Android中measure过程、WRAP_CONTENT详解以及xml布局文件解析流程浅析(上)
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Android中measure过程、WRAP_CONTENT详解以及xml布局文件解析流程浅析(上)
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在之前一篇博文中<<?Android中View繪制流程以及invalidate()等相關方法分析>>
，簡單的闡述 了
Android View?
? ? ? ? ? ? ? ? ? ? ? 3、 draw 過程 ? ? ?--- 繪制過程
? ? ? 要想對Android 中View這塊深入理解，對這三個步驟地學習是必不可少的 。
android:layout_weight="wrap_content"???//自適應大小?? android:layout_weight="match_parent"???//與父視圖等高?? android:layout_weight="fill_parent"????//與父視圖等高?? android:layout_weight="100dip"?????????//精確設置高度值為?100dip??
? ? ?意思大概是說：?View通過LayoutParams類告訴其父視圖它想要地大小(即，長度和寬度)。
public?class?View?implements?Drawable.Callback,?KeyEvent.Callback,?AccessibilityEventSource?{?? ??...?? ??/**? ???*?The?layout?parameters?associated?with?this?view?and?used?by?the?parent? ???*?{@link?android.view.ViewGroup}?to?determine?how?this?view?should?be? ???*?laid?out.? ???*?{@hide}? ???*/?? ??//該View擁有的?LayoutParams屬性，父試圖添加該View時，會為其賦值，特別注意，其類型為ViewGroup.LayoutParams。?? ??protected?ViewGroup.LayoutParams?mLayoutParams;???? ??...?? }??
public?abstract?class?ViewGroup?extends?View?implements?ViewParent,?ViewManager?{?? ????...?? ?????public?static?class?LayoutParams?{?? ????????/**? ?????????*?Special?value?for?the?height?or?width?requested?by?a?View.? ?????????*?FILL_PARENT?means?that?the?view?wants?to?be?as?big?as?its?parent,? ?????????*?minus?the?parent's?padding,?if?any.?This?value?is?deprecated? ?????????*?starting?in?API?Level?8?and?replaced?by?{@link?#MATCH_PARENT}.? ?????????*/?? ????????@Deprecated?? ????????public?static?final?int?FILL_PARENT?=?-1;??//?注意值為-1，Android2.2版本不建議使用?? ????????/**? ?????????*?Special?value?for?the?height?or?width?requested?by?a?View.? ?????????*?MATCH_PARENT?means?that?the?view?wants?to?be?as?big?as?its?parent,? ?????????*?minus?the?parent's?padding,?if?any.?Introduced?in?API?Level?8.? ?????????*/?? ????????public?static?final?int?MATCH_PARENT?=?-1;?//?注意值為-1?? ????????/**? ?????????*?Special?value?for?the?height?or?width?requested?by?a?View.? ?????????*?WRAP_CONTENT?means?that?the?view?wants?to?be?just?large?enough?to?fit? ?????????*?its?own?internal?content,?taking?its?own?padding?into?account.? ?????????*/?? ????????public?static?final?int?WRAP_CONTENT?=?-2;?//?注意值為-2?? ????????/**? ?????????*?Information?about?how?wide?the?view?wants?to?be.?Can?be?one?of?the? ?????????*?constants?FILL_PARENT?(replaced?by?MATCH_PARENT?,? ?????????*?in?API?Level?8)?or?WRAP_CONTENT.?or?an?exact?size.? ?????????*/?? ????????public?int?width;??//該View的寬度，可以為WRAP_CONTENT/MATCH_PARENT?或者一個具體值?? ????????/**? ?????????*?Information?about?how?tall?the?view?wants?to?be.?Can?be?one?of?the? ?????????*?constants?FILL_PARENT?(replaced?by?MATCH_PARENT?,? ?????????*?in?API?Level?8)?or?WRAP_CONTENT.?or?an?exact?size.? ?????????*/?? ????????public?int?height;?//該View的高度，可以為WRAP_CONTENT/MATCH_PARENT?或者一個具體值?? ????????/**? ?????????*?Used?to?animate?layouts.? ?????????*/?? ????????public?LayoutAnimationController.AnimationParameters?layoutAnimationParameters;?? ????????/**? ?????????*?Creates?a?new?set?of?layout?parameters.?The?values?are?extracted?from? ?????????*?the?supplied?attributes?set?and?context.?The?XML?attributes?mapped? ?????????*?to?this?set?of?layout?parameters?are:、? ?????????*/?? ????????public?LayoutParams(Context?c,?AttributeSet?attrs)?{?? ????????????TypedArray?a?=?c.obtainStyledAttributes(attrs,?R.styleable.ViewGroup_Layout);?? ????????????setBaseAttributes(a,?? ????????????????????R.styleable.ViewGroup_Layout_layout_width,?? ????????????????????R.styleable.ViewGroup_Layout_layout_height);?? ????????????a.recycle();?? ????????}?? ?? ????????/**? ?????????*?Creates?a?new?set?of?layout?parameters?with?the?specified?width? ?????????*?and?height.? ?????????*/?? ????????public?LayoutParams(int?width,?int?height)?{?? ????????????this.width?=?width;?? ????????????this.height?=?height;?? ????????}?? ????????/**? ?????????*?Copy?constructor.?Clones?the?width?and?height?values?of?the?source.? ?????????*? ?????????*?@param?source?The?layout?params?to?copy?from.? ?????????*/?? ????????public?LayoutParams(LayoutParams?source)?{?? ????????????this.width?=?source.width;?? ????????????this.height?=?source.height;?? ????????}?? ????????/**? ?????????*?Used?internally?by?MarginLayoutParams.? ?????????*?@hide? ?????????*/?? ????????LayoutParams()?{?? ????????}?? ????????/**? ?????????*?Extracts?the?layout?parameters?from?the?supplied?attributes.? ?????????*? ?????????*?@param?a?the?style?attributes?to?extract?the?parameters?from? ?????????*?@param?widthAttr?the?identifier?of?the?width?attribute? ?????????*?@param?heightAttr?the?identifier?of?the?height?attribute? ?????????*/?? ????????protected?void?setBaseAttributes(TypedArray?a,?int?widthAttr,?int?heightAttr)?{?? ????????????width?=?a.getLayoutDimension(widthAttr,?"layout_width");?? ????????????height?=?a.getLayoutDimension(heightAttr,?"layout_height");?? ????????}?? }??
//Adds?a?child?view.?????? void?addView(View?child,?int?index)?? //Adds?a?child?view?with?this?ViewGroup's?default?layout?parameters??? //and?the?specified?width?and?height.?? void?addView(View?child,?int?width,?int?height)?? //Adds?a?child?view?with?the?specified?layout?parameters.????????? void?addView(View?child,?ViewGroup.LayoutParams?params)??
