简介
自定义View必须要知道View的工作原理，我们都知道View的工作流程是measure->layout->draw,今天我们就逐个分析一下三个步骤。
首先要知道ViewRoot和Window和DecorView三者的关系，ViewRoot对应的是ViewRootImpl类，它是连接WindoManage和DecorView的纽带，View的整个工作流程都是在ViewRootImpl的performTraversals中完成的。
今天我们一起看看Measure的工作流程。
理解MeasureSpec
在View的Measure过程MeasureSpec是一个很重要的内容，它描述了要测量的尺寸和模式。View在Measure的过程是根据自身的LayoutParams和父容器的MeasureSpec转换成对应自身的MeasureSpec，然后再根据这个MeasureSpec测量出View的宽和高。
我们先看一下MeasureSpec的定义
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;
public static int makeMeasureSpec( int size, @MeasureSpecMode int mode) {
if (sUseBrokenMakeMeasureSpec) {
return size + mode;
} else {
return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}
@UnsupportedAppUsage
public static int makeSafeMeasureSpec(int size, int mode) {
if (sUseZeroUnspecifiedMeasureSpec && mode == UNSPECIFIED) {
return 0;
}
return makeMeasureSpec(size, mode);
}
@MeasureSpecMode
public static int getMode(int measureSpec) {
//noinspection ResourceType
return (measureSpec & MODE_MASK);
}
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
}
MeasureSpec 代表一个32位的int值，高2位代表SpecMode，低30位代表SpecSize。
通过静态方法makeMeasureSpec将SpecMode和SpecSize打包成一个int值，避免过多的对象内存分配，另外还提供了解包方法。
SpecMode有三种模式：
**UNSPECIFIED **
父容器对view没有任何约束，view想要多大就多大，这种模式一般系统使用或者是scrollView使用。
EXACTLY
父容器检测出View需要的精确大小，View的大小是SpecSize指定的值。它对应于LayoutParams中的match_parent和具体的数值。
AT_MOST
父容器指定一个最大值SpecSize，View的大小不能超过该值，它对应的是LayoutParams的wrap_content
MeasureSpec在DecorView和View以及ViewGroup中的表现是不一样的，我们分别分析一下三者的区别。
DecorView的MeasureSpec
DecorView的MeasureSpec是由Window的尺寸和自身的LayoutParams共同决定的。在ViewRootImpl的measureHierarchy中展示了MeasureSpec的创建过程
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
desiredWindowWidth和desiredWindowHeight表示屏幕的尺寸，接着看一下getRootMeasureSpec
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
int measureSpec;
switch (rootDimension) {
case ViewGroup.LayoutParams.MATCH_PARENT:
// Window can't resize. Force root view to be windowSize.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
case ViewGroup.LayoutParams.WRAP_CONTENT:
// Window can resize. Set max size for root view.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break;
default:
// Window wants to be an exact size. Force root view to be that size.
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
到此可以明确的看出DecorView的MeasureSpec产生遵守如下规则：
- MATCH_PARENT：精确模式，大小是窗口大小
- WRAP_CONTENT: 最大模式，最大大小是窗口大小
- 其他：精确模式，大小是LayoutParams指定的具体值。
View的MeasureSpec
先看一下ViewGroup的measureChildWithMargins
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);
}
上述方法对子元素进行measure，measure前的childXXXMeasureSpec是通过父容器的MeasureSpec，自身(ViewGroup)的被占用的空间(padding和margin)，子View的LayoutParams综合测量计算出来的。我们看一下getChildMeasureSpec的实现
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) {
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
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) {
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
//noinspection ResourceType
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
方法内部主要是根据父容器的MeasureSpec和自身的LayoutParams来确定子元素的MeasureSpec，其中size是根据父容器的尺寸减去已经被占用的空间尺寸,及可用的尺寸。为了更清楚的展示getChildMeasureSpec的流程我们绘制一个表格：
MeasureSpec.png
如果只是一个View的measure会调用onMeasure方法完成测量
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
重点是getDefaultSize
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;
}
从这里可以看出View的layoutParams是wrap_content的时候，getDefaultSize得到的是specSize，根据上边的图表分析知道specSize是父容器的剩余空间，这种情况和match_content的大小一样，所以我们在自定义View的时候需要处理specMode是AT_MOST的情况。
ViewGroup的measure过程
对于ViewGroup除了完成自身的测量还要完成子元素的测量，ViewGroup没有onMeasure方法但是它有一个meausreChildren方法
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);
}
}
}
只要子元素是可见的都进行测量,我们看一下measureChild的内容
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);
}
measureChild的思想就是根据子元素的LayoutParams,然后再通过getChildMeasureSpec计算出子元素的MeasureSpec，将这个MeasureSpec传递给子元素的measure方法。
至此View,ViewGroup，DecorView的meausure都介绍完了。
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