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我们在自定义View时通常会去重写View的onMeasure方法，此方法提供的有一个默认实现：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
这个方法有两个int型的参数：widthMeasureSpec,heightMeasureSpec。这篇文章将会分析它们的确定过程。首先看一下MeasureSpec是什么？
MeasureSpec
MeasureSpec是View类的一个静态内部类，源码不太多，这里贴出分析之：
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);
}
static int adjust(int measureSpec, int delta) {
final int mode = getMode(measureSpec);
if (mode == UNSPECIFIED) {
// No need to adjust size for UNSPECIFIED mode.
return makeMeasureSpec(0, UNSPECIFIED);
}
int size = getSize(measureSpec) + 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);
}
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();
}
}
widthMeasureSpec 与 heightMeasureSpec是一个int型的变量，java中int型变量由4个字节(32bit)组成，其中高2位用来封装MeasureMode,MeasureMode一共有3种:
- UNSPECIFIED = 0 << MODE_SHIFT; 即: 00
000000 00000000 00000000 00000000父容器不对子View有任何限制 - EXACTLY = 1 << MODE_SHIFT; 即: 01
000000 00000000 00000000 00000000父容器已经测量出子View所需要的大小,即measureSpec中封装的specsize - AT_MOST = 2 << MODE_SHIFT; 即: 10
000000 00000000 00000000 00000000父窗口限定了一个最大值给子View即specsize
低30位用来封装size.
- public static int makeMeasureSpec(int size, int mode) 此方法用来根据size与mode组合出特定的measureSpec.可以看到在api17之后是通过二进制 & | 计算的，这样效率更高。
- static int adjust(int measureSpec, int delta) 此方法是用来对measurespec中的size进行调整的 和
- public static String toString(int measureSpec) 此方法大概是用来方便开发者从一个int值中打印mode与size，方便调试。
过程分析
MeasureSpec大概就是这样，为了说明MeasureSpec的创建过程，我们写一个CustomLinearLayout里面套一个CustomView,通过debug来分析一下，布局代码大致如下:
然后我们给CustomLinearLayout,CustomView的onMeasure方法加入一些log,如下：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
Log.e("ghui", "CustomLinearLayout." + "widthMeasureSpec:"
+ MeasureSpec.toString(widthMeasureSpec)
+ ", heightMeasureSpec: " + MeasureSpec.toString(heightMeasureSpec));
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
Log.e("ghui", "CustomView." + "widthMeasureSpec:"
+ MeasureSpec.toString(widthMeasureSpec)
+ ", heightMeasureSpec: " + MeasureSpec.toString(heightMeasureSpec));
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
我们在CustomView的onMeasure方法中加入一个断点，通过android studio观察onMeasure方法的调用栈信息，截图如下：
从上面可以发现CustomView的onMeasure方法调用过程源起其Parent CustomLinearLayout中的onMeasure.在这条调用链上最后一个ViewGroup的方法是
measureChildWithMargins(),源码如下:
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);
}
可以看到此方法的最后一行中会调用child.measure方法将计算好的childWidthMeasureSpec 与 childHeightMeasureSpec传给子View,再往上看会发现这两个参数的计算过程被完全封装到了
getChildMeasureSpec中，此方法的源码如下:
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);
}
从中可以看出子View的MeasureSpec的计算过程被分为了3部分：
-
parentMeasureMode = EXACTLY
a. childDimension >=0 即子View设置的有具体的大小(如：100px)，这种情况下childSpecSize即为其自身LayoutParams里指定的大小，childSpecMode为EXACTLY
b. childDimension = MATCH_PARENT 即子View的LayoutParams里设置的为
MATCH_PARENT,这种情况下childSpecSize即为parentLeftSize(父容器剩余的空间)，childSpecMode为EXACTLYc. childDimension = WRAP_CONTENT 即子View的LayoutParams里设置的为
WRAP_CONTENT,这种情况下childSpecSize为parentLeftSize(父容器剩余的空间),childSpecMode为AT_MOST -
parentMeasureMode = AT_MOST
a. childDimension >=0 即子View设置的有具体的大小(如：100px)，这种情况下childSpecSize即为其自身LayoutParams里指定的大小，childSpecMode为EXACTLY (同 1.a)
b. childDimension = MATCH_PARENT 即子View的LayoutParams里设置的为
MATCH_PARENT,这种情况下childSpecSize即为parentLeftSize(父容器剩余的空间)，childSpecMode为AT_MOST(同 1.c)c. childDimension = WRAP_CONTENT 即子View的LayoutParams里设置的为
WRAP_CONTENT,这种情况下childSpecSize即为parentLeftSize(父容器剩余的空间),childSpecMode为AT_MOST(同1.c) -
parentMeasureMode = UNSPECIFIED
a. childDimension >=0 即子View设置的有具体的大小(如：100px)，这种情况下childSpecSize即为其自身LayoutParams里指定的大小，childSpecMode为EXACTLY (同 1.a)
b. childDimension = MATCH_PARENT 即子View的LayoutParams里设置的为
MATCH_PARENT,这种情况下childSpecSize为0，childSpecMode为UNSPECIFIEDc. childDimension = WRAP_CONTENT 即子View的LayoutParams里设置的为
WRAP_CONTENT,这种情况下childSpecSize为0，childSpecMode为UNSPECIFIED(同3.b)
整理一下可以得到如下表格:
总结
- View的
measureSpec由其父容器的measureSpec及自身的LayoutParams共同决定 - 若子View为具体的大小(如100px),则不管其父容器的specMode为哪种，子View对应的specMode均为
EXACTLY,specSize均为childSize - 若子View的LayoutParams中的宽或高为
wrap_content,则不管其父容器的specMode为哪种，子View对应的specMode均为AT_MOST,specSize均为parentLeftSize - 子View的前期measure过程实际上在其父容器的onMeasure中就基本完成了，父容器会把计算好的measureSpec传递给子View，子View在自己的onMeasure中可以得到这些值，子View可以根据这些值设置自己的大小，当然也可以不参考它们。最终在onMeasure方法通过调用
setMeasuredDimension方法设置view的最终measureSize。但View的真实大小是在Layout阶段才确定下来的，通过child.layout(left,top,right,bottom).View的measure size与 layout size不必相等，但绝大多数情况下是相等的。
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