? ? ? ? ?三個重載方法的區別只是添加View時構造LayoutParams對象的方式不同而已，稍后我們探尋一下它們的源碼。
? ? ? 2、 通過xml布局文件指定某個View的屬性為：android:layout_heigth=””以及android:layout_weight=”” 時。
? ? 總的來說，這兩種方式都會設定View的LayoutParams屬性值----指定的或者Default值。
public?abstract?class?ViewGroup?extends?View?implements?ViewParent,?ViewManager?{?? ????...?? ????/**? ?????*?Adds?a?child?view.?If?no?layout?parameters?are?already?set?on?the?child,?the? ?????*?default?parameters?for?this?ViewGroup?are?set?on?the?child.? ?????*? ?????*?@param?child?the?child?view?to?add? ?????*? ?????*?@see?#generateDefaultLayoutParams()? ?????*/?? ????public?void?addView(View?child)?{?? ????????addView(child,?-1);?? ????}?? ????/**? ?????*?Adds?a?child?view.?If?no?layout?parameters?are?already?set?on?the?child,?the? ?????*?default?parameters?for?this?ViewGroup?are?set?on?the?child.? ?????*? ?????*?@param?child?the?child?view?to?add? ?????*?@param?index?the?position?at?which?to?add?the?child? ?????*? ?????*?@see?#generateDefaultLayoutParams()? ?????*/?? ????public?void?addView(View?child,?int?index)?{?? ????????LayoutParams?params?=?child.getLayoutParams();?? ????????if?(params?==?null)?{?? ????????????params?=?generateDefaultLayoutParams();?//返回默認地LayoutParams類，作為該View的屬性值?? ????????????if?(params?==?null)?{//如果不能獲取到LayoutParams對象，則拋出異常。?? ????????????????throw?new?IllegalArgumentException("generateDefaultLayoutParams()?cannot?return?null");?? ????????????}?? ????????}?? ????????addView(child,?index,?params);?? ????}?? ????/**? ?????*?Adds?a?child?view?with?this?ViewGroup's?default?layout?parameters?and?the? ?????*?specified?width?and?height.? ?????*? ?????*?@param?child?the?child?view?to?add? ?????*/?? ????public?void?addView(View?child,?int?width,?int?height)?{?? ????????//返回默認地LayoutParams類，作為該View的屬性值?? ????????final?LayoutParams?params?=?generateDefaultLayoutParams();??? ????????params.width?=?width;???//重新設置width值?? ????????params.height?=?height;?//重新設置height值?? ????????addView(child,?-1,?params);?//這兒，我們有指定width、height的大小了。?? ????}?? ????/**? ?????*?Adds?a?child?view?with?the?specified?layout?parameters.? ?????*? ?????*?@param?child?the?child?view?to?add? ?????*?@param?params?the?layout?parameters?to?set?on?the?child? ?????*/?? ????public?void?addView(View?child,?LayoutParams?params)?{?? ????????addView(child,?-1,?params);?? ????}?? ????/**? ?????*?Adds?a?child?view?with?the?specified?layout?parameters.? ?????*? ?????*?@param?child?the?child?view?to?add? ?????*?@param?index?the?position?at?which?to?add?the?child? ?????*?@param?params?the?layout?parameters?to?set?on?the?child? ?????*/?? ????public?void?addView(View?child,?int?index,?LayoutParams?params)?{?? ????????...?? ????????//?addViewInner()?will?call?child.requestLayout()?when?setting?the?new?LayoutParams?? ????????//?therefore,?we?call?requestLayout()?on?ourselves?before,?so?that?the?child's?request?? ????????//?will?be?blocked?at?our?level?? ????????requestLayout();?? ????????invalidate();?? ????????addViewInner(child,?index,?params,?false);?? ????}?? ????/**? ?????*?Returns?a?set?of?default?layout?parameters.?These?parameters?are?requested? ?????*?when?the?View?passed?to?{@link?#addView(View)}?has?no?layout?parameters? ?????*?already?set.?If?null?is?returned,?an?exception?is?thrown?from?addView.? ?????*? ?????*?@return?a?set?of?default?layout?parameters?or?null? ?????*/?? ????protected?LayoutParams?generateDefaultLayoutParams()?{?? ????????//width?為?WRAP_CONTENT大小?，?height?為WRAP_CONTENT??? ????????//ViewGroup的子類可以重寫該方法，達到其特定要求。稍后會以LinearLayout類為例說明。?? ????????return?new?LayoutParams(LayoutParams.WRAP_CONTENT,?LayoutParams.WRAP_CONTENT);?? ????}?? ????private?void?addViewInner(View?child,?int?index,?LayoutParams?params,?? ????????????boolean?preventRequestLayout)?{?? ?? ????????if?(!checkLayoutParams(params))?{?//params對象是否為null?? ????????????params?=?generateLayoutParams(params);?//如果params對象是為null，重新構造個LayoutParams對象?? ????????}?? ????????//preventRequestLayout值為false?? ????????if?(preventRequestLayout)?{???? ????????????child.mLayoutParams?=?params;?//為View的mLayoutParams屬性賦值?? ????????}?else?{?? ????????????child.setLayoutParams(params);//為View的mLayoutParams屬性賦值，但會調用requestLayout()請求重新布局?? ????????}?? ????????//if?else?語句會設置View為mLayoutParams屬性賦值?? ????????...?? ????}?? ????...?? }??
