Android進階 - View 工作原理探究

前言

探究分析了View繪製的總體流程：onMeasure、onLayout、onDraw三大方法。

知識準備

ViewRoot

ViewRoot對應ViewRootImpl類，是連接WindowManager與DecorView的紐帶。View的三大流程都是通過ViewRoot完成的。ActivityThread中，Activity對象被回收時，會將DecorView添加到Window中，同時創建ViewRootImpl對象，並將ViewRootImpl對象和DecorView對象建立關聯。
代碼示例：

//創建ViewRootImpl對象
root = new ViewRootImpl(view.getContext(),display);
//添加關聯
root.setView(view,wparams,panelparentView);

View繪製流程：從ViewRoot的performTraversals開始，經過measure、layout、draw三大流程後纔將View繪製出來。
performTraversals方法（8W多行代碼），會依次調用performMeasure、performLayout、performDraw方法（這三個方法分別完成頂級View - DecorView 的measure、layout、draw方法）

其中performMeasure方法會調用measure方法，在measure方法中又會調用onMeasure方法，onMeasure方法則會對所有子元素進行measure，從而達到measure流程從父容器傳遞到子元素中的目的。接着子元素會重複父容器的measure過程，如此反覆完成整個View樹的遍歷。

最後，perfromLayout、performDraw的傳遞過程也是類似的，唯一不同，而performDraw的傳遞過程在draw方法中通過dispatchDraw來實現。

DecorView - 繼承FrameLayout

DecorView作爲頂級View，一般情況下會包含一個LinearLayout，該LinearLayout分爲上下兩部分，titlebar部分、android.R.id.content部分（所以，setContentView方法其實就是將佈局添加到id爲content的FrameLayout中）正如小標題，DecorView本質是FrameLayout，View層時間都必先經過DecorView然後在傳遞給其中的View。

MeasureSpec - 很大程度決定View的尺寸規格

MeasureSpec代表一個32位int值，高2位代表SpecMode（測量模式），低30位代表SpecSize（規格大小）
**小白科普：**Java中int爲4個字節，Android使用第一個高位字節存儲Mode，剩下三個字節存儲Size

MeasureSpec內部實現原理

private static final int MODE_SHIFT = 30;
private static final int MODE_MASK  = 0x3 << MODE_SHIFT;

//下面是三種測量模式
/**
* 父控件不對子控件施加任何約束，一般用於系統內部
*/
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
/**
* 父控件已爲子控件指定精確大小，對應於LayoutParams中的 match_parenet和具體數值這兩種模式
*/
public static final int EXACTLY     = 1 << MODE_SHIFT;
/**
* 子控件可隨意大小，但不可大於父控件大小，對應於LayoutParams中的 wrap_content
*/
public static final int AT_MOST     = 2 << MODE_SHIFT;

/**
* 根據提供的測量模式以及大小創建 measure specification
*/
public static int makeMeasureSpec(int size, int mode) {
            if (sUseBrokenMakeMeasureSpec) {
                return size + mode;
            } else {
                return (size & ~MODE_MASK) | (mode & MODE_MASK);
            }
        }

public static int getMode(int measureSpec) {
            return (measureSpec & MODE_MASK);
        }

public static int getSize(int measureSpec) {
            return (measureSpec & ~MODE_MASK);
        }

小白科普：<< 是移位運算，3<<30表示的是首先把3變成二進制的11然後右邊補30個0所組成的一個二進制的數。

MearsureSpec和LayoutParams對應關係
系統內部是通過MeasureSpec進行View測量的，但正常情況下，都是使View指定MeasureSpec。View測量時候系統會將LayoutParams在父容器約束下轉換成對應的MeasureSpec，然後再根據這個MeasureSpec來確定View測量後的寬高。因此，Measure需要由LayoutParams和父容器一起決定。

另外，對於頂級View（DecorView），其MeasureSpec由窗口尺寸和其自身的LayoutParams共同決定。Measure一旦決定後，onMeasure中in個即可獲得View的測量寬高。

/**
* DecorView中，MeasureSpec產生過程
* 根據LayoutParams劃分，併產生MeasureSpec
*/
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
        int measureSpec;
        switch (rootDimension) {

        case ViewGroup.LayoutParams.MATCH_PARENT:
            // Window can't resize. Force root view to be windowSize.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
            break;
        case ViewGroup.LayoutParams.WRAP_CONTENT:
            // Window can resize. Set max size for root view.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
            break;
        default:
            // Window wants to be an exact size. Force root view to be that size.
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
            break;
        }
        return measureSpec;
    }

