clipChildren属性和MeasureSpec

clipChildren属性和MeasureSpec

1、背景

之前遇到一个问题，想要实现一个子view超出父view的效果，用了clipChildren属性，但是它不生效。

2、分析

写了demo改了一些条件，发现子view的宽度固定的时候，clipChildren才会生效，子View的宽能够超出父View，wrap_content和match_parent时就不行。
好像可以理解，wrap_content和match_parent时测量的结果都和父view的宽的有关，测量的时候子view的宽度就限定不能超出父view，绘制出来的子view怎么可能超出呢。

3、源码

3.1 measure

measure过程中有一个非常重要的类MeasureSpec：

public static class MeasureSpec {
        private static final int MODE_SHIFT = 30;
        private static final int MODE_MASK  = 0x3 << MODE_SHIFT;

        /** @hide */
        @IntDef({UNSPECIFIED, EXACTLY, AT_MOST})
        @Retention(RetentionPolicy.SOURCE)
        public @interface MeasureSpecMode {}

        /**
         * Measure specification mode: The parent has not imposed any constraint
         * on the child. It can be whatever size it wants.
         */
        public static final int UNSPECIFIED = 0 << MODE_SHIFT;

        /**
         * Measure specification mode: The parent has determined an exact size
         * for the child. The child is going to be given those bounds regardless
         * of how big it wants to be.
         */
        public static final int EXACTLY     = 1 << MODE_SHIFT;

        /**
         * Measure specification mode: The child can be as large as it wants up
         * to the specified size.
         */
        public static final int AT_MOST     = 2 << MODE_SHIFT;

        /**
         * Creates a measure specification based on the supplied size and mode.
         *
         * The mode must always be one of the following:
         * 

         *  
{@link android.view.View.MeasureSpec#UNSPECIFIED}
         *  
{@link android.view.View.MeasureSpec#EXACTLY}
         *  
{@link android.view.View.MeasureSpec#AT_MOST}
         * 
         *
         * 
Note: On API level 17 and lower, makeMeasureSpec's
         * implementation was such that the order of arguments did not matter
         * and overflow in either value could impact the resulting MeasureSpec.
         * {@link android.widget.RelativeLayout} was affected by this bug.
         * Apps targeting API levels greater than 17 will get the fixed, more strict
         * behavior.
         *
         * @param size the size of the measure specification
         * @param mode the mode of the measure specification
         * @return the measure specification based on size and mode
         */
        public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size,
                                          @MeasureSpecMode int mode) {
            if (sUseBrokenMakeMeasureSpec) {
                return size + mode;
            } else {
                return (size & ~MODE_MASK) | (mode & MODE_MASK);
            }
        }

        /**
         * Like {@link #makeMeasureSpec(int, int)}, but any spec with a mode of UNSPECIFIED
         * will automatically get a size of 0. Older apps expect this.
         *
         * @hide internal use only for compatibility with system widgets and older apps
         */
        @UnsupportedAppUsage
        public static int makeSafeMeasureSpec(int size, int mode) {
            if (sUseZeroUnspecifiedMeasureSpec && mode == UNSPECIFIED) {
                return 0;
            }
            return makeMeasureSpec(size, mode);
        }

        /**
         * Extracts the mode from the supplied measure specification.
         *
         * @param measureSpec the measure specification to extract the mode from
         * @return {@link android.view.View.MeasureSpec#UNSPECIFIED},
         *         {@link android.view.View.MeasureSpec#AT_MOST} or
         *         {@link android.view.View.MeasureSpec#EXACTLY}
         */
        @MeasureSpecMode
        public static int getMode(int measureSpec) {
            //noinspection ResourceType
            return (measureSpec & MODE_MASK);
        }

        /**
         * Extracts the size from the supplied measure specification.
         *
         * @param measureSpec the measure specification to extract the size from
         * @return the size in pixels defined in the supplied measure specification
         */
        public static int getSize(int measureSpec) {
            return (measureSpec & ~MODE_MASK);
        }

        static int adjust(int measureSpec, int delta) {
            final int mode = getMode(measureSpec);
            int size = getSize(measureSpec);
            if (mode == UNSPECIFIED) {
                // No need to adjust size for UNSPECIFIED mode.
                return makeMeasureSpec(size, UNSPECIFIED);
            }
            size += delta;
            if (size < 0) {
                Log.e(VIEW_LOG_TAG, "MeasureSpec.adjust: new size would be negative! (" + size +
                        ") spec: " + toString(measureSpec) + " delta: " + delta);
                size = 0;
            }
            return makeMeasureSpec(size, mode);
        }

