Android GUI之View测量

最新推荐文章于 2021-05-26 18:49:00 发布

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最新推荐文章于 2021-05-26 18:49:00 发布

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本文链接：https://blog.youkuaiyun.com/JerehEdu/article/details/47272715

　　在上篇文章（http://blog.youkuaiyun.com/jerehedu/article/details/47081849）中，根据源码探索了View的绘制过程，过程有三个主要步骤，分别为测量、布局、绘制。系统对绘制已经做了很好的封装，我们主要对测量和布局过程进行分析，看一看android是如何对view进行测量和布局的。

　　根据上篇文章的分析，我们知道在ViewRootImpl的performMeasure方法中，实际上调用了mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);方法。根据源码我们找到了该方法的原型，此方法在View类中，并且是final方法，不可被子类重写，方法的具体源码如下：

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
          ……
        }
        // Suppress sign extension for the low bytes
        long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
        if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);

        if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
                widthMeasureSpec != mOldWidthMeasureSpec ||
                heightMeasureSpec != mOldHeightMeasureSpec) {

           ……
            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;
            }
……

            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
        }

        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;

        mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
                (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
    }

　　根据方法内容和说明，可以知道本方法就是用来测量View的大小的，而需要的两个参数是由父View构建的，用于说明父View对子View的测量的规格要求，实际上在这个方法中真正完成测量大小的是方法onMeasure，此方法我们稍后分析。在此之前我们先要明白measure方法中的两个参数的含义，刚才有提到参数是父View对子View的测量规格要求，那么Android是如何描述的呢，这里用到了一个类MeasureSpec，此类为View中的一个内部类，关键源码如下：

public static class MeasureSpec {
        private static final int MODE_SHIFT = 30;
        private static final int MODE_MASK  = 0x3 << MODE_SHIFT;
        public static final int UNSPECIFIED = 0 << MODE_SHIFT;
        public static final int EXACTLY     = 1 << MODE_SHIFT;
        public static final int AT_MOST     = 2 << MODE_SHIFT;
        public static int makeMeasureSpec(int size, int mode) {
            if (sUseBrokenMakeMeasureSpec) {
                return size + mode;
            } else {
                return (size & ~MODE_MASK) | (mode & MODE_MASK);
            }
        }
        public static int getMode(int measureSpec) {
            return (measureSpec & MODE_MASK);
        }
        public static int getSize(int measureSpec) {
            return (measureSpec & ~MODE_MASK);
        }

      ……
}

　　根据SDK，此类封装了父View对子View的布局要求，每个实例都代表了对子View的高度或者宽度的要求，测量要求包含两个部分，分别为尺寸和模式。模式主要由三种，具体如下：

1、 UNSPECIFIED：代表父View对子View没有约束，子View可以为任意大小。

2、 EXACTLY：父View确定子View的大小，子View被限定在给定的边界中，忽咯本身的大小。

3、 AT_MOST：子View最大可以达到指定大小的值。

　　该类中提供了用来计算和生成测量要求的方法，具体如下：

1、 public static int makeMeasureSpec(int size, int mode)，此方法最终生成一个32位二进制数用来表明测量规格要求，其中32和31位用来表明模式，后30位代表了大小。

2、 public static int getMode(int measureSpec)，此方法可以根据测量说明，计算模式。

3、 public static int getSize(int measureSpec)，此方法根据测量说明，计算大小。

　　明白了MeasureSpec，我们在回过头来，看一看onMeasure方法，该方法的源码如下：

    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}

　　此方法的默认实现非常简单，调用了setMeasuredDimersion方法将测量好的尺寸保存到mMeasuredWidth和mMeasuredHeight。而在setMeasuredDimersion方法中调用了getDefaultSize用来计算，该方法具体如下：

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;
            break;
        case MeasureSpec.AT_MOST:
        case MeasureSpec.EXACTLY:
            result = specSize;
            break;
        }
        return result;
    }

　　很明显，此方法根据提供的默认大小和测量要求计算View的实际大小。到此为止，View完了测量过程。不过大多数情况下，当我们自定义ViewGroup的时候，我们需要重写onMeasure方法，在此方法中，可以遍历所有的子View并要求他们对自己的大小进行测量，同时不要忘记调用setMeasuredDimension进行保存测量结果，在ViewGroup是通过如下三个方法实现的，关键代码如下：

　　方法mesureChildren，遍历所有的非隐藏的子View，并调用measureChild方法设置子View的测量要求。

protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
        final int size = mChildrenCount;
        final View[] children = mChildren;
        for (int i = 0; i < size; ++i) {
            final View child = children[i];
            if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
                measureChild(child, widthMeasureSpec, heightMeasureSpec);
            }
        }
}

　　方法measureChild，获取子View的测量规格，并调用measure进行测量实际大小。

protected void measureChild(View child, int parentWidthMeasureSpec,
            int parentHeightMeasureSpec) {
        final LayoutParams lp = child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

　　方法getChildMeasureSpec用于获取View的测量规格要求。

    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);
    }

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