;
        }

        // Apply auto-brightness.
        boolean slowChange = false;
        if (brightness < 0) {
            if (autoBrightnessEnabled) {
                brightness = mAutomaticBrightnessController.getAutomaticScreenBrightness();
            }
            if (brightness >= 0) {
                // Use current auto-brightness value and slowly adjust to changes.
                brightness = clampScreenBrightness(brightness);
                if (mAppliedAutoBrightness && !autoBrightnessAdjustmentChanged) {
                    slowChange = true; // slowly adapt to auto-brightness
                }
                mAppliedAutoBrightness = true;
            } else {
                mAppliedAutoBrightness = false;
            }
        } else {
            mAppliedAutoBrightness = false;
        }

        // Use default brightness when dozing unless overridden.
        if (brightness < 0 && (state == Display.STATE_DOZE
                || state == Display.STATE_DOZE_SUSPEND)) {
            brightness = mScreenBrightnessDozeConfig;
        }

        // Apply manual brightness.
        // Use the current brightness setting from the request, which is expected
        // provide a nominal default value for the case where auto-brightness
        // is not ready yet.
        if (brightness < 0) {
            brightness = clampScreenBrightness(mPowerRequest.screenBrightness);
        }

        // Apply dimming by at least some minimum amount when user activity
        // timeout is about to expire.
        if (mPowerRequest.policy == DisplayPowerRequest.POLICY_DIM) {
            if (brightness > mScreenBrightnessRangeMinimum) {
                brightness = Math.max(Math.min(brightness - SCREEN_DIM_MINIMUM_REDUCTION,
                        mScreenBrightnessDimConfig), mScreenBrightnessRangeMinimum);
            }
            if (!mAppliedDimming) {
                slowChange = false;
            }
            mAppliedDimming = true;
        }

        // If low power mode is enabled, cut the brightness level by half
        // as long as it is above the minimum threshold.
        if (mPowerRequest.lowPowerMode) {//低功耗模式
            if (brightness > mScreenBrightnessRangeMinimum) {
                brightness = Math.max(brightness / 2, mScreenBrightnessRangeMinimum);
            }
            if (!mAppliedLowPower) {
                slowChange = false;
            }
            mAppliedLowPower = true;
        }

        // Animate the screen brightness when the screen is on or dozing.
        // Skip the animation when the screen is off or suspended.
        if (!mPendingScreenOff) {
            if (state == Display.STATE_ON || state == Display.STATE_DOZE) {
                animateScreenBrightness(brightness,
                        slowChange ? BRIGHTNESS_RAMP_RATE_SLOW : BRIGHTNESS_RAMP_RATE_FAST);
            } else {
                animateScreenBrightness(brightness, 0);
            }
        }

        // Determine whether the display is ready for use in the newly requested state.
        // Note that we do not wait for the brightness ramp animation to complete before
        // reporting the display is ready because we only need to ensure the screen is in the
        // right power state even as it continues to converge on the desired brightness.
        final boolean ready = mPendingScreenOnUnblocker == null
                && !mColorFadeOnAnimator.isStarted()
                && !mColorFadeOffAnimator.isStarted()
                && mPowerState.waitUntilClean(mCleanListener);
        final boolean finished = ready
                && !mScreenBrightnessRampAnimator.isAnimating();

        // Grab a wake lock if we have unfinished business.
        if (!finished && !mUnfinishedBusiness) {
            if (DEBUG) {
                Slog.d(TAG, "Unfinished business...");
            }
            mCallbacks.acquireSuspendBlocker();//回撥持cpu鎖
            mUnfinishedBusiness = true;//使用該變數，只能進一次該函式
        }

        // Notify the power manager when ready.
        if (ready && mustNotify) {
            // Send state change.
            synchronized (mLock) {
                if (!mPendingRequestChangedLocked) {
                    mDisplayReadyLocked = true;

                    if (DEBUG) {
                        Slog.d(TAG, "Display ready!");
                    }
                }
            }
            sendOnStateChangedWithWakelock();//重新在PowerManagerService中呼叫updatePowerState函式
        }

        // Release the wake lock when we have no unfinished business.
        if (finished && mUnfinishedBusiness) {
            if (DEBUG) {
                Slog.d(TAG, "Finished business...");
            }
            mUnfinishedBusiness = false;
            mCallbacks.releaseSuspendBlocker();//釋放cpu鎖
        }
    }

