一、MeasureSpc類說明
SDK的介紹:MeasureSpc類封裝了父View傳遞給子View的佈局(layout)要求。每個MeasureSpc實例代表寬度或者高度
它有三種模式:
①、UNSPECIFIED(未指定),父元素部隊自元素施加任何束縛,子元素可以得到任意想要的大小;
②、EXACTLY(完全),父元素決定自元素的確切大小,子元素將被限定在給定的邊界裏而忽略它本身大小;
③、AT_MOST(至多),子元素至多達到指定大小的值。
常用的三個函數:
static int getMode(int measureSpec) : 根據提供的測量值(格式),提取模式(上述三個模式之一)
static int getSize(int measureSpec) : 根據提供的測量值(格式),提取大小值(這個大小也就是我們通常所說的大小)
static int makeMeasureSpec(int size,int mode) : 根據提供的大小值和模式,創建一個測量值(格式)
MeasureSpc類源碼分析 其爲View.java類的內部類,路徑:frameworksbasecorejavaandroidviewView.java
public class View implements ... {
...
public static class MeasureSpec {
private static final int MODE_SHIFT = 30; //移位位數爲30
//int類型佔32位,向右移位30位,該屬性表示掩碼值,用來與size和mode進行"&"運算,獲取對應值。
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
//向右移位30位,其值爲00 + (30位0) , 即 0x0000(16進製表示)
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
//向右移位30位,其值爲01 + (30位0) , 即0x1000(16進製表示)
public static final int EXACTLY = 1 << MODE_SHIFT;
//向右移位30位,其值爲02 + (30位0) , 即0x2000(16進製表示)
public static final int AT_MOST = 2 << MODE_SHIFT;
//創建一個整形值,其高兩位代表mode類型,其餘30位代表長或寬的實際值。可以是WRAP_CONTENT、MATCH_PARENT或具體大小exactly size
public static int makeMeasureSpec(int size, int mode) {
return size + mode;
}
//獲取模式 ,與運算
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
//獲取長或寬的實際值 ,與運算
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
}
...
MeasureSpec類的處理思路是:
右移運算,使int 類型的高兩位表示模式的實際值,其餘30位表示其餘30位代表長或寬的實際值----可以是WRAP_CONTENT、MATCH_PARENT或具體大小exactly size。 通過掩碼MODE_MASK進行與運算 “&”,取得模式(mode)以及長或寬(value)的實際值。
MeasureSpec.makeMeasureSpec方法,實際上這個方法很簡單:
public static int makeMeasureSpec(int size, int mode) {
return size + mode;
}
其用法如下:
int w = View.MeasureSpec.makeMeasureSpec(0,View.MeasureSpec.UNSPECIFIED);
int h = View.MeasureSpec.makeMeasureSpec(0,View.MeasureSpec.UNSPECIFIED);
ssidtext.measure(w, h);
int width =ssidtext.getMeasuredWidth();
int height =ssidtext.getMeasuredHeight();
二、measure過程詳解
UI框架開始繪製時,皆是從ViewRoot.java類開始繪製的:
ViewRoot類簡要說明: 任何顯示在設備中的窗口,例如:Activity、Dialog等,都包含一個ViewRoot實例,該類主要用來與遠端 WindowManagerService交互以及控制(開始/銷燬)繪製。
1、開始UI繪製 , 具體繪製方法則是:
//開始View繪製流程
private void performTraversals(){
...
//這兩個值我們在後面討論時,在回過頭來看看是怎麼賦值的。現在只需要記住其值MeasureSpec.makeMeasureSpec()構建的。
int childWidthMeasureSpec; //其值由MeasureSpec類構建 , makeMeasureSpec
int childHeightMeasureSpec;//其值由MeasureSpec類構建 , makeMeasureSpec
// Ask host how big it wants to be
host.measure(childWidthMeasureSpec, childHeightMeasureSpec);
...
}
...
2、調用measure()方法去做一些前期準備 measure()方法原型定義在View.java類中,final修飾符修飾,其不能被重載:
public class View implements ... {
...
/**
* This is called to find out how big a view should be. The parent
* supplies constraint information in the width and height parameters.