public?class?LinearLayout?extends?ViewGroup?{?? ????...?? ????@Override?? ????public?LayoutParams?generateLayoutParams(AttributeSet?attrs)?{?? ????????return?new?LinearLayout.LayoutParams(getContext(),?attrs);?? ????}?? ????@Override?? ????protected?LayoutParams?generateDefaultLayoutParams()?{?? ????????//該LinearLayout是水平方向還是垂直方向?? ????????if?(mOrientation?==?HORIZONTAL)?{??? ????????????return?new?LayoutParams(LayoutParams.WRAP_CONTENT,?LayoutParams.WRAP_CONTENT);?? ????????}?else?if?(mOrientation?==?VERTICAL)?{?? ????????????return?new?LayoutParams(LayoutParams.MATCH_PARENT,?LayoutParams.WRAP_CONTENT);?? ????????}?? ????????return?null;?? ????}?? ????@Override?? ????protected?LayoutParams?generateLayoutParams(ViewGroup.LayoutParams?p)?{?? ????????return?new?LayoutParams(p);?? ????}?? ????/**? ?????*?Per-child?layout?information?associated?with?ViewLinearLayout.? ?????*?? ?????*?@attr?ref?android.R.styleable#LinearLayout_Layout_layout_weight? ?????*?@attr?ref?android.R.styleable#LinearLayout_Layout_layout_gravity? ?????*/?//自定義的LayoutParams類?? ????public?static?class?LayoutParams?extends?ViewGroup.MarginLayoutParams?{?? ????????/**? ?????????*?Indicates?how?much?of?the?extra?space?in?the?LinearLayout?will?be? ?????????*?allocated?to?the?view?associated?with?these?LayoutParams.?Specify? ?????????*?0?if?the?view?should?not?be?stretched.?Otherwise?the?extra?pixels? ?????????*?will?be?pro-rated?among?all?views?whose?weight?is?greater?than?0.? ?????????*/?? ????????@ViewDebug.ExportedProperty(category?=?"layout")?? ????????public?float?weight;??????//??見于屬性，android:layout_weight=""??;?? ????????/**? ?????????*?Gravity?for?the?view?associated?with?these?LayoutParams.? ?????????*? ?????????*?@see?android.view.Gravity? ?????????*/?? ????????public?int?gravity?=?-1;??//?見于屬性，?android:layout_gravity=""??;??? ????????/**? ?????????*?{@inheritDoc}? ?????????*/?? ????????public?LayoutParams(Context?c,?AttributeSet?attrs)?{?? ????????????super(c,?attrs);?? ????????????TypedArray?a?=c.obtainStyledAttributes(attrs,?com.android.internal.R.styleable.LinearLayout_Layout);?? ????????????weight?=?a.getFloat(com.android.internal.R.styleable.LinearLayout_Layout_layout_weight,?0);?? ????????????gravity?=?a.getInt(com.android.internal.R.styleable.LinearLayout_Layout_layout_gravity,?-1);?? ?? ????????????a.recycle();?? ????????}?? ????????/**? ?????????*?{@inheritDoc}? ?????????*/?? ????????public?LayoutParams(int?width,?int?height)?{?? ????????????super(width,?height);?? ????????????weight?=?0;?? ????????}?? ????????/**? ?????????*?Creates?a?new?set?of?layout?parameters?with?the?specified?width,?height? ?????????*?and?weight.? ?????????*? ?????????*?@param?width?the?width,?either?{@link?#MATCH_PARENT},? ?????????*????????{@link?#WRAP_CONTENT}?or?a?fixed?size?in?pixels? ?????????*?@param?height?the?height,?either?{@link?#MATCH_PARENT},? ?????????*????????{@link?#WRAP_CONTENT}?or?a?fixed?size?in?pixels? ?????????*?@param?weight?the?weight? ?????????*/?? ????????public?LayoutParams(int?width,?int?height,?float?weight)?{?? ????????????super(width,?height);?? ????????????this.weight?=?weight;?? ????????}?? ????????public?LayoutParams(ViewGroup.LayoutParams?p)?{?? ????????????super(p);?? ????????}?? ????????public?LayoutParams(MarginLayoutParams?source)?{?? ????????????super(source);?? ????????}?? ????}?? ????...?? }??
public?class?LinearLayout?extends?ViewGroup?{?? ????...?? ????@Override??//onMeasure方法。?? ????protected?void?onMeasure(int?widthMeasureSpec,?int?heightMeasureSpec)?{?? ????????//判斷是垂直方向還是水平方向，這兒我們假設是VERTICAL垂直方向，?? ????????if?(mOrientation?==?VERTICAL)?{?? ????????????measureVertical(widthMeasureSpec,?heightMeasureSpec);?? ????????}?else?{?? ????????????measureHorizontal(widthMeasureSpec,?heightMeasureSpec);?? ????????}?? ????}?? ?????/**? ?????*?Measures?the?children?when?the?orientation?of?this?LinearLayout?is?set? ?????*?to?{@link?#VERTICAL}.? ?????*? ?????*?@param?widthMeasureSpec?Horizontal?space?requirements?as?imposed?by?the?parent.? ?????*?@param?heightMeasureSpec?Vertical?space?requirements?as?imposed?by?the?parent.? ?????*? ?????*?@see?#getOrientation()? ?????*?@see?#setOrientation(int)? ?????*?@see?#onMeasure(int,?int)? ?????*/?? ??????void?measureVertical(int?widthMeasureSpec,?int?heightMeasureSpec)?{?? ????????????mTotalLength?=?0;?? ????????????...?? ????????????//?See?how?tall?everyone?is.?Also?remember?max?width.?? ????????????for?(int?i?=?0;?i?<?count;?++i)?{?? ????????????????final?View?child?=?getVirtualChildAt(i);?//獲得索引處為i的子VIew????? ????????????????...?? ????????????????//注意，我們將類型為?ViewGroup.LayoutParams的實例對象強制轉換為了LinearLayout.LayoutParams，?? ????????????????//即父對象轉換為了子對象，能這樣做的原因就是LinearLayout的所有子View的LayoutParams類型都為?? ????????????????//LinearLayout.LayoutParams?? ????????????????LinearLayout.LayoutParams?lp?=?(LinearLayout.LayoutParams)?child.getLayoutParams();?? ????????????????...?? ????????}?? ????...?? }??
public?class?CellLayout?extends?ViewGroup?{?? ????...??? ????public?static?class?LayoutParams?extends?ViewGroup.MarginLayoutParams?{?? ????????????/**? ?????????????*?Horizontal?location?of?the?item?in?the?grid.? ?????????????*/?? ????????????public?int?cellX;???//X方向的單元格索引?? ????????????/**? ?????????????*?Vertical?location?of?the?item?in?the?grid.? ?????????????*/?? ????????????public?int?cellY;???//Y方向的單元格索引?? ????????????/**? ?????????????*?Number?of?cells?spanned?horizontally?by?the?item.? ?????????????*/?? ????????????public?int?cellHSpan;??//水平方向所占高度?? ????????????/**? ?????????????*?Number?of?cells?spanned?vertically?by?the?item.? ?????????????*/?? ????????????public?int?cellVSpan;??//垂直方向所占高度?? ????????????...?? ????????????public?LayoutParams(Context?c,?AttributeSet?attrs)?{?? ????????????????super(c,?attrs);?? ????????????????cellHSpan?=?1;??//默認為高度?1?? ????????????????cellVSpan?=?1;?? ????????????}?? ?? ????????????public?LayoutParams(ViewGroup.LayoutParams?source)?{?? ????????????????super(source);?//默認為高度?1?? ????????????????cellHSpan?=?1;?? ????????????????cellVSpan?=?1;?? ????????????}?? ?????????????? ????????????public?LayoutParams(int?cellX,?int?cellY,?int?cellHSpan,?int?cellVSpan)?{?? ????????????????super(LayoutParams.MATCH_PARENT,?LayoutParams.MATCH_PARENT);?? ????????????????this.cellX?=?cellX;?? ????????????????this.cellY?=?cellY;?? ????????????????this.cellHSpan?=?cellHSpan;?? ????????????????this.cellVSpan?=?cellVSpan;?? ????????????}?? ????????????...?? ????????}?? ????...?? }??
