View工作原理(三)视图大小计算过程(measure过程)
一、android中view的measure过程总概
视图大小计算的过程是从根视图measure()方法开始,接着该方法会调用根视图的onMeasure()方法,onMeasure()方法会对所包含的子视图逐一执行measure()方法,如果子视图是ViewGroup子类对象(LinearLayout、FrameLayout、RelativeLayout等布局),则继续调用子视图的measure()方法,重复这一过程。如果子视图是View子类对象(Button、EditText、TextView、ImageView等),则在子视图重载的onMeasure方法内部不需要进行对子视图进行measure操作,从而一次measure过程完成。过程如下图所示:
二、measure详细过程
View中的measure()方法源码(ViewGroup类继承了View类,measure过程先从ViewGroup子类开始):
- public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
- if ((mPrivateFlags & FORCE_LAYOUT) == FORCE_LAYOUT ||
- widthMeasureSpec != mOldWidthMeasureSpec ||
- heightMeasureSpec != mOldHeightMeasureSpec) {
- // first clears the measured dimension flag
- mPrivateFlags &= ~MEASURED_DIMENSION_SET;
- if (ViewDebug.TRACE_HIERARCHY) {
- ViewDebug.trace(this, ViewDebug.HierarchyTraceType.ON_MEASURE);
- }
- // measure ourselves, this should set the measured dimension flag back
- onMeasure(widthMeasureSpec, heightMeasureSpec);
- // flag not set, setMeasuredDimension() was not invoked, we raise
- // an exception to warn the developer
- if ((mPrivateFlags & MEASURED_DIMENSION_SET) != MEASURED_DIMENSION_SET) {
- throw new IllegalStateException("onMeasure() did not set the"
- + " measured dimension by calling"
- + " setMeasuredDimension()");
- }
- mPrivateFlags |= LAYOUT_REQUIRED;
- }
- mOldWidthMeasureSpec = widthMeasureSpec;
- mOldHeightMeasureSpec = heightMeasureSpec;
- }
注:通过源码,我们看到该方法的定义中用了final关键字,说明该方法是不能被重写的,即View系统定义的这个measure框架不能被修改。参数widthMeasureSpec和heightMeasureSpec分别对应宽和高的measureSpec,当父视图对子视图进行measure操作时,会调用子视图的measure()方法,该参数得意思是父视图所提供的measure的“规格”,因为父视图为子视图提供的窗口尺寸是由父视图和子视图共同决定。该参数有两部分组成,第一部分:高16位表示specMode,定义在MeasureSpec类中,有三种类型:MeasureSpec.EXACTLY:表示明确的尺寸大小, MeasureSpec.AT_MOST:表示最大大小, MeasureSpec.UNSPECIFIED:不确定大小。第二部分:低16位表示size,即父view的大小,这就是为什么我们在重写onmeasure方法是需要:int specMode = MeasureSpec.getMod(spec); int specSize = MeasureSpec.getSize(spec)这样调用。specMode一般都为MeasureSpec.EXACTLY ,而size分别对应屏幕宽,高。也就是Window第一次掉用的view,一般都是这个值,而对于子view来说,这个值就是你在xml定义的属性 android:layout_width和android:layout_height的值。
下面我们看看源码执行过程,看注释就能很明白,首先清 除测量尺寸的标识。接着将重新测量自己的尺寸,即调用onMeasure()方法。最后是判断测量尺寸大小的标识是否已经重新赋值,如果没有则不执行setMeasuredDimension()方法。方法结束。这个方法里面主要就是调用自己的onMeasure()方法,对自己的大小尺寸进行测量。下面来介绍onMeasure()方法。
ViewGroup中的onMeasure方法介绍
其实在ViewGroup类中并没有重写该方法,一般在他的子类中进行重写,比如LinearLayout、RelativeLayout,下面我们以Linearlayout来分析。LinearLayout中onMeasure方法源码如下:
- @Override
- protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
- if (mOrientation == VERTICAL) {
- measureVertical(widthMeasureSpec, heightMeasureSpec);
- } else {
- measureHorizontal(widthMeasureSpec, heightMeasureSpec);
- }
- }
注:通过源码我们可以知道,首先onMeasure会判断这个布局是纵向布局还是横向布局,即对应android:orientation=""属性。下面以纵向布局来分析,源码如下,有点长:
- /**
- * 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;
- int maxWidth = 0;
- int alternativeMaxWidth = 0;
- int weightedMaxWidth = 0;
- boolean allFillParent = true;
- float totalWeight = 0;
- final int count = getVirtualChildCount();
- final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
- final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
- boolean matchWidth = false;
- final int baselineChildIndex = mBaselineAlignedChildIndex;
- final boolean useLargestChild = mUseLargestChild;
- int largestChildHeight = Integer.MIN_VALUE;
- // 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);
- continue;
- }
- if (child.getVisibility() == View.GONE) {
- i += getChildrenSkipCount(child, i);
- 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.