我們先來看下在上面函式中呼叫的幾個mCallbacks函式：

    private final DisplayManagerInternal.DisplayPowerCallbacks mDisplayPowerCallbacks =
            new DisplayManagerInternal.DisplayPowerCallbacks() {
        private int mDisplayState = Display.STATE_UNKNOWN;

        @Override
        public void onStateChanged() {//只是重新呼叫下powerManagerService中的updatePowerStateLocked函式
            synchronized (mLock) {
                mDirty |= DIRTY_ACTUAL_DISPLAY_POWER_STATE_UPDATED;
                updatePowerStateLocked();
            }
        }

        @Override
        public void onProximityPositive() {
            synchronized (mLock) {
                mProximityPositive = true;
                mDirty |= DIRTY_PROXIMITY_POSITIVE;
                updatePowerStateLocked();
            }
        }

        @Override
        public void onProximityNegative() {
            synchronized (mLock) {
                mProximityPositive = false;
                mDirty |= DIRTY_PROXIMITY_POSITIVE;
                userActivityNoUpdateLocked(SystemClock.uptimeMillis(),
                        PowerManager.USER_ACTIVITY_EVENT_OTHER, 0, Process.SYSTEM_UID);
                updatePowerStateLocked();
            }
        }

        @Override
        public void onDisplayStateChange(int state) {
            // This method is only needed to support legacy display blanking behavior
            // where the display's power state is coupled to suspend or to the power HAL.
            // The order of operations matters here.
            synchronized (mLock) {
                if (mDisplayState != state) {
                    mDisplayState = state;
                    if (state == Display.STATE_OFF) {
                        if (!mDecoupleHalInteractiveModeFromDisplayConfig) {
                            setHalInteractiveModeLocked(false);
                        }
                        if (!mDecoupleHalAutoSuspendModeFromDisplayConfig) {
                            setHalAutoSuspendModeLocked(true);
                        }
                    } else {
                        if (!mDecoupleHalAutoSuspendModeFromDisplayConfig) {
                            setHalAutoSuspendModeLocked(false);
                        }
                        if (!mDecoupleHalInteractiveModeFromDisplayConfig) {
                            setHalInteractiveModeLocked(true);
                        }
                    }
                }
            }
        }

        @Override
        public void acquireSuspendBlocker() {
            mDisplaySuspendBlocker.acquire();
        }

        @Override
        public void releaseSuspendBlocker() {
            mDisplaySuspendBlocker.release();
        }

而sendOnStateChangedWithWakelock程式碼如下：

    private void sendOnStateChangedWithWakelock() {
        mCallbacks.acquireSuspendBlocker();
        mHandler.post(mOnStateChangedRunnable);
    }

    private final Runnable mOnStateChangedRunnable = new Runnable() {直接在兩個執行緒中，這個handler是PowerManagerService中的
        @Override
        public void run() {
            mCallbacks.onStateChanged();
            mCallbacks.releaseSuspendBlocker();
        }
    };

第一次呼叫requestPowerState時返回的mDisplayReadyLocked = false，然後在DisplayPowerControler中的updatePowerState函式全都好了，把變數mDisplayReadyLocked = true ，再呼叫PowerManagerService中的updatePowerLocked，再調的話mDisplayReadyLocked 就返回true了。

接下來我們來看animateScreenStateChange函式，主要是設定display的state。

    private void animateScreenStateChange(int target, boolean performScreenOffTransition) {
        // If there is already an animation in progress, don't interfere with it.
        if (mColorFadeOnAnimator.isStarted()
                || mColorFadeOffAnimator.isStarted()) {
            return;
        }

        if (mPendingScreenOff && target != Display.STATE_OFF) {
            setScreenState(Display.STATE_OFF);//設定display的state
            mPendingScreenOff = false;
        }

        if (target == Display.STATE_ON) {
            if (!setScreenState(Display.STATE_ON)) {
                return; // screen on blocked
            }
            if (USE_COLOR_FADE_ON_ANIMATION && mPowerRequest.isBrightOrDim()) {
                // Perform screen on animation.
                if (mPowerState.getColorFadeLevel() == 1.0f) {
                    mPowerState.dismissColorFade();
                } else if (mPowerState.prepareColorFade(mContext,
                        mColorFadeFadesConfig ?
                                ColorFade.MODE_FADE :
                                        ColorFade.MODE_WARM_UP)) {
                    mColorFadeOnAnimator.start();
                } else {
                    mColorFadeOnAnimator.end();
                }
            } else {
                // Skip screen on animation.
                mPowerState.setColorFadeLevel(1.0f);
                mPowerState.dismissColorFade();
            }
        } else if (target == Display.STATE_DOZE) {
            // Want screen dozing.
            // Wait for brightness animation to complete beforehand when entering doze
            // from screen on to prevent a perceptible jump because brightness may operate
            // differently when the display is configured for dozing.
            if (mScreenBrightnessRampAnimator.isAnimating()
                    && mPowerState.getScreenState() == Display.STATE_ON) {
                return;
            }

