前言:
通過Android 自定義View及其在布局文件中的使用示例和Android 自定義View及其在布局文件中的使用示例(二),我們知道了如何使用自定義的View,以及Android繪制View的理論基礎,其包含三個過程,測量View大小(通過onMeasure()方法實現),計算View位置(通過onLayout()方法實現),最後開始繪制(通過onDraw()方法實現),本篇,我們將結合Android 4.4.2_r1源碼詳細分析測量過程的具體實現.
在第一篇裡,我們提供了一個自定義的View的源代碼,現在引用一下該代碼與測量相關的部分:
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1 @Override
2 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
3 setMeasuredDimension(measureWidth(widthMeasureSpec),
4 measureHeight(heightMeasureSpec));
5 }
6
7 /**
8 * Determines the width of this view
9 *
10 * @param measureSpec
11 * A measureSpec packed into an int
12 * @return The width of the view, honoring constraints from measureSpec
13 */
14 private int measureWidth(int measureSpec) {
15 int result = 0;
16 int specMode = MeasureSpec.getMode(measureSpec);
17 int specSize = MeasureSpec.getSize(measureSpec);
18
19 if (specMode == MeasureSpec.EXACTLY) {
20 // We were told how big to be
21 result = specSize;
22 } else {
23 // Measure the text
24 result = (int) mTextPaint.measureText(mText) + getPaddingLeft()
25 + getPaddingRight();
26 if (specMode == MeasureSpec.AT_MOST) {
27 // Respect AT_MOST value if that was what is called for by
28 // measureSpec
29 result = Math.min(result, specSize);
30 }
31 }
32
33 return result;
34 }
35
36 /**
37 * Determines the height of this view
38 *
39 * @param measureSpec
40 * A measureSpec packed into an int
41 * @return The height of the view, honoring constraints from measureSpec
42 */
43 private int measureHeight(int measureSpec) {
44 int result = 0;
45 int specMode = MeasureSpec.getMode(measureSpec);
46 int specSize = MeasureSpec.getSize(measureSpec);
47
48 mAscent = (int) mTextPaint.ascent();
49 if (specMode == MeasureSpec.EXACTLY) {
50 // We were told how big to be
51 result = specSize;
52 } else {
53 // Measure the text (beware: ascent is a negative number)
54 result = (int) (-mAscent + mTextPaint.descent()) + getPaddingTop()
55 + getPaddingBottom();
56 if (specMode == MeasureSpec.AT_MOST) {
57 // Respect AT_MOST value if that was what is called for by
58 // measureSpec
59 result = Math.min(result, specSize);
60 }
61 }
62 return result;
63 }
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我們可以看到:protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)是一個override的方法,它接收兩個參數,通過字面意思,我們知道,這兩個參數分別為寬度測量規格,高度測量規格,此時,我們會有一個疑問,這兩個參數是從哪裡來的?這個疑問咱們先記下來,給它編個號:Q01,暫時略過,到本文下一部分,我們就知道它的來龍去脈了.接著,我們來看onMeasure方法在本地的實現:
setMeasuredDimension(measureWidth(widthMeasureSpec),measureHeight(heightMeasureSpec));
我們跟進setMeasuredDimension(int,int)方法,看看它到底都做了些什麼事情:
因為我們自定義的View是繼承自View,所以我們進入View.java(源碼位置:/frameworks/base/core/java/android/view/View.java)去看看有沒有這個方法:
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16575 /**
16576 * <p>This method must be called by {@link #onMeasure(int, int)} to store the
16577 * measured width and measured height. Failing to do so will trigger an
16578 * exception at measurement time.</p>
16579 *
16580 * @param measuredWidth The measured width of this view. May be a complex
16581 * bit mask as defined by {@link #MEASURED_SIZE_MASK} and
16582 * {@link #MEASURED_STATE_TOO_SMALL}.
16583 * @param measuredHeight The measured height of this view. May be a complex
16584 * bit mask as defined by {@link #MEASURED_SIZE_MASK} and
16585 * {@link #MEASURED_STATE_TOO_SMALL}.
