上一篇寫(xiě)了View的測(cè)量學(xué)個(gè)明白--View的測(cè)量
這一篇寫(xiě)View的layout過(guò)程。

這一篇主要是總結(jié)View的Layout過(guò)程。
我們?cè)谶M(jìn)行View的Layout的時(shí)候，直接調(diào)用View的layout方法。

layout(int l, int t, int r, int b)
// left,top, right,bottom

但是里面發(fā)生了什么呢？

我在學(xué)習(xí)了查閱了資料后寫(xiě)下了這篇文章，主要內(nèi)容主要摘錄自《Android開(kāi)發(fā)藝術(shù)探索》和源碼，加入自己的見(jiàn)解和理解。本人才疏學(xué)淺，不恰當(dāng)之處望批評(píng)指正。

Layout過(guò)程

Layout的作用是ViewGroup用來(lái)確定子元素的位置，當(dāng)ViewGroup的位置被確定后，它在onLayout中會(huì)遍歷所有的子元素并調(diào)用其layout的方法,在layout方法中onLayout方法又會(huì)被調(diào)用。Layout過(guò)程和measure過(guò)程相比就簡(jiǎn)單多了，layout方法確定View本身的位置，而onLayout方法則會(huì)確定所有子元素的位置。
View的layout()方法

//android-25 layout方法
 /**
     * Assign a size and position to a view and all of its
     * descendants
     *
     * <p>This is the second phase of the layout mechanism.
     * (The first is measuring). In this phase, each parent calls
     * layout on all of its children to position them.
     * This is typically done using the child measurements
     * that were stored in the measure pass().</p>
     *
     * <p>Derived classes should not override this method.
     * Derived classes with children should override
     * onLayout. In that method, they should
     * call layout on each of their children.</p>
     *
     * @param l Left position, relative to parent
     * @param t Top position, relative to parent
     * @param r Right position, relative to parent
     * @param b Bottom position, relative to parent
     */
@SuppressWarnings({"unchecked"})
    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;
        }

        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);

            if (shouldDrawRoundScrollbar()) {
                if(mRoundScrollbarRenderer == null) {
                    mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
                }
            } else {
                mRoundScrollbarRenderer = null;
            }

            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;
    }

大致流程如下：首先會(huì)通過(guò)setFrame方法來(lái)設(shè)定View的四個(gè)頂點(diǎn)的位置，即初始化mLeft， mTopm，Rightm，Bottom這四個(gè)值，View的四個(gè)頂點(diǎn)一旦確定，那么View在父容器中的位置也就確定了;
接著會(huì)調(diào)用onLayout方法，這個(gè)方法的用途是父容器確定子元素的位置。
和onMeasure方法類(lèi)似，onLayout的具體實(shí)現(xiàn)同樣和具體的布局有關(guān)系，所以View和ViewGroup均沒(méi)有真正實(shí)現(xiàn)。

 /**
     * Called from layout when this view should
     * assign a size and position to each of its children.
     *
     * Derived classes with children should override
     * this method and call layout on each of
     * their children.
     * @param changed This is a new size or position for this view
     * @param left Left position, relative to parent
     * @param top Top position, relative to parent
     * @param right Right position, relative to parent
     * @param bottom Bottom position, relative to parent
     */
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    }

我們來(lái)看看LinearLayout 的onLayout方法吧。

 @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        if (mOrientation == VERTICAL) {
            layoutVertical(l, t, r, b); 
        } else {
            layoutHorizontal(l, t, r, b);
        }
    }

下面貼出layoutVertical(), layoutHorizontal()代碼：(似乎純屬占篇幅)

/**
     * Position the children during a layout pass if the orientation of this
     * LinearLayout is set to {@link #VERTICAL}.
     *
     * @see #getOrientation()
     * @see #setOrientation(int)
     * @see #onLayout(boolean, int, int, int, int)
     * @param left
     * @param top
     * @param right
     * @param bottom
     */
    void layoutVertical(int left, int top, int right, int bottom) {
        final int paddingLeft = mPaddingLeft;

        int childTop;
        int childLeft;
        
        // Where right end of child should go
        final int width = right - left;
        int childRight = width - mPaddingRight;
        
        // Space available for child
        int childSpace = width - paddingLeft - mPaddingRight;
        
        final int count = getVirtualChildCount();

        final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
        final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;

        switch (majorGravity) {
           case Gravity.BOTTOM:
               // mTotalLength contains the padding already
               childTop = mPaddingTop + bottom - top - mTotalLength;
               break;

