自定義
view的流程分為measure政钟、layout、draw三個主要步驟潮秘，今天我們通過源碼來分下下measure的過程我們從頂級view開始琼开，頂級view即DecorView，view的事件都是先經(jīng)過這個DecorView,接下來我們來看看這個DecorView的MeasureSpec的創(chuàng)建過程：
進入ViewRootImpl中枕荞，查看measureHierarchy方法柜候，有如下代碼：
final DisplayMetrics packageMetrics = res.getDisplayMetrics();
res.getValue(com.android.internal.R.dimen.config_prefDialogWidth, mTmpValue, true);
int baseSize = 0;
if (mTmpValue.type == TypedValue.TYPE_DIMENSION) {
baseSize = (int)mTmpValue.getDimension(packageMetrics);
}
if (DEBUG_DIALOG) Log.v(mTag, "Window " + mView + ": baseSize=" + baseSize
+ ", desiredWindowWidth=" + desiredWindowWidth);
if (baseSize != 0 && desiredWindowWidth > baseSize) {
childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
這里只是截選一部分的源碼， 我們看到這個baseSize躏精， 其實就是屏幕的尺寸大小渣刷， 獲取寬的MeasureSpc的方法：
childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
這里傳入的參數(shù)是屏幕尺寸以及DecorView自身的大小， 接著我們來看 getRootMeasureSpec方法：
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;
}
就是這個方法確定了DecorView的MeasureSpec矗烛， 這里分了三種情況辅柴，
- 如果傳入的view大小為
math_parent，那么這個view的mode為EXACTLY瞭吃， 大小為屏幕的尺寸.
- 如果傳入的view大小為
wrap_content碌嘀，那么這個view的mode為AT_MOST,大小為屏幕的尺寸. - 如果傳入的view大小為一個具體的值，那么這個view的mode為
EXACTLY虱而，大小為view本身大小筏餐。
以上就是DecorView的MeaureSpec的整個創(chuàng)建的過程了。
看了頂級view之后我們來看普通的view牡拇， 普通的view的measure過程是由viewgroup傳遞過來的，接著我們來看看viewgroup的measureChildWithMargins方法：
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);
}
這個方法獲得了子view的MeasureSpec穆律，并且將其傳入子view的measure方法中惠呼， 這里重點來看下viewgroup是如何創(chuàng)建子view的MeasuerSpec的。來看getChildMeasureSpec方法內(nèi)部的實現(xiàn)：
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 = 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);
}
這個方法很長峦耘，但是我們只需要注意到AT_MOST跟EXACTLY這兩種情況就行剔蹋，稍微分析下這個過程：
首先要理解這個方法的三個參數(shù)， 第一個是父view的MeasureSpec, 第二個是父view已占用的大小辅髓，第三個是view的LayoutParams的大小泣崩，如果不理解可以看看ViewGroup的MeasureChildWithMargins方法中的調(diào)用：
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);
第二個參數(shù)很長 ，
mPaddingLeft+mPaddingRight+lp.leftMargin+lp.rightMargin+withUsed洛口，這些所有的值都有一個共同特點矫付，就是這些位置是不能擺放任何view的，即父view已經(jīng)占用的地盤第焰，現(xiàn)在是不是對參數(shù)更加理解了呢买优。
接著我們回到getChildMeasureSpec方法中繼續(xù)看看viewGroup到底是怎么創(chuàng)建view的MeasureSpec的。
第一步： 根據(jù)參數(shù)一，即傳入的父view的MeasureSpec獲得父view的Mode和Size杀赢。這里的第三行代碼：
int size = Math.max(0, specSize - padding);
這個size表示取0與父容器中可占用的位置的最大值烘跺，可以直接理解為父view的大小。
第二步：根據(jù)父view的Mode分情況處理脂崔， 到這一步我們應(yīng)該就清楚為什么說view的大小是由父view的MeasureSpec與本身LayoutParmas大小共同決定的吧滤淳。
這里我們依然只看AT_MOST跟EXACTLY兩種情況，
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;
這里有個比較難以理解的值就是childDimension > 0 砌左，這個其實就表示view的大小是一個具體的值比如100dp , 因為view的match_parent和 wrap_content在系統(tǒng)內(nèi)部定義的都是負數(shù)脖咐，一個是-1. 一個是-2 ，所以判斷childDimension > 0即绊困，view的大小為一個具體的值文搂。
接著就比較好理解了， 我們來稍微總結(jié)下：
無論父view是
match_parent還是wrap_content秤朗，只要view是一個具體的值煤蹭，view的Mode永遠都是EXACTLY, 大小均是view本身定義的大小。
父view模式如果是
EXACTLY, ---> 子view如果是mathch_parent取视，那么子view的大小是父view的大小硝皂，模式也跟父view一樣為EXACTLY. 子view如果是wrap_content， 大小還是父view的大小作谭， 模式為AT_MOST
父view模式如果是
AT_MOST, --- > 子view如果是math_parent稽物， 那么子view大小為父view大小， 模式與父view一樣都是AT_MOST, 子view如果是wrap_content折欠， 子view大小為父view大小贝或， 模式為AT_MOST
上面說的有點繞，但其實我們只需要記住一點锐秦， 無論上面那種情況咪奖，子view在wrap_content下，大小都是父view的大小酱床， 到這里我們是不是就能理解為什么在自定義view的過程中如果不重寫onMeasure羊赵， wrap_content是和match_parent是一個效果了吧。
以上過程是viewGroup中創(chuàng)建子view的
MeasureSpec的過程扇谣，
有了這個MeasureSpec昧捷，測量子view大小就很簡單了，我們可以看到在ViewGroup獲取到子view的MeasureSpec之后罐寨，傳入到子view的measure方法中：
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
進入到view的measure方法,
不知不覺我們已經(jīng)從viewgroup進入到了view的測量過程靡挥，
這里是不是突然意識到，ViewGroup根本沒有測繪自己本身啊衩茸， 只是獲取到子view的MeasureSpec然后傳入子view的measure方法里去芹血，這是因為ViewGroup是個抽象類贮泞，本身并沒有定義測量的過程， ViewGroup的onMeasure需要各個子類去實現(xiàn)幔烛，比如LinearLayout 啃擦、 RelativeLayout等等，并且每個子類的測量過程都不一樣饿悬，這個我們后面會講令蛉， 現(xiàn)在我們還是接著看view的Measure過程。
上面說到viewgroup將創(chuàng)建的子view的MeasureSpec傳入到了view的Measure方法中狡恬， 那么我們就來看看View的Measure方法：
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);
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()");
}
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}
這個方法真是又臭又長珠叔。。弟劲。講道理的話其實我也看不懂祷安， 但是我們只需要注意到一點， 就是這個方法調(diào)用了OnMeasure方法兔乞！
也就是說measure-->OnMeasure
OnMeasure就簡單了 ：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
很簡潔對不汇鞭，但是簡潔并不代表簡單， 這里套了好幾層庸追。霍骄。。 不要被迷惑 淡溯， 我們看最外層其實就是setMeasureDimension().
設(shè)置寬和高读整， 這個寬和高是在 getDefaultSize方法里返回的， 所以我們來看看getDefaultSize的具體代碼：
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;
}
如果我們忽略掉UNSPECIFIED情況的話咱娶， 我們會發(fā)現(xiàn)第一個參數(shù)size根本用不到米间。。膘侮。
也就是說view的大小其實就是父view給他創(chuàng)建的MeasureSpec中的size大小车伞。
這也進一步說明，view在wrap_content情況下 喻喳，大小還是會跟父view大小一樣， 所以我們需要在自定義view的時候重寫OnMeasure困曙。
