LinearLayout是android中最常用的布局之一。简单了解一下LinearLayout的源码实现过程，可以加深其各属性的用法，以及这么用的原因。比如常用的layout_weight等。
LinearLayout源码如下(以垂直布局为例，截取部分代码):
public class LinearLayout extends ViewGroup {
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
if (mOrientation == VERTICAL) {
// 分析垂直布局
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
// measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}
/**
* Measures the children when the orientation of this LinearLayout is set to
* {@link #VERTICAL}.
*
* @param widthMeasureSpec
* Horizontal space requirements as imposed by the parent.
* @param heightMeasureSpec
* Vertical space requirements as imposed by the parent.
*
* @see #getOrientation()
* @see #setOrientation(int)
* @see #onMeasure(int, int)
*/
void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
mTotalLength = 0;//子view的高度和
int maxWidth = 0;
//子视图的最大宽度(不包含layout_weight>0的子view)
int alternativeMaxWidth = 0;
//子视图的最大宽度(仅包含layout_weight>0的子view)
int weightedMaxWidth = 0;
//子视图的宽度是否全是fillParent的，用于后续判断是否需要重新计算
boolean allFillParent = true;
//所有子view的weight之和
float totalWeight = 0;
//子view的个数(仅包含直接子view，如LinearLayout1中
//分别有Textview1，LinearLayout2,TextView2，
//而LinearLayout2中又有多个子view。此时LinearLayout1的getVirtualChildCount();为3)
final int count = getVirtualChildCount();
//LinearLayout宽度模式
final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
//LinearLayout高度模式
final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
//子view的宽度是否要由父确定。如父LinearLayout为layout_width=wrap_content，
//子view为fill_parent则matchWidth =true
boolean matchWidth = false;
//以LinearLayout中第几个子view的baseLine作为LinearLayout的基准线
final int baselineChildIndex = mBaselineAlignedChildIndex;
// See how tall everyone is. Also remember max width.
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null) {
mTotalLength += measureNullChild(i);
//measureNullChild(i)默认都返回0，扩展预留，此处没用
continue;
}
if (child.getVisibility() == View.GONE) {
i += getChildrenSkipCount(child, i);
//getChildrenSkipCount(child, i)默认都返回空，扩展预留，此处没用
continue;
}
LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();
totalWeight += lp.weight;
if (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0) {
// Optimization: don't bother measuring children who are going to use
// leftover space. These views will get measured again down below if
// there is any leftover space.
//如果LinearLayout高度是已经确定的。并且这个子view的height=0，weight>0，
//则mTotalLength只需要加上margin即可，
//由于是weight>0；该view的具体高度等会还要计算
mTotalLength += lp.topMargin + lp.bottomMargin;
} else {
int oldHeight = Integer.MIN_VALUE;
if (lp.height == 0 && lp.weight > 0) {
// heightMode is either UNSPECIFIED OR AT_MOST, and this child
// wanted to stretch to fill available space. Translate that to
// WRAP_CONTENT so that it does not end up with a height of 0
oldHeight = 0;
lp.height = LayoutParams.WRAP_CONTENT;
}
// Determine how big this child would like to. 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).
measureChildBeforeLayout(
child, i, widthMeasureSpec, 0, heightMeasureSpec,
totalWeight == 0 ? mTotalLength : 0);
if (oldHeight != Integer.MIN_VALUE) {
lp.height = oldHeight;
}
mTotalLength += child.getMeasuredHeight() + lp.topMargin +
lp.bottomMargin + getNextLocationOffset(child);
}
/**
* 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) {
//为什么i < baselineChildIndex && lp.weight > 0不行。
//假如行的话，如果LinearLayout与其他view视图对其的话，
//由于weight>0的作用，会影响其他所有的view位置
//应该是由于效率的原因才不允许这样。
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.FILL_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
//如果LinearLayout宽度不是已确定的，如是wrap_content,而子view是FILL_PARENT，
//则做标记matchWidth=true； matchWidthLocally = true;
matchWidth = true;
matchWidthLocally = true;
}
final int margin = lp.leftMargin + lp.rightMargin;
final int measuredWidth = child.getMeasuredWidth() + margin;
maxWidth = Math.max(maxWidth, measuredWidth);//最大子view的宽度
//子view宽度是否全是FILL_PARENT
allFillParent = allFillParent && lp.width == LayoutParams.FILL_PARENT;
if (lp.weight > 0) {
/*
* Widths of weighted Views are bogus if we end up
* remeasuring, so keep them separate.
