上一篇文章講解了，從setContentView方法到了解View是如何繪制的：傳送門
在這篇博客講述了躏碳， 在ViewRootImpl類中performTraversals方法中具體的繪制過程，其中里面就有 performMeasure()榕莺、performLayout()、performDraw() 三個方法的調(diào)用棵介， 那么要了解View 的測量钉鸯、布局、繪制邮辽，就分別跟這三個方法有關(guān)系唠雕。

先來看看performMeasure()方法的調(diào)用過程

private void performTraversals() {
   // 這里的mView就是decorView對象
  final View host = mView;
   //... 通過查看代碼可以得知mWindowAttributes 的值，后面會用到
  WindowManager.LayoutParams lp = mWindowAttributes;
  // ... 
  int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
  int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
  // Ask host how big it wants to be
  performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);

  // ...
  performLayout(lp, mWidth, mHeight);
  
  //...
  performDraw();
}

1吨述、測量 performMeasure

先看performMeasure方法岩睁，這個方法有兩個參數(shù)，都是通過getRootMeasureSpec()方法計算得到

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;
}

這里有一個關(guān)鍵類MeasureSpec锐极，在這里需要了解下這個類的原理笙僚。
這里要感謝這位博主，他講述的很清晰灵再，我自己動手測算了肋层，很容易理解。大概就是用一個數(shù)字通過高位記錄Mode,地位記錄size的方式翎迁，記錄兩個數(shù)據(jù)栋猖，都是通過一個掩碼做位移運(yùn)算得來。就是說這個變量（measureSpec）的值可以通過掩碼分別得到測量mode 和 測量size汪榔。
繼續(xù)查看performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);方法：

private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
    if (mView == null) {
        return;
    }
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
    try {
        mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

這里mView是DecorView對象蒲拉，那么他調(diào)用的實際上是View的measure方法，查詢DecorView和FrameLayout都沒有measure方法痴腌，所以他調(diào)用的是View的measure方法

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
    boolean optical = isLayoutModeOptical(this);
    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);
    }

    // ...
    if (forceLayout || needsLayout) {
        // first clears the measured dimension flag
        mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;

        resolveRtlPropertiesIfNeeded();

        int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
        if (cacheIndex < 0 || sIgnoreMeasureCache) {
            // measure ourselves, this should set the measured dimension flag back
            // 調(diào)用DecorView的onMeasure方法雌团，進(jìn)行測量
            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;
        }

        mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
    }

    mOldWidthMeasureSpec = widthMeasureSpec;
    mOldHeightMeasureSpec = heightMeasureSpec;

    mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
            (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}

DecorView.onMeasure()方法如下：

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    final DisplayMetrics metrics = getContext().getResources().getDisplayMetrics();
    final boolean isPortrait =
            getResources().getConfiguration().orientation == ORIENTATION_PORTRAIT;
    final int widthMode = getMode(widthMeasureSpec);
    final int heightMode = getMode(heightMeasureSpec);

    // ...  其他的各種計算

    // 調(diào)用父級onMeasure方法，即FrameLayout.onMeasure
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);

    int width = getMeasuredWidth();
    boolean measure = false;

    widthMeasureSpec = MeasureSpec.makeMeasureSpec(width, EXACTLY);

    if (!fixedWidth && widthMode == AT_MOST) {
        final TypedValue tv = isPortrait ? mWindow.mMinWidthMinor : mWindow.mMinWidthMajor;
        if (tv.type != TypedValue.TYPE_NULL) {
            final int min;
            if (tv.type == TypedValue.TYPE_DIMENSION) {
                min = (int)tv.getDimension(metrics);
            } else if (tv.type == TypedValue.TYPE_FRACTION) {
                min = (int)tv.getFraction(mAvailableWidth, mAvailableWidth);
            } else {
                min = 0;
            }
            if (DEBUG_MEASURE) Log.d(mLogTag, "Adjust for min width: " + min + ", value::"
                    + tv.coerceToString() + ", mAvailableWidth=" + mAvailableWidth);

            if (width < min) {
                widthMeasureSpec = MeasureSpec.makeMeasureSpec(min, EXACTLY);
                measure = true;
            }
        }
    }

