View工作原理（四）view的layout过程
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View工作原理（四）view的layout过程
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/**? ????*?Assign?a?size?and?position?to?a?view?and?all?of?its? ????*?descendants? ????*? ????*?<p>This?is?the?second?phase?of?the?layout?mechanism.? ????*?(The?first?is?measuring).?In?this?phase,?each?parent?calls? ????*?layout?on?all?of?its?children?to?position?them.? ????*?This?is?typically?done?using?the?child?measurements? ????*?that?were?stored?in?the?measure?pass().? ????*? ????*?Derived?classes?with?children?should?override? ????*?onLayout.?In?that?method,?they?should? ????*?call?layout?on?each?of?their?their?children.? ????*? ????*?@param?l?Left?position,?relative?to?parent? ????*?@param?t?Top?position,?relative?to?parent? ????*?@param?r?Right?position,?relative?to?parent? ????*?@param?b?Bottom?position,?relative?to?parent? ????*/?? ???public?final?void?layout(int?l,?int?t,?int?r,?int?b)?{?? ???????boolean?changed?=?setFrame(l,?t,?r,?b);?? ???????if?(changed?||?(mPrivateFlags?&?LAYOUT_REQUIRED)?==?LAYOUT_REQUIRED)?{?? ???????????if?(ViewDebug.TRACE_HIERARCHY)?{?? ???????????????ViewDebug.trace(this,?ViewDebug.HierarchyTraceType.ON_LAYOUT);?? ???????????}?? ?? ???????????onLayout(changed,?l,?t,?r,?b);?? ???????????mPrivateFlags?&=?~LAYOUT_REQUIRED;?? ???????}?? ???????mPrivateFlags?&=?~FORCE_LAYOUT;?? ???}??
?protected?boolean?setFrame(int?left,?int?top,?int?right,?int?bottom)?{?? ????????boolean?changed?=?false;?? ????????if?(DBG)?{?? ????????????Log.d("View",?this?+?"?View.setFrame("?+?left?+?","?+?top?+?","?? ????????????????????+?right?+?","?+?bottom?+?")");?? ????????}?? ????????if?(mLeft?!=?left?||?mRight?!=?right?||?mTop?!=?top?||?mBottom?!=?bottom)?{?? ????????????changed?=?true;?? ????????????//?Remember?our?drawn?bit?? ????????????int?drawn?=?mPrivateFlags?&?DRAWN;?? ????????????//?Invalidate?our?old?position?? ????????????invalidate();?? ????????????int?oldWidth?=?mRight?-?mLeft;?? ????????????int?oldHeight?=?mBottom?-?mTop;?? ????????????mLeft?=?left;?? ????????????mTop?=?top;?? ????????????mRight?=?right;?? ????????????mBottom?=?bottom;?? ????????????mPrivateFlags?|=?HAS_BOUNDS;?? ????????????int?newWidth?=?right?-?left;?? ????????????int?newHeight?=?bottom?-?top;?? ????????????if?(newWidth?!=?oldWidth?||?newHeight?!=?oldHeight)?{?? ????????????????onSizeChanged(newWidth,?newHeight,?oldWidth,?oldHeight);?? ????????????}?? ????????????if?((mViewFlags?&?VISIBILITY_MASK)?==?VISIBLE)?{?? ????????????????//?If?we?are?visible,?force?the?DRAWN?bit?to?on?so?that?? ????????????????//?this?invalidate?will?go?through?(at?least?to?our?parent).?? ????????????????//?This?is?because?someone?may?have?invalidated?this?view?? ????????????????//?before?this?call?to?setFrame?came?in,?therby?clearing?? ????????????????//?the?DRAWN?bit.?? ????????????????mPrivateFlags?|=?DRAWN;?? ????????????????invalidate();?? ????????????}?? ????????????//?Reset?drawn?bit?to?original?value?(invalidate?turns?it?off)?? ????????????mPrivateFlags?|=?drawn;?? ????????????mBackgroundSizeChanged?=?true;?? ????????}?? ????????return?changed;?? ????}??