            // Set screen state.
            if (!setScreenState(Display.STATE_DOZE)) {
                return; // screen on blocked
            }

。。。。。。。。。。

</pre><p>再看setScreenState函式</p><pre class="java" name="code">    private boolean setScreenState(int state) {
        if (mPowerState.getScreenState() != state) {
            final boolean wasOn = (mPowerState.getScreenState() != Display.STATE_OFF);
            mPowerState.setScreenState(state);//呼叫DisplayPowerState的setScreenState

            // Tell battery stats about the transition.
            try {
                mBatteryStats.noteScreenState(state);
            } catch (RemoteException ex) {
                // same process
            }

            // Tell the window manager what's happening.
            // Temporarily block turning the screen on until the window manager is ready
            // by leaving a black surface covering the screen.  This surface is essentially
            // the final state of the color fade animation.
            boolean isOn = (state != Display.STATE_OFF);
            if (wasOn && !isOn) {
                unblockScreenOn();
                mWindowManagerPolicy.screenTurnedOff();//通知window Manager
            } else if (!wasOn && isOn) {
                if (mPowerState.getColorFadeLevel() == 0.0f) {
                    blockScreenOn();
                } else {
                    unblockScreenOn();
                }
                mWindowManagerPolicy.screenTurningOn(mPendingScreenOnUnblocker);
            }
        }
        return mPendingScreenOnUnblocker == null;
    }

好，接下來我們就主要分析DisPlayPowerRequest，先看setScreenState

    public void setScreenState(int state) {
        if (mScreenState != state) {
            if (DEBUG) {
                Slog.d(TAG, "setScreenState: state=" + state);
            }

            mScreenState = state;
            mScreenReady = false;
            scheduleScreenUpdate();
        }
    }
    
    private void scheduleScreenUpdate() {
        if (!mScreenUpdatePending) {
            mScreenUpdatePending = true;
            postScreenUpdateThreadSafe();
        }
    
    private void postScreenUpdateThreadSafe() {
        mHandler.removeCallbacks(mScreenUpdateRunnable);
        mHandler.post(mScreenUpdateRunnable);
    }

    private final Runnable mScreenUpdateRunnable = new Runnable() {
        @Override
        public void run() {
            mScreenUpdatePending = false;

            int brightness = mScreenState != Display.STATE_OFF
                    && mColorFadeLevel > 0f ? mScreenBrightness : 0;
            if (mPhotonicModulator.setState(mScreenState, brightness)) {//最後呼叫了執行緒的setState
                if (DEBUG) {
                    Slog.d(TAG, "Screen ready");
                }
                mScreenReady = true;
                invokeCleanListenerIfNeeded();
            } else {
                if (DEBUG) {
                    Slog.d(TAG, "Screen not ready");
                }
            }
        }
    };    

下面我們分析下PhotonicModulator這個執行緒類：

    private final class PhotonicModulator extends Thread {
        private static final int INITIAL_SCREEN_STATE = Display.STATE_OFF; // unknown, assume off
        private static final int INITIAL_BACKLIGHT = -1; // unknown

        private final Object mLock = new Object();

        private int mPendingState = INITIAL_SCREEN_STATE;
        private int mPendingBacklight = INITIAL_BACKLIGHT;
        private int mActualState = INITIAL_SCREEN_STATE;
        private int mActualBacklight = INITIAL_BACKLIGHT;
        private boolean mChangeInProgress;

        public boolean setState(int state, int backlight) {
            synchronized (mLock) {
                if (state != mPendingState || backlight != mPendingBacklight) {
                    if (DEBUG) {
                        Slog.d(TAG, "Requesting new screen state: state="
                                + Display.stateToString(state) + ", backlight=" + backlight);
                    }

                    mPendingState = state;//設定state
                    mPendingBacklight = backlight;

                    if (!mChangeInProgress) {
                        mChangeInProgress = true;
                        mLock.notifyAll();
                    }
                }
                return !mChangeInProgress;
            }
        }

