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View工作原理（三）视图大小计算过程（measure过程）
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一、android中view的measure過程總概
視圖大小計算的過程是從根視圖measure()方法開始，接著該方法會調用根視圖的onMeasure()方法，onMeasure()方法會對所包含的子視圖逐一執行measure()方法，如果子視圖是ViewGroup子類對象（LinearLayout、FrameLayout、RelativeLayout等布局），則繼續調用子視圖的measure()方法，重復這一過程。如果子視圖是View子類對象（Button、EditText、TextView、ImageView等），則在子視圖重載的onMeasure方法內部不需要進行對子視圖進行measure操作，從而一次measure過程完成。過程如下圖所示：
二、measure詳細過程
View中的measure()方法源碼（ViewGroup類繼承了View類，measure過程先從ViewGroup子類開始）:
[java] view plaincopy
public?final?void?measure(int?widthMeasureSpec,?int?heightMeasureSpec)?{?????????if?((mPrivateFlags?&?FORCE_LAYOUT)?==?FORCE_LAYOUT?||?????????????????widthMeasureSpec?!=?mOldWidthMeasureSpec?||?????????????????heightMeasureSpec?!=?mOldHeightMeasureSpec)?{????????????????????????????mPrivateFlags?&=?~MEASURED_DIMENSION_SET;???????????????if?(ViewDebug.TRACE_HIERARCHY)?{?????????????????ViewDebug.trace(this,?ViewDebug.HierarchyTraceType.ON_MEASURE);?????????????}????????????????????????????onMeasure(widthMeasureSpec,?heightMeasureSpec);?????????????????????????????????????????if?((mPrivateFlags?&?MEASURED_DIMENSION_SET)?!=?MEASURED_DIMENSION_SET)?{?????????????????throw?new?IllegalStateException("onMeasure()?did?not?set?the"?????????????????????????+?"?measured?dimension?by?calling"?????????????????????????+?"?setMeasuredDimension()");?????????????}???????????????mPrivateFlags?|=?LAYOUT_REQUIRED;?????????}???????????mOldWidthMeasureSpec?=?widthMeasureSpec;?????????mOldHeightMeasureSpec?=?heightMeasureSpec;?????}??
注：通過源碼，我們看到該方法的定義中用了final關鍵字，說明該方法是不能被重寫的，即View系統定義的這個measure框架不能被修改。參數widthMeasureSpec和heightMeasureSpec分別對應寬和高的measureSpec，當父視圖對子視圖進行measure操作時，會調用子視圖的measure()方法，該參數得意思是父視圖所提供的measure的“規格”，因為父視圖為子視圖提供的窗口尺寸是由父視圖和子視圖共同決定。該參數有兩部分組成，第一部分：高16位表示specMode，定義在MeasureSpec類中，有三種類型：MeasureSpec.EXACTLY：表示明確的尺寸大小，?MeasureSpec.AT_MOST：表示最大大小，?MeasureSpec.UNSPECIFIED：不確定大小。第二部分：低16位表示size，即父view的大小，這就是為什么我們在重寫onmeasure方法是需要：int?specMode?=?MeasureSpec.getMod(spec);?int?specSize?=?MeasureSpec.getSize(spec)這樣調用。specMode一般都為MeasureSpec.EXACTLY?，而size分別對應屏幕寬，高。也就是Window第一次掉用的view，一般都是這個值，而對于子view來說，這個值就是你在xml定義的屬性??android:layout_width和android:layout_height的值。
下面我們看看源碼執行過程，看注釋就能很明白，首先清?除測量尺寸的標識。接著將重新測量自己的尺寸，即調用onMeasure()方法。最后是判斷測量尺寸大小的標識是否已經重新賦值，如果沒有則不執行setMeasuredDimension()方法。方法結束。這個方法里面主要就是調用自己的onMeasure（）方法，對自己的大小尺寸進行測量。下面來介紹onMeasure（）方法。
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ViewGroup中的onMeasure方法介紹
其實在ViewGroup類中并沒有重寫該方法，一般在他的子類中進行重寫，比如LinearLayout、RelativeLayout，下面我們以Linearlayout來分析。LinearLayout中onMeasure方法源碼如下：
[java] view plaincopy
@Override??protected?void?onMeasure(int?widthMeasureSpec,?int?heightMeasureSpec)?{??????if?(mOrientation?==?VERTICAL)?{??????????measureVertical(widthMeasureSpec,?heightMeasureSpec);??????}?else?{??????????measureHorizontal(widthMeasureSpec,?heightMeasureSpec);??????}??}??