? ? ?對該自定義CellLayout.LayoutParams類的使用可以參考LinearLayout.LayoutParams類，我也不再贅述了。
public?class?MainActivity?extends?Activity?{?? ????/**?Called?when?the?activity?is?first?created.?*/?? ????@Override?? ????public?void?onCreate(Bundle?savedInstanceState)?{?? ????????super.onCreate(savedInstanceState);?? ????????//1、該方法最終也會調用到?LayoutInflater的inflate()方法中去解析。?? ????????setContentView(R.layout.main);?? ?????????? ????????//2、使用常見的API方法去解析xml布局文件，?? ????????LayoutInflater?layoutInflater?=?(LayoutInflater)getSystemService();?? ????????View?root?=?layoutInflater.inflate(R.layout.main,?null);?? ????}?? }??
/**? ?*?Common?implementation?of?Context?API,?which?provides?the?base? ?*?context?object?for?Activity?and?other?application?components.? ?*/?? class?ContextImpl?extends?Context?{?? ????if?(WINDOW_SERVICE.equals(name))?{?? ????????return?WindowManagerImpl.getDefault();?? ????}?else?if?(LAYOUT_INFLATER_SERVICE.equals(name))?{?? ????????synchronized?(mSync)?{?? ????????????LayoutInflater?inflater?=?mLayoutInflater;?? ????????????//是否已經賦值，如果是，直接返回引用?? ????????????if?(inflater?!=?null)?{?? ????????????????return?inflater;?? ????????????}?? ????????????//返回一個LayoutInflater對象，getOuterContext()指的是我們的Activity、Service或者Application引用?? ????????????mLayoutInflater?=?inflater?=?PolicyManager.makeNewLayoutInflater(getOuterContext());?? ????????????return?inflater;?? ????????}?? ????}?else?if?(ACTIVITY_SERVICE.equals(name))?{?? ????????return?getActivityManager();?? ????}...?? }??
public?final?class?PolicyManager?{?? ????private?static?final?String?POLICY_IMPL_CLASS_NAME?=?"com.android.internal.policy.impl.Policy";?? ????private?static?final?IPolicy?sPolicy;???//?這可不是Binder機制額，這只是是一個接口，別想多啦?? ????static?{?? ????????//?Pull?in?the?actual?implementation?of?the?policy?at?run-time?? ????????try?{?? ????????????Class?policyClass?=?Class.forName(POLICY_IMPL_CLASS_NAME);?? ????????????sPolicy?=?(IPolicy)policyClass.newInstance();?? ????????}?? ????????...?? ????}?? ????...?? ????public?static?LayoutInflater?makeNewLayoutInflater(Context?context)?{?? ????????return?sPolicy.makeNewLayoutInflater(context);?//繼續去實現類中去查找?? ????}?? }??
? ? IPolicy接口的實現對為Policy類。路徑：/frameworks/base/policy/src/com/android/internal/policy/impl/Policy.java
//Simple?implementation?of?the?policy?interface?that?spawns?the?right?? //set?of?objects?? public?class?Policy?implements?IPolicy{?? ????...?? ????public?PhoneLayoutInflater?makeNewLayoutInflater(Context?context)?{?? ????????//實際上返回的是PhoneLayoutInflater類。?? ????????return?new?PhoneLayoutInflater(context);?? ????}?? }?? //PhoneLayoutInflater繼承至LayoutInflater類?? public?class?PhoneLayoutInflater?extends?LayoutInflater?{?? ????...?? ????/**? ?????*?Instead?of?instantiating?directly,?you?should?retrieve?an?instance? ?????*?through?{@link?Context#getSystemService}? ?????*?? ?????*?@param?context?The?Context?in?which?in?which?to?find?resources?and?other? ?????*????????????????application-specific?things.? ?????*?? ?????*?@see?Context#getSystemService? ?????*/?? ????public?PhoneLayoutInflater(Context?context)?{?? ????????super(context);?? ????}?? ????...?? }??
? ?Step 2、調用inflate()方法去解析布局文件。 [java] view plaincopyprint?public?abstract?class?LayoutInflater?{?? ????...?? ????public?View?inflate(int?resource,?ViewGroup?root)?{?? ????????//繼續看下個函數，注意root為null?? ????????return?inflate(resource,?root,?root?!=?null);??? ????}?? ?????? ????public?View?inflate(int?resource,?ViewGroup?root,?boolean?attachToRoot)?{?? ????????//獲取一個XmlResourceParser來解析XML文件---布局文件。?? ????????//XmlResourceParser類以及xml是如何解析的，大家自己有興趣找找。?? ????????XmlResourceParser?parser?=?getContext().getResources().getLayout(resource);?? ????????try?{?? ????????????return?inflate(parser,?root,?attachToRoot);?? ????????}?finally?{?? ????????????parser.close();?? ????????}?? ????}?? }?? /**? ?*?The?XML?parsing?interface?returned?for?an?XML?resource.??This?is?a?standard? ?*?XmlPullParser?interface,?as?well?as?an?extended?AttributeSet?interface?and? ?*?an?additional?close()?method?on?this?interface?for?the?client?to?indicate? ?*?when?it?is?done?reading?the?resource.? ?*/?? public?interface?XmlResourceParser?extends?XmlPullParser,?AttributeSet?{?? ????/**? ?????*?Close?this?interface?to?the?resource.??Calls?on?the?interface?are?no? ?????*?longer?value?after?this?call.? ?????*/?? ????public?void?close();?? }??
