LinearLayout 主要是水平或者垂直排列子View，所以在调用三大方法时都是分为水平或者垂直方向的。
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}
@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);
}
}
一般ViewGroup是不会做绘制操作的，LinearLayout因为有分割线 divider 的属性，所以会在 onDraw() 中绘制分割线。
@Override
protected void onDraw(Canvas canvas) {
if (mDivider == null) {
return;
}
if (mOrientation == VERTICAL) {
drawDividersVertical(canvas);
} else {
drawDividersHorizontal(canvas);
}
}
measureVertical()，水平垂直逻辑类似。
- 通过 mTotalLength 计算所有childView 占用的高度，(高度为wrap_content时用于计算LinearLayout的高度), 最后根据LinearLayout的高度计算出剩余空间，用于weight属性分配剩余空间
- 使用 maxWidth 记录 childView 最大的宽度，用于计算LinearLayout的宽度
- 第一次循环，如果childView的属性 height=0，weight>0；LinearLayout 的 heightMode 为固定大小
···
final boolean useExcessSpace = lp.height == 0 && lp.weight > 0;
if (heightMode == MeasureSpec.EXACTLY && useExcessSpace) {
···
则会跳过childView的第一次measure过程，所以这样设置属性可以提升性能
- 第一次循环，除了3这种情况的childView，所有的childView都会measure 一次
final int usedHeight = totalWeight == 0 ? mTotalLength : 0;
measureChildBeforeLayout(child, i, widthMeasureSpec, 0,
heightMeasureSpec, usedHeight);
- 第二次循环，设置 measureWithLargestChild="true" 属性 并且高度为 warp_content ，则会进入第二次循环，用最大 childView 的高度 * childView 个数 再次计算 mTotalLength,防止LinearLayout高度出现问题。
if (useLargestChild && (heightMode == MeasureSpec.AT_MOST || heightMode == MeasureSpec.UNSPECIFIED)) {
mTotalLength = 0;
for (int i = 0; i < count; ++i) {
...
mTotalLength = Math.max(totalLength, totalLength + largestChildHeight + lp.topMargin +
lp.bottomMargin + getNextLocationOffset(child));
...
}
}
根据 mTotalLength 与 LinearLayout的高度，计算出剩余空间，第三次循环会weight >0 的 childView 会调用 measure 计算大小
高度为 wrap_content 时，因为没有剩余空间，进入不到第三次循环，所有这个情况 weight 属性无效。
android:measureWithLargestChild="true" 属性需要weight属性配合使用，不然只会增加 LinearLayout的高度。
代码只会进入第二个循环，用最大childView 的大小计算 LinearLayout的高度，但不会进入第三个循环来重新设置childView的大小。
float childExtra = lp.weight;
if (childExtra > 0) {
child.measure(MeasureSpec.makeMeasureSpec(child.getMeasuredWidth(),MeasureSpec.EXACTLY),
MeasureSpec.makeMeasureSpec(largestChildHeight,MeasureSpec.EXACTLY));
}
最后，当LinearLayout 宽度为wrap_content, childView 高度为 match_parent, LinearLayout 会最后再次 measure 宽度为match_parent 的childView,所以开发中要避免这种情况。
当LinearLayout 宽度为wrap_content，childView 的宽度，一个为 wrap_content ,一个为 match_parent,以wrap_content 为准
if (!allFillParent && widthMode != MeasureSpec.EXACTLY) {
maxWidth = alternativeMaxWidth; //wrap_content view的宽度
}
- android:measureWithLargestChild="true" 与 divider 属性冲突，因为第二次循环没有记录 divider 高度，在展示时会出现问题
layoutVertical()，水平垂直逻辑类似。
onLayout() 主要用于确认View展示的位置，一般有父容器确认，所以这个方法主要用于确认childView 展示的位置。
protected void onLayout(boolean changed, int l, int t, int r, int b) {
}
确认位置其实只要知道左上角位置即可,width,height 在measure 过程已经获得。
child.layout(left, top, left + width, top + height);
所以主要的逻辑是获取到 top left。
- 根据 LinearLayout 的 gravity属性 获取 childTop
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;
}
- 根据 childView 的 layout_garvity 属性获得 left
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;
}
- 就能确定 childView 的位置了
childTop += lp.topMargin;
setChildFrame(child, childLeft, childTop + getLocationOffset(child),
childWidth, childHeight);
drawDividersVertical()，水平垂直逻辑类似。
void drawDividersVertical(Canvas canvas) {
final int count = getVirtualChildCount();
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child != null && child.getVisibility() != GONE) {
//判断当前View之前是否要绘制分割线
if (hasDividerBeforeChildAt(i)) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final int top = child.getTop() - lp.topMargin - mDividerHeight;
drawHorizontalDivider(canvas, top);
}
}
}
//判断是否要绘制最后的分割线
if (hasDividerBeforeChildAt(count)) {
final View child = getLastNonGoneChild();
int bottom = 0;
if (child == null) {
bottom = getHeight() - getPaddingBottom() - mDividerHeight;
} else {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