16586 */
16587 protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
16588 boolean optical = isLayoutModeOptical(this);
16589 if (optical != isLayoutModeOptical(mParent)) {
16590 Insets insets = getOpticalInsets();
16591 int opticalWidth = insets.left + insets.right;
16592 int opticalHeight = insets.top + insets.bottom;
16593
16594 measuredWidth += optical ? opticalWidth : -opticalWidth;
16595 measuredHeight += optical ? opticalHeight : -opticalHeight;
16596 }
16597 mMeasuredWidth = measuredWidth;
16598 mMeasuredHeight = measuredHeight;
16599
16600 mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
16601 }
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果然,我們在View.java中找到了這個方法的具體實現,通過方法說明,得知此方法必須被onMeasure()方法調用 ,來保存測量到的寬度和高度,否則的話,會在測量時引發異常.通過代碼主線 ,我們知道它將傳進去的兩個參數賦給本地的mMeasuredWidth和mMeasuredHeight變量,以便在View類中使用;好了,此時我們該抽離出來,回到我們出發的地方:
setMeasuredDimension(measureWidth(widthMeasureSpec),measureHeight(heightMeasureSpec));
有了上面的分析過程,我們知道這個方法中的measureWidth(widthMeasureSpec)是作為測量到的寬度,measureHeight(heightMeasureSpec)是作為測量到的高度,而這兩個是需要我們在自定義的View中去實現的,由於測量寬度與高度的過程類似,我們在此文中僅分析measureWidth()的過程,很自然地,我們看看本地的measureWidth()是如何實現的:
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1 /**
2 * Determines the width of this view
3 *
4 * @param measureSpec
5 * A measureSpec packed into an int
6 * @return The width of the view, honoring constraints from measureSpec
7 */
8 private int measureWidth(int measureSpec) {
9 int result = 0;
10 int specMode = MeasureSpec.getMode(measureSpec);
11 int specSize = MeasureSpec.getSize(measureSpec);
12
13 if (specMode == MeasureSpec.EXACTLY) {
14 // We were told how big to be
15 result = specSize;
16 } else {
17 // Measure the text
18 result = (int) mTextPaint.measureText(mText) + getPaddingLeft()
19 + getPaddingRight();
20 if (specMode == MeasureSpec.AT_MOST) {
21 // Respect AT_MOST value if that was what is called for by
22 // measureSpec
23 result = Math.min(result, specSize);
24 }
25 }
26
27 return result;
28 }
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該方法用來確定我們自定義的這個View的寬度,它接收onMeasure()的widthMeasureSpec參數,接著
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
MeasureSpec.getMode(measureSpec),getMode()?我們在上一篇文章中的最後,有如下描述:
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MeasureSpec:
該對象封裝了父容器傳遞給子元素的布局要求,它有三種模式：
1)
UNSPECIFIED:父容器對子元素沒有要求，子元素可以得到任意值；
2)
EXACTLY:父窗口決定子元素的大小，子元素將被限定在給定的邊界裡而忽略它本身大小;
3)
AT MOST:子元素至多達到父窗口指定的大小,子元素不能超過這個邊界;
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所以我們會想,getMode()方法,應該就是獲取上述這三種模式之一吧?我們跟進源碼,看看getMode()都做了哪些事情:
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18341 /**
18342 * Extracts the mode from the supplied measure specification.
18343 *
18344 * @param measureSpec the measure specification to extract the mode from
18345 * @return {@link android.view.View.MeasureSpec#UNSPECIFIED},
18346 * {@link android.view.View.MeasureSpec#AT_MOST} or
18347 * {@link android.view.View.MeasureSpec#EXACTLY}
18348 */
18349 public static int getMode(int measureSpec) {
18350 return (measureSpec & MODE_MASK);
18351 }
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由此方法的文字描述部分,我們得知,該方法從接收的參數measureSpec中,獲取到對應的三種模式之一,即返回measureSpec & MODE_MASK,這裡的MODE_MASK又是個什麼東西呢?在View.java中,我們找到在View這個類中,有個內部類MeasureSpec類
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18289 public static class MeasureSpec {
18290 private static final int MODE_SHIFT = 30;
18291 private static final int MODE_MASK = 0x3 << MODE_SHIFT;
..............................................................
18297 public static final int UNSPECIFIED = 0 << MODE_SHIFT;
18298
18299 /**
18300 * Measure specification mode: The parent has determined an exact size
18301 * for the child. The child is going to be given those bounds regardless
18302 * of how big it wants to be.