[java] view plaincopyprint?public?abstract?class?LayoutInflater?{?? ????...?? ????/**? ?????*?Inflate?a?new?view?hierarchy?from?the?specified?XML?node.?Throws? ?????*?{@link?InflateException}?if?there?is?an?error.? ?????*/?? ????//我們傳遞過來的參數如下：?root?為null?，?attachToRoot為false?。?? ????public?View?inflate(XmlPullParser?parser,?ViewGroup?root,?boolean?attachToRoot)?{?? ????????synchronized?(mConstructorArgs)?{?? ????????????final?AttributeSet?attrs?=?Xml.asAttributeSet(parser);?? ????????????Context?lastContext?=?(Context)mConstructorArgs[0];?? ????????????mConstructorArgs[0]?=?mContext;??//該mConstructorArgs屬性最后會作為參數傳遞給View的構造函數?? ????????????View?result?=?root;??//根View?? ?? ????????????try?{?? ????????????????//?Look?for?the?root?node.?? ????????????????int?type;?? ????????????????while?((type?=?parser.next())?!=?XmlPullParser.START_TAG?&&?? ????????????????????????type?!=?XmlPullParser.END_DOCUMENT)?{?? ????????????????????//?Empty?? ????????????????}?? ????????????????...?? ????????????????final?String?name?=?parser.getName();??//節點名，即API中的控件或者自定義View完整限定名。?? ????????????????if?(TAG_MERGE.equals(name))?{?//?處理<merge?/>標簽?? ????????????????????if?(root?==?null?||?!attachToRoot)?{?? ????????????????????????throw?new?InflateException("<merge?/>?can?be?used?only?with?a?valid?"?? ????????????????????????????????+?"ViewGroup?root?and?attachToRoot=true");?? ????????????????????}?? ????????????????????//將<merge?/>標簽的View樹添加至root中，該函數稍后講到。?? ????????????????????rInflate(parser,?root,?attrs);?? ????????????????}?else?{?? ????????????????????//?Temp?is?the?root?view?that?was?found?in?the?xml?? ????????????????????//創建該xml布局文件所對應的根View。?? ????????????????????View?temp?=?createViewFromTag(name,?attrs);??? ?? ????????????????????ViewGroup.LayoutParams?params?=?null;?? ?? ????????????????????if?(root?!=?null)?{?? ????????????????????????//?Create?layout?params?that?match?root,?if?supplied?? ????????????????????????//根據AttributeSet屬性獲得一個LayoutParams實例，記住調用者為root。?? ????????????????????????params?=?root.generateLayoutParams(attrs);??? ????????????????????????if?(!attachToRoot)?{?//重新設置temp的LayoutParams?? ????????????????????????????//?Set?the?layout?params?for?temp?if?we?are?not?? ????????????????????????????//?attaching.?(If?we?are,?we?use?addView,?below)?? ????????????????????????????temp.setLayoutParams(params);?? ????????????????????????}?? ????????????????????}?? ????????????????????//?Inflate?all?children?under?temp?? ????????????????????//添加所有其子節點，即添加所有字View?? ????????????????????rInflate(parser,?temp,?attrs);?? ?????????????????????? ????????????????????//?We?are?supposed?to?attach?all?the?views?we?found?(int?temp)?? ????????????????????//?to?root.?Do?that?now.?? ????????????????????if?(root?!=?null?&&?attachToRoot)?{?? ????????????????????????root.addView(temp,?params);?? ????????????????????}?? ????????????????????//?Decide?whether?to?return?the?root?that?was?passed?in?or?the?? ????????????????????//?top?view?found?in?xml.?? ????????????????????if?(root?==?null?||?!attachToRoot)?{?? ????????????????????????result?=?temp;?? ????????????????????}?? ????????????????}?? ????????????}??? ????????????...?? ????????????return?result;?? ????????}?? ????}?? ?????? ????/*? ?????*?default?visibility?so?the?BridgeInflater?can?override?it.? ?????*/?? ????View?createViewFromTag(String?name,?AttributeSet?attrs)?{?? ????????//節點是否為View，如果是將其重新賦值，形如?<View?class="com.qin.xxxView"></View>?? ????????if?(name.equals("view"))?{???? ????????????name?=?attrs.getAttributeValue(null,?"class");?? ????????}?? ????????try?{?? ????????????View?view?=?(mFactory?==?null)???null?:?mFactory.onCreateView(name,?? ????????????????????mContext,?attrs);??//沒有設置工廠方法?? ?? ????????????if?(view?==?null)?{?? ????????????????//通過這個判斷是Android?API的View，還是自定義View?? ????????????????if?(-1?==?name.indexOf('.'))?{?? ????????????????????view?=?onCreateView(name,?attrs);?//創建Android?API的View實例?? ????????????????}?else?{?? ????????????????????view?=?createView(name,?null,?attrs);//創建一個自定義View實例?? ????????????????}?? ????????????}?? ????????????return?view;?? ????????}??? ????????...?? ????}?? ????//獲得具體視圖的實例對象?? ????public?final?View?createView(String?name,?String?prefix,?AttributeSet?attrs)?{?? ????????Constructor?constructor?=?sConstructorMap.get(name);?? ????????Class?clazz?=?null;?? ????????//以下功能主要是獲取如下三個類對象：?? ????????//1、類加載器??ClassLoader?? ????????//2、Class對象?? ????????//3、類的構造方法句柄?Constructor?? ????????try?{?? ????????????if?(constructor?==?null)?{?? ????????????//?Class?not?found?in?the?cache,?see?if?it's?real,?and?try?to?add?it?? ????????????clazz?=?mContext.getClassLoader().loadClass(prefix?!=?null???(prefix?+?name)?:?name);?? ????????????...?? ????????????constructor?=?clazz.getConstructor(mConstructorSignature);?? ????????????sConstructorMap.put(name,?constructor);?? ????????}?else?{?? ????????????//?If?we?have?a?filter,?apply?it?to?cached?constructor?? ????????????if?(mFilter?!=?null)?{?? ????????????????...????? ????????????}?? ????????}?? ????????????//傳遞參數獲得該View實例對象?? ????????????Object[]?args?=?mConstructorArgs;?? ????????????args[1]?=?attrs;?? ????????????return?(View)?constructor.newInstance(args);?? ????????}??? ????????...?? ????}?? ?? }??
//我們傳遞過來的參數如下：?root?為null?，?attachToRoot為false?。?? public?View?inflate(XmlPullParser?parser,?ViewGroup?root,?boolean?attachToRoot)?{?? ????synchronized?(mConstructorArgs)?{?? ????????...?? ????????try?{?? ?????????????? ????????????...?? ????????????if?(TAG_MERGE.equals(name))?{?//?處理<merge?/>標簽?? ????????????????...?? ????????????}?else?{?? ????????????????//?Temp?is?the?root?view?that?was?found?in?the?xml?? ????????????????//創建該xml布局文件所對應的根View。?? ????????????????View?temp?=?createViewFromTag(name,?attrs);??? ????????????????ViewGroup.LayoutParams?params?=?null;?? ?? ????????????????//注意！！！?root為null時，temp變量的LayoutParams屬性不會被賦值的。?? ????????????????if?(root?!=?null)?{?? ????????????????????//?Create?layout?params?that?match?root,?if?supplied?? ????????????????????//根據AttributeSet屬性獲得一個LayoutParams實例，記住調用者為root。?? ????????????????????params?=?root.generateLayoutParams(attrs);??? ????????????????????if?(!attachToRoot)?{?//重新設置temp的LayoutParams?? ????????????????????????//?Set?the?layout?params?for?temp?if?we?are?not?? ????????????????????????//?attaching.?(If?we?are,?we?use?addView,?below)?? ????????????????????????temp.setLayoutParams(params);?? ????????????????????}?? ????????????????}?? ????????????????...?? ????????????}?? ????????}??? ????????...?? ????}?? }??
