super_status_bar.xml是systemUI的一個總的佈局檔案。
下面是super_status_bar.xml的原始碼：
<com.android.systemui.statusbar.phone.StatusBarWindowView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:systemui="http://schemas.android.com/apk/res/com.android.systemui"
    android:focusable="true"
    android:descendantFocusability="afterDescendants"
    android:fitsSystemWindows="true"
    android:background="@android:color/transparent"
    >

    <include layout="@layout/status_bar"
        android:layout_width="match_parent"
        android:layout_height="@*android:dimen/status_bar_height"
        />
    <com.android.systemui.statusbar.phone.PanelHolder
        android:id="@+id/panel_holder"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        >
        <include layout="@layout/status_bar_expanded"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            />
        <ViewStub android:id="@+id/quick_settings_stub"
            android:layout="@layout/quick_settings"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            />
    </com.android.systemui.statusbar.phone.PanelHolder>

</com.android.systemui.statusbar.phone.StatusBarWindowView>

從上面的原始碼可以知道，systemUI佈局大概分為三個部分
第一：status_bar，這部分是狀態列。
第二：status_bar_expanded，這部分是下拉通知欄。
第三：quick_settings_stub,這部分是下拉快捷設定欄。
其中，本文中是重點分析下拉通知欄。

*************************************************分割線**************************************
下面是下拉通知欄的佈局檔案status_bar_expended.xml:
<com.android.systemui.statusbar.phone.NotificationPanelView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:systemui="http://schemas.android.com/apk/res/com.android.systemui"
    android:id="@+id/notification_panel"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:background="@drawable/notification_panel_bg"
    android:paddingTop="@dimen/notification_panel_padding_top"
    android:layout_marginStart="@dimen/notification_panel_margin_left"
    >

    <View
        android:id="@+id/handle"
        android:layout_width="match_parent"
        android:layout_height="@dimen/close_handle_height"
        android:background="@drawable/status_bar_close"
        android:visibility="invisible"
        />

    <include
        layout="@layout/carrier_label"
        android:layout_height="@dimen/carrier_label_height"
        android:layout_width="match_parent"
        android:layout_marginBottom="@dimen/close_handle_height"
        android:layout_gravity="bottom"
        />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="@dimen/close_handle_underlap"
        android:orientation="vertical"
        android:animateLayoutChanges="false"
        >

        <include layout="@layout/status_bar_expanded_header"
            android:layout_width="match_parent"
            android:layout_height="@dimen/notification_panel_header_height"
            />

        <TextView
            android:id="@+id/emergency_calls_only"
            android:textAppearance="@style/TextAppearance.StatusBar.Expanded.Network.EmergencyOnly"
            android:layout_height="wrap_content"
            android:layout_width="match_parent"
            android:padding="4dp"
            android:gravity="center"
            android:visibility="gone"
            />

        <FrameLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            >
            <ViewStub android:id="@+id/flip_settings_stub"
                android:layout="@layout/flip_settings"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                />
    
            <ScrollView
                android:id="@+id/scroll"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:fadingEdge="none"
                android:overScrollMode="ifContentScrolls"
                >
                <com.android.systemui.statusbar.policy.NotificationRowLayout
                    android:id="@+id/latestItems"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    systemui:rowHeight="@dimen/notification_row_min_height"
                    />
            </ScrollView>
        </FrameLayout>
    </LinearLayout>
</com.android.systemui.statusbar.phone.NotificationPanelView>

其中，程式碼段： <include layout="@layout/status_bar_expanded_header"
            android:layout_width="match_parent"
            android:layout_height="@dimen/notification_panel_header_height"
            />
該程式碼段是下拉通知欄的頂部的佈局，包括時間，日期和一個清除通知訊息的按鈕。


程式碼段：<com.android.systemui.statusbar.policy.NotificationRowLayout
                    android:id="@+id/latestItems"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    systemui:rowHeight="@dimen/notification_row_min_height"
                    />
該程式碼段是用於存放下拉通知欄裡面的各種通知訊息。


