android之layout_weight属性-优快云博客

本文链接：https://blog.youkuaiyun.com/tangyisen18/article/details/78021910

本文深入探讨了Android LinearLayout布局中的layout_weight属性的工作原理。详细分析了子视图如何通过两次测量过程确定最终尺寸，特别是在剩余空间分配方面的计算逻辑。

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我们知道在xml布局的时候，LinearLayout下面的子控件可以使用layout_weight属性，那么我们根据源码，来分析下这个属性的一些知识点。

首先，在系统代码attrs.xml，我们可以知道对这个的定义，还有一个跟weight有关的weightSum的定义。

<declare-styleable name="LinearLayout_Layout">

<attr name="layout_weight" format="float" /> 这个就是layout_weight的定义

</declare-styleable>

<declare-styleable name="LinearLayout">

<attr name="weightSum" format="float" /> weightSum是LinearLayout的属性值

然后我们在LinearLayout.java中

找到LinearLayout配对的 LayoutParams，里面

public LayoutParams(Context c, AttributeSet attrs) {

super(c, attrs);

TypedArray a =

c.obtainStyledAttributes(attrs, com.android.internal.R.styleable.LinearLayout_Layout);

weight = a.getFloat(com.android.internal.R.styleable.LinearLayout_Layout_layout_weight, 0);

gravity = a.getInt(com.android.internal.R.styleable.LinearLayout_Layout_layout_gravity, -1);

a.recycle();

}

所以从这里我们知道layout_weight是专属于LinearLayout的子控件所用，也只有在LinearLayout下才起作用。我们在xml中在LinearLayout下定义其子view，会自动为该子view生成LinearLayout. LayoutParams的布局参数的。在代码中，要显示定义。

layout_weight属性是跟控件大小相关的，属于measure，所以我们可以看下LinearLayout的onMeasure方法

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);//我们选择水平布局来看
}
}

measureHorizontal代码很长，我们只看跟weight相关的代码

voidmeasureHorizontal(int widthMeasureSpec, int heightMeasureSpec) {
mTotalLength = 0;//LinearLayout已经被使用了的大小
.......
float totalWeight = 0;//LinearLayout的所有子view总的weight

final int count = getVirtualChildCount();

final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
final int heightMode = MeasureSpec.getMode(heightMeasureSpec);

final boolean isExactly = widthMode == MeasureSpec.EXACTLY;

int usedExcessSpace = 0;

// See how wide everyone is. Also remember max height.
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;
}

if (hasDividerBeforeChildAt(i)) {
mTotalLength += mDividerWidth;
}

final LayoutParams lp = (LayoutParams) child.getLayoutParams();
totalWeight += lp.weight;

final boolean useExcessSpace = lp.width == 0 && lp.weight > 0;//我们useExcessSpace 为false，以为lp.width是-1和-2这两种情况
if (widthMode == MeasureSpec.EXACTLY && useExcessSpace) {
......
} else {
if (useExcessSpace) {
.......
}

// 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).
final int usedWidth = totalWeight == 0 ? mTotalLength : 0;
measureChildBeforeLayout(child, i, widthMeasureSpec, usedWidth,
heightMeasureSpec, 0);//第一次测量子view的大小，因为有weight，所以会有第二次测量

final int childWidth = child.getMeasuredWidth();
if (useExcessSpace) {
......
}

if (isExactly) {//我们默认LinearLayout是占据屏幕宽度来分析
mTotalLength += childWidth + lp.leftMargin + lp.rightMargin
+ getNextLocationOffset(child);
} else {
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + childWidth + lp.leftMargin
+ lp.rightMargin + getNextLocationOffset(child));
}

.......

}

if (mTotalLength > 0 && hasDividerBeforeChildAt(count)) {
mTotalLength += mDividerWidth;
}

if (useLargestChild &&//useLargestChild 默认为false
(widthMode == MeasureSpec.AT_MOST || widthMode == MeasureSpec.UNSPECIFIED)) {
......
}

// Add in our padding
mTotalLength += mPaddingLeft + mPaddingRight;

int widthSize = mTotalLength;

// Check against our minimum width
widthSize = Math.max(widthSize, getSuggestedMinimumWidth());

// Reconcile our calculated size with the widthMeasureSpec
int widthSizeAndState = resolveSizeAndState(widthSize, widthMeasureSpec, 0);//根据父亲的MeasureSpec参数测量LinearLayout的大小，这里是占据整个屏幕
widthSize = widthSizeAndState & 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.
int remainingExcess = widthSize - mTotalLength//下面的为0，根据注释我们也知道，这个值就是剩余可用空间的大小
+ (mAllowInconsistentMeasurement ? 0 : usedExcessSpace);
if (skippedMeasure || remainingExcess != 0 && totalWeight > 0.0f) {
float remainingWeightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;//这里mWeightSum 为0，所以remainingWeightSum 就是刚刚遍历所有子view后所有的weight的相加值
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;
if (childWeight > 0) {
final int share = (int) (childWeight * remainingExcess / remainingWeightSum);//这里就是重新测量，剩余可用空间的百分比
remainingExcess -= share;
remainingWeightSum -= childWeight;

final int childWidth;
if (mUseLargestChild && widthMode != MeasureSpec.EXACTLY) {
childWidth = largestChildWidth;
} else if (lp.width == 0 && (!mAllowInconsistentMeasurement
|| widthMode == MeasureSpec.EXACTLY)) {
// This child needs to be laid out from scratch using
// only its share of excess space.
childWidth = share;
} else {
// This child had some intrinsic width to which we
// need to add its share of excess space.
childWidth = child.getMeasuredWidth() + share;
}

final int childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
Math.max(0, childWidth), MeasureSpec.EXACTLY);
final int childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin,
lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);//第二次设置子view的大小

}