對於普通View（佈局中的View），View的measure方法需要由ViewGroup傳遞過來
再看看ViewGroup中的measureChildWithMargins方法

   protected void measureChildWithMargins(View child,
            int parentWidthMeasureSpec, int widthUsed,
            int parentHeightMeasureSpec, int heightUsed) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
                        + widthUsed, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
                        + heightUsed, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

上述方法，對子元素進行measure，調喲in個子元素measure之前會獲取子元素的MeasureSpec。顯然，子元素MeasureSpec的創建與父容器的MeasureSpec和子元素本身的LayoutParams有關，還與View的margin以及padding有關（具體需要研究ViewGroup的getChildMeasureSpec方法）。

public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);

        int size = Math.max(0, specSize - padding);

        int resultSize = 0;
        int resultMode = 0;

        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size. So be it.
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent has imposed a maximum size on us
        case MeasureSpec.AT_MOST:
            if (childDimension >= 0) {
                // Child wants a specific size... so be it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size, but our size is not fixed.
                // Constrain child to not be bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent asked to see how big we want to be
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                // Child wants a specific size... let him have it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size... find out how big it should
                // be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size.... find out how
                // big it should be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

上述方法，主要根據父容器的MeasureSpec同時結合View本身的LayoutParams來確定子元素的MeasureSpec。
另外注意，子元素可用大小爲父容器尺寸減去padding。

View工作流程

measure 確定View寬高
layout確定View最終寬高和四個頂點位置
draw將View繪製到屏幕

View的生命週期與工作流程

View生命週期示意圖

View工作流程示意圖

探究Measure過程

兩種情況，若只是一個原始的View，通過measure方法就完成了測量過程；如果是ViewGroup，除了完成自身的measure過程，還需要遍歷子元素的measure方法，各個子元素遞歸去執行這個部分。（如上面的示意圖所述）

View的measure方法是一個final類型的方法 - 意味着子類不能重寫該方法,因此仔細研究onMeasure方法的實現效果會更好。

這裏貼出View中Measure方法，有部分註釋，供有興趣的讀者閱讀研究。

/**
* View中的Measure方法
*/
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
        boolean optical = isLayoutModeOptical(this);
        //先判斷當前Mode是不是特例LAYOUT_MODE_OPTICAL_BOUNDS
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int oWidth  = insets.left + insets.right;
            int oHeight = insets.top  + insets.bottom;
            widthMeasureSpec  = MeasureSpec.adjust(widthMeasureSpec,  optical ? -oWidth  : oWidth);
            heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
        }

        // Suppress sign extension for the low bytes
        long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
        if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);

        if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
                widthMeasureSpec != mOldWidthMeasureSpec ||
                heightMeasureSpec != mOldHeightMeasureSpec) {

            // first clears the measured dimension flag
            mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;

            resolveRtlPropertiesIfNeeded();

            int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :
                    mMeasureCache.indexOfKey(key);
            if (cacheIndex < 0 || sIgnoreMeasureCache) {
                // measure ourselves, this should set the measured dimension flag back
                onMeasure(widthMeasureSpec, heightMeasureSpec);
                mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            } else {
                long value = mMeasureCache.valueAt(cacheIndex);
                // Casting a long to int drops the high 32 bits, no mask needed
                setMeasuredDimensionRaw((int) (value >> 32), (int) value);
                mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            }

            // flag not set, setMeasuredDimension() was not invoked, we raise
            // an exception to warn the developer

            //如果自定義View重寫了onMeasure方法而沒有調用setMeasureDimension()方法,將會在這裏拋出異常
            //判斷原理：通過解析狀態位mPrivateFlags，setMeasureDimension()方法會將mPrivateFlags設置爲已計算狀態（PFLAG_MEASURED_DIMENSION_SET），只需要檢查mPrivateFlags是否含有PFLAG_MEASURED_DIMENSION_SET即可。
            if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
                throw new IllegalStateException("View with id " + getId() + ": "
                        + getClass().getName() + "#onMeasure() did not set the"
                        + " measured dimension by calling"
                        + " setMeasuredDimension()");
            }