下面主要分析com.android.systemui.statusbar.phone.NotificationPanelView下拉通知欄的實現類的原始碼：
public class NotificationPanelView extends PanelView {}；由類定義的程式碼可以知道該類是繼承PanelView類，PanelView類則是繼承於FrameLayout。
下拉選單欄的主要實現都是在NotificationPanelView的父類PanelView中實現。
看以下的程式碼段：
protected void onFinishInflate() {
        super.onFinishInflate();
        mHandleView = findViewById(R.id.handle);

        loadDimens();

        if (DEBUG) logf("handle view: " + mHandleView);
        if (mHandleView != null) {
            mHandleView.setOnTouchListener(new View.OnTouchListener() {
                @Override
                public boolean onTouch(View v, MotionEvent event) {
                            Log.i("Garment30", "onTouch");
                    int pointerIndex = event.findPointerIndex(mTrackingPointer);
                    if (pointerIndex < 0) {
                        pointerIndex = 0;
                        mTrackingPointer = event.getPointerId(pointerIndex);
                    }
                    final float y = event.getY(pointerIndex);
                    final float rawDelta = event.getRawY() - event.getY();
                    final float rawY = y + rawDelta;
                    if (DEBUG) logf("handle.onTouch: a=%s p=[%d,%d] y=%.1f rawY=%.1f off=%.1f",
                            MotionEvent.actionToString(event.getAction()),
                            mTrackingPointer, pointerIndex,
                            y, rawY, mTouchOffset);
                    PanelView.this.getLocationOnScreen(mAbsPos);

                    switch (event.getActionMasked()) {
                        case MotionEvent.ACTION_DOWN:
                            Log.i("Garment30", "ACTION_DOWN");
                            mTracking = true;
                            mHandleView.setPressed(true);
                            postInvalidate(); // catch the press state change
                            mInitialTouchY = y;
                            mVelocityTracker = FlingTracker.obtain();
                            trackMovement(event);
                            mTimeAnimator.cancel(); // end any outstanding animations
                            mBar.onTrackingStarted(PanelView.this);
                            mTouchOffset = (rawY - mAbsPos[1]) - mExpandedHeight;
                            if (mExpandedHeight == 0) {
                                mJustPeeked = true;
                                runPeekAnimation();
                            }
                            break;

                        case MotionEvent.ACTION_POINTER_UP:
                            Log.i("Garment30", "ACTION_POINTER_UP");
                            final int upPointer = event.getPointerId(event.getActionIndex());
                            if (mTrackingPointer == upPointer) {
                                // gesture is ongoing, find a new pointer to track
                                final int newIndex = event.getPointerId(0) != upPointer ? 0 : 1;
                                final float newY = event.getY(newIndex);
                                final float newRawY = newY + rawDelta;
                                mTrackingPointer = event.getPointerId(newIndex);
                                mTouchOffset = (newRawY - mAbsPos[1]) - mExpandedHeight;
                                mInitialTouchY = newY;
                            }
                            break;

                        case MotionEvent.ACTION_MOVE:
                            Log.i("Garment30", "ACTION_MOVE");

                                                Log.i("Garment28", "PanelView---ACTION_MOVE");
                            final float h = rawY - mAbsPos[1] - mTouchOffset;
                            if (h > mPeekHeight) {
                                                Log.i("Garment28", "PanelView---h > mPeekHeight");
                                if (mPeekAnimator != null && mPeekAnimator.isStarted()) {
                                    mPeekAnimator.cancel();
                                }
                                mJustPeeked = false;
                            }
                            if (!mJustPeeked) {
                                                            Log.i("Garment28", "PanelView---!mJustPeeked--h:"+h);
                                                            //下面兩段程式碼需要同時使用
                              PanelView.this.setExpandedHeightInternal(h);//遮蔽了這段程式碼，下拉選單無法隨手的移動一起下拉，鬆手後選單移動到預設的長度
                              mBar.panelExpansionChanged(PanelView.this, mExpandedFraction);//遮蔽了這段程式碼，下拉通知欄顯示為透明，鬆手後才正常
                            }

                            trackMovement(event);
                            break;

                        case MotionEvent.ACTION_UP:
                        case MotionEvent.ACTION_CANCEL:
                            Log.i("Garment30", "ACTION_CANCEL");
                            mFinalTouchY = y;
                            mTracking = false;
                            mTrackingPointer = -1;
                            mHandleView.setPressed(false);
                            postInvalidate(); // catch the press state change
                            mBar.onTrackingStopped(PanelView.this);
                            trackMovement(event);

                            float vel = 0, yVel = 0, xVel = 0;
                            boolean negative = false;

                            if (mVelocityTracker != null) {
                                // the velocitytracker might be null if we got a bad input stream
                                mVelocityTracker.computeCurrentVelocity(1000);