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注：通過源碼我們可以知道，首先onMeasure會判斷這個布局是縱向布局還是橫向布局，即對應android:orientation=""屬性。下面以縱向布局來分析，源碼如下，有點長：
[java] view plaincopy
??????????????void?measureVertical(int?widthMeasureSpec,?int?heightMeasureSpec)?{????????mTotalLength?=?0;????????int?maxWidth?=?0;????????int?alternativeMaxWidth?=?0;????????int?weightedMaxWidth?=?0;????????boolean?allFillParent?=?true;????????float?totalWeight?=?0;??????????final?int?count?=?getVirtualChildCount();???????????????final?int?widthMode?=?MeasureSpec.getMode(widthMeasureSpec);????????final?int?heightMode?=?MeasureSpec.getMode(heightMeasureSpec);????????boolean?matchWidth?=?false;????????final?int?baselineChildIndex?=?mBaselineAlignedChildIndex;????????????????final?boolean?useLargestChild?=?mUseLargestChild;????????int?largestChildHeight?=?Integer.MIN_VALUE;????????????????for?(int?i?=?0;?i?<?count;?++i)?{????????????final?View?child?=?getVirtualChildAt(i);??????????????if?(child?==?null)?{????????????????mTotalLength?+=?measureNullChild(i);????????????????continue;????????????}????????????if?(child.getVisibility()?==?View.GONE)?{???????????????i?+=?getChildrenSkipCount(child,?i);???????????????continue;????????????}????????????LinearLayout.LayoutParams?lp?=?(LinearLayout.LayoutParams)?child.getLayoutParams();????????????totalWeight?+=?lp.weight;????????????????????????if?(heightMode?==?MeasureSpec.EXACTLY?&&?lp.height?==?0?&&?lp.weight?>?0)?{????????????????????????????????????????????????????????????????final?int?totalLength?=?mTotalLength;????????????????mTotalLength?=?Math.max(totalLength,?totalLength?+?lp.topMargin?+?lp.bottomMargin);????????????}?else?{????????????????int?oldHeight?=?Integer.MIN_VALUE;????????????????if?(lp.height?==?0?&&?lp.weight?>?0)?{????????????????????????????????????????????????????????????????????????????????????????????????????oldHeight?=?0;????????????????????lp.height?=?LayoutParams.WRAP_CONTENT;????????????????}????????????????????????????????????????????????????????????????????????????????measureChildBeforeLayout(???????????????????????child,?i,?widthMeasureSpec,?0,?heightMeasureSpec,???????????????????????totalWeight?==?0???mTotalLength?:?0);??????????????????if?(oldHeight?!=?Integer.MIN_VALUE)?{???????????????????lp.height?=?oldHeight;????????????????}????????????????final?int?childHeight?=?child.getMeasuredHeight();????????????????final?int?totalLength?=?mTotalLength;????????????????mTotalLength?=?Math.max(totalLength,?totalLength?+?childHeight?+?lp.topMargin?+???????????????????????lp.bottomMargin?+?getNextLocationOffset(child));??????????????????if?(useLargestChild)?{????????????????????largestChildHeight?=?Math.max(childHeight,?largestChildHeight);????????????????}????????????}???????????????????????????if?((baselineChildIndex?>=?0)?&&?(baselineChildIndex?==?i?+?1))?{???????????????mBaselineChildTop?=?mTotalLength;????????????}????????????????????????????????????????????????if?(i?<?baselineChildIndex?&&?lp.weight?>?0)?{????????????????throw?new?RuntimeException("A?child?of?LinearLayout?with?index?"????????????????????????+?"less?than?mBaselineAlignedChildIndex?has?weight?>?0,?which?"????????????????????????+?"won't?work.??Either?remove?the?weight,?or?don't?set?"????????????????????????+?"mBaselineAlignedChildIndex.");????????????}????????????boolean?matchWidthLocally?