        @Override
        public void run() {
            for (;;) {//執行緒一直跑，狀態改變就去PowerManagerService中呼叫setHalInteractiveModeLocked設定螢幕狀態
                // Get pending change.
                final int state;
                final boolean stateChanged;
                final int backlight;
                final boolean backlightChanged;
                synchronized (mLock) {
                    state = mPendingState;
                    stateChanged = (state != mActualState);//state是否改變
                    backlight = mPendingBacklight;
                    backlightChanged = (backlight != mActualBacklight);
                    if (!stateChanged && !backlightChanged) {
                        // All changed applied, notify outer class and wait for more.
                        mChangeInProgress = false;
                        postScreenUpdateThreadSafe();
                        try {
                            mLock.wait();
                        } catch (InterruptedException ex) { }
                        continue;
                    }
                    mActualState = state;
                    mActualBacklight = backlight;
                }

                // Apply pending change.
                if (DEBUG) {
                    Slog.d(TAG, "Updating screen state: state="
                            + Display.stateToString(state) + ", backlight=" + backlight);
                }
                boolean suspending = Display.isSuspendedState(state);
                if (stateChanged && !suspending) {改變了就呼叫requestDisplayState函式
                    requestDisplayState(state);
                }
                if (backlightChanged) {
                    setBrightness(backlight);
                }
                if (stateChanged && suspending) {
                    requestDisplayState(state);
                }
            }
        }

        private void requestDisplayState(int state) {
            Trace.traceBegin(Trace.TRACE_TAG_POWER, "requestDisplayState("
                    + Display.stateToString(state) + ")");
            try {
                mBlanker.requestDisplayState(state);//其中的呼叫了PowerManagerService的函式，詳細看下面
            } finally {
                Trace.traceEnd(Trace.TRACE_TAG_POWER);
            }
        }

        private void setBrightness(int backlight) {
            Trace.traceBegin(Trace.TRACE_TAG_POWER, "setBrightness(" + backlight + ")");
            try {
                mBacklight.setBrightness(backlight);
            } finally {
                Trace.traceEnd(Trace.TRACE_TAG_POWER);
            }
        }
    }

當display的state改變的時候呼叫了mBlanker的requestDisplayState函式，而這個blanker如下：

    private final class LocalService extends DisplayManagerInternal {
        @Override
        public void initPowerManagement(final DisplayPowerCallbacks callbacks, Handler handler,
                SensorManager sensorManager) {
            synchronized (mSyncRoot) {
                DisplayBlanker blanker = new DisplayBlanker() {
                    @Override
                    public void requestDisplayState(int state) {
                        // The order of operations is important for legacy reasons.
                        if (state == Display.STATE_OFF) {
                            requestGlobalDisplayStateInternal(state);
                        }

                        callbacks.onDisplayStateChange(state);//呼叫了PowerManagerService中的onDisplayStateChange

                        if (state != Display.STATE_OFF) {
                            requestGlobalDisplayStateInternal(state);
                        }
                    }
                };
                mDisplayPowerController = new DisplayPowerController(
                        mContext, callbacks, handler, sensorManager, blanker);
            }
        }

最終還是呼叫了PowerManagerService中的onDisplayStateChange，

        @Override
        public void onDisplayStateChange(int state) {
            // This method is only needed to support legacy display blanking behavior
            // where the display's power state is coupled to suspend or to the power HAL.
            // The order of operations matters here.
            synchronized (mLock) {
                if (mDisplayState != state) {
                    mDisplayState = state;
                    if (state == Display.STATE_OFF) {
                        if (!mDecoupleHalInteractiveModeFromDisplayConfig) {
                            setHalInteractiveModeLocked(false);
                        }
                        if (!mDecoupleHalAutoSuspendModeFromDisplayConfig) {
                            setHalAutoSuspendModeLocked(true);
                        }
                    } else {
                        if (!mDecoupleHalAutoSuspendModeFromDisplayConfig) {
                            setHalAutoSuspendModeLocked(false);
                        }
                        if (!mDecoupleHalInteractiveModeFromDisplayConfig) {
                            setHalInteractiveModeLocked(true);
                        }
                    }
                }
            }
        }

其中setHalInteractiveModeLocked是一個native函式，設定螢幕是否亮。

    private void setHalInteractiveModeLocked(boolean enable) {
        if (enable != mHalInteractiveModeEnabled) {
            if (DEBUG) {
                Slog.d(TAG, "Setting HAL interactive mode to " + enable);
            }
            mHalInteractiveModeEnabled = enable;
            Trace.traceBegin(Trace.TRACE_TAG_POWER, "setHalInteractive(" + enable + ")");
            try {
                nativeSetInteractive(enable);
            } finally {
                Trace.traceEnd(Trace.TRACE_TAG_POWER);
            }
        }
    }