               // mTotalLength contains the padding already
           case Gravity.CENTER_VERTICAL:
               childTop = mPaddingTop + (bottom - top - mTotalLength) / 2;
               break;

           case Gravity.TOP:
           default:
               childTop = mPaddingTop;
               break;
        }

        for (int i = 0; i < count; i++) {
            final View child = getVirtualChildAt(i);
            if (child == null) {
                childTop += measureNullChild(i);
            } else if (child.getVisibility() != GONE) {
                final int childWidth = child.getMeasuredWidth();
                final int childHeight = child.getMeasuredHeight();
                
                final LinearLayout.LayoutParams lp =
                        (LinearLayout.LayoutParams) child.getLayoutParams();
                
                int gravity = lp.gravity;
                if (gravity < 0) {
                    gravity = minorGravity;
                }
                final int layoutDirection = getLayoutDirection();
                final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
                switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                    case Gravity.CENTER_HORIZONTAL:
                        childLeft = paddingLeft + ((childSpace - childWidth) / 2)
                                + lp.leftMargin - lp.rightMargin;
                        break;

                    case Gravity.RIGHT:
                        childLeft = childRight - childWidth - lp.rightMargin;
                        break;

                    case Gravity.LEFT:
                    default:
                        childLeft = paddingLeft + lp.leftMargin;
                        break;
                }

                if (hasDividerBeforeChildAt(i)) {
                    childTop += mDividerHeight;
                }

                childTop += lp.topMargin;
                setChildFrame(child, childLeft, childTop + getLocationOffset(child),
                        childWidth, childHeight);
                childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);

                i += getChildrenSkipCount(child, i);
            }
        }
    }

/**
     * Position the children during a layout pass if the orientation of this
     * LinearLayout is set to {@link #HORIZONTAL}.
     *
     * @see #getOrientation()
     * @see #setOrientation(int)
     * @see #onLayout(boolean, int, int, int, int)
     * @param left
     * @param top
     * @param right
     * @param bottom
     */
    void layoutHorizontal(int left, int top, int right, int bottom) {
        final boolean isLayoutRtl = isLayoutRtl();
        final int paddingTop = mPaddingTop;

        int childTop;
        int childLeft;
        
        // Where bottom of child should go
        final int height = bottom - top;
        int childBottom = height - mPaddingBottom; 
        
        // Space available for child
        int childSpace = height - paddingTop - mPaddingBottom;

        final int count = getVirtualChildCount();

        final int majorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
        final int minorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;

        final boolean baselineAligned = mBaselineAligned;

        final int[] maxAscent = mMaxAscent;
        final int[] maxDescent = mMaxDescent;

        final int layoutDirection = getLayoutDirection();
        switch (Gravity.getAbsoluteGravity(majorGravity, layoutDirection)) {
            case Gravity.RIGHT:
                // mTotalLength contains the padding already
                childLeft = mPaddingLeft + right - left - mTotalLength;
                break;

            case Gravity.CENTER_HORIZONTAL:
                // mTotalLength contains the padding already
                childLeft = mPaddingLeft + (right - left - mTotalLength) / 2;
                break;

            case Gravity.LEFT:
            default:
                childLeft = mPaddingLeft;
                break;
        }

        int start = 0;
        int dir = 1;
        //In case of RTL, start drawing from the last child.
        if (isLayoutRtl) {
            start = count - 1;
            dir = -1;
        }

        for (int i = 0; i < count; i++) {
            final int childIndex = start + dir * i;
            final View child = getVirtualChildAt(childIndex);
            if (child == null) {
                childLeft += measureNullChild(childIndex);
            } else if (child.getVisibility() != GONE) {
                final int childWidth = child.getMeasuredWidth();
                final int childHeight = child.getMeasuredHeight();
                int childBaseline = -1;

                final LinearLayout.LayoutParams lp =
                        (LinearLayout.LayoutParams) child.getLayoutParams();

                if (baselineAligned && lp.height != LayoutParams.MATCH_PARENT) {
                    childBaseline = child.getBaseline();
                }
                
                int gravity = lp.gravity;
                if (gravity < 0) {
                    gravity = minorGravity;
                }
                
                switch (gravity & Gravity.VERTICAL_GRAVITY_MASK) {
                    case Gravity.TOP:
                        childTop = paddingTop + lp.topMargin;
                        if (childBaseline != -1) {
                            childTop += maxAscent[INDEX_TOP] - childBaseline;
                        }
                        break;

                    case Gravity.CENTER_VERTICAL:
                        // Removed support for baseline alignment when layout_gravity or
                        // gravity == center_vertical. See bug #1038483.
                        // Keep the code around if we need to re-enable this feature
                        // if (childBaseline != -1) {
                        //     // Align baselines vertically only if the child is smaller than us
                        //     if (childSpace - childHeight > 0) {
                        //         childTop = paddingTop + (childSpace / 2) - childBaseline;
                        //     } else {
                        //         childTop = paddingTop + (childSpace - childHeight) / 2;
                        //     }
                        // } else {
                        childTop = paddingTop + ((childSpace - childHeight) / 2)
                                + lp.topMargin - lp.bottomMargin;
                        break;

