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：

1. 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。
2. 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。
3. 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。
4. 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。
5. 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。