/**? ?*?Recursive?method?used?to?descend?down?the?xml?hierarchy?and?instantiate? ?*?views,?instantiate?their?children,?and?then?call?onFinishInflate().? ?*/?? //遞歸調用每個字節點?? private?void?rInflate(XmlPullParser?parser,?View?parent,?final?AttributeSet?attrs)?? ????????throws?XmlPullParserException,?IOException?{?? ?? ????final?int?depth?=?parser.getDepth();?? ????int?type;?? ?? ????while?(((type?=?parser.next())?!=?XmlPullParser.END_TAG?||?? ????????????parser.getDepth()?>?depth)?&&?type?!=?XmlPullParser.END_DOCUMENT)?{?? ?? ????????if?(type?!=?XmlPullParser.START_TAG)?{?? ????????????continue;?? ????????}?? ????????final?String?name?=?parser.getName();?? ?????????? ????????if?(TAG_REQUEST_FOCUS.equals(name))?{?//處理<requestFocus?/>標簽?? ????????????parseRequestFocus(parser,?parent);?? ????????}?else?if?(TAG_INCLUDE.equals(name))?{?//處理<include?/>標簽?? ????????????if?(parser.getDepth()?==?0)?{?? ????????????????throw?new?InflateException("<include?/>?cannot?be?the?root?element");?? ????????????}?? ????????????parseInclude(parser,?parent,?attrs);//解析<include?/>節點?? ????????}?else?if?(TAG_MERGE.equals(name))?{?//處理<merge?/>標簽?? ????????????throw?new?InflateException("<merge?/>?must?be?the?root?element");?? ????????}?else?{?? ????????????//根據節點名構建一個View實例對象?? ????????????final?View?view?=?createViewFromTag(name,?attrs);??? ????????????final?ViewGroup?viewGroup?=?(ViewGroup)?parent;?? ????????????//調用generateLayoutParams()方法返回一個LayoutParams實例對象，?? ????????????final?ViewGroup.LayoutParams?params?=?viewGroup.generateLayoutParams(attrs);?? ????????????rInflate(parser,?view,?attrs);?//繼續遞歸調用?? ????????????viewGroup.addView(view,?params);?//OK，將該View以特定LayoutParams值添加至父View中?? ????????}?? ????}?? ????parent.onFinishInflate();??//完成了解析過程，通知....?? }??
public?abstract?class?ViewGroup?extends?View?implements?ViewParent,?ViewManager?{?? ????...?? ?????? ????public?LayoutParams?generateLayoutParams(AttributeSet?attrs)?{?? ????????return?new?LayoutParams(getContext(),?attrs);?? ????}?? ????public?static?class?LayoutParams?{?? ????????...?//會調用這個構造函數?? ????????public?LayoutParams(Context?c,?AttributeSet?attrs)?{?? ????????????TypedArray?a?=?c.obtainStyledAttributes(attrs,?R.styleable.ViewGroup_Layout);?? ????????????setBaseAttributes(a,?? ????????????????????R.styleable.ViewGroup_Layout_layout_width,?? ????????????????????R.styleable.ViewGroup_Layout_layout_height);?? ????????????a.recycle();?? ????????}?? ????????protected?void?setBaseAttributes(TypedArray?a,?int?widthAttr,?int?heightAttr)?{?? ????????????width?=?a.getLayoutDimension(widthAttr,?"layout_width");?? ????????????height?=?a.getLayoutDimension(heightAttr,?"layout_height");?? ????????}?? ?????? }??
public?class?TypedArray?{?? ????...?? ????/**? ?????*?Special?version?of?{@link?#getDimensionPixelSize}?for?retrieving? ?????*?{@link?android.view.ViewGroup}'s?layout_width?and?layout_height? ?????*?attributes.??This?is?only?here?for?performance?reasons;?applications? ?????*?should?use?{@link?#getDimensionPixelSize}.? ?????*?? ?????*?@param?index?Index?of?the?attribute?to?retrieve.? ?????*?@param?name?Textual?name?of?attribute?for?error?reporting.? ?????*?? ?????*?@return?Attribute?dimension?value?multiplied?by?the?appropriate?? ?????*?metric?and?truncated?to?integer?pixels.? ?????*/?? ????public?int?getLayoutDimension(int?index,?String?name)?{?? ????????index?*=?AssetManager.STYLE_NUM_ENTRIES;?? ????????final?int[]?data?=?mData;?? ????????//獲得屬性對應的標識符?，?Identifies，目前還沒有仔細研究相關類。?? ????????final?int?type?=?data[index+AssetManager.STYLE_TYPE];?? ????????if?(type?>=?TypedValue.TYPE_FIRST_INT?? ????????????????&&?type?<=?TypedValue.TYPE_LAST_INT)?{?? ????????????return?data[index+AssetManager.STYLE_DATA];?? ????????}?else?if?(type?==?TypedValue.TYPE_DIMENSION)?{?//類型為dimension類型?? ????????????return?TypedValue.complexToDimensionPixelSize(?? ????????????????data[index+AssetManager.STYLE_DATA],?mResources.mMetrics);?? ????????}?? ????????//沒有提供layout_weight和layout_height會來到此處?，這兒會報異常！?? ????????//因此布局文件中的View包括自定義View必須加上屬性layout_weight和layout_height。?? ????????throw?new?RuntimeException(getPositionDescription()?? ????????????????+?":?You?must?supply?a?"?+?name?+?"?attribute.");?? ????}?? ????...?? }??
