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，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.接著調用setChildFrame()，該方法內部實際上就是調用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對象調調用根視圖的layout()方法開始，接著layout()方法調用根視圖的onLayout()方法，onLayout()方法會對所包含的子視圖逐一執行layout操作，如果子視圖是ViewGroup子類對象，則繼續調用子視圖的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。四個參數看注釋，左、上、右、下分別相距父視圖的距離。
b)?調用setFrame(l,t,r,b)將位置保存起來，這些參數將保存到View內部變量?（mLeft、mTop、mRight、mBottom）中。保存完變量前，會先對比這些參數是否和原來的相同，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，同時調用invalidate()通知View系統原來占用的位置需要重繪。
c)?調用onLayout(),View中定義的onLayout()方法默認什么都不做，View系統提供onLayout()方法的目的是為了使系統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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