bottom = child.getBottom() + lp.bottomMargin;
}
drawHorizontalDivider(canvas, bottom);
}
}
所有代码与注释,可以通过自定义LinearLayout的方式打断点调试
void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
mTotalLength = 0; // 所有 childView 的高度和 + 本身的 padding，注意：它和 LinearLayout 本身的高度是不同的
int maxWidth = 0; // 所有 childView 中宽度的最大值
int childState = 0;
int alternativeMaxWidth = 0; // 所有 layout_weight <= 0 的 childView 中宽度的最大值 用于获取最大款第
int weightedMaxWidth = 0; // 所有 layout_weight >0 的 childView 中宽度的最大值 用于获取最大款第
boolean allFillParent = true; //是否 layout_width 全是 matchParent
float totalWeight = 0; // 所有 childView 的 weight 之和
final int count = getVirtualChildCount(); //getCount()
final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
boolean matchWidth = false; //是否填充 LinearLayout,最后 measure一次child判断使用
boolean skippedMeasure = false; //是否跳过Measure过程
final int baselineChildIndex = mBaselineAlignedChildIndex;
final boolean useLargestChild = mUseLargestChild; //可以通过 xml 属性 android:measureWithLargestChild 设置的
int largestChildHeight = Integer.MIN_VALUE;
int consumedExcessSpace = 0; //使用的剩余空间
int nonSkippedChildCount = 0; // 没有跳过的 childView 个数，用于分割线的逻辑
// See how tall everyone is. Also remember max width.
// 看看每个 View 有多高,记录最大宽度
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i); //getChildAt()
if (child == null) {
mTotalLength += measureNullChild(i); //0
continue;
}
if (child.getVisibility() == View.GONE) {
i += getChildrenSkipCount(child, i); //0
continue;
}
nonSkippedChildCount++; // 没有跳过的 childView 个数
if (hasDividerBeforeChildAt(i)) { //是否有分割线
mTotalLength += mDividerHeight; //加上分割线高度
}
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
totalWeight += lp.weight; //计算总权重 totalWeight
final boolean useExcessSpace = lp.height == 0 && lp.weight > 0; //是否使用剩余空间
//LinearLayout 的高度是固定的，child 的 lp.height == 0 && lp.weight > 0，跳过 else
//步骤 skippedMeasure=true
if (heightMode == MeasureSpec.EXACTLY && useExcessSpace) {
// Optimization: don't bother measuring children who are only
// laid out using excess space. These views will get measured
// later if we have space to distribute.
//优化:不要麻烦测量那些只使用多余空间的孩子。如果我们有空间分布，这些视图将在稍后得到度量。
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
skippedMeasure = true;
} else {
if (useExcessSpace) {
// The heightMode is either UNSPECIFIED or AT_MOST, and
// this child is only laid out using excess space. Measure
// using WRAP_CONTENT so that we can find out the view's
// optimal height. We'll restore the original height of 0
// after measurement.
//heightMode 是 UNSPECIFIED or AT_MOST， childView 只使用剩余空间
//使用WRAP_CONTENT进行测量，以便找出视图的最佳高度。测量后恢复原来高度 0
lp.height = LayoutParams.WRAP_CONTENT;
}
// Determine how big this child would like to be. If this or
// previous children have given a weight, then we allow it to
// use all available space (and we will shrink things later
// if needed).
//确定这个孩子想要多大。如果这个或前面的孩子给了一个权重，那么我们允许它使用所有可用的空间
//(如果需要，我们将在稍后缩小内容)。
final int usedHeight = totalWeight == 0 ? mTotalLength : 0;
//计算 childView 高度
measureChildBeforeLayout(child, i, widthMeasureSpec, 0,
heightMeasureSpec, usedHeight);
final int childHeight = child.getMeasuredHeight();
if (useExcessSpace) {
// Restore the original height and record how much space
// we've allocated to excess-only children so that we can
// match the behavior of EXACTLY measurement.
//恢复原来的高度，并记录我们分配给 excess-only children 的空间大小，以便我们能够准确地匹配测量的行为。
lp.height = 0;
consumedExcessSpace += childHeight;
}
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
lp.bottomMargin + getNextLocationOffset(child)); //getNextLocationOffset() 0
if (useLargestChild) {
largestChildHeight = Math.max(childHeight, largestChildHeight); //最大childHeight
}
}
/**
* If applicable, compute the additional offset to the child's baseline
* we'll need later when asked {@link #getBaseline}.
