前言
将一个view绘制在屏幕上我们需要知道这个view的大小,所以我们要先measure -》 然后根据规则放置view layout -》最后才是我们的draw 绘制出来。而这里measure是整个绘制流程的第一个,也是自定义View必须要了解的知识点。
MeasureSpec
定义
测量规格,封装了父容器对View的布局限制,内部提供了宽高的信息(SpecSize、SpecMode),SpecSize是指在某种SpecMode下的参考尺寸,其中SpecMode 有如下3种:
- UNSPECIFIED 父控件不对你有任何限制,你想要多大就给你多大。这种情况一般用于系统内部(这个模式主要用于系统内部多次Measure情形,并不是真的说你想要多大最后真的有多大)
- EXACTLY父控件已经知道你所需的精确大小,你的最终大小应该就这么大
- AT_MOST你的大小不能大于父控件给你指定size的大小,具体多少有你来定
意义
通过将SpecMode 和 SpecSize 打包成一个int 值可以避免过多的对象分配内存,为了方便操作,其提供了打包和解包方法
MeasureSpec.getMode() //解包方法
MeasureSpec.getSize() //解包方法
MeasureSpec.makeMeasureSpec() //打包方法
MeasureSpec
子View的MeasureSpec值是根据子View的布局参数(LayoutParams)和父容器的MeasureSpec值来计算得来的,具体计算逻辑封装在getChildMeasureSpec函数中。下面是ViewGroup 里面的获取子View MeasureSpec的方法:
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec); // 父控件的测量模式
int specSize = MeasureSpec.getSize(spec); // 父控件的大小
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// 父控件的测量模式是EXACTLY模式, 也就是有精确的尺寸
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
// 当child的布局参数有固定值, 比如 layout_height = “100dp”
// 那么child的规格也就确定下来了 大小为 size = 100dp; mode = MeasureSpec.EXACTLY
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child 占满父控件,而父控件是EXACTLY 模式,所以模式和大小就确定了
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// child 大小自己控制,但是不能超过父大小,测量模式是AT_MOST
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// 父控件也不知道自己的大小,需要计算,但是大小不能超过size
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// child 指定尺寸
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child 想要跟父控件一样大,但是父控件也未知,所以child 大小未知 大小不能超过size
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// child 大小自己控制,但是不能超过父大小,测量模式是AT_MOST
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 = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
//noinspection ResourceType
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
View Measure 过程
measure
/**
* View.java 文件中的测量方法,这个
*/
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int oWidth = insets.left + insets.right;
int oHeight = insets.top + insets.bottom;
widthMeasureSpec = MeasureSpec.adjust(widthMeasureSpec, optical ? -oWidth : oWidth);
heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
}
// Suppress sign extension for the low bytes
long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
// Optimize layout by avoiding an extra EXACTLY pass when the view is
// already measured as the correct size. In API 23 and below, this
// extra pass is required to make LinearLayout re-distribute weight.
final boolean specChanged = widthMeasureSpec != mOldWidthMeasureSpec
|| heightMeasureSpec != mOldHeightMeasureSpec;
final boolean isSpecExactly = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY
&& MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;
final boolean matchesSpecSize = getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec)
&& getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);
final boolean needsLayout = specChanged
&& (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize);
if (forceLayout || needsLayout) {
// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
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);
// 更新flag3标志位,在layout的时候需要调用onMeasure。这样做的目的猜想-》“有时候我们的mesure方法会多次调用(尤其是在特殊的父控件 会多次进行子View的测量),这样避免我们多次执行onMeasure 方法 减少计算量,提高性能。直到你真正layout的时候发现你未调用onMeasure 才执行一次 ”
mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
// flag not set, setMeasuredDimension() was not invoked, we raise
// an exception to warn the developer
if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
throw new IllegalStateException("View with id " + getId() + ": "
+ getClass().getName() + "#onMeasure() did not set the"
+ " measured dimension by calling"
+ " setMeasuredDimension()");
}
//将我们layout的标志位置为1,表示我已经测量计算过View大小了,layout需要跟着变化
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}
requestLayout
调用requestLayout之后我们Measure流程就会调用onMeasure方法
public void requestLayout() {
if (mMeasureCache != null) mMeasureCache.clear();
if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == null) {
// Only trigger request-during-layout logic if this is the view requesting it,
// not the views in its parent hierarchy
ViewRootImpl viewRoot = getViewRootImpl();
if (viewRoot != null && viewRoot.isInLayout()) {
if (!viewRoot.requestLayoutDuringLayout(this)) {
return;
}
}
mAttachInfo.mViewRequestingLayout = this;
}
//这里将我们Flag的forceLayout标志为设为1
mPrivateFlags |= PFLAG_FORCE_LAYOUT;
mPrivateFlags |= PFLAG_INVALIDATED;
if (mParent != null && !mParent.isLayoutRequested()) {
mParent.requestLayout();
}
if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == this) {
mAttachInfo.mViewRequestingLayout = null;
}
}