    // TODO: Support height?

    if (measure) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }
}

FrameLayout.onMeasure方法如下：（這個方法里面都很重要）

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int count = getChildCount();

    final boolean measureMatchParentChildren =
            MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
            MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
    mMatchParentChildren.clear();

    int maxHeight = 0;
    int maxWidth = 0;
    int childState = 0;
    //這里是遍歷所有的子view士聪，并動態(tài)計算
    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        if (mMeasureAllChildren || child.getVisibility() != GONE) {
            // 測量計算子view 
            measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            maxWidth = Math.max(maxWidth,
                    child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
            maxHeight = Math.max(maxHeight,
                    child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
            childState = combineMeasuredStates(childState, child.getMeasuredState());
            if (measureMatchParentChildren) {
                if (lp.width == LayoutParams.MATCH_PARENT ||
                        lp.height == LayoutParams.MATCH_PARENT) {
                    mMatchParentChildren.add(child);
                }
            }
        }
    }

    // Account for padding too
    maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
    maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

    // Check against our minimum height and width
    maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
    maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

    // Check against our foreground's minimum height and width
    final Drawable drawable = getForeground();
    if (drawable != null) {
        maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
        maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
    }

    setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
            resolveSizeAndState(maxHeight, heightMeasureSpec,
                    childState << MEASURED_HEIGHT_STATE_SHIFT));

    count = mMatchParentChildren.size();
    if (count > 1) {
        for (int i = 0; i < count; i++) {
            final View child = mMatchParentChildren.get(i);
            final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

            final int childWidthMeasureSpec;
            if (lp.width == LayoutParams.MATCH_PARENT) {
                final int width = Math.max(0, getMeasuredWidth()
                        - getPaddingLeftWithForeground() - getPaddingRightWithForeground()
                        - lp.leftMargin - lp.rightMargin);
                childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
                        width, MeasureSpec.EXACTLY);
            } else {
                childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                        getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                        lp.leftMargin + lp.rightMargin,
                        lp.width);
            }

            final int childHeightMeasureSpec;
            if (lp.height == LayoutParams.MATCH_PARENT) {
                final int height = Math.max(0, getMeasuredHeight()
                        - getPaddingTopWithForeground() - getPaddingBottomWithForeground()
                        - lp.topMargin - lp.bottomMargin);
                childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                        height, MeasureSpec.EXACTLY);
            } else {
                childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                        getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                        lp.topMargin + lp.bottomMargin,
                        lp.height);
            }
            // 關(guān)鍵點在這里锦援，繼續(xù)調(diào)用child的measure方法
            child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        }
    }
}

我們先看 ， measureChildWithMargins 方法剥悟，

protected void measureChildWithMargins(View child,
        int parentWidthMeasureSpec, int widthUsed,
        int parentHeightMeasureSpec, int heightUsed) {
    final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
    // 這個方法是獲取子view的測量模式
    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);
   // 最終調(diào)用的還是View.measure 通過這種循環(huán)迭代把所有的子view全部測量完畢
    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

getChildMeasureSpec 方法內(nèi)容如下：

  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;
    }
    //noinspection ResourceType
    return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

那么假如我們寫的布局根節(jié)點是LinearLayout灵寺，那么就會在執(zhí)行到View.measure方法里面的onMeasure方法時，就會調(diào)用到LinearLayout.onMeasure方法区岗，具體內(nèi)容如下：

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    if (mOrientation == VERTICAL) {
        measureVertical(widthMeasureSpec, heightMeasureSpec);
    } else {
        measureHorizontal(widthMeasureSpec, heightMeasureSpec);
    }
}

通過源碼可以看到略板，還是會循環(huán)調(diào)用子View , 就這樣循環(huán)遞歸的測量完最里面的一個view，這個過程中onMeasure方法可能會被多次執(zhí)行慈缔。

2叮称、布局 performLayout

還是從ViewRootImpl.performTraversals開始

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
        int desiredWindowHeight) {
    mLayoutRequested = false;
    mScrollMayChange = true;
    mInLayout = true;

    final View host = mView;
    if (host == null) {
        return;
    }
    try {
        // 這里調(diào)用decorView的layout方法，實際上最終調(diào)用的是View.layout方法
        host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