                    case Gravity.BOTTOM:
                        childTop = childBottom - childHeight - lp.bottomMargin;
                        if (childBaseline != -1) {
                            int descent = child.getMeasuredHeight() - childBaseline;
                            childTop -= (maxDescent[INDEX_BOTTOM] - descent);
                        }
                        break;
                    default:
                        childTop = paddingTop;
                        break;
                }

                if (hasDividerBeforeChildAt(childIndex)) {
                    childLeft += mDividerWidth;
                }

                childLeft += lp.leftMargin;
                setChildFrame(child, childLeft + getLocationOffset(child), childTop,
                        childWidth, childHeight);
                childLeft += childWidth + lp.rightMargin +
                        getNextLocationOffset(child);

                i += getChildrenSkipCount(child, childIndex);
            }
        }
    }

(是不是看到這些代碼瑟瑟發(fā)抖。只要你堅(jiān)信代碼是寫(xiě)出的就行了。(什么? AI 寫(xiě)代碼。草，你找茬是不是？))
高潮來(lái)了：
簡(jiǎn)化一下layoutVertical代碼，注意看// Tudou add annotation的地方

 void layoutVertical(int left, int top, int right, int bottom) {
        final int paddingLeft = mPaddingLeft;

        int childTop;
        int childLeft;
        
        // Where right end of child should go
        final int width = right - left;
        int childRight = width - mPaddingRight;
        
        // Space available for child
        int childSpace = width - paddingLeft - mPaddingRight;
        // Tudou add annotation
        // get count
        final int count = getVirtualChildCount();

        final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
        final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;

        switch (majorGravity) {
           case Gravity.BOTTOM:
               // mTotalLength contains the padding already
               childTop = mPaddingTop + bottom - top - mTotalLength;
               break;

               // mTotalLength contains the padding already
           case Gravity.CENTER_VERTICAL:
               childTop = mPaddingTop + (bottom - top - mTotalLength) / 2;
               break;

           case Gravity.TOP:
           default:
               childTop = mPaddingTop;
               break;
        }
        // Tudou add annotation
        // 遍歷
        for (int i = 0; i < count; i++) {
            final View child = getVirtualChildAt(i);
            if (child == null) {
                childTop += measureNullChild(i);
            } else if (child.getVisibility() != GONE) {
                // Tudou add annotation
                // get width,height
                final int childWidth = child.getMeasuredWidth();
                final int childHeight = child.getMeasuredHeight();
                
                final LinearLayout.LayoutParams lp =
                        (LinearLayout.LayoutParams) child.getLayoutParams();
                
               ....

                if (hasDividerBeforeChildAt(i)) {
                    childTop += mDividerHeight;
                }

                childTop += lp.topMargin;
                //Tudou add annotation
                // 在setChildFrame的childWidth, childHeight，我們可以看出來(lái)，就是子元素測(cè)量寬/高.
                setChildFrame(child, childLeft, childTop + getLocationOffset(child),
                        childWidth, childHeight);
                childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);

                i += getChildrenSkipCount(child, i);
            }
        }
    }

簡(jiǎn)單分析一下layoutVertical的代碼邏輯，此方法會(huì)遍歷所有子元素并調(diào)用setChildFrame方法來(lái)為子元素指定對(duì)應(yīng)的位置。
其中childTop變量會(huì)逐漸增大，這就意味著后面的子元素會(huì)被放置在靠下的位置，正好符合LinearLayout 方向是豎直方向的特點(diǎn)。
setChildFrame僅僅是調(diào)用子元素的layout方法而已， 這樣父元素在layout中完成自己的定位后，就通過(guò)onLayout方法去調(diào)用子元素的layout方法，子元素又會(huì)通過(guò)自己的layout方法來(lái)確定自己的位置，這樣一層一層地傳遞下去就完成了整個(gè)View樹(shù)的layout過(guò)程。

看一下setChildFrame

//這里的width，height就是就是子元素測(cè)量寬/高.
 private void setChildFrame(View child, int left, int top, int width, int height) {        
       child.layout(left, top, left + width, top + height);
   }

而在layout方法中會(huì)通過(guò)setFrame去設(shè)置子元素的四個(gè)頂點(diǎn)的位置，在setFrame中有如下賦值語(yǔ)句，這樣一來(lái)子元素的位置就確定了。

mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;

參考資料：
developer.android.com
Android開(kāi)發(fā)藝術(shù)探索