- final int totalLength = mTotalLength;
- mTotalLength = Math.max(totalLength, totalLength + 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 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).
- measureChildBeforeLayout(
- child, i, widthMeasureSpec, 0, heightMeasureSpec,
- totalWeight == 0 ? mTotalLength : 0);
- if (oldHeight != Integer.MIN_VALUE) {
- lp.height = oldHeight;
- }
- final int childHeight = child.getMeasuredHeight();
- final int totalLength = mTotalLength;
- mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
- lp.bottomMargin + getNextLocationOffset(child));
- if (useLargestChild) {
- largestChildHeight = Math.max(childHeight, largestChildHeight);
- }
- }
- /**
- * 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.
- matchWidth = true;
- matchWidthLocally = true;
- }
- final int margin = lp.leftMargin + lp.rightMargin;
- final int measuredWidth = child.getMeasuredWidth() + margin;
- maxWidth = Math.max(maxWidth, measuredWidth);
- 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.
- */
- weightedMaxWidth = Math.max(weightedMaxWidth,
- matchWidthLocally ? margin : measuredWidth);
- } else {
- alternativeMaxWidth = Math.max(alternativeMaxWidth,
- matchWidthLocally ? margin : measuredWidth);
- }
- i += getChildrenSkipCount(child, i);
- }
- if (useLargestChild && heightMode == MeasureSpec.AT_MOST) {
- 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
- final int totalLength = mTotalLength;
- mTotalLength = Math.max(totalLength, totalLength + largestChildHeight +
- lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
- }
- }
- // 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.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);
- }
- 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);
- }
- }
注:
- 获取所有的子view数量,对每个子view开始处理,如果子view是GONE的,则直接跳过。
- 获取子view的LayoutParams,在xml中定义的参数,通过layout_weight定义的值累加到变量totalWeight中,然后判断如果view的height设置为零,但weight设置的大于0,则将height的值设置为LayoutParams.WRAP_CONTENT。
- 然后调用measureChildWithMargins方法,该方法处理的逻辑:计算子view的measureSpec,即specMode和specSize,调用方法为:getChildMeasureSpec,调用两次,分别 计算宽和高,getChildMeasureSpec内部根据父view的measure和子view的layout_width和layout_height属性计算子view的measure。getChildMeasureSpec计算子view的measure,总结如下:1.如果在xml中指定了子view的具体大小,那么计算结果不管父的measure是什么,结果都是EXACITY+child_size,2.如果子view的height指定的值为FILL_PARENT,则返回的结果为:EXACITY+size,原因很简单:因为FILL_PARENT的意思是充满这个父view,所以返回的子view的measure就是view的大小。3.如果子view的大小为wrap_content,那么返回的结果都为AT_MOST+size,原因是:最大不能超过父view的大小。
- 子view的measure确定好以后,然后调用子view的measure方法,如果子view是View对象,则该view的大小测量结束,开始下一个子view的循环,如果子view是ViewGroup那么,又开始一个新的递归,处理逻辑和上面一样,直到所有的view对象测量结束。
- 所有的子view测量结束后,才开始对layout_weight计算,这样我们可能想到,如果父view已经被占满了,那么有可能layout_weight大于0的view对象是不会显示的,而计算layout_weight的方法也很简单,就是用总高度减去上面分析完mTotalLength的值,就是剩下,然后去平分给view对象,注意计算权重时优先去android:android:weightSum(LinearLayout的xml属性)的值,如果不设置该值会计算和,所以该值既然设置了,就一定要子view的weight的总和相等,否则平分可能不能得到预期效果。
过程分析完毕,这篇文章这里提到了LinearLayout中的layout_weight属性,这个属性对很对人来说是又恨又爱,下篇文章,我们将来总结改属性的详细用法,让大家彻底理解这个属性。
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