18303 */
18304 public static final int EXACTLY = 1 << MODE_SHIFT;
18305
18306 /**
18307 * Measure specification mode: The child can be as large as it wants up
18308 * to the specified size.
18309 */
18310 public static final int AT_MOST = 2 << MODE_SHIFT;
................................
}
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所以,MODE_MASK的值為0x3左移了MODE_SHIFT(30)位,那麼,用32位的二進制來表示的話,MODE_MASK為:1100 0000 0000 0000 0000 0000 0000 0000;如果非要探究此時的measureSpec & MODE_MASK後的值是多少,那麼我們不妨用Debug模式調試一下我們的代碼來獲取getMode方法中傳進來的參數measureSpec是什麼值, 首先,從上面的源碼中,可以知道三種MeasureSpec三種模式的值:
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UNSPECIFIED = 0 << MODE_SHIFT;即:UNSPECIFIED為:0000 0000 0000 0000 0000 0000 0000 0000
其實我們也可以參閱官方文檔對此值的定義:
public static final int UNSPECIFIED
Added in API level 1
Measure specification mode: The parent has not imposed any constraint on the child. It can be whatever size it wants.
Constant Value: 0 (0x00000000)
注:只不過官方文檔此處用十六進制表示而已,以下兩個模式也都用十六進制表示而已.
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EXACTLY = 1 << MODE_SHIFT;即 EXACTLY為:0100 0000 0000 0000 0000 0000 0000 0000
public static final int EXACTLY
Added in API level 1
Measure specification mode: The parent has determined an exact size for the child. The child is going to be given those bounds regardless of how big it wants to be.
Constant Value: 1073741824 (0x40000000)
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AT_MOST = 2 << MODE_SHIFT;即 AT_MOST為:1000 0000 0000 0000 0000 0000 0000 0000
public static final int AT_MOST
Added in API level 1
Measure specification mode: The child can be as large as it wants up to the specified size.
Constant Value: -2147483648 (0x80000000)
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MODE_MASK為:1100 0000 0000 0000 0000 0000 0000 0000
好,我們來看一下debug前,自定義的View在布局文件中的layout_width的配置及我所調試的設備的屏幕像素為480*800,也就是我的顯示屏寬為480像素;
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<com.project.summary.customview.CustomView
android:id="@+id/customView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:colorValue="@color/textRed"
app:textSize="20sp"
app:textString="This the Custom View1!!!" />
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調試結果出來,此時傳入的measureSpec的值是-2147483648,到了這裡,我們又會產生一個疑問 ,為什麼是它?為什麼是這個值?我們先把這個疑問做個標記:Q02;到了文章最後,這個疑問就能解開了,這裡先把思路跳出來,繼續分析我們的measureWidth()這個本地方法的代碼;
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1 /**
2 * Determines the width of this view
3 *
4 * @param measureSpec
5 * A measureSpec packed into an int
6 * @return The width of the view, honoring constraints from measureSpec
7 */
8 private int measureWidth(int measureSpec) {
9 int result = 0;
10 int specMode = MeasureSpec.getMode(measureSpec);
11 int specSize = MeasureSpec.getSize(measureSpec);
12
13 if (specMode == MeasureSpec.EXACTLY) {
14 // We were told how big to be
15 result = specSize;
16 } else {
17 // Measure the text
18 result = (int) mTextPaint.measureText(mText) + getPaddingLeft()
19 + getPaddingRight();
20 if (specMode == MeasureSpec.AT_MOST) {
21 // Respect AT_MOST value if that was what is called for by
22 // measureSpec
23 result = Math.min(result, specSize);
24 }
25 }
26
27 return result;
28 }
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上面我們已經分析到第10行,由於第11行是獲取傳入的measureSpec的大小,過程與獲取傳入的measureSpec的模式類似,這裡暫時先略過,接下來看第13行代碼,這裡要對獲取到的模式進行判斷,由上一篇文章,我們知道,如果自定義的View在布局文件中指定固定大小,那麼,它的模式就是屬於MeasureSepc.EXACTLY,此時,measureWidth()這個本地方法就返回11行所得的大小,否則進入另外一個分支,因為本系列中我們實現的實現上是一個類似於TextView的自定義控件,那麼,這個View的大小就應該由它所繪制的文字長度來決定,此時,我們先計算出文字的寬度,然後再對其模式進行判斷,如果模式是屬於measureSpec.AT_MOST,我們通過數學運算,比較文字長度與通過傳入的measureSpec所包含的大小,它們之中更小的那個做為我們控件的寬度.