*/
if ((baselineChildIndex >= 0) && (baselineChildIndex == i + 1)) {
mBaselineChildTop = mTotalLength;
}
// if we are trying to use a child index for our baseline, the above
// book keeping only works if there are no children above it with
// weight. fail fast to aid the developer.
if (i < baselineChildIndex && lp.weight > 0) {
throw new RuntimeException("A child of LinearLayout with index "
+ "less than mBaselineAlignedChildIndex has weight > 0, which "
+ "won't work. Either remove the weight, or don't set "
+ "mBaselineAlignedChildIndex.");
}
boolean matchWidthLocally = false;
if (widthMode != MeasureSpec.EXACTLY && lp.width == LayoutParams.MATCH_PARENT) {
// The width of the linear layout will scale, and at least one
// child said it wanted to match our width. Set a flag
// indicating that we need to remeasure at least that view when
// we know our width.
// 线性布局的宽度会缩放，并且至少有一个孩子说它想要匹配我们的宽度。
// 设置一个标志，表明当我们知道宽度时，至少需要重新测量该视图。
//LinearLayout 的宽度是 WRAP_CONTENT
matchWidth = true;
matchWidthLocally = true;
}
final int margin = lp.leftMargin + lp.rightMargin;
final int measuredWidth = child.getMeasuredWidth() + margin;
maxWidth = Math.max(maxWidth, measuredWidth);
childState = combineMeasuredStates(childState, child.getMeasuredState());
allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;
if (lp.weight > 0) {
/*
* Widths of weighted Views are bogus if we end up
* remeasuring, so keep them separate.
*/
//如果我们最终重新测量加权视图的宽度，那么它就是假的，所以要把它们分开。
weightedMaxWidth = Math.max(weightedMaxWidth,
matchWidthLocally ? margin : measuredWidth);
} else {
alternativeMaxWidth = Math.max(alternativeMaxWidth,
matchWidthLocally ? margin : measuredWidth);
}
i += getChildrenSkipCount(child, i); //0
}//第一次循环结束 获得 alternative weightedMaxWidth maxWidth matchWidth largestChildHeight
if (nonSkippedChildCount > 0 && hasDividerBeforeChildAt(count)) {
mTotalLength += mDividerHeight;
}
//useLargestChild 属性指定
//所以接下来根据 largestChildHeight 重新计算高度
if (useLargestChild &&
(heightMode == MeasureSpec.AT_MOST || heightMode == MeasureSpec.UNSPECIFIED)) {
mTotalLength = 0;
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null) {
mTotalLength += measureNullChild(i);
continue;
}
if (child.getVisibility() == GONE) {
i += getChildrenSkipCount(child, i);
continue;
}
final LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams)
child.getLayoutParams();
// Account for negative margins
// 负 margins
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + largestChildHeight +
lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
}
}//第二个循环 useLargestChild 使用 largestChildHeight 计算 mTotalLength
// Add in our padding
mTotalLength += mPaddingTop + mPaddingBottom;
int heightSize = mTotalLength;
// Check against our minimum height
//检查 最低高度
heightSize = Math.max(heightSize, getSuggestedMinimumHeight()); //(mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());
// Reconcile our calculated size with the heightMeasureSpec
//将我们计算出的尺寸与高度测量值进行比对
int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);
// 通过 heightMeasureSpec，调整 heightSize 的大小，具体的过程需要
// 看一下 resolveSizeAndState() 方法的实现
// 经过调整之后就是 LinearLayout 的大小了
heightSize = heightSizeAndState & MEASURED_SIZE_MASK;
// Either expand children with weight to take up available space or
// shrink them if they extend beyond our current bounds. If we skipped
// measurement on any children, we need to measure them now.
// 重新计算有 weight 属性的 childView 大小，
// 如果还有可用的空间，则扩展 childView，计算其大小
// 如果 childView 超出了 LinearLayout 的边界，则收缩 childView
int remainingExcess = heightSize - mTotalLength
+ (mAllowInconsistentMeasurement ? 0 : consumedExcessSpace); //剩余空间
if (skippedMeasure
|| ((sRemeasureWeightedChildren || remainingExcess != 0) && totalWeight > 0.0f)) {
//// 根据 mWeightSum 计算得到 remainingWeightSum，mWeightSum 是通过
// `android:weightSum` 属性设置的，totalWeight 是通过第一次 for 循环计算得到的
float remainingWeightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;
mTotalLength = 0;
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null || child.getVisibility() == View.GONE) {
continue;
}
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final float childWeight = lp.weight;
// 这是设置了 weight 的情况下，最重要的一行代码
// remainingExcess 剩余高度 * ( childView 的 weight / remainingWeightSum)
// share 便是此 childView 通过这个公式计算得到的高度，
// 并重新计算剩余高度 remainingExcess 和剩余权重总和 remainingWeightSum
if (childWeight > 0) {
final int share = (int) (childWeight * remainingExcess / remainingWeightSum);
remainingExcess -= share;
remainingWeightSum -= childWeight;
final int childHeight;
if (mUseLargestChild && heightMode != MeasureSpec.EXACTLY) {
childHeight = largestChildHeight;
} else if (lp.height == 0 && (!mAllowInconsistentMeasurement
|| heightMode == MeasureSpec.EXACTLY)) {
// This child needs to be laid out from scratch using
// only its share of excess space.
