1. 概述

layout 过程是为了确定View的最终宽高和四个顶点的位置。理解layout过程的重点在于理解View的layout方法和onLayout方法

layout 方法：
首先会确定View自身四个顶点的位置，即初始化mLeft、mRight、mTop和mBottom这四个值。接着会调用onLayout方法。

onLayout 方法：
遍历所有子元素并调用子元素的layout方法，完成子元素的layout过程

2. View的layout方法

源码如下：

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;

    //通过setFrame方法来设定View的四个顶点的位置
    boolean changed = isLayoutModeOptical(mParent) ?
            setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

    if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
        //确定子元素的位置
        //onLayout和ViewGroup均没有实现onLayout方法
        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方法首先会通过setFrame方法来设定View的四个顶点的View，即初始化mLeft、mRight、mTop和mBottom这四个值，setFrame方法源码如下：

protected boolean setFrame(int left, int top, int right, int bottom) {
    boolean changed = false;

    if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
        changed = true;

        // Remember our drawn bit
        int drawn = mPrivateFlags & PFLAG_DRAWN;

        int oldWidth = mRight - mLeft;
        int oldHeight = mBottom - mTop;
        int newWidth = right - left;
        int newHeight = bottom - top;
        boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);

        // Invalidate our old position
        invalidate(sizeChanged);

        mLeft = left;
        mTop = top;
        mRight = right;
        mBottom = bottom;
        mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);

        mPrivateFlags |= PFLAG_HAS_BOUNDS;


        if (sizeChanged) {
            sizeChange(newWidth, newHeight, oldWidth, oldHeight);
        }

        if ((mViewFlags & VISIBILITY_MASK) == VISIBLE || mGhostView != null) {
            // If we are visible, force the DRAWN bit to on so that
            // this invalidate will go through (at least to our parent).
            // This is because someone may have invalidated this view
            // before this call to setFrame came in, thereby clearing
            // the DRAWN bit.
            mPrivateFlags |= PFLAG_DRAWN;
            invalidate(sizeChanged);
            // parent display list may need to be recreated based on a change in the bounds
            // of any child
            invalidateParentCaches();
        }

        // Reset drawn bit to original value (invalidate turns it off)
        mPrivateFlags |= drawn;

        mBackgroundSizeChanged = true;
        mDefaultFocusHighlightSizeChanged = true;
        if (mForegroundInfo != null) {
            mForegroundInfo.mBoundsChanged = true;
        }

        notifySubtreeAccessibilityStateChangedIfNeeded();
    }
    return changed;
}

layout方法在通过调用setFrame方法设定自己四个顶点的位置后，接着会调用onLayout方法，这个方法的用途是父容器确定子元素的位置。和onMeasure方法类似，onLayout的具体实现同样和具体的布局有关，所以View和ViewGroup均没有真正实现onLayout方法。关于onLayout方法的实现，可以参考LinearLayout对于onLayout方法的实现。

3. LinearLayout的onLayout方法

LinearLayout的onLayout源码如下：

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);
    }
}

同样的，选择layoutVertical方法分析竖直LinearLayout的layout过程：

void layoutVertical(int left, int top, int right, int bottom) {

    int childTop;
    int childLeft;

    //遍历所有子元素
    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);

            //获取子元素left的位置
            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;

            //调用child.layout方法，完成子元素的layout
            //child.layout(left, top, left + width, top + height);
            setChildFrame(child, childLeft, childTop + getLocationOffset(child),
                    childWidth, childHeight);
            
            //计算下一个子元素的top位置
            childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);

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

layoutVertical方法会遍历所有子元素，根据Gravity计算子元素left的值，根据子元素的高度和margin计算每个子元素的top，计算出left和top后，再根据宽高计算出right和bottom，最后通过setChildFrame调用子元素的layout方法，子元素就会通过自己的layout方法确定自己的位置。