            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
        }

        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;

        //計算出的key作爲鍵，量算結果作爲值，將該鍵值對放入成員變量mMeasureCache中,實現本次計算結果的環緩存
        mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
                (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
    }

好啦，接下來，繼續研究onMeasure

/**
* View中的onMeasure方法
*/
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

setMeasuredDimension()方法會設置View寬高測量值，接下來進一步深入，研究getDefaultSize()方法。

/**
* View中的getDefaultSize方法
*/
public static int getDefaultSize(int size, int measureSpec) {
        int result = size;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);

        switch (specMode) {
        case MeasureSpec.UNSPECIFIED://一般是系統內部的測量過程，重點注意另外兩種模式
            result = size;
            break;
        case MeasureSpec.AT_MOST://這兩種模式都是做一樣的事情
        case MeasureSpec.EXACTLY:
            result = specSize;
            break;
        }
        return result;
    }

getDefaultSize方法直接就是根據測量模式返回measureSpec中的specSize，而這個specSize就是View測量後的大小。

注意：**View測量後大小與 View最終大小需要區分，是兩個東西，因爲View最終大小是在**layout階段確定的，但兩者幾乎所有情況都是相等的。

接下來，再繼續探究getDefaultSize方法的第一個參數，從onMeasure方法中可知，該參數來源於下面兩個方法

protected int getSuggestedMinimumWidth() {
        return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
    }
protected int getSuggestedMinimumHeight() {
        return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());

    }

上述兩個方法實現原理都一致，判斷有沒有背景，如果有，返回兩者較大的寬高，沒有則返回自己的寬高（android:minwith這個屬性指定的值）。

那麼，問題來了，背景最小寬高原理是什麼？

public int getMinimumWidth() {
        final int intrinsicWidth = getIntrinsicWidth();
        return intrinsicWidth > 0 ? intrinsicWidth : 0;
    }

上述代碼中可見，Drawable的原始寬度，如果沒有原始寬度，則返回0。

**小白科普：**ShapeDrawable無原始寬高，而BimapDrawable有原始寬高（即圖片尺寸）

再談談ViewGroup的measure過程
主要區別：
1. 除了完成自己measure過程還要遍歷調用子元素的measure方法，各個子元素再遞歸執行該過程。
2. ViewGroup是抽象類，沒有重寫View的onMeasure方法，而是提供了一個measureChildren的方法

protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
        final int size = mChildrenCount;
        final View[] children = mChildren;
        for (int i = 0; i < size; ++i) {
            final View child = children[i];
            if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
                measureChild(child, widthMeasureSpec, heightMeasureSpec);
            }
        }
    }
protected void measureChild(View child, int parentWidthMeasureSpec,
            int parentHeightMeasureSpec) {
        final LayoutParams lp = child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

View在measure過程中會對每一個子元素進行measure。
再細說下，measureChild方法的思路：
1. 取出子元素的LayoutParams
2. 通過getChildMeasureSpec離開創建子元素的MeasureSpec
3. 將MeasureSpec傳遞給View的Measure方法進行測量。

問題：爲什麼ViewGroup不像View一樣對其onMeasure方法做統一實現？
因爲不同的ViewGroup子類會有不同的特性，因此其中的onMeasure細節不相同。

獲取View寬高方法不當，可能會獲取錯誤。
原因：**View的**measure過程和Activity生命週期方法執行順序是不確定的，無法保證Activity執行了onCreate、onStart、onReasume時，View測量完畢。

如果View還沒有完成測量，則獲取的寬高會是0
給出四種方法解決：

onWindowFocusChanged - 該方法被調用時候，View已經測量完畢，能夠正確獲取View寬高。
注意:該方法會被調用多次，Activity窗口得到焦點與失去焦點時均會被調用一次（繼續執行，暫停執行）。

public void onWindowFocusChanged(boolean hasWindowFocus) {
        super.onWindowFocusChanged(hasWindowFocus);
        if(hasWindowFocus){
        //獲取寬高
        int with = view.getMeasuredWidth();
        int height = view.getMeasuredHeight();
        }
    }

view.post(runnable)
通過post將一個runnable投遞到消息隊列隊尾，等待Looper調用此runnable時，View已初始化完畢。

protected void onStart(){
    super.onStart();
    view.post(new Runnable(){
        @override
        public void run(){
        //獲取寬高
        int with = view.getMeasuredWidth();
        int height = view.getMeasuredHeight();
        }
    })
}

