文章标题

measureChildWithMargins：ViewGroup中测量子控件大小的方法（measureChild方法类似）

 /**   * Ask one of the children of this view to measure itself, taking into account both the MeasureSpec   * requirements for this view and its padding and margins. The child must have MarginLayoutParams The    * heavy lifting is done in getChildMeasureSpec.   * 要求子控件测量自己，考虑因素包括子控件的MeasureSpec和padding、margin，要求子控件必须有LayoutParams   * @param child The child to measure   * @param parentWidthMeasureSpec The width requirements for this view   * @param widthUsed Extra space that has been used up by the parent horizontally (possibly by other children of    * the parent) 父控件中已经使用掉的空间   * @param parentHeightMeasureSpec The height requirements for this view    * @param heightUsed Extra space that has been used up by the parent vertically (possibly by other children of    * the parent)   */   protected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed,    int parentHeightMeasureSpec, int heightUsed) {       // 获取child的LayoutParams       final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();       // 分别获取child宽高的MeasureSpec，传入parent的MeasureSpec、parent已用空间大小，child的LayoutParams宽高       // 此处LayoutParams.MATCH_PARENT = -1，LayoutParams.WRAP_CONTENT = -2，所以指定宽高时才会得到非负数        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);       // 使用获取到的childMeasureSpec为参数调用子控件的measure方法        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);    }

getChildMeasureSpec：获取child的MeasureSpec

/**     * Does the hard part of measureChildren: figuring out the MeasureSpec to pass to a particular child.      * This method figures out the right MeasureSpec for one dimension (height or width) of one child view.     * The goal is to combine information from our MeasureSpec with the LayoutParams of the child to get the best      * possible results. For example, if the this view knows its size (because its MeasureSpec has a mode of     * EXACTLY), and the child has indicated in its LayoutParams that it wants to be the same size as the parent,      * the parent should ask the child to layout given an exact size.     * 该方法用于计算要传递给子控件的MeasureSpec，一次计算一个子控件一个方向上的MeasureSpec，目标就是根据父控件的MeasureSpec     * 以及子控件的LayoutParams算出最好的可能结果     * @param spec The requirements for this view 父控件自身的MeasureSpec     * @param padding The padding of this view for the current dimension and margins, if applicable 父控件的     * padding和margin     * @param childDimension How big the child wants to be in the current dimension 子控件要求的大小     * @return a MeasureSpec integer for the child     */    public static int getChildMeasureSpec(int spec, int padding, int childDimension) {        int specMode = MeasureSpec.getMode(spec);        int specSize = MeasureSpec.getSize(spec);        // 父控件剩余大小        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: // 1.父控件本身受到精确测量的要求            if (childDimension >= 0) {                 // 1.1子控件要求的大小为一个精确值，则子控件的MeasureSpec就是精确测量到指定大小                resultSize = childDimension;                resultMode = MeasureSpec.EXACTLY;            } else if (childDimension == LayoutParams.MATCH_PARENT) {                 // Child wants to be our size. So be it.                       // 1.2子控件要求是充满父控件，而父控件剩余大小是确定的，所以子控件的MeasureSpec就是精确测量到父控件剩余大小                   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.                 // 1.3子控件要求自定义大小，所以子控件的MeasureSpec就是最大限制到父控件剩余大小                resultSize = size;                resultMode = MeasureSpec.AT_MOST;            }            break;        // Parent has imposed a maximum size on us        case MeasureSpec.AT_MOST: // 2.父控件本身受到最大限制，大小不确定            if (childDimension >= 0) { // 2.1子控件要求的大小为一个精确值，则子控件的MeasureSpec就是精确测量到指定大小                // 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.                // 2.2子控件要求与父控件同大小，父控件大小不确定，则子控件的MeasureSpec就是最大限制到父控件剩余大小                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.                 // 2.3子控件要求自定义大小，则子控件的MeasureSpec就是最大限制到父控件剩余大小                resultSize = size;                resultMode = MeasureSpec.AT_MOST;            }            break;        // Parent asked to see how big we want to be        case MeasureSpec.UNSPECIFIED:// 3.父控件本身未指定大小，大小不确定            if (childDimension >= 0) {// 3.1子控件要求的大小为一个精确值，则子控件的MeasureSpec就是精确测量到指定大小                // 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                // 3.2子控件要求与父控件同大小，父控件大小不确定，则子控件的MeasureSpec就是未指定，根据flag决定返回0或父剩余大小                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;                resultMode = MeasureSpec.UNSPECIFIED;             } else if (childDimension == LayoutParams.WRAP_CONTENT) {                // Child wants to determine its own size.... find out how big it should be                // 3.3子控件要求自定义大小，则子控件的MeasureSpec就是未指定，根据flag决定返回0或父控件剩余大小                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;                resultMode = MeasureSpec.UNSPECIFIED;            }            break;        }        //noinspection ResourceType // 合成MeasureSpec        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);    }