                                yVel = mVelocityTracker.getYVelocity();
                                negative = yVel < 0;

                                xVel = mVelocityTracker.getXVelocity();
                                if (xVel < 0) {
                                    xVel = -xVel;
                                }
                                if (xVel > mFlingGestureMaxXVelocityPx) {
                                    xVel = mFlingGestureMaxXVelocityPx; // limit how much we care about the x axis
                                }

                                vel = (float)Math.hypot(yVel, xVel);
                                if (vel > mFlingGestureMaxOutputVelocityPx) {
                                    vel = mFlingGestureMaxOutputVelocityPx;
                                }

                                mVelocityTracker.recycle();
                                mVelocityTracker = null;
                            }

                            // if you've barely moved your finger, we treat the velocity as 0
                            // preventing spurious flings due to touch screen jitter
                            final float deltaY = Math.abs(mFinalTouchY - mInitialTouchY);
                            if (deltaY < mFlingGestureMinDistPx
                                    || vel < mFlingExpandMinVelocityPx
                                    ) {
                                vel = 0;
                            }

                            if (negative) {
                                vel = -vel;
                            }

                            if (DEBUG) logf("gesture: dy=%f vel=(%f,%f) vlinear=%f",
                                    deltaY,
                                    xVel, yVel,
                                    vel);

                            fling(vel, true);    //鬆手後下拉通知欄回彈的動畫

                            break;
                    }
                    return true;
                }});
        }
    }
其中，mHandleView = findViewById(R.id.handle);程式碼用於初始化下拉通知欄最下面的那一條直線，可以知道該view的觸控事件用於控制下拉通知欄的拉動時的高度的變化。
在MotionEvent.ACTION_MOVE事件中PanelView.this.setExpandedHeightInternal(h);這段程式碼實現了拉動過程在中下拉通知欄的大小的改變，傳入的h變數就是下拉通知欄的高度。看該方法的實現：
public void setExpandedHeightInternal(float h) {
        if (Float.isNaN(h)) {
            // If a NaN gets in here, it will freeze the Animators.
            if (DEBUG_NAN) {
                Log.v(TAG, "setExpandedHeightInternal: warning: h=NaN, using 0 instead",
                        new Throwable());
            }
            h = 0;
        }

        float fh = getFullHeight();
        if (fh == 0) {
            // Hmm, full height hasn't been computed yet
        }

        if (h < 0) h = 0;
        if (!(mRubberbandingEnabled && (mTracking || mRubberbanding)) && h > fh) h = fh;

        mExpandedHeight = h; //更新最新的通知欄的長度，遮蔽了這段程式碼，通知欄無法拉出

        if (DEBUG) logf("setExpansion: height=%.1f fh=%.1f tracking=%s rubber=%s", h, fh, mTracking?"T":"f", mRubberbanding?"T":"f");
        Log.i("Garment30", "PanelView---setExpandedHeightInternal--h:"+h);
        requestLayout(); //觸發OnMeasure()和onLayout()進行View的重繪， 遮蔽這段程式碼，下拉通知欄無法顯示，初始時h=0，預設狀態時，h=729，最大為1919
//        FrameLayout.LayoutParams lp = (FrameLayout.LayoutParams) getLayoutParams();
//        lp.height = (int) mExpandedHeight;
//        setLayoutParams(lp);

        mExpandedFraction = Math.min(1f, (fh == 0) ? 0 : h / fh);
    }
在setExpandedHeightInternal（）函式中，mExpandedHeight = h;這段程式碼用於更新需要重畫的最新的下拉選單欄的高度。
 requestLayout()；該函式觸發了OnMeasure()和onLayout()函式，對該View進行重新測量並重新佈局。