childHeight = share;
} else {
// This child had some intrinsic height to which we
// need to add its share of excess space.
childHeight = child.getMeasuredHeight() + share;
}
final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
Math.max(0, childHeight), MeasureSpec.EXACTLY);
final int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin,
lp.width);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
// Child may now not fit in vertical dimension.
childState = combineMeasuredStates(childState, child.getMeasuredState()
& (MEASURED_STATE_MASK>>MEASURED_HEIGHT_STATE_SHIFT));
} //child measure
final int margin = lp.leftMargin + lp.rightMargin;
final int measuredWidth = child.getMeasuredWidth() + margin;
maxWidth = Math.max(maxWidth, measuredWidth);
boolean matchWidthLocally = widthMode != MeasureSpec.EXACTLY &&
lp.width == LayoutParams.MATCH_PARENT;
alternativeMaxWidth = Math.max(alternativeMaxWidth,
matchWidthLocally ? margin : measuredWidth);
allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + child.getMeasuredHeight() +
lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
}
// Add in our padding
mTotalLength += mPaddingTop + mPaddingBottom;
// TODO: Should we recompute the heightSpec based on the new total length?
} else {
alternativeMaxWidth = Math.max(alternativeMaxWidth,
weightedMaxWidth);
// We have no limit, so make all weighted views as tall as the largest child.
// Children will have already been measured once.
if (useLargestChild && heightMode != MeasureSpec.EXACTLY) {
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null || child.getVisibility() == View.GONE) {
continue;
}
final LinearLayout.LayoutParams lp =
(LinearLayout.LayoutParams) child.getLayoutParams();
float childExtra = lp.weight;
if (childExtra > 0) {
child.measure(
MeasureSpec.makeMeasureSpec(child.getMeasuredWidth(),
MeasureSpec.EXACTLY),
MeasureSpec.makeMeasureSpec(largestChildHeight,
MeasureSpec.EXACTLY));
}
}
}
}
if (!allFillParent && widthMode != MeasureSpec.EXACTLY) {
maxWidth = alternativeMaxWidth;
}
maxWidth += mPaddingLeft + mPaddingRight;
// Check against our minimum width
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
heightSizeAndState);
if (matchWidth) {
forceUniformWidth(count, heightMeasureSpec);
}
}
/**
* 再次计算宽度
*/
private void forceUniformWidth(int count, int heightMeasureSpec) {
// Pretend that the linear layout has an exact size.
int uniformMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth(),
MeasureSpec.EXACTLY);
for (int i = 0; i< count; ++i) {
final View child = getVirtualChildAt(i);
if (child != null && child.getVisibility() != GONE) {
LinearLayout.LayoutParams lp = ((LinearLayout.LayoutParams)child.getLayoutParams());
if (lp.width == LayoutParams.MATCH_PARENT) {
// Temporarily force children to reuse their old measured height
// FIXME: this may not be right for something like wrapping text?
int oldHeight = lp.height;
lp.height = child.getMeasuredHeight();
// Remeasue with new dimensions
measureChildWithMargins(child, uniformMeasureSpec, 0, heightMeasureSpec, 0);
lp.height = oldHeight;
}
}
}
}
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);
}
}
}
void drawDividersVertical(Canvas canvas) {
final int count = getVirtualChildCount();
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child != null && child.getVisibility() != GONE) {
//判断当前View之前是否要绘制分割线
if (hasDividerBeforeChildAt(i)) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final int top = child.getTop() - lp.topMargin - mDividerHeight;
drawHorizontalDivider(canvas, top);
}
}
}
//判断是否要绘制最后的分割线
if (hasDividerBeforeChildAt(count)) {
final View child = getLastNonGoneChild();
int bottom = 0;
if (child == null) {
bottom = getHeight() - getPaddingBottom() - mDividerHeight;
} else {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
bottom = child.getBottom() + lp.bottomMargin;
}
drawHorizontalDivider(canvas, bottom);
}
}