总结一下获取子控件MeasureSpec的情况有几种：
1. 当子控件的LayoutParams为指定值时，不管父控件的测量模式和剩余空间大小，返回的MeasureSpec就是指定值精确测量
2. 当子控件的LayoutParams为MATCH_PARENT时，子控件的测量模式跟随父控件的测量模式，size等于父控件的剩余空间
3. 当子控件的LayoutParams为WRAP_CONTENT时，子控件的基本模式就是不超过父控件大小

Measure方法

 /**   * This is called to find out how big a view should be. The parent   * supplies constraint information in the width and height parameters. 实际测量工作在onMeasure方法中完成   * The actual measurement work of a view is performed in {@link #onMeasure(int, int)}, called by this method.    * Therefore, only {@link #onMeasure(int, int)} can and must be overridden by subclasses.   * @param widthMeasureSpec Horizontal space requirements as imposed by the parent 父控件要求的水平空间   * @param heightMeasureSpec Vertical space requirements as imposed by the parent 父控件要求的垂直空间   * @see #onMeasure(int, int)   */    public final void measure(int widthMeasureSpec, int heightMeasureSpec) {        boolean optical = isLayoutModeOptical(this);        if (optical != isLayoutModeOptical(mParent)) {            Insets insets = getOpticalInsets();            int oWidth  = insets.left + insets.right;            int oHeight = insets.top  + insets.bottom;            widthMeasureSpec  = MeasureSpec.adjust(widthMeasureSpec,  optical ? -oWidth  : oWidth);            heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);        }        // Suppress sign extension for the low bytes        long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;        if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);        final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;        // Optimize layout by avoiding an extra EXACTLY pass when the view is        // already measured as the correct size. In API 23 and below, this        // extra pass is required to make LinearLayout re-distribute weight.        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);        if (forceLayout || needsLayout) {            // first clears the measured dimension flag            mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;            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;        }        mOldWidthMeasureSpec = widthMeasureSpec;        mOldHeightMeasureSpec = heightMeasureSpec;        mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |                (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension    }

onMeasure方法

/**  * Measure the view and its content to determine the measured width and the measured height. This method is   * invoked by {@link #measure(int, int)} and should be overridden by subclasses to provide accurate and   * efficient measurement of their contents.  * <strong>CONTRACT:</strong> When overriding this method, you <em>must</em> call {@link   * #setMeasuredDimension(int, int)} to store the measured width and height of this view. Failure to do so will   * trigger an <code>IllegalStateException</code>, thrown by {@link #measure(int, int)}. Calling the superclass'  * {@link #onMeasure(int, int)} is a valid use.  * The base class implementation of measure defaults to the background size, unless a larger size is allowed by   * the MeasureSpec. Subclasses should override {@link #onMeasure(int, int)} to provide better measurements of  * their content.  * If this method is overridden, it is the subclass's responsibility to make sure the measured height and width   * are at least the view's minimum height and width ({@link #getSuggestedMinimumHeight()} and  *  {@link #getSuggestedMinimumWidth()}).  * @param widthMeasureSpec horizontal space requirements as imposed by the parent. The requirements are encoded   * with {@link android.view.View.MeasureSpec}.  * @param heightMeasureSpec vertical space requirements as imposed by the parent. The requirements are encoded   * with {@link android.view.View.MeasureSpec}.  * @see #getMeasuredWidth()  * @see #getMeasuredHeight()  * @see #setMeasuredDimension(int, int)  * @see #getSuggestedMinimumHeight()  * @see #getSuggestedMinimumWidth()  * @see android.view.View.MeasureSpec#getMode(int)  * @see android.view.View.MeasureSpec#getSize(int)  */    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));    }