       // ...
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
    mInLayout = false;
}

這里跟measure 調(diào)用流程其實一樣藐鹤，DecorView和FrameLayout沒有重寫layout方法颅拦，所以調(diào)用的是View.layout方法，

public void layout(int l, int t, int r, int b) {
    // setFrame 方法就是給mLeft\mTop\mRight\mBottom賦值
    boolean changed = isLayoutModeOptical(mParent) ?
            setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
    if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
        // 這里調(diào)用的是onLayout方法
        onLayout(changed, l, t, r, b);
        // ...
    }
 }

由于當(dāng)前對象是decorView教藻，所以調(diào)用的是DecorView.onLayout方法：

@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    // 關(guān)鍵的就是這個方法距帅，調(diào)用的是FrameLayout.onLayout
    super.onLayout(changed, left, top, right, bottom);
    getOutsets(mOutsets);
    // ...
}

FrameLayout.onLayout 方法如下：

@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
    final int count = getChildCount();

    final int parentLeft = getPaddingLeftWithForeground();
    final int parentRight = right - left - getPaddingRightWithForeground();

    final int parentTop = getPaddingTopWithForeground();
    final int parentBottom = bottom - top - getPaddingBottomWithForeground();

    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        if (child.getVisibility() != GONE) {
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();

            final int width = child.getMeasuredWidth();
            final int height = child.getMeasuredHeight();

            int childLeft;
            int childTop;

            int gravity = lp.gravity;
            if (gravity == -1) {
                gravity = DEFAULT_CHILD_GRAVITY;
            }

            final int layoutDirection = getLayoutDirection();
            final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
            final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;

            switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                case Gravity.CENTER_HORIZONTAL:
                    childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
                    lp.leftMargin - lp.rightMargin;
                    break;
                case Gravity.RIGHT:
                    if (!forceLeftGravity) {
                        childLeft = parentRight - width - lp.rightMargin;
                        break;
                    }
                case Gravity.LEFT:
                default:
                    childLeft = parentLeft + lp.leftMargin;
            }

            switch (verticalGravity) {
                case Gravity.TOP:
                    childTop = parentTop + lp.topMargin;
                    break;
                case Gravity.CENTER_VERTICAL:
                    childTop = parentTop + (parentBottom - parentTop - height) / 2 +
                    lp.topMargin - lp.bottomMargin;
                    break;
                case Gravity.BOTTOM:
                    childTop = parentBottom - height - lp.bottomMargin;
                    break;
                default:
                    childTop = parentTop + lp.topMargin;
            }
            // 這里最終調(diào)用的是 子view的layout方法，遞歸調(diào)用括堤，直到布局完畢所有的view
            child.layout(childLeft, childTop, childLeft + width, childTop + height);
        }
    }
}

3碌秸、繪制 performDraw

private void performDraw() {
  // ...
  try {
        // 調(diào)用繪制方法
        boolean canUseAsync = draw(fullRedrawNeeded);
        if (usingAsyncReport && !canUseAsync) {
            mAttachInfo.mThreadedRenderer.setFrameCompleteCallback(null);
            usingAsyncReport = false;
        }
    } finally {
        mIsDrawing = false;
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
  // ... 
}

private boolean draw(boolean fullRedrawNeeded) {
    //...
    if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) {
        if (mAttachInfo.mThreadedRenderer != null && mAttachInfo.mThreadedRenderer.isEnabled()) {
             if (updated) {
                requestDrawWindow();
            }
        }else{
            // 符合條件再次調(diào)用這個關(guān)鍵方法（從新測量、布局悄窃、繪制）
            scheduleTraversals();
        }
        // 這個drawSoftware方法很關(guān)鍵
        if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
              scalingRequired, dirty, surfaceInsets)) {
              return false;
        }
    }

 }


  private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
        boolean scalingRequired, Rect dirty, Rect surfaceInsets) {
      // ...
      try {
        dirty.offset(-dirtyXOffset, -dirtyYOffset);
        final int left = dirty.left;
        final int top = dirty.top;
        final int right = dirty.right;
        final int bottom = dirty.bottom;

        canvas = mSurface.lockCanvas(dirty);