? ? ? ? ?從上面得知， ? 我們將View的AttributeSet屬性傳遞給generateLayoutParams()方法，讓其構建合適地
? ? ? ?首先，獲得了了布局文件地root View，即布局文件中最頂層的View。
? 繪制流程的三個步驟，即：
? ? ? ? ? ? ? ? ? ? ? 1、 ?measure過程 --- 測量過程
? ? ? ? ? ? ? ? ? ? ? 2、 layout 過程 ? ? --- 布局過程? ? ? ? ? ? ? ? ? ? ? 3、 draw 過程 ? ? ?--- 繪制過程
? ? ? 要想對Android 中View這塊深入理解，對這三個步驟地學習是必不可少的 。
? ? ? 今天，我著重講解下如下三個內容：
? ? ? ? ? ? 1、?measure過程
? ? ? ? ? ? 2、WRAP_CONTENT、MATCH_PARENT/FILL_PARENT屬性的原理說明
? ? ? ? ? ? 3、xml布局文件解析成View樹的流程分析。
? ? ?希望對大家能有幫助。- -??分析版本基于Android 2.3 。
?1、WRAP_CONTENT、MATCH_PARENT/FILL_PARENT?
? ? ? ?初入Android殿堂的同學們，對這三個屬性一定又愛又恨。愛的是使用起來挺爽地---照葫蘆畫瓢即可，恨的
? 卻是時常混淆這幾個屬性地意義，需要三思而后行。在帶著大家重溫下這幾個屬性的用法吧(希望我沒有啰嗦)。
? ? ? 這三個屬性都用來適應視圖的水平或垂直大小，一個以視圖的內容或尺寸為基礎的布局比精確地指定視圖范圍
? 更加方便。
? ? ? ? ① ?fill_parent? ? ? ? ? ? ? ? 設置一個視圖的布局為fill_parent將強制性地使視圖擴展至父元素大小。
? ? ? ? ② match_parent
? ? ? ? ? ? ? ?Android 中match_parent和fill_parent意思一樣，但match_parent更貼切，于是從2.2開始兩個詞都可以
? ? ? ? ? 用，但2.3版本后建議使用match_parent。
? ? ? ?③ wrap_content
? ? ? ? ? ? ? 自適應大小，強制性地使視圖擴展以便顯示其全部內容。以TextView和ImageView控件為例，設置為
? ? ? ? ?wrap_content將完整顯示其內部的文本和圖像。布局元素將根據內容更改大小。
? ? ? ?
? ? ? 可不要重復造輪子，以上摘自<<Android?fill_parent、wrap_content和match_parent的區別>>。
? ? ? 當然，我們可以設置View的確切寬高，而不是由以上屬性指定。
[java] view plaincopyprint?
? ? ? 接下來，我們需要轉換下視角，看看ViewGroup.LayoutParams類及其派生類。
?2、ViewGroup.LayoutParams類及其派生類
? ? 2.1、 ?ViewGroup.LayoutParams類說明
? ? ? ? ? ? Android API中如下介紹：? ? ? ? ? ? ??? LayoutParams are used by views to tell their parents how they want to be laid out.
? ? ?意思大概是說：?View通過LayoutParams類告訴其父視圖它想要地大小(即，長度和寬度)。
? ? 因此，每個View都包含一個ViewGroup.LayoutParams類或者其派生類，View類依賴于ViewGroup.LayoutParams。
? ? ? ? 路徑：frameworks\base\core\java\android\view\View.java
[java] view plaincopyprint?? ? ?2.2、 ?ViewGroup.LayoutParams源碼分析
? ? ? 路徑位于：frameworks\base\core\java\android\view\ViewGroup.java
[java] view plaincopyprint?
? ? ? ?我們發現FILL_PARENT/MATCH_PARENT值為 -1 ，WRAP_CONETENT值為-2，是不是有點詫異？ 將值
? 設置為負值的目的是為了區別View的具體值(an exact size) 總是大于0的。
? ? ? ?ViewGroup子類可以實現自定義LayoutParams，自定義LayoutParams提供了更好地擴展性，例如LinearLayout
?就有LinearLayout.?LayoutParams自定義類(見下文)。整個LayoutParams類家族還是挺復雜的。
? ? ? ViewGroup.LayoutParams及其常用派生類的類圖(部分類圖)如下：
? ? ? ? ? ? ? ? ??
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 該類圖是在太龐大了，大家有興趣的去看看Android API吧。
? ? ? ? ? ?
? ? ? 前面我們說過，每個View都包含一個ViewGroup.LayoutParams類或者其派生類，下面我們的疑問是Android框架
?中時如何為View設置其LayoutParams屬性的。
? ? ?有兩種方法會設置View的LayoutParams屬性：
? ? ? ?1、 直接添加子View時，常見于如下幾種方法：ViewGroup.java
[java] view plaincopyprint?? ? ? ? ?三個重載方法的區別只是添加View時構造LayoutParams對象的方式不同而已，稍后我們探尋一下它們的源碼。
? ? ? 2、 通過xml布局文件指定某個View的屬性為：android:layout_heigth=””以及android:layout_weight=”” 時。
? ? 總的來說，這兩種方式都會設定View的LayoutParams屬性值----指定的或者Default值。
? 方式1流程分析：
? ? ?直接添加子View時，比較容易理解，我們先來看看這種方式設置LayoutParams的過程：
? ? ? ? ?路徑：\frameworks\base\core\java\android\view\ViewGroup.java
[java] view plaincopyprint?
? ? ? 主要功能就是在添加子View時為其構建了一個LayoutParams對象。但更重要的是，ViewGroup的子類可以重載
?上面的幾個方法，返回特定的LayoutParams對象，例如：對于LinearLayout而言，則是LinearLayout.LayoutParams
?對象。這么做地目的是，能在其他需要它的地方，可以將其強制轉換成LinearLayout.LayoutParams對象。
? ? ??LinearLayout重寫函數地實現為：
[java] view plaincopyprint?
? ? ? ?LinearLayout.LayoutParams類繼承至ViewGroup.MarginLayoutParams類，添加了對android:layout_weight以及
? ?android:layout_gravity這兩個屬性的獲取和保存。而且它的重寫函數返回的都是LinearLayout.LayoutParams
? ?類型。這樣，我們可以再對子View進行其他操作時，可以將將其強制轉換成LinearLayout.LayoutParams對象進行
? ?使用。
? ? ? ? ?例如，LinearLayout進行measure過程，使用了LinearLayout.LayoutParam對象，有如下代碼：
[java] view plaincopyprint?
? ? ? ? 超類ViewGroup.LayoutParams強制轉換為了子類LinearLayout.LayoutParams，因為LinearLayout的每個
? ”直接“子View的LayoutParams屬性都是LinearLayout.LayoutParams類型，因此可以安全轉換。
? ? ? ?PS : Android 2.3源碼Launcher2中也實現了自定義的LayoutParams類，在IDLE界面的每個View至少包含如下
? 信息：所在X方向的單元格索引和高度、所在Y方向的單元格索引和高度等。
? ? ? ? ? ??路徑:?packages\apps\Launcher2\src\com\android\launcher2\CellLayout.java
[java] view plaincopyprint?
