android/view/ViewRootImpl.java
private void performTraversals() {
    performLayout(lp, mWidth, mHeight);
}
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
        int desiredWindowHeight) {
    final View host = mView;
    //執(zhí)行DecorView的layout方法，傳入的是measuredWidth,measuredHeight 因此得先measure，后layout
    host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
}

android/view/View.java
public void layout(int l, int t, int r, int b) {
    //判斷是否需要重新測量
    if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
        onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
        mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
    }
    //存儲之前的l t r b信息
    int oldL = mLeft;
   int oldT = mTop;
   int oldB = mBottom;
   int oldR = mRight;
    //確定布局是否發(fā)生了變化，并存儲新的l t r b
   boolean changed = isLayoutModeOptical(mParent) ?
           setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
       if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
        //執(zhí)行布局相應(yīng)的onLayout()
           onLayout(changed, l, t, r, b);
        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) {
               //當(dāng)布局發(fā)生變化時，通知觀察者布局變化通知
               listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
           }
       }
    }
}

android/view/View.java
protected boolean setFrame(int left, int top, int right, int bottom) {
    boolean changed = false;
    //如果傳入的位置信息跟之前的位置信息不同
    if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
        //則說明布局發(fā)生改變
       changed = true;
        //重新存儲位置信息
        //這里注意一點(diǎn)，mLeft,mTop,mRight,mBottom都是相對位置，不是坐標(biāo)系的絕對位置
        //是相對于父view的位置大小
        mLeft = left;
       mTop = top;
       mRight = right;
       mBottom = bottom;
    }
    //返回布局是否發(fā)生改變
    return changed;
}

android/view/View.java
//空實(shí)現(xiàn)
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}

android/view/ViewGroup.java
//抽象方法
protected abstract void onLayout(boolean changed,
            int l, int t, int r, int b);

public final int getWidth() {
    //根據(jù)布局傳入的right - left得到寬
    return mRight - mLeft;
}
public final int getHeight() {
    //根據(jù)布局傳入的bottom - top得到高
    return mBottom - mTop;
}

android/widget/LinearLayout.java
protected void onLayout(boolean changed, int l, int t, int r, int b) {
    //根據(jù)LinearLayout的方向來決定子view的布局
    if (mOrientation == VERTICAL) {
        layoutVertical(l, t, r, b);
    } else {
        layoutHorizontal(l, t, r, b);
    }
}

void layoutVertical(int left, int top, int right, int bottom) {
    //獲取子view個數(shù)
    final int count = getVirtualChildCount();
    //遍歷子view
    for (int i = 0; i < count; i++) {
       //這里我們不關(guān)注子view的left top right bottom是如何計(jì)算的 主要關(guān)注整體布局流程
       setChildFrame(child, childLeft, childTop + getLocationOffset(child),
           childWidth, childHeight);
    }
}

private void setChildFrame(View child, int left, int top, int width, int height) {
    //執(zhí)行child.layout，繼續(xù)完成子類的布局設(shè)置
    child.layout(left, top, left + width, top + height);
}