*/
//如父width是wrap_content，子是fill_parent，则子的宽度需要在父确定后才能确定。这里并不是真实的宽度
weightedMaxWidth = Math.max(weightedMaxWidth,
matchWidthLocally ? margin : measuredWidth);
} else {
alternativeMaxWidth = Math.max(alternativeMaxWidth,
matchWidthLocally ? margin : measuredWidth);
}
i += getChildrenSkipCount(child, i);
}
// Add in our padding
mTotalLength += mPaddingTop + mPaddingBottom;
int heightSize = mTotalLength;
// Check against our minimum height
heightSize = Math.max(heightSize, getSuggestedMinimumHeight());
// Reconcile our calculated size with the heightMeasureSpec
heightSize = resolveSize(heightSize, heightMeasureSpec);
// Either expand children with weight to take up available space or
// shrink them if they extend beyond our current bounds
int delta = heightSize - mTotalLength;
if (delta != 0 && totalWeight > 0.0f) {
float weightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;
mTotalLength = 0;
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child.getVisibility() == View.GONE) {
continue;
}
LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();
float childExtra = lp.weight;
if (childExtra > 0) {
// Child said it could absorb extra space -- give him his share
int share = (int) (childExtra * delta / weightSum);
weightSum -= childExtra;
delta -= share;
final int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
mPaddingLeft + mPaddingRight +
lp.leftMargin + lp.rightMargin, lp.width);
// TODO: Use a field like lp.isMeasured to figure out if this
// child has been previously measured
if ((lp.height != 0) || (heightMode != MeasureSpec.EXACTLY)) {
// child was measured once already above...
// base new measurement on stored values
int childHeight = child.getMeasuredHeight() + share;
if (childHeight < 0) {
childHeight = 0;
}
child.measure(childWidthMeasureSpec,
MeasureSpec.makeMeasureSpec(childHeight, MeasureSpec.EXACTLY));
} else {
// child was skipped in the loop above.
// Measure for this first time here
child.measure(childWidthMeasureSpec,
MeasureSpec.makeMeasureSpec(share > 0 ? share : 0,
MeasureSpec.EXACTLY));
}
}
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.FILL_PARENT;
alternativeMaxWidth = Math.max(alternativeMaxWidth,
matchWidthLocally ? margin : measuredWidth);
allFillParent = allFillParent && lp.width == LayoutParams.FILL_PARENT;
mTotalLength += child.getMeasuredHeight() + lp.topMargin +
lp.bottomMargin + getNextLocationOffset(child);
}
// Add in our padding
mTotalLength += mPaddingTop + mPaddingBottom;
} else {
alternativeMaxWidth = Math.max(alternativeMaxWidth,
weightedMaxWidth);
}
if (!allFillParent && widthMode != MeasureSpec.EXACTLY) {
maxWidth = alternativeMaxWidth;
}
maxWidth += mPaddingLeft + mPaddingRight;
// Check against our minimum width
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
setMeasuredDimension(resolveSize(maxWidth, widthMeasureSpec), heightSize);
if (matchWidth) {
forceUniformWidth(count, heightMeasureSpec);
}
}
}
根据源码，对layout_weight的用法进行说明，
以下均是对第二个LinearLayout 即linear1的调试过程