? ? ?對該自定義CellLayout.LayoutParams類的使用可以參考LinearLayout.LayoutParams類，我也不再贅述了。
?方法2流程分析：
? ? ? ? 使用屬性android:layout_heigth=””以及android:layout_weight=”” 時，為某個View設置LayoutParams值。
?
? ? ? ?其實這種賦值方法其實也如同前面那種，只不過它需要一個前期孵化過程---需要利用XML解析將布局文件
? 解析成一個完整的View樹，可別小看它了，所有Xxx.xml的布局文件都需要解析成一個完整的View樹。下面，
? 我們就來仔細走這個過程，重點關注如下兩個方面
? ? ? ? ?①、xml布局是如何解析成View樹的 ;
? ? ? ? ?②、android:layout_heigth=””和android:layout_weight=””的解析。
? ? ? ? PS： 一直以來，我都想當然android:layout_heigth以及android:layout_weight這兩個屬性的解析過程是在
? ?View.java內部完成的，但當我真正去找尋時，卻一直沒有在View.java類或者ViewGroup.java類找到。直到一位
? ?網友的一次提問，才發現它們的藏身之地。
3、布局文件解析流程分析
? ? ? ?解析布局文件時，使用的類為LayoutInflater。 關于該類的使用請參考如下博客：
? ? ? ? ? ? ? ? ? ? ? ? ??? <android中LayoutInflater的使用?>>
? ? ? 主要有如下API方法：
? ? ? ? ?public?View?inflate?(XmlPullParser?parser,?ViewGroup?root, boolean attachToRoot)? ? ? ? ??public?View?inflate?(int resource,?ViewGroup?root)
? ? ? ? ??public?View?inflate?(int resource,?ViewGroup?root, boolean attachToRoot)
? ? ?這三個類主要迷惑之處在于地三個參數attachToRoot，即是否將該View樹添加到root中去。具體可看這篇博客：
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?<<關于inflate的第3個參數>>
? ? 當然還有LayoutInflater的inflate()的其他重載方法，大家可以自行了解下。
? ? ?
? ? 我利用下面的例子給大家走走這個流程 ：
[java] view plaincopyprint?
? ?Step 1、獲得LayoutInflater的引用。
? ? ? ? ?路徑：\frameworks\base\core\java\android\app\ContextImpl.java
[java] view plaincopyprint?? ? ? ? ?繼續去PolicyManager查詢對應函數，看看內部實現。 ? ?
? ? ? ? ? ?路徑：frameworks\base\core\java\com\android\internal\policy\PolicyManager.java
[java] view plaincopyprint?
[java] view plaincopyprint?
? ? ? ?LayoutInflater是個抽象類，實際上我們返回的是PhoneLayoutInflater類，但解析過程的操作基本上是在
? LayoutInflater中完成地。
? ?Step 2、調用inflate()方法去解析布局文件。 [java] view plaincopyprint?
? ? ? 我們獲得了一個當前應用程序環境的XmlResourceParser對象，該對象的主要作用就是來解析xml布局文件的。
??XmlResourceParser類是個接口類，更多關于XML解析的，大家可以參考下面博客：
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? <<android之XmlResourceParser類使用實例>>
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? <<android解析xml文件的方式(其一)>>
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ??<<android解析xml文件的方式(其二)>>
?? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?<<android解析xml文件的方式(其三)>>
[java] view plaincopyprint?
? ?? 這段代碼的作用是獲取xml布局文件的root View，做了如下兩件事情
? ? ? ? ? 1、獲取xml布局的View實例，通過createViewFromTag()方法獲取，該方法會判斷節點名是API 控件
? ? ? ? ? ? 還是自定義控件，繼而調用合適的方法去實例化View。
? ? ? ? ? 2、判斷root以及attachToRoot參數，重新設置root View值以及temp變量的LayoutParams值。
? ? ?? ?如果仔細看著段代碼，不知大家心里有沒有疑惑：當root為null時，我們的temp變量的LayoutParams值是為
? null的，即它不會被賦值？有個View的LayoutParams值為空，那么，在系統中不會報異常嗎？見下面部分
? 代碼：
[java] view plaincopyprint?? ? ? ? 關于這個問題的詳細答案，我會在后面講到。這兒我簡單說下，任何View樹的頂層View被添加至窗口時，
? 一般調用WindowManager.addView()添加至窗口時，在這個方法中去做進一步處理。即使，LayoutParams
? 值為空，UI框架每次measure()時都忽略該View的LayoutParams值，而是直接傳遞MeasureSpec值至View樹。
??? ? ?接下來，我們關注另外一個函數，rInflate()，該方法會遞歸調用每個View下的子節點，以當前View作為根View
?形成一個View樹。
[java] view plaincopyprint?? ? ? ? ? 值得注意的是，每次addView前都調用了viewGroup.generateLayoutParams(attrs)去構建一個LayoutParams
? 實例，然后在addView()方法中為其賦值。參見如下代碼：ViewGroup.java
??
[java] view plaincopyprint?? ??好吧 ~~ 我們還是探尋根底，去TypeArray類的getLayoutDimension()看看。
? ? ? ? ?路徑：/frameworks/base/core/java/android/content/res/TypedArray.java
[java] view plaincopyprint?
? ? ? ? ?從上面得知， ? 我們將View的AttributeSet屬性傳遞給generateLayoutParams()方法，讓其構建合適地
? ?LayoutParams對象，并且初始化屬性值weight和height。同時我們也得知?布局文件中的View包括自定義View
? ?必須加上屬性layout_weight和layout_height，否則會報異常。
? ? ? ?首先，獲得了了布局文件地root View，即布局文件中最頂層的View。
? ? ? ?其次，通過遞歸調用，我們形成了整個View樹以及設置了每個View的LayoutParams對象。
? ??
? ??總結：通過對布局文件的解析流程的學習，也就是轉換為View樹的過程，我們明白了解析過程的個中奧妙，以及
設置ViewLayoutParams對象的過程。但是，我們這兒只是簡單的浮光掠影，更深層次的內容希望大家能深入學習。
? ? ? 本來是準備接下去往下寫的，但無奈貼出來的代碼太多，文章有點長而且自己也有點凌亂了，因此決定做兩篇
? 博客發表吧。下篇內容包括如下方面：
? ? ? ? 1、MeasureSpec類說明 ;
? ? ? ? 2、measure過程中如何正確設置每個View的長寬 ;
? ? ? ? 3、UI框架正確設置頂層View的LayoutParams對象，對Activity而言，頂層View則是DecorView，
? ?其他的皆是普通View了。
總結
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