View框架之measure()流程

先来张View体系中measure()流程的时序图

注意：1.以下MeasureSpec用ms来代替（ms具体会在后面介绍）     2.该时序图描述的是从ViewRootImpl开始的measure()过程，因为ViewRootImpl是window与view连接的枢纽     3.该时序图着重于大概流程及重要方法的作用，具体代码细节会放到后面介绍     4.DecorView作为顶级View，本身是一个frameLayout,所以也是一个ViewGroup

MesureSpec简介 (以下简称ms)

在view的测量过程中，ms起着非常重要的作用，ms在某种程度上来说更像是一种测量的标准。ms是一个32位的int值，前2位代表测量模式，后30位代表测量大小测量模式（Specmode）一共分为3种：    UNSPECIFIED: 父容器不对子view限制，这种情况一般我们用不到，系统内部使用    EXACTLY :  父容器可以算出View的具体的大小，一般对应于 具体值或者match_parent    AT_MOST : 父容器指定一个值，子view不能超过这个值，一般对应于 wrap_content

View关于MesureSpec的计算

一般情况下，一个view的ms的测量是根据父容器的ms和本身的LayoutParams来决定的，根据一些规则，在父容器中算出ms，然后父容器调用此view的measure()方法将结果传给该view,该view接着会回掉onMeasure()将结果传给该view的实现类。  但是DecorView作为顶级View是没有父容器的，所以DecorView的ms是根据窗口的大小和自身的LayoutParams来决定的。

1.DecorView的ms计算过程：

--         /**           *     desireWindowWidth : 代表屏幕的宽带 ，同理，dwh 代表屏幕的高度          **/      private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp,        final Resources res, final int desiredWindowWidth, final int desiredWindowHeight) {              *************************省略           //计算出DecorView的ms            ** childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);               childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height); **             *************************省略}    /**    * 计算出decorView的ms*    **/  private static int getRootMeasureSpec(int windowSize, int rootDimension) {    int measureSpec;    switch (rootDimension) {    case ViewGroup.LayoutParams.MATCH_PARENT:        //如果设置的是math_parent,屏幕长度就是view长度，模式为EXACTLY        measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);        break;    case ViewGroup.LayoutParams.WRAP_CONTENT:       //如果设置的是wrap_conentt,屏幕长度就是view长度，模式为AT_MOST        measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);        break;    default:        // 如果设置的是具体的数值，这个值就是View的长度，模式威EXANTLY        measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);        break;    }    return measureSpec;

2**.一般View的ms计算过程：**

 protected void measureChildWithMargins(View child,        int parentWidthMeasureSpec, int widthUsed,        int parentHeightMeasureSpec, int heightUsed) {    final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();          //求出view的ms,           //第二个参数的意思 在父容器中最大可用大小    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,            mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin                    + widthUsed, lp.width);    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,            mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin                    + heightUsed, lp.height);   //将测量好的ms,传递给子view    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);}//根据父容器的ms和自身的LayoutParams求出该view的ms的具体操作 public static int getChildMeasureSpec(int spec, int padding, int childDimension) {    int specMode = MeasureSpec.getMode(spec);    int specSize = MeasureSpec.getSize(spec);    int size = Math.max(0, specSize - padding);    int resultSize = 0;    int resultMode = 0;    switch (specMode) {    // Parent has imposed an exact size on us    case MeasureSpec.EXACTLY:        if (childDimension >= 0) {            resultSize = childDimension;            resultMode = MeasureSpec.EXACTLY;        } else if (childDimension == LayoutParams.MATCH_PARENT) {            // Child wants to be our size. So be it.            resultSize = size;            resultMode = MeasureSpec.EXACTLY;        } else if (childDimension == LayoutParams.WRAP_CONTENT) {            // Child wants to determine its own size. It can't be            // bigger than us.            resultSize = size;            resultMode = MeasureSpec.AT_MOST;        }        break;    // Parent has imposed a maximum size on us    case MeasureSpec.AT_MOST:        if (childDimension >= 0) {            // Child wants a specific size... so be it            resultSize = childDimension;            resultMode = MeasureSpec.EXACTLY;        } else if (childDimension == LayoutParams.MATCH_PARENT) {            // Child wants to be our size, but our size is not fixed.            // Constrain child to not be bigger than us.            resultSize = size;            resultMode = MeasureSpec.AT_MOST;        } else if (childDimension == LayoutParams.WRAP_CONTENT) {            // Child wants to determine its own size. It can't be            // bigger than us.            resultSize = size;            resultMode = MeasureSpec.AT_MOST;        }        break;    // Parent asked to see how big we want to be    case MeasureSpec.UNSPECIFIED:        if (childDimension >= 0) {            // Child wants a specific size... let him have it            resultSize = childDimension;            resultMode = MeasureSpec.EXACTLY;        } else if (childDimension == LayoutParams.MATCH_PARENT) {            // Child wants to be our size... find out how big it should            // be            resultSize = 0;            resultMode = MeasureSpec.UNSPECIFIED;        } else if (childDimension == LayoutParams.WRAP_CONTENT) {            // Child wants to determine its own size.... find out how            // big it should be            resultSize = 0;            resultMode = MeasureSpec.UNSPECIFIED;        }        break;    }    return MeasureSpec.makeMeasureSpec(resultSize, resultMode);}