如下布局1：在垂直线性布局中放三个textview，第三个高度90，余下的高度由第一个占2/3，第二个占1/3
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent" android:layout_height="480dip">
<LinearLayout android:id="@+id/linear1" xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="fill_parent"
android:background="#ff888888" android:layout_height="fill_parent">
<TextView android:id="@+id/id1" android:layout_width="fill_parent"
android:layout_height="0dip" android:layout_weight="2"
android:background="#ff765423" android:text="11111" />
<TextView android:id="@+id/id2" android:layout_width="fill_parent"
android:layout_height="0dip" android:layout_weight="1"
android:background="#ffff0000" android:text="aaaaa" />
<TextView android:id="@+id/id3" android:layout_width="fill_parent"
android:background="#ff234532" android:layout_height="90dip"
android:text="2222222" />
</LinearLayout >
</LinearLayout>
运行结果，高度分别为260、130、90符合预期。
在linear1运行到源码171行时，heightSize =480(480由mTotalLength以及linear1的高度确定)，mTotalLength=90(90=0+0+90)；此时剩余的高度int delta = heightSize - mTotalLength=390;
由于totalWeight=2；由189行计算得tv1的附加高度为260；同理tv2高度130；
布局2：当将tv1与tv2的layout_height改成fill_parent结果又如何呢？
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent" android:layout_height="480dip">
<LinearLayout
android:id="@+id/linear"
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="fill_parent"
android:background="#ff888888" android:layout_height="fill_parent">
<TextView android:id="@+id/id1" android:layout_width="fill_parent"
android:layout_height="fill_parent" android:layout_weight="2"
android:background="#ff765423" android:text="11111" />
<TextView android:id="@+id/id2" android:layout_width="fill_parent"
android:layout_height="fill_parent" android:layout_weight="1"
android:background="#ffff0000" android:text="aaaaa" />
<TextView android:id="@+id/id3" android:layout_width="fill_parent"
android:background="#ff234532" android:layout_height="90dip"
android:text="2222222" />
</LinearLayout>
</LinearLayout>
运行结果，三者高度分别为100，290，90；结果为什么会是这样？tv1与tv2比例为什么不是2:1或者1：2？
linear运行到第171行时，heightSize=480，mTotalLength=1050（tv1与tv2的两个fill_parent都为480，tv3高度90；1050=480+480+90）
因此剩余的高度delta = heightSize - mTotalLength=-570;
因此在189行， int share = (int) (childExtra * delta / weightSum);
对tv1来说share=2*（-570）/3=-380 对tv2来说share=1*(-190)/1=-190;
由行202，int childHeight = child.getMeasuredHeight() + share;
计算的tv1高度为100，tv2高度为290；
总结：
其他模式如tv1为fill_parent,tv2为wrap_content时，这两个的layout_weight又起到什么作用？都可以从源码中分析出来。以上说的高度均是指mMeasuredHeight，因此有可能为负数，总和也可能大于手机屏幕高度。
在线性布局中，需要用到layout_weight的地方，均需要将对应的layout_height(垂直时)或者layout_width(水平时)
设置为0dip，这样他们的高度或者宽度就是把剩余的长度按layout_weight成正比分配给对应的view。如果不是设置为0dip， 剩余空间如何按比例分配是不确定的。根据计算，有可能刚好按layout_weight的比例分配(如子view的layout_weight都设置为1，并且设置为fill_parent,这种情况下，各子view刚好平分。)。
一般地， layout_weight>0时，将layout_height或者layout_width设置为0dip，不但能正确的按比例分配剩余空间，而且还会提高效率(由源码67行可见）。
layout_weight的作用是把剩余的部分，按比例分。
如布局2，总共是480的高度，tv1与tv2 由于layout_height="fill_parent"则高度都为480，tv3高度为90.剩余的高度为-570.将-570按比例2比1分配给tv1与tv2，则tv1的高度为100，tv2高度为290.
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