=?false;????????????if?(widthMode?!=?MeasureSpec.EXACTLY?&&?lp.width?==?LayoutParams.MATCH_PARENT)?{????????????????????????????????????????????????????????????????????????????????matchWidth?=?true;????????????????matchWidthLocally?=?true;????????????}????????????final?int?margin?=?lp.leftMargin?+?lp.rightMargin;????????????final?int?measuredWidth?=?child.getMeasuredWidth()?+?margin;????????????maxWidth?=?Math.max(maxWidth,?measuredWidth);????????????allFillParent?=?allFillParent?&&?lp.width?==?LayoutParams.MATCH_PARENT;????????????if?(lp.weight?>?0)?{???????????????????????????????????weightedMaxWidth?=?Math.max(weightedMaxWidth,????????????????????????matchWidthLocally???margin?:?measuredWidth);????????????}?else?{????????????????alternativeMaxWidth?=?Math.max(alternativeMaxWidth,????????????????????????matchWidthLocally???margin?:?measuredWidth);????????????}????????????i?+=?getChildrenSkipCount(child,?i);????????}????????if?(useLargestChild?&&?heightMode?==?MeasureSpec.AT_MOST)?{????????????mTotalLength?=?0;????????????for?(int?i?=?0;?i?<?count;?++i)?{????????????????final?View?child?=?getVirtualChildAt(i);????????????????if?(child?==?null)?{????????????????????mTotalLength?+=?measureNullChild(i);????????????????????continue;????????????????}????????????????if?(child.getVisibility()?==?GONE)?{????????????????????i?+=?getChildrenSkipCount(child,?i);????????????????????continue;????????????????}????????????????final?LinearLayout.LayoutParams?lp?=?(LinearLayout.LayoutParams)????????????????????????child.getLayoutParams();????????????????????????????????final?int?totalLength?=?mTotalLength;????????????????mTotalLength?=?Math.max(totalLength,?totalLength?+?largestChildHeight?+????????????????????????lp.topMargin?+?lp.bottomMargin?+?getNextLocationOffset(child));????????????}????????}????????????????mTotalLength?+=?mPaddingTop?+?mPaddingBottom;????????int?heightSize?=?mTotalLength;????????????????heightSize?=?Math.max(heightSize,?getSuggestedMinimumHeight());????????????????????????heightSize?=?resolveSize(heightSize,?heightMeasureSpec);????????????????????????????????int?delta?=?heightSize?-?mTotalLength;????????if?(delta?!=?0?&&?totalWeight?>?0.0f)?{????????????float?weightSum?=?mWeightSum?>?0.0f???mWeightSum?:?totalWeight;????????????mTotalLength?=?0;????????????for?(int?i?=?0;?i?<?count;?++i)?{????????????????final?View?child?=?getVirtualChildAt(i);????????????????if?(child.getVisibility()?==?View.GONE)?{????????????????????continue;????????????????}????????????????????????????????LinearLayout.LayoutParams?lp?=?(LinearLayout.LayoutParams)?child.getLayoutParams();????????????????????????????????float?childExtra?=?lp.weight;????????????????if?(childExtra?>?0)?{????????????????????????????????????????int?share?=?(int)?(childExtra?*?delta?/?weightSum);????????????????????weightSum?-=?childExtra;????????????????????delta?-=?share;????????????????????final?int?childWidthMeasureSpec?=?getChildMeasureSpec(widthMeasureSpec,????????????????????????????mPaddingLeft?+?mPaddingRight?+????????????????????????????????????lp.leftMargin?+?lp.rightMargin,?lp.width);??????????????????????????????????????????????????????????????if?((lp.height?!=?0)?||?(heightMode?!=?MeasureSpec.EXACTLY))?{????????????????????????????????????????????????????????????????????????int?childHeight?