Layout布局流程

Layout作用是ViewGroup用來確定子元素的位置的。如果當(dāng)前是一個(gè)View，則會(huì)通過layout方法確定當(dāng)前View的位置；如果當(dāng)前是一個(gè)ViewGroup，除了在layout方法中確定當(dāng)前ViewGroup的位置外，還會(huì)調(diào)用onLayout方法分別確定它c(diǎn)hild的位置，如果child中有ViewGroup，還會(huì)繼續(xù)執(zhí)行該過程，直到完成該View樹的layout。

View的布局

View的layout方法

@SuppressWarnings({"unchecked"})
public void layout(int l, int t, int r, int b) {
    //...
    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);
    //...
  }
//...
}

首先用局部變量記錄了當(dāng)前View的左上右下的位置，然后調(diào)用setFrame方法

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);
        }
                //...
    }
    return changed;
}

將新傳入的左上右下的值和成員變量的值對(duì)比，如果有一個(gè)不一樣就證明當(dāng)前布局發(fā)生了變化，changed置為true，changed作為方法的返回值返回。計(jì)算新的寬高和原來的寬高比較，如果有不同就證明View的尺寸也發(fā)生了改變，將新的位置賦值給成員變量，并且回調(diào)sizeChange方法，這樣View的位置也就確定了。

ViewGroup的布局

ViewGroup的layout方法

@Override
public final void layout(int l, int t, int r, int b) {
    if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) {
        if (mTransition != null) {
            mTransition.layoutChange(this);
        }
        super.layout(l, t, r, b);
    } else {
        // record the fact that we noop'd it; request layout when transition finishes
        mLayoutCalledWhileSuppressed = true;
    }
}

調(diào)用父類View的layout方法，根據(jù)上面的分析，先確定當(dāng)前ViewGroup的位置。然后就調(diào)用了onLayout方法進(jìn)行子child的布局，由于各種ViewGroup的布局特性也不同，所以ViewGroup并沒有實(shí)現(xiàn)onLayout方法，而是由各ViewGroup子類實(shí)現(xiàn)具體的布局方法。以LinearLayout為例，其onLayout如下

@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);
    }
}

與Measure測(cè)量時(shí)一樣，也是分布局方向進(jìn)行處理，垂直方向的布局調(diào)用layoutVertical方法

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);
        }
    }
}

根據(jù)LinearLayout垂直方向的Gravity計(jì)算出child垂直方向布局的起點(diǎn)childTop，然后遍歷child，如果當(dāng)前child是可見的話，會(huì)計(jì)算每個(gè)子child的childLeft的位置，同時(shí)childTop隨著遍歷也變大，這樣下一個(gè)child就會(huì)在當(dāng)前child下面。調(diào)用setChildFrame方法

private void setChildFrame(View child, int left, int top, int width, int height) {
    child.layout(left, top, left + width, top + height);
}

該方法調(diào)用child的layout方法，為子child指定位置。這樣，ViewGroup在layout方法中完成自己的定位后，又調(diào)用onLayout方法調(diào)用子child的layout方法，子child通過自己的layout確定了自己的位置，這樣一層層傳遞下去就完成了整個(gè)View樹的layout過程。

setChildFrame的width和height參數(shù)分別是View的測(cè)量寬高，我們知道在layout方法中會(huì)為成員變量賦值，即

//widht為測(cè)量寬度
mRight = left + width;
//height為測(cè)量高度
mBottom = top + height;

而View的最終寬高計(jì)算為

public final int getWidth() {
    return mRight - mLeft;
}

public final int getHeight() {
    return mBottom - mTop;
}

由此可見，在View的默認(rèn)實(shí)現(xiàn)中，View的測(cè)量寬高和最終寬高是相同的。區(qū)別在于測(cè)量寬高形成于measure測(cè)量過程，View的最終寬高形成于layout過程中，兩者的賦值時(shí)機(jī)不同。