View及ViewGroup的measure()操作

一般view的measure()方法如下：public final void measure(int widthMeasureSpec, int heightMeasureSpec) {    boolean optical = isLayoutModeOptical(this);    final boolean specChanged = widthMeasureSpec != mOldWidthMeasureSpec            || heightMeasureSpec != mOldHeightMeasureSpec;    final boolean isSpecExactly = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY            && MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;    final boolean matchesSpecSize = getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec)            && getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);    final boolean needsLayout = specChanged            && (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize);        //判断view的ms与存好的是否一样，以及需要根据标志，需要重新测量吗    if (forceLayout || needsLayout) {        resolveRtlPropertiesIfNeeded();        int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);        if (cacheIndex < 0 || sIgnoreMeasureCache) {            // measure ourselves, this should set the measured dimension flag back            onMeasure(widthMeasureSpec, heightMeasureSpec);            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;        } else {            long value = mMeasureCache.valueAt(cacheIndex);            // Casting a long to int drops the high 32 bits, no mask needed            setMeasuredDimensionRaw((int) (value >> 32), (int) value);            mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;        }        // flag not set, setMeasuredDimension() was not invoked, we raise        // an exception to warn the developer        if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {            throw new IllegalStateException("View with id " + getId() + ": "                    + getClass().getName() + "#onMeasure() did not set the"                    + " measured dimension by calling"                    + " setMeasuredDimension()");        }        mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;    }

// 将ms值保存起来，为了提高性能，如果下次进来并且标志没变，就不会再重复执行onMeasure()
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;

    mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |            (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension}ViewGroup 的 measure(); View是一个容器，他不但要完成自己的测量方法，还需要遍历onMeasure()计算出下一层级的ms,下面以FrameLayout的onMeasure方法为讲解 @Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {    int count = getChildCount();    final boolean measureMatchParentChildren =            MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||            MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;    mMatchParentChildren.clear();    int maxHeight = 0;    int maxWidth = 0;    int childState = 0;    for (int i = 0; i < count; i++) {    //遍历计算childView的ms        final View child = getChildAt(i);        if (mMeasureAllChildren || child.getVisibility() != GONE) {            measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);            final LayoutParams lp = (LayoutParams) child.getLayoutParams();            maxWidth = Math.max(maxWidth,                    child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);            maxHeight = Math.max(maxHeight,                    child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);            childState = combineMeasuredStates(childState, child.getMeasuredState());            if (measureMatchParentChildren) {                if (lp.width == LayoutParams.MATCH_PARENT ||                        lp.height == LayoutParams.MATCH_PARENT) {                    mMatchParentChildren.add(child);                }            }        }    }    // Account for padding too    maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();    maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();    // Check against our minimum height and width    maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());    maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());    // Check against our foreground's minimum height and width    final Drawable drawable = getForeground();    if (drawable != null) {        maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());        maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());    }    setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),            resolveSizeAndState(maxHeight, heightMeasureSpec,                    childState << MEASURED_HEIGHT_STATE_SHIFT));    count = mMatchParentChildren.size();    if (count > 1) {        for (int i = 0; i < count; i++) {            final View child = mMatchParentChildren.get(i);            final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();            int childWidthMeasureSpec;            int childHeightMeasureSpec;            if (lp.width == LayoutParams.MATCH_PARENT) {                childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() -                        getPaddingLeftWithForeground() - getPaddingRightWithForeground() -                        lp.leftMargin - lp.rightMargin,                        MeasureSpec.EXACTLY);            } else {                childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,                        getPaddingLeftWithForeground() + getPaddingRightWithForeground() +                        lp.leftMargin + lp.rightMargin,                        lp.width);            }            if (lp.height == LayoutParams.MATCH_PARENT) {                childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredHeight() -                        getPaddingTopWithForeground() - getPaddingBottomWithForeground() -                        lp.topMargin - lp.bottomMargin,                        MeasureSpec.EXACTLY);            } else {                childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,                        getPaddingTopWithForeground() + getPaddingBottomWithForeground() +                        lp.topMargin + lp.bottomMargin,                        lp.height);            }            //调用子类的measure()方法进行传递            child.measure(childWidthMeasureSpec, childHeightMeasureSpec);        }    }}