?View中的onLayout()方法如下：
[java] view plaincopy protected?void?onLayout(boolean?changed,?int?left,?int?top,?int?right,?int?bottom)?{?? ????}??
@Override?? ????protected?abstract?void?onLayout(boolean?changed,?? ????????????int?l,?int?t,?int?r,?int?b);??
@Override?? ???protected?void?onLayout(boolean?changed,?int?l,?int?t,?int?r,?int?b)?{?? ???????if?(mOrientation?==?VERTICAL)?{?? ???????????layoutVertical();?? ???????}?else?{?? ???????????layoutHorizontal();?? ???????}?? ???}??
void?layoutVertical()?{?? ????????final?int?paddingLeft?=?mPaddingLeft;?? ????????int?childTop?=?mPaddingTop;?? ????????int?childLeft;?? ????????//?Where?right?end?of?child?should?go?? ????????final?int?width?=?mRight?-?mLeft;?? ????????int?childRight?=?width?-?mPaddingRight;?? ????????//?Space?available?for?child?? ????????int?childSpace?=?width?-?paddingLeft?-?mPaddingRight;?? ????????final?int?count?=?getVirtualChildCount();?? ????????final?int?majorGravity?=?mGravity?&?Gravity.VERTICAL_GRAVITY_MASK;?? ????????final?int?minorGravity?=?mGravity?&?Gravity.HORIZONTAL_GRAVITY_MASK;?? ????????if?(majorGravity?!=?Gravity.TOP)?{?? ???????????switch?(majorGravity)?{?? ???????????????case?Gravity.BOTTOM:?? ???????????????????//?mTotalLength?contains?the?padding?already,?we?add?the?top?? ???????????????????//?padding?to?compensate?? ???????????????????childTop?=?mBottom?-?mTop?+?mPaddingTop?-?mTotalLength;?? ???????????????????break;?? ???????????????case?Gravity.CENTER_VERTICAL:?? ???????????????????childTop?+=?((mBottom?-?mTop)??-?mTotalLength)?/?2;?? ???????????????????break;?? ???????????}?? ????????}?? ????????for?(int?i?=?0;?i?<?count;?i++)?{?? ????????????final?View?child?=?getVirtualChildAt(i);?? ????????????if?(child?==?null)?{?? ????????????????childTop?+=?measureNullChild(i);?? ????????????}?else?if?(child.getVisibility()?!=?GONE)?{?? ????????????????final?int?childWidth?=?child.getMeasuredWidth();?? ????????????????final?int?childHeight?=?child.getMeasuredHeight();?? ????????????????final?LinearLayout.LayoutParams?lp?=?? ????????????????????????(LinearLayout.LayoutParams)?child.getLayoutParams();?? ????????????????int?gravity?=?lp.gravity;?? ????????????????if?(gravity?<?0)?{?? ????????????????????gravity?=?minorGravity;?? ????????????????}?? ????????????????switch?(gravity?&?Gravity.HORIZONTAL_GRAVITY_MASK)?{?? ????????????????????case?Gravity.LEFT:?? ????????????????????????childLeft?=?paddingLeft?+?lp.leftMargin;?? ????????????????????????break;?? ????????????????????case?Gravity.CENTER_HORIZONTAL:?? ????????????????????????childLeft?=?paddingLeft?+?((childSpace?-?childWidth)?/?2)?? ????????????????????????????????+?lp.leftMargin?-?lp.rightMargin;?? ????????????????????????break;?? ????????????????????case?Gravity.RIGHT:?? ????????????????????????childLeft?=?childRight?-?childWidth?-?lp.rightMargin;?? ????????????????????????break;?? ????????????????????default:?? ????????????????????????childLeft?=?paddingLeft;?? ????????????????????????break;?? ????????????????}?? ????????????????childTop?+=?lp.topMargin;?? ????????????????setChildFrame(child,?childLeft,?childTop?+?getLocationOffset(child),?? ????????????????????????childWidth,?childHeight);?? ????????????????childTop?+=?childHeight?+?lp.bottomMargin?+?getNextLocationOffset(child);?? ????????????????i?+=?getChildrenSkipCount(child,?i);?? ????????????}?? ????????}?? ????}??
