Android GUI之View测量

　　在上篇文章（http://www.cnblogs.com/jerehedu/p/4607599.html#gui）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：


　　该类中提供了用来计算和生成测量要求的方法，具体如下：


　　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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作者：杰瑞教育
出处：http://www.cnblogs.com/jerehedu/
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