當手鬆開的時候，通知欄會有一個回彈的動作，看到fling(vel, true);這段程式碼：
public void fling(float vel, boolean always) {
        if (DEBUG) logf("fling: vel=%.3f, this=%s", vel, this);
        mVel = vel;

        if (always||mVel != 0) {
            animationTick(0); // begin the animation  開始執行回彈的動畫
        }
    }
其中animationTick(0);是回彈動畫的實現方法：
 private void animationTick(long dtms) {
        if (!mTimeAnimator.isStarted()) {
            // XXX HAX to work around bug in TimeAnimator.end() not resetting its last time
            mTimeAnimator = new TimeAnimator();
            mTimeAnimator.setTimeListener(mAnimationCallback);

            if (mPeekAnimator != null) mPeekAnimator.cancel();

            mTimeAnimator.start();

            mRubberbanding = mRubberbandingEnabled // is it enabled at all?
                    && mExpandedHeight > getFullHeight() // are we past the end?
                    && mVel >= -mFlingGestureMinDistPx; // was this not possibly a "close" gesture?
            if (mRubberbanding) {
                mClosing = true;
            } else if (mVel == 0) {
                // if the panel is less than halfway open, close it
                mClosing = (mFinalTouchY / getFullHeight()) < 0.5f;
            } else {
                mClosing = mExpandedHeight > 0 && mVel < 0;
            }
        } else if (dtms > 0) {
            final float dt = dtms * 0.001f;                  // ms -> s
            if (DEBUG) logf("tick: v=%.2fpx/s dt=%.4fs", mVel, dt);
            if (DEBUG) logf("tick: before: h=%d", (int) mExpandedHeight);

            final float fh = getFullHeight();
            boolean braking = false;
            if (BRAKES) {
                if (mClosing) {
                    braking = mExpandedHeight <= mCollapseBrakingDistancePx;
                    mAccel = braking ? 10*mCollapseAccelPx : -mCollapseAccelPx;
                } else {
                    braking = mExpandedHeight >= (fh-mExpandBrakingDistancePx);
                    mAccel = braking ? 10*-mExpandAccelPx : mExpandAccelPx;
                }
            } else {
                mAccel = mClosing ? -mCollapseAccelPx : mExpandAccelPx;
            }

            mVel += mAccel * dt;

            if (braking) {
                if (mClosing && mVel > -mBrakingSpeedPx) {
                    mVel = -mBrakingSpeedPx;
                } else if (!mClosing && mVel < mBrakingSpeedPx) {
                    mVel = mBrakingSpeedPx;
                }
            } else {
                if (mClosing && mVel > -mFlingCollapseMinVelocityPx) {
                    mVel = -mFlingCollapseMinVelocityPx;
                } else if (!mClosing && mVel > mFlingGestureMaxOutputVelocityPx) {
                    mVel = mFlingGestureMaxOutputVelocityPx;
                }
            }

            float h = mExpandedHeight + mVel * dt;

            if (mRubberbanding && h < fh) {
                h = fh;
            }

            if (DEBUG) logf("tick: new h=%d closing=%s", (int) h, mClosing?"true":"false");
            Log.i("Garment30", "PanelView---animationTick--h:"+h);  //end at 729

            setExpandedHeightInternal(h);

            mBar.panelExpansionChanged(PanelView.this, mExpandedFraction);

            if (mVel == 0
                    || (mClosing && mExpandedHeight == 0)
                    || ((mRubberbanding || !mClosing) && mExpandedHeight == fh)) {
                post(mStopAnimator);       //回彈動畫停止
            }
        } else {
            Log.v(TAG, "animationTick called with dtms=" + dtms + "; nothing to do (h="
                    + mExpandedHeight + " v=" + mVel + ")");
        }
    }
可以看到，setExpandedHeightInternal(h); mBar.panelExpansionChanged(PanelView.this, mExpandedFraction);該方法中有呼叫這兩個方法對下拉選單欄的長度進行更新的操作。mTimeAnimator = new TimeAnimator();mTimeAnimator.setTimeListener(mAnimationCallback);也初始化了一個TimeAnimator物件，並設定了監聽器。
private final TimeListener mAnimationCallback = new TimeListener() {
        @Override
        public void onTimeUpdate(TimeAnimator animation, long totalTime, long deltaTime) {
            animationTick(deltaTime);
        }
    };
該監聽器中的回撥函式呼叫了animationTick(deltaTime)下拉通知欄的回彈動畫的方法，所以我們鬆開手後通知欄會有一個回彈的動作。
當滿足一定條件時，回彈動畫停止。
if (mVel == 0|| (mClosing && mExpandedHeight == 0)|| ((mRubberbanding || !mClosing) && mExpandedHeight == fh)) {
                post(mStopAnimator);       //回彈動畫停止
            }

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注意：下拉快捷設定欄的實現和下拉通知欄的實現是差不多，只是裝載的內容不同。所以也可以通過類似的方法來分析下拉快速設定欄。