=?child.getMeasuredHeight()?+?share;????????????????????????if?(childHeight?<?0)?{????????????????????????????childHeight?=?0;????????????????????????}????????????????????????child.measure(childWidthMeasureSpec,????????????????????????????????MeasureSpec.makeMeasureSpec(childHeight,?MeasureSpec.EXACTLY));????????????????????}?else?{????????????????????????????????????????????????????????????????????????child.measure(childWidthMeasureSpec,????????????????????????????????MeasureSpec.makeMeasureSpec(share?>?0???share?:?0,????????????????????????????????????????MeasureSpec.EXACTLY));????????????????????}????????????????}????????????????final?int?margin?=??lp.leftMargin?+?lp.rightMargin;????????????????final?int?measuredWidth?=?child.getMeasuredWidth()?+?margin;????????????????maxWidth?=?Math.max(maxWidth,?measuredWidth);????????????????boolean?matchWidthLocally?=?widthMode?!=?MeasureSpec.EXACTLY?&&????????????????????????lp.width?==?LayoutParams.MATCH_PARENT;????????????????alternativeMaxWidth?=?Math.max(alternativeMaxWidth,????????????????????????matchWidthLocally???margin?:?measuredWidth);????????????????allFillParent?=?allFillParent?&&?lp.width?==?LayoutParams.MATCH_PARENT;????????????????final?int?totalLength?=?mTotalLength;????????????????mTotalLength?=?Math.max(totalLength,?totalLength?+?child.getMeasuredHeight()?+????????????????????????lp.topMargin?+?lp.bottomMargin?+?getNextLocationOffset(child));????????????}????????????????????????mTotalLength?+=?mPaddingTop?+?mPaddingBottom;????????????????????}?else?{????????????alternativeMaxWidth?=?Math.max(alternativeMaxWidth,???????????????????????????????????????????weightedMaxWidth);????????}????????if?(!allFillParent?&&?widthMode?!=?MeasureSpec.EXACTLY)?{????????????maxWidth?=?alternativeMaxWidth;????????}????????maxWidth?+=?mPaddingLeft?+?mPaddingRight;????????????????maxWidth?=?Math.max(maxWidth,?getSuggestedMinimumWidth());????????setMeasuredDimension(resolveSize(maxWidth,?widthMeasureSpec),?heightSize);????????if?(matchWidth)?{????????????forceUniformWidth(count,?heightMeasureSpec);????????}????}??
注：
獲取所有的子view數量，對每個子view開始處理，如果子view是GONE的，則直接跳過。獲取子view的LayoutParams，在xml中定義的參數，通過layout_weight定義的值累加到變量totalWeight中，然后判斷如果view的height設置為零，但weight設置的大于0，則將height的值設置為LayoutParams.WRAP_CONTENT。然后調用measureChildWithMargins方法，該方法處理的邏輯：計算子view的measureSpec，即specMode和specSize，調用方法為：getChildMeasureSpec，調用兩次，分別?計算寬和高，getChildMeasureSpec內部根據父view的measure和子view的layout_width和layout_height屬性計算子view的measure。getChildMeasureSpec計算子view的measure，總結如下：1.如果在xml中指定了子view的具體大小，那么計算結果不管父的measure是什么，結果都是EXACITY+child_size，2.如果子view的height指定的值為FILL_PARENT，則返回的結果為：EXACITY+size，原因很簡單：因為FILL_PARENT的意思是充滿這個父view，所以返回的子view的measure就是view的大小。3.如果子view的大小為wrap_content,那么返回的結果都為AT_MOST+size，原因是：最大不能超過父view的大小。子view的measure確定好以后，然后調用子view的measure方法，如果子view是View對象，則該view的大小測量結束，開始下一個子view的循環，如果子view是ViewGroup那么，又開始一個新的遞歸，處理邏輯和上面一樣，直到所有的view對象測量結束。所有的子view測量結束后，才開始對layout_weight計算，這樣我們可能想到，如果父view已經被占滿了，那么有可能layout_weight大于0的view對象是不會顯示的，而計算layout_weight的方法也很簡單，就是用總高度減去上面分析完mTotalLength的值，就是剩下，然后去平分給view對象，注意計算權重時優先去android:android:weightSum（LinearLayout的xml屬性）的值，如果不設置該值會計算和，所以該值既然設置了，就一定要子view的weight的總和相等，否則平分可能不能得到預期效果。
?過程分析完畢，這篇文章這里提到了LinearLayout中的layout_weight屬性，這個屬性對很對人來說是又恨又愛，下篇文章，我們將來總結改屬性的詳細用法，讓大家徹底理解這個屬性。
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