*
* @param widthMeasureSpec Horizontal space requirements as imposed by the
* parent
* @param heightMeasureSpec Vertical space requirements as imposed by the
* parent
* @see #onMeasure(int, int)
*/
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
//判斷是否爲強制佈局,即帶有“FORCE_LAYOUT”標記 以及 widthMeasureSpec或heightMeasureSpec發生了改變
if ((mPrivateFlags & FORCE_LAYOUT) == FORCE_LAYOUT ||
widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec) {
// first clears the measured dimension flag
//清除MEASURED_DIMENSION_SET標記 ,該標記會在onMeasure()方法後被設置
mPrivateFlags &= ~MEASURED_DIMENSION_SET;
// measure ourselves, this should set the measured dimension flag back
// 1、 測量該View本身的大小 ; 2 、 設置MEASURED_DIMENSION_SET標記,否則接寫來會報異常。
onMeasure(widthMeasureSpec, heightMeasureSpec);
// flag not set, setMeasuredDimension() was not invoked, we raise
// an exception to warn the developer
if ((mPrivateFlags & MEASURED_DIMENSION_SET) != MEASURED_DIMENSION_SET) {
throw new IllegalStateException("onMeasure() did not set the"
+ " measured dimension by calling" + " setMeasuredDimension()");
}
mPrivateFlags |= LAYOUT_REQUIRED; //下一步是layout了,添加LAYOUT_REQUIRED標記
}
mOldWidthMeasureSpec = widthMeasureSpec; //保存值
mOldHeightMeasureSpec = heightMeasureSpec; //保存值
}
...
參數widthMeasureSpec和heightMeasureSpec 由父View構建,表示父View給子View的測量要求。其值地構建如下: measure()方法顯示判斷是否需要重新調用設置改View大小,即調用onMeasure()方法,然後操作兩個標識符: ①、重置MEASURED_DIMENSION_SET : onMeasure()方法中,需要添加該標識符,否則,會報異常; ②、添加LAYOUT_REQUIRED : 表示需要進行layout操作。最後,保存當前的widthMeasureSpec和heightMeasureSpec值。
3、調用onMeasure()方法去真正設置View的長寬值,其默認實現爲:
//設置該View本身地大小
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
//@param size參數一般表示設置了android:minHeight屬性或者該View背景圖片的大小值
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
//根據不同的mode值,取得寬和高的實際值。
switch (specMode) {
case MeasureSpec.UNSPECIFIED: //表示該View的大小父視圖未定,設置爲默認值
result = size;
break;
case MeasureSpec.AT_MOST: //表示該View的大小由父視圖指定了
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
//獲得設置了android:minHeight屬性或者該View背景圖片的大小值, 最爲該View的參考值
protected int getSuggestedMinimumWidth() {
int suggestedMinWidth = mMinWidth; // android:minHeight
if (mBGDrawable != null) { // 背景圖片對應地Width。
final int bgMinWidth = mBGDrawable.getMinimumWidth();
if (suggestedMinWidth < bgMinWidth) {
suggestedMinWidth = bgMinWidth;
}
}
return suggestedMinWidth;
}
//設置View在measure過程中寬和高
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= MEASURED_DIMENSION_SET; //設置了MEASURED_DIMENSION_SET標記
}
主要功能就是根據該View屬性(android:minWidth和背景圖片大小)和父View對該子View的"測量要求",設置該View的 mMeasuredWidth 和 mMeasuredHeight 值。
這兒只是一般的View類型地實現方法。一般來說,父View,也就是ViewGroup類型,都需要在重寫onMeasure()方法,遍歷所有子View,設置每個子View的大小。
基本思想如下:遍歷所有子View,設置每個子View的大小。僞代碼表示爲:
//某個ViewGroup類型的視圖
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//必須調用super.ononMeasure()或者直接調用setMeasuredDimension()方法設置該View大小,否則會報異常。
super.onMeasure(widthMeasureSpec , heightMeasureSpec)
//遍歷每個子View
for(int i = 0 ; i < getChildCount() ; i++){
View child = getChildAt(i);
//調用子View的onMeasure,設置他們的大小。childWidthMeasureSpec , childHeightMeasureSpec ?