ViewTreeObserver
該類有衆多回調接口，其中的OnGlobalLayoutListener接口，當View樹狀態發生變化或者View樹內部的View的可見性發生改變，該方法都會被回調，利用此特性，可獲得寬高。

protected void onStart(){
    super.onStart();
    viewTreeObserver observer = view.getViewTreeObserver();
    observer.addOnGlobalLayoutListener(new OnGlobalListener(){
        public void onGlobalLayout(){
        view.getViewTreeObserver().removeGlobalOnlayoutListener(this);
        //獲取寬高
        int with = view.getMeasuredWidth();
        int height = view.getMeasuredHeight();
        }
    })
}

view.measure(int widthMeasureSpec,int heightMeasureSpec)
手動對View進行獲取，根據View的LayoutParams不同，而採取不同手段。（因不常用，這裏就不詳細說明）

探究layout過程

layout主要作用是ViewGroup用來確定子元素位置（遞歸）。

 public void layout(int l, int t, int r, int b) {
        if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
            onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        }

        //設定四個頂點位置
        int oldL = mLeft;
        int oldT = mTop;
        int oldB = mBottom;
        int oldR = mRight;

        boolean changed = isLayoutModeOptical(mParent) ?
                setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

        if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
            onLayout(changed, l, t, r, b);
            mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;

            ListenerInfo li = mListenerInfo;
            if (li != null && li.mOnLayoutChangeListeners != null) {
                ArrayList<OnLayoutChangeListener> listenersCopy =
                        (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
                int numListeners = listenersCopy.size();
                for (int i = 0; i < numListeners; ++i) {
                    listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
                }
            }
        }

        mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
        mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
    }

layout方法流程：
1. setFrame方法設定View四個頂點位置
2. 調用onLayout方法，父容器確定子元素位置
另外，與onMeasure方法相似，onLayout方法也是各不相同的。

onLayout方法（LinearLayout、RelativeLayout等基本控件可自行嘗試研究下）

探究draw過程

draw主要作用是將View繪製到屏幕上
繪製過程：
1. 繪製背景(background.draw(canvas))
2. 繪製自己(onDraw)
3. 繪製children(dispatchDraw)
4. 繪製裝飾(onDrawScrollBars)

/**
* View中的draw方法
*/
public void draw(Canvas canvas) {
        final int privateFlags = mPrivateFlags;
        final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
                (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
        mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;

        /*
         * Draw traversal performs several drawing steps which must be executed
         * in the appropriate order:
         *
         *      1. Draw the background
         *      2. If necessary, save the canvas' layers to prepare for fading
         *      3. Draw view's content
         *      4. Draw children
         *      5. If necessary, draw the fading edges and restore layers
         *      6. Draw decorations (scrollbars for instance)
         */

        // Step 1, draw the background, if needed
        int saveCount;

        if (!dirtyOpaque) {
            drawBackground(canvas);
        }

        // skip step 2 & 5 if possible (common case)
        final int viewFlags = mViewFlags;
        boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
        boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
        if (!verticalEdges && !horizontalEdges) {
            // Step 3, draw the content
            if (!dirtyOpaque) onDraw(canvas);

            // Step 4, draw the children
            dispatchDraw(canvas);

            // Overlay is part of the content and draws beneath Foreground
            if (mOverlay != null && !mOverlay.isEmpty()) {
                mOverlay.getOverlayView().dispatchDraw(canvas);
            }

            // Step 6, draw decorations (foreground, scrollbars)
            onDrawForeground(canvas);

            // we're done...
            return;
        }

        /*
         * Here we do the full fledged routine...
         * (this is an uncommon case where speed matters less,
         * this is why we repeat some of the tests that have been
         * done above)
         */

        boolean drawTop = false;
        boolean drawBottom = false;
        boolean drawLeft = false;
        boolean drawRight = false;

        float topFadeStrength = 0.0f;
        float bottomFadeStrength = 0.0f;
        float leftFadeStrength = 0.0f;
        float rightFadeStrength = 0.0f;