        // TODO: Do this in native
        canvas.setDensity(mDensity);
    } {...}

    try{
        canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);
        // 這里開始調(diào)用view的draw方法
        mView.draw(canvas);
    }{...}
}

繼續(xù)查看 View.draw方法

public void draw(Canvas canvas) {
    final int privateFlags = mPrivateFlags;
    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;

    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
        onDraw(canvas);

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

        drawAutofilledHighlight(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);

        // Step 7, draw the default focus highlight
        drawDefaultFocusHighlight(canvas);

        if (debugDraw()) {
            debugDrawFocus(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 topSaveCount = -1;
    int bottomSaveCount = -1;
    int leftSaveCount = -1;
    int rightSaveCount = -1;

    int solidColor = getSolidColor();
    if (solidColor == 0) {
        if (drawTop) {
            topSaveCount = canvas.saveUnclippedLayer(left, top, right, top + length);
        }

        if (drawBottom) {
            bottomSaveCount = canvas.saveUnclippedLayer(left, bottom - length, right, bottom);
        }

        if (drawLeft) {
            leftSaveCount = canvas.saveUnclippedLayer(left, top, left + length, bottom);
        }

        if (drawRight) {
            rightSaveCount = canvas.saveUnclippedLayer(right - length, top, right, bottom);
        }
    } else {
        scrollabilityCache.setFadeColor(solidColor);
    }

    // Step 3, draw the content
    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;

    // must be restored in the reverse order that they were saved
    if (drawRight) {
        matrix.setScale(1, fadeHeight * rightFadeStrength);
        matrix.postRotate(90);
        matrix.postTranslate(right, top);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        if (solidColor == 0) {
            canvas.restoreUnclippedLayer(rightSaveCount, p);

        } else {
            canvas.drawRect(right - length, top, right, bottom, p);
        }
    }

    if (drawLeft) {
        matrix.setScale(1, fadeHeight * leftFadeStrength);
        matrix.postRotate(-90);
        matrix.postTranslate(left, top);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        if (solidColor == 0) {
            canvas.restoreUnclippedLayer(leftSaveCount, p);
        } else {
            canvas.drawRect(left, top, left + length, bottom, p);
        }
    }

    if (drawBottom) {
        matrix.setScale(1, fadeHeight * bottomFadeStrength);
        matrix.postRotate(180);
        matrix.postTranslate(left, bottom);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        if (solidColor == 0) {
            canvas.restoreUnclippedLayer(bottomSaveCount, p);
        } else {
            canvas.drawRect(left, bottom - length, right, bottom, p);
        }
    }

    if (drawTop) {
        matrix.setScale(1, fadeHeight * topFadeStrength);
        matrix.postTranslate(left, top);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        if (solidColor == 0) {
            canvas.restoreUnclippedLayer(topSaveCount, p);
        } else {
            canvas.drawRect(left, top, right, top + length, p);
        }
    }

    canvas.restoreToCount(saveCount);

    drawAutofilledHighlight(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);

    if (debugDraw()) {
        debugDrawFocus(canvas);
    }
}

在這個方法里面讥电，所有重要的方法都在里面，onDraw轧抗、dispatchDraw 等等都在里面恩敌，我看了下這個方法里面都挺重要就沒刪減，也都能看得懂横媚。

到此View的整個繪制流程就搞清楚了纠炮。

總結(jié)：

關(guān)于子view測量
1月趟、不管父View是何模式，若子View有確切數(shù)值恢口，則子View大小就是其本身大小孝宗，且mode是EXACTLY
2、若子View是match_parent耕肩，則模式與父View相同因妇，且大小同父View（若父View是UNSPECIFIED，則子View大小為0）
3猿诸、若子View是wrap_content婚被，則模式是AT_MOST，大小同父View梳虽，表示不可超過父View大兄沸尽（若父View是UNSPECIFIED，則子View大小為0）

關(guān)于繪制流程
我們自定義的view怖辆，基本上只需要重寫 onMeasure是复、onLayout、onDraw即可