child.onMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
如何去設置每個子View的大小,基本思想也如同我們之前描述的思想:遍歷所有子View,設置每個子View的大小。
//widthMeasureSpec 和 heightMeasureSpec 表示該父View的佈局要求
//遍歷每個子View,然後調用measureChild()方法去實現每個子View大小
protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) != GONE) { // 不處於 “GONE” 狀態
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}
//測量每個子View高寬時,清楚了該View本身的邊距大小,即android:padding屬性 或android:paddingLeft等屬性標記
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams(); // LayoutParams屬性
//設置子View的childWidthMeasureSpec屬性,去除了該父View的邊距值 mPaddingLeft + mPaddingRight
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
//設置子View的childHeightMeasureSpec屬性,去除了該父View的邊距值 mPaddingTop + mPaddingBottom
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
measureChildren()方法:遍歷所有子View,調用measureChild()方法去設置該子View的屬性值。
measureChild() 方法 : 獲取特定子View的widthMeasureSpec、heightMeasureSpec,調用measure()方法設置子View的實際寬高值。
getChildMeasureSpec()就是獲取子View的widthMeasureSpec、heightMeasureSpec值。
// spec參數 表示該父View本身所佔的widthMeasureSpec 或 heightMeasureSpec值
// padding參數 表示該父View的邊距大小,見於android:padding屬性 或android:paddingLeft等屬性標記
// childDimension參數 表示該子View內部LayoutParams屬性的值,可以是wrap_content、match_parent、一個精確指(an exactly size),
// 例如:由android:width指定等。
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec); //獲得父View的mode
int specSize = MeasureSpec.getSize(spec); //獲得父View的實際值
int size = Math.max(0, specSize - padding); //父View爲子View設定的大小,減去邊距值,
int resultSize = 0; //子View對應地 size 實際值 ,由下面的邏輯條件賦值
int resultMode = 0; //子View對應地 mode 值 , 由下面的邏輯條件賦值
switch (specMode) {
// Parent has imposed an exact size on us
//1、父View是EXACTLY的 !
case MeasureSpec.EXACTLY:
//1.1、子View的width或height是個精確值 (an exactly size)
if (childDimension >= 0) {
resultSize = childDimension; //size爲精確值
resultMode = MeasureSpec.EXACTLY; //mode爲 EXACTLY 。
}
//1.2、子View的width或height爲 MATCH_PARENT/FILL_PARENT
else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size; //size爲父視圖大小
resultMode = MeasureSpec.EXACTLY; //mode爲 EXACTLY 。
}
//1.3、子View的width或height爲 WRAP_CONTENT
else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size; //size爲父視圖大小
resultMode = MeasureSpec.AT_MOST; //mode爲AT_MOST 。
}
break;
// Parent has imposed a maximum size on us
//2、父View是AT_MOST的 !
case MeasureSpec.AT_MOST:
//2.1、子View的width或height是個精確值 (an exactly size)
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension; //size爲精確值
resultMode = MeasureSpec.EXACTLY; //mode爲 EXACTLY 。
}
//2.2、子View的width或height爲 MATCH_PARENT/FILL_PARENT
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; //size爲父視圖大小
resultMode = MeasureSpec.AT_MOST; //mode爲AT_MOST
}
//2.3、子View的width或height爲 WRAP_CONTENT
else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size; //size爲父視圖大小
resultMode = MeasureSpec.AT_MOST; //mode爲AT_MOST
}
break;
// Parent asked to see how big we want to be
//3、父View是UNSPECIFIED的 !
case MeasureSpec.UNSPECIFIED:
//3.1、子View的width或height是個精確值 (an exactly size)
if (childDimension >= 0) {
// Child wants a specific size... let him have it
resultSize = childDimension; //size爲精確值
resultMode = MeasureSpec.EXACTLY; //mode爲 EXACTLY
}
//3.2、子View的width或height爲 MATCH_PARENT/FILL_PARENT
else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
resultSize = 0; //size爲0! ,其值未定
resultMode = MeasureSpec.UNSPECIFIED; //mode爲 UNSPECIFIED
}
//3.3、子View的width或height爲 WRAP_CONTENT
else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = 0; //size爲0! ,其值未定
resultMode = MeasureSpec.UNSPECIFIED; //mode爲 UNSPECIFIED
}
break;
}
//根據上面邏輯條件獲取的mode和size構建MeasureSpec對象。
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
每個View大小的設定都事由其父View以及該View共同決定的。但這只是一個期望的大小,每個View在測量時最終大小的設定是由setMeasuredDimension()最終決定的。因此,最終確定一個View的“測量長寬“是由以下幾個方面影響:
1、父View的MeasureSpec屬性; 2、子View的LayoutParams屬性 ; 3、setMeasuredDimension()或者其它類似設定 mMeasuredWidth 和 mMeasuredHeight 值的方法。
//設置View在measure過程中寬和高
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= MEASURED_DIMENSION_SET; //設置了MEASURED_DIMENSION_SET標記
}
本文出自:http://www.cnblogs.com/nanxiaojue/p/3536381.html?utm_source=tuicool