        /**
         * Returns a String representation of the specified measure
         * specification.
         *
         * @param measureSpec the measure specification to convert to a String
         * @return a String with the following format: "MeasureSpec: MODE SIZE"
         */
        public static String toString(int measureSpec) {
            int mode = getMode(measureSpec);
            int size = getSize(measureSpec);

            StringBuilder sb = new StringBuilder("MeasureSpec: ");

            if (mode == UNSPECIFIED)
                sb.append("UNSPECIFIED ");
            else if (mode == EXACTLY)
                sb.append("EXACTLY ");
            else if (mode == AT_MOST)
                sb.append("AT_MOST ");
            else
                sb.append(mode).append(" ");

            sb.append(size);
            return sb.toString();
        }
    }

分别有3中模式：UNSPECIFIED、EXACTLY、AT_MOST，这三种模式可以用两个bit来表示，分别是00，01，10。
因为int型是32位，所以，可以用高两位表示mode，剩下的30位表示size。getSize和getMode方法乍一看有点摸不着头脑，但是用二进的思路再去梳理一遍就很容易可以理解了。

再看看测量过程中是如何使用这个类的。
这里以LinearLayout的垂直布局为例：

    // 入参是它的父类传递进来的
    void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
        mTotalLength = 0;

        // 父类的mode
        final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        final int heightMode = MeasureSpec.getMode(heightMeasureSpec);

        // 遍历子view
        for (int i = 0; i < count; ++i) {
            final View child = getVirtualChildAt(i);
            ...
            nonSkippedChildCount++;
            if (hasDividerBeforeChildAt(i)) {
                mTotalLength += mDividerHeight;
            }

            final LayoutParams lp = (LayoutParams) child.getLayoutParams();

            totalWeight += lp.weight;

            // 子view根据weight设置高度
            final boolean useExcessSpace = lp.height == 0 && lp.weight > 0;
            if (heightMode == MeasureSpec.EXACTLY && useExcessSpace) {
                // viewGroup高度固定+子view的weight已经可以确定每个子view的高度了，不需要再去测量
                final int totalLength = mTotalLength;
                mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
                skippedMeasure = true;
            } else {

                // 已经用掉的高度
                final int usedHeight = totalWeight == 0 ? mTotalLength : 0;
                measureChildBeforeLayout(child, i, widthMeasureSpec, 0,
                        heightMeasureSpec, usedHeight);
            }
        }
    }

measureChildBeforeLayout会调用到measureChildWithMargins，可以理解成是把parentMeasureSpec转换成childMeasuresSpec的过程：

    protected void measureChildWithMargins(View child,
            int parentWidthMeasureSpec, int widthUsed,
            int parentHeightMeasureSpec, int heightUsed) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

        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);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

getChildMeasureSpec是这样的（为了便于理解，换了一下参数名）：

    public static int getChildMeasureSpec(int parentSpec, int widthUsed, int childDimension) {
        int parentSpecMode = MeasureSpec.getMode(parentSpec);
        int parentSpecSize = MeasureSpec.getSize(parentSpec);

        // 子view可用size
        int size = Math.max(0, parentSpecSize - widthUsed);

        int resultSize = 0;
        int resultMode = 0;

        switch (parentSpecMode) {
        case MeasureSpec.EXACTLY:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // 子view的size不能超过父view提供给它的size
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;
        case MeasureSpec.AT_MOST:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

可以看到，除了父view是UNSPECIFIED的情况，在子view设置为match_parent或者wrap_content的时候，子view的宽度都不能超过父view给它提供的宽度。也就支持了第2点的分析。

这段代码里我们也可以总结出来父view和子view的MeasureSpec的一些关系：

子View Size/ 父级SpecMode	EXACTLY	AT_MOST	UNSPECIFIED
具体尺寸设置	EXACTLY	EXACTLY	EXACTLY
match_parent	EXACTLY	AT_MOST	UNSPECIFIED
wrap_content	AT_MOST	AT_MOST	UNSPECIFIED

3.2 ClipChildren

理解了为什么clipChildren不生效，现在来看看这个属性是如何生效的。
其实也就是父view没有将画布裁剪了，子view可以在画布上完整绘制出来。

#View.java
    boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
        if (!drawingWithRenderNode) {
            //若是父布局需要裁剪子布局，也就是说clipChildren==true，那么就需要对canvas进行裁剪
            if ((parentFlags & ViewGroup.FLAG_CLIP_CHILDREN) != 0 && cache == null) {
                if (offsetForScroll) {
                    //裁剪canvas与子布局大小一致
                    //sx,sy 是scroll值，没设置scroll时sx,sy都为0
                    canvas.clipRect(sx, sy, sx + getWidth(), sy + getHeight());
                } else {
                    ...
                }
            }
            ...
        }
    }

4、 总结

使用clipChilren，子view必须设置一个固定的size