我們再看下DisPlayPowerControler如何設定ScreenBrightness,是呼叫animateScreenBrightness函式：

    private void animateScreenBrightness(int target, int rate) {
        if (DEBUG) {
            Slog.d(TAG, "Animating brightness: target=" + target +", rate=" + rate);
        }
        if (mScreenBrightnessRampAnimator.animateTo(target, rate)) {
            try {
                mBatteryStats.noteScreenBrightness(target);
            } catch (RemoteException ex) {
                // same process
            }
        }
    }

直接看類RampAnimator<T> 的animateTo函式，其中呼叫了mProperty.setValue(mObject, target);

    public boolean animateTo(int target, int rate) {
        // Immediately jump to the target the first time.
        if (mFirstTime || rate <= 0) {
            if (mFirstTime || target != mCurrentValue) {
                mFirstTime = false;
                mRate = 0;
                mTargetValue = target;
                mCurrentValue = target;
                mProperty.setValue(mObject, target);
                if (mAnimating) {
                    mAnimating = false;
                    cancelAnimationCallback();
                }
                if (mListener != null) {
                    mListener.onAnimationEnd();
                }
                return true;
            }
            return false;
        }

最後還是呼叫了DisplayPowerState的setScreenState函式

    public void setScreenState(int state) {
        if (mScreenState != state) {
            if (DEBUG) {
                Slog.d(TAG, "setScreenState: state=" + state);
            }

            mScreenState = state;
            mScreenReady = false;
            scheduleScreenUpdate();
        }
    }

這個函式分析大體和上面的setState差不多，最後也是線上程中呼叫setBrightness函式

        public void run() {
            for (;;) {
                // Get pending change.
                final int state;
                final boolean stateChanged;
                final int backlight;
                final boolean backlightChanged;
                synchronized (mLock) {
                    state = mPendingState;
                    stateChanged = (state != mActualState);
                    backlight = mPendingBacklight;
                    backlightChanged = (backlight != mActualBacklight);
                    if (!stateChanged && !backlightChanged) {
                        // All changed applied, notify outer class and wait for more.
                        mChangeInProgress = false;
                        postScreenUpdateThreadSafe();
                        try {
                            mLock.wait();
                        } catch (InterruptedException ex) { }
                        continue;
                    }
                    mActualState = state;
                    mActualBacklight = backlight;
                }

                // Apply pending change.
                if (DEBUG) {
                    Slog.d(TAG, "Updating screen state: state="
                            + Display.stateToString(state) + ", backlight=" + backlight);
                }
                boolean suspending = Display.isSuspendedState(state);
                if (stateChanged && !suspending) {
                    requestDisplayState(state);
                }
                if (backlightChanged) {
                    setBrightness(backlight);
                }
                if (stateChanged && suspending) {
                    requestDisplayState(state);
                }
            }

        private void setBrightness(int backlight) {
            Trace.traceBegin(Trace.TRACE_TAG_POWER, "setBrightness(" + backlight + ")");
            try {
                mBacklight.setBrightness(backlight);
            } finally {
                Trace.traceEnd(Trace.TRACE_TAG_POWER);
            }
        }

而mBackLigth是LocalServices.getService(LightsManager.class);

最後調到LightsService中的setLightLocked函式。

        private void setLightLocked(int color, int mode, int onMS, int offMS, int brightnessMode) {
            if (color != mColor || mode != mMode || onMS != mOnMS || offMS != mOffMS) {
                if (DEBUG) Slog.v(TAG, "setLight #" + mId + ": color=#"
                        + Integer.toHexString(color));
                mColor = color;
                mMode = mode;
                mOnMS = onMS;
                mOffMS = offMS;
                Trace.traceBegin(Trace.TRACE_TAG_POWER, "setLight(" + mId + ", " + color + ")");
                try {
                    setLight_native(mNativePointer, mId, color, mode, onMS, offMS, brightnessMode);//native函式設定亮度
                } finally {
                    Trace.traceEnd(Trace.TRACE_TAG_POWER);
                }
            }
        }

總結下：PowerManagerService中updateDisplayPowerStateLocked函式呼叫DisplayPowerControler中的requestPowerState函式，直接返回一個false，然後再發送一個訊號，呼叫DisplayPowerControler中的updatePowerstate，然後和DisplayPowerState進行互動，結束後將mDisplayReadyLocked = true ，再呼叫PowerManagerService的updatePowerStateLocked，重新呼叫DisplayPowerControler中的requestPowerState函式然後再返回true。