android-View工作原理(三)视图大小计算过程(measure过程)
2016-06-06 15:33
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本文转载自:http://blog.csdn.net/ff20081528/article/details/17560135
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):
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:
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:
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。
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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