a)?LinearLayout中的子視圖有兩種布局方式，一個是縱向的，一個是橫向的，這里我們以縱向的分析。
b)?獲得子視圖的寬度。
c)?根據(jù)父視圖中的grarity屬性，來判斷子視圖的起始位置。
d) 開始for()循環(huán)，為每個子視圖分配位置。對于每個子視圖首先取出子視圖的LayoutParams屬性，并且獲得gravity的值。根據(jù)gravity的值確定水平方向的起始位置，三種值分別為：LEFT,CENTER_HORIZONTAL和RIGHT.接著調(diào)用setChildFrame()，該方法內(nèi)部實際上就是調(diào)用child.layout()為子視圖設置布局位置。
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一、android中view的layout過程總概
Layout過程其實就是父視圖按照子視圖的大小及布局參數(shù)將子視圖放在窗口的合適的位置上。 視圖的布局過程是從ViewRoot對象調(diào)調(diào)用根視圖的layout()方法開始，接著layout()方法調(diào)用根視圖的onLayout()方法，onLayout()方法會對所包含的子視圖逐一執(zhí)行layout操作，如果子視圖是ViewGroup子類對象，則繼續(xù)調(diào)用子視圖的layout()，重復這一過程。如果子視圖是View子類對象，則在子視圖重載的onLayout()內(nèi)部只需要將自己布局到視圖中，不需要對子視圖進行layout操作，這樣一次layout過程結束。過程如下圖: ?轉載請說明出處：http://blog.csdn.net/ff20081528/article/details/17784911
二、layout詳細過程
View中的layout()方法源碼（ViewGroup類繼承了View類，layout過程先從ViewGroup子類開始）: [java] view plaincopya)?首先我們看這個方法的定義，用了關鍵字final，說明此方法是不可被重寫的，這樣也就保證了View的layout過程是不變的。四個參數(shù)看注釋，左、上、右、下分別相距父視圖的距離。
b)?調(diào)用setFrame(l,t,r,b)將位置保存起來，這些參數(shù)將保存到View內(nèi)部變量?（mLeft、mTop、mRight、mBottom）中。保存完變量前，會先對比這些參數(shù)是否和原來的相同，如果相同，則什么都不做，如果不同則進行重新賦值，并在賦值前給mPrivateFlags中添加DRAWN標識，同時調(diào)用invalidate()通知View系統(tǒng)原來占用的位置需要重繪。
c)?調(diào)用onLayout(),View中定義的onLayout()方法默認什么都不做，View系統(tǒng)提供onLayout()方法的目的是為了使系統(tǒng)包含的子視圖的父視圖能夠在onLayout()方法對子視圖進行位置分配，正因為如此，如果是父視圖，則必須重寫onLayout()，也正因為如此ViewGroup類才會把onLayout重載改成了abstract類型。
d）清除mPrivateFlags中的LAYOUT_REQUIRED標識，因為layout操作已經(jīng)完成。
上面提到的setFrame方法源碼如下：
[java] view plaincopy而ViewGroup中的onLayout()方法如下：
[java] view plaincopy?
轉載請說明出處：http://blog.csdn.net/ff20081528/article/details/17784911?
因為ViewGroup中的onLayout()方法是一個抽象方法，所以下面我們用他的子類LinearLayout中的onLayout()方法來分析。源碼如下：
onlayout()方法：
[java] view plaincopylayoutVertical()方法源碼：
[java] view plaincopy總結
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