        // Step 2, save the canvas' layers
        int paddingLeft = mPaddingLeft;

        final boolean offsetRequired = isPaddingOffsetRequired();
        if (offsetRequired) {
            paddingLeft += getLeftPaddingOffset();
        }

        int left = mScrollX + paddingLeft;
        int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
        int top = mScrollY + getFadeTop(offsetRequired);
        int bottom = top + getFadeHeight(offsetRequired);

        if (offsetRequired) {
            right += getRightPaddingOffset();
            bottom += getBottomPaddingOffset();
        }

        final ScrollabilityCache scrollabilityCache = mScrollCache;
        final float fadeHeight = scrollabilityCache.fadingEdgeLength;
        int length = (int) fadeHeight;

        // clip the fade length if top and bottom fades overlap
        // overlapping fades produce odd-looking artifacts
        if (verticalEdges && (top + length > bottom - length)) {
            length = (bottom - top) / 2;
        }

        // also clip horizontal fades if necessary
        if (horizontalEdges && (left + length > right - length)) {
            length = (right - left) / 2;
        }

        if (verticalEdges) {
            topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
            drawTop = topFadeStrength * fadeHeight > 1.0f;
            bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
            drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
        }

        if (horizontalEdges) {
            leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
            drawLeft = leftFadeStrength * fadeHeight > 1.0f;
            rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
            drawRight = rightFadeStrength * fadeHeight > 1.0f;
        }

        saveCount = canvas.getSaveCount();

        int solidColor = getSolidColor();
        if (solidColor == 0) {
            final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;

            if (drawTop) {
                canvas.saveLayer(left, top, right, top + length, null, flags);
            }

            if (drawBottom) {
                canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
            }

            if (drawLeft) {
                canvas.saveLayer(left, top, left + length, bottom, null, flags);
            }

            if (drawRight) {
                canvas.saveLayer(right - length, top, right, bottom, null, flags);
            }
        } else {
            scrollabilityCache.setFadeColor(solidColor);
        }

        // Step 3, draw the content
        if (!dirtyOpaque) onDraw(canvas);

        // Step 4, draw the children
        dispatchDraw(canvas);

        // Step 5, draw the fade effect and restore layers
        final Paint p = scrollabilityCache.paint;
        final Matrix matrix = scrollabilityCache.matrix;
        final Shader fade = scrollabilityCache.shader;

        if (drawTop) {
            matrix.setScale(1, fadeHeight * topFadeStrength);
            matrix.postTranslate(left, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, top, right, top + length, p);
        }

        if (drawBottom) {
            matrix.setScale(1, fadeHeight * bottomFadeStrength);
            matrix.postRotate(180);
            matrix.postTranslate(left, bottom);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, bottom - length, right, bottom, p);
        }

        if (drawLeft) {
            matrix.setScale(1, fadeHeight * leftFadeStrength);
            matrix.postRotate(-90);
            matrix.postTranslate(left, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, top, left + length, bottom, p);
        }

        if (drawRight) {
            matrix.setScale(1, fadeHeight * rightFadeStrength);
            matrix.postRotate(90);
            matrix.postTranslate(right, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(right - length, top, right, bottom, p);
        }

        canvas.restoreToCount(saveCount);

        // Overlay is part of the content and draws beneath Foreground
        if (mOverlay != null && !mOverlay.isEmpty()) {
            mOverlay.getOverlayView().dispatchDraw(canvas);
        }

        // Step 6, draw decorations (foreground, scrollbars)
        onDrawForeground(canvas);
    }

View的繪製過程傳遞是通過dispatchDraw來實現，dispatchDraw會遍歷所有子元素的draw方法，另外，View還有一個特殊的方法setWillNotDraw

public void setWillNotDraw(boolean willNotDraw) {
        setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK);
    }

setFlags - 該方法可設置優化標記
如果View不需要繪製任何內容，那麼將設置標記爲true，系統會相應優化。默認情況下，View不啓用這個標記位，但ViewGroup會默認啓動該優化標記。
而實際開發意義：當我們自定義控件繼承於ViewGroup並且本身不具備繪製功能，則開啓標記。而如果明確知道一個ViewGroup需要通過onDraw來繪製內容時候，則需要顯式關閉WILL_NOT_DRAW這個標記位。

Android進階 - View 工作原理探究

前言

知識準備

ViewRoot

DecorView - 繼承FrameLayout

MeasureSpec - 很大程度決定View的尺寸規格

View工作流程

View的生命週期與工作流程

探究Measure過程

探究layout過程

探究draw過程

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