一，寫在前面
當Android原生控件無法滿足開發需求時，需要自己來創造view，自定義控件。自定義控件分三步來完成：測量(onMeasure)，佈局(onLayout)，繪製(onDraw)。今天主要介紹自定義流程的第一步-測量，通常一個佈局文件的控件的簡單嵌套，顯示如下：
LinearLayout裏面有兩個子view：TextView，RelativeLayout(裏面又有兩個TextView)。我們知道自定義控件無非是extends View, extends ViewGroup, extends 容器控件/非容器控件，因此提取出其中的View.java, FrameLayout.java, TextView.java進行分析。下面展示涉及類，方法的結構圖，看不懂互相之間關係沒事，可以看完後面的分析，然後再回過頭來看結構圖。
結構圖：
由上圖可知，語法角度：子類可以重寫onMeasure，只能繼承View的measure，setMeasuredDimension方法。測量流程分爲兩種情況討論：容器控件ViewGroup，原始的View(非容器控件)。原始的View測量，只需要測量自己的寬高；而容器控件需要先測量所有的子View的寬高，然後再測量自己的寬高。看懂本篇文章，還需要大家自己先去研究下類View$MeasureSpec，相對比較簡單，本文不描述MeasureSpec相關知識。
二，源碼分析之View
先分析原始的View，打開View.java文件，查看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);
}
if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec) {
// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
// 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("onMeasure() did not set the"
+ " measured dimension by calling"
+ " setMeasuredDimension()");
}
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
}主要分析measure(w,h)的兩個關鍵點：
一，字段mPrivateFlags
1.1 字段mPrivateFlags在調用onMeasure(w,h)前，執行mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET，設置成員變量mPrivateFlags的MEASURED_DIMENSION_SET位設置爲0；
1.2 在onMeasure(w,h)執行完成後，判斷if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET)決定是否拋出IllegalStateException異常；
二，實際測量方法onMeasure(w,h)，進入該方法查看源碼：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int opticalWidth = insets.left + insets.right;
int opticalHeight = insets.top + insets.bottom;
measuredWidth += optical ? opticalWidth : -opticalWidth;
measuredHeight += optical ? opticalHeight : -opticalHeight;
}
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}mPrivateFlags更像是一個標誌位，在onMeasure測量前設置一個值，在onMeasure執行的最後設置一個值，測量完成後判斷mPrivateFlags的值。若前面沒有執行setMeasuredDimension(w,h)完成測量，那麼mPrivateFlags值則不會重新設置，判斷mPrivateFlags時會執行if語句中內容，拋出IllegalStateException異常。繼續分析前面提到：參數measuredWidth,measuredHeight是如何計算得來？只分析寬度(高度計算方式類似)，分析這樣一段代碼：getDefaultSize(getSuggestedMinimumWidth(),widthMeasureSpec)，於是進入方法getDefaultSize查看源碼：
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;
}protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}判斷specMode，1，如果是模式AT_MOST / EXACTLY，返回specSize；2，如果specMode是UNSPECIFIED，則父容器不對子view做任何限制，返回size。(UNSPECIFIED這種測量模式一般不做分析，不用管它)
注意，specMode是EXACTLY，說明父控件已經知道子view需要的精確值，那麼直接使用specSize容易理解；那specMode是AT_MOST時，說明父控件給的建議值是一個子view可以使用的最大值(<=specSize)，爲什麼直接返回specSize呢？這裏肯定需要修改，因爲測量模式爲EXACTLY，說明子view寬度：要麼是match_parent，要麼是具體的值(100dp)。測量模式爲AT_MOST，說明子View寬度：只可能是wrap_content。在使用這個自定義的view時，不能讓match_parent和wrap_content的體現的效果一樣。
於是可以得出結論：在extends View的自定義控件中，需要重寫onMeasure(w,h)，並單獨判斷MeasureSpec.getMode(w)爲AT_MOST時，返回一個寬度值(具體邏輯按需求來吧)，高度同理！查看TextView源碼，onMeasure方法有對specMode爲AT_MOST進行處理。
原始的View(非容器控件)的測量，代碼流程圖大致如下，保存該圖片到本地可以清晰展示信息哦！
三，源碼分析之容器控件
接下來分析容器控件測量流程，前面說到容器控件(繼承ViewGroup)的測量過程：先測量所有子view，然後再測量容器控件本身。每種容器控件測量的細節不盡相同，但都遵循上面的方式。閱讀LinearLayout源碼時，發現裏面if條件判斷極爲噁心，於是本篇以FrameLayout爲例子分析容器控件的測量流程。首先貼上涉及的類，方法結構圖如下，可以看完後面分析回過頭來看此圖。
查看FrameLayout源碼，分析容器控件的測量過程：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
// ...code
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
maxWidth = Math.max(maxWidth,
child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
maxHeight = Math.max(maxHeight,
child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
childState = combineMeasuredStates(childState, child.getMeasuredState());
if (measureMatchParentChildren) {
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
mMatchParentChildren.add(child);
}
}
}
}
// Account for padding too
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
// Check against our minimum height and width
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
// Check against our foreground's minimum height and width
final Drawable drawable = getForeground();
if (drawable != null) {
maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
}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);
}執行getChildMeasureSpec(parentWidthMeasureSpec,mPaddingLeft+mPaddingRight+ lp.leftMargin +lp.rightMargin+widthUsed, lp.width)獲取子View的寬度測量建議值，這個值最終要傳入到child.measure(w,h)中，開始測量子View。 從該方法參數可知：測量時，子View的MeasureSpec值由兩部分組成：(parentWidthMeasureSpec-->父控件的MeasureSpec)，以及子View本身的佈局參數LayoutParams決定的。
其中LayoutParams中，涉及子view的寬高值，外邊距margin。至於mPaddingLeft 是指FrameLayout與其內容之間的距離，字段繼承於類View。
接下來分析如何合成子View的MeasureSpec，上面已經得出結論，這裏從代碼角度具體分析，查看getChildMeasureSpec的源碼：
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);
}1，當childDimension >= 0(android:layout_width="50dp")，resultSize爲50dp，resultMode爲精確EXACTLY；
2，當childDimension == LayoutParams.MATCH_PARENT時，由於父容器specMode是精確的，子view又填充所有空間，那麼resultSize大小就爲size
= Math.max(0, specSize - padding)，有具體數值，屬於精確模式Exactly；
3，當childDimension == LayoutParams.WRAP_CONTENT時，子view要小於等於size，於是大小爲size，模式爲AT_MOST。爸爸FrameLayout的測量模式爲AT_MOST,UNSPECIFIED的情況，就不再具體分析了。最後，使用MeasureSpec合併resultSize,resultMode。
至於，child.measure(w,h)的繼續分析，無非就是兩種情況：child如果是容器控件，則繼續重複上面的測量流程；如果child是一個原始的view，那就是進入文章前半部分的測量流程。
容器控件測量自己，調用方法setMeasuredDimension，由方法resolveSizeAndState的返回值，得到測量的寬高大小(不包含測量模式specMode)。這裏不再分析resolveSizeAndState方法，跟原始的view在測量時的getDefaultSize方法很類似，前面已經分析了getDefaultSize方法。注：子view的測量後寬高，影響到了容器控件測量自己，這也是爲什麼要先測量所有子view，然後才測量容器控件自己。
下面展示了代碼走向流程圖：
這裏，原始的view(非容器控件)與容器控件的測量流程分析完畢了。