getDefaultSize

 /**     * Utility to return a default size. Uses the supplied size if the MeasureSpec imposed no constraints. Will      * get larger if allowed by the MeasureSpec.     *      * @param size Default size for this view // view的默认大小，view的最小宽度或者背景的最小宽度两者最大值     * @param measureSpec Constraints imposed by the parent     * @return The size this view should be.     */    public static int getDefaultSize(int size, int measureSpec) {        int result = size;        int specMode = MeasureSpec.getMode(measureSpec);        int specSize = MeasureSpec.getSize(measureSpec);        switch (specMode) {        case MeasureSpec.UNSPECIFIED:            result = size; // 没有限制时使用默认大小            break;        case MeasureSpec.AT_MOST: // 最大限制或者精确大小时使用指定大小        case MeasureSpec.EXACTLY:            result = specSize;            break;        }        return result;    }

getSuggestedMinHeight

 /**     * Returns the suggested minimum width that the view should use. This returns the maximum of the view's      * minimum width and the background's minimum width      * ({@link android.graphics.drawable.Drawable#getMinimumWidth()}).     * When being used in {@link #onMeasure(int, int)}, the caller should still ensure the returned width is      * within the requirements of the parent.     *     * @return The suggested minimum width of the view.     */ // 获取建议最小宽度，View的最小宽度可以由xml属性指定或者调用setMinimumWidth指定    protected int getSuggestedMinimumWidth() { // 如果没有背景则返回最小宽度，有背景则返回最小宽度和背景最小值两者最大值        return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());    }     /** // 设置view的最小宽度，但是不保证view可以达到这个最小宽度，如果父布局限制到更小的宽度     * Sets the minimum width of the view. It is not guaranteed the view will be able to achieve this minimum      * width (for example, if its parent layout constrains it with less available width).     * @param minWidth The minimum width the view will try to be.     * @see #getMinimumWidth()     * @attr ref android.R.styleable#View_minWidth     */    public void setMinimumWidth(int minWidth) {        mMinWidth = minWidth;        requestLayout();    }

setMeasuredDimension:

/**     * <p>This method must be called by {@link #onMeasure(int, int)} to store the measured width and measured      * height. Failing to do so will trigger an exception at measurement time.</p>     * @param measuredWidth The measured width of this view.  May be a complex     * bit mask as defined by {@link #MEASURED_SIZE_MASK} and {@link #MEASURED_STATE_TOO_SMALL}.     * @param measuredHeight The measured height of this view.  May be a complex     * bit mask as defined by {@link #MEASURED_SIZE_MASK} and {@link #MEASURED_STATE_TOO_SMALL}.     */    protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {        boolean optical = isLayoutModeOptical(this);        if (optical != isLayoutModeOptical(mParent)) {            Insets insets = getOpticalInsets();            int opticalWidth  = insets.left + insets.right;            int opticalHeight = insets.top  + insets.bottom;            measuredWidth  += optical ? opticalWidth  : -opticalWidth;            measuredHeight += optical ? opticalHeight : -opticalHeight;        }        setMeasuredDimensionRaw(measuredWidth, measuredHeight);    }      /**     * Sets the measured dimension without extra processing for things like optical bounds.     * Useful for reapplying consistent values that have already been cooked with adjustments     * for optical bounds, etc. such as those from the measurement cache.     *     * @param measuredWidth The measured width of this view.  May be a complex     * bit mask as defined by {@link #MEASURED_SIZE_MASK} and     * {@link #MEASURED_STATE_TOO_SMALL}.     * @param measuredHeight The measured height of this view.  May be a complex     * bit mask as defined by {@link #MEASURED_SIZE_MASK} and     * {@link #MEASURED_STATE_TOO_SMALL}.     */    private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {        mMeasuredWidth = measuredWidth;        mMeasuredHeight = measuredHeight;        mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;    }