文章開頭的相關代碼中,本地方法:getMeasureHeight()的過程與本地方法getMeasureWidth()類似,在此不再分析.
在此總結一下,文章開頭引用的代碼是我們在編寫自定義View時,在重寫onMeasure()這個方法時的一般步驟,那麼,本文中的分析過程中還留有兩個疑問:
Q01:
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)是一個override的方法,它接收兩個參數,通過字面意思,我們知道,這兩個參數分別為寬度測量規格,高度測量規格,此時,我們會有一個疑問,這兩個參數是從哪裡來的?
Q02:
調試結果出來,此時傳入的measureSpec的值是-2147483648,到了這裡,我們又會產生一個疑問 ,為什麼是它?為什麼是這個值?
要探究這兩個疑問,我們在本系列第二篇文章中,曾經提過Android繪制View的理論基礎,從那篇文章中,我們明白,Android要繪制View的時候,必須要先遍歷View的樹形結構,並且先從最頂端的結點開始遍歷,通過查找官方文檔,我們進入
ViewRootImpl.java(文件位於:/frameworks/base/core/java/android/view/ViewRootImpl.java),一起找出上面的那兩個疑問.........
/*********************************友情提醒:開始下面的探究前,最好先休息一下*********************************/
我們先大致浏覽一下ViewRootImpl.java,這個文件代碼有6707行有沒有,不用怕,我們先找到一個叫performtraversals()的方法,看這字面意思,它是要開始遍歷的節奏啊,果斷跟進去看一下,順便找找幾個有用的干貨:
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private void performTraversals()
{
.......................................
1122 WindowManager.LayoutParams lp = mWindowAttributes;//詳見分析PERFORMTRAVERSALS()點1
.........................................................
1155 Rect frame = mWinFrame;//詳見分析PERFORMTRAVERSALS()點2
.......................................................
1563 if (mWidth != frame.width() || mHeight != frame.height()) {
1564 mWidth = frame.width();
1565 mHeight = frame.height();
1566 }
1567
.......................................................................
PERFORMTRAVERSALS()點3:
1634 if (!mStopped) {
1635 boolean focusChangedDueToTouchMode = ensureTouchModeLocally(
1636 (relayoutResult&WindowManagerGlobal.RELAYOUT_RES_IN_TOUCH_MODE) != 0);
1637 if (focusChangedDueToTouchMode || mWidth != host.getMeasuredWidth()
1638 || mHeight != host.getMeasuredHeight() || contentInsetsChanged) {
1639 int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);//詳見getRootMeasureSpec()方法的分析
1640 int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
1641
1642 if (DEBUG_LAYOUT) Log.v(TAG, "Ooops, something changed! mWidth="
1643 + mWidth + " measuredWidth=" + host.getMeasuredWidth()
1644 + " mHeight=" + mHeight
1645 + " measuredHeight=" + host.getMeasuredHeight()
1646 + " coveredInsetsChanged=" + contentInsetsChanged);
1647
1648 // Ask host how big it wants to be
1649 performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
}
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/************************************分析PERFORMTRAVERSALS()點1 開始**********************************/
這裡的lp用得還挺多,也許對我們有用,
因為
WindowManager.LayoutParams lp = mWindowAttributes;
所以我們分析一下這個mWindowAttributes是何方神聖:
分析PERFORMTRAVERSALS()點1:mWindowAttributes相關代碼：
final WindowManager.LayoutParams mWindowAttributes = new WindowManager.LayoutParams();
我們進入WindowManager類的內部類LayoutParams的構造方法
1 public LayoutParams() {
2 super(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);
3 type = TYPE_APPLICATION;
4 format = PixelFormat.OPAQUE;
5 }
其中有這麼一句:注意兩個參數都為LayoutParams.MATCH_PARENT
super(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);
因為WindowManager類的內部類LayoutParams繼承自ViewGroup.LayoutParams,所以進入ViewGroup的內部類LayoutParams看一下
/frameworks/base/core/java/android/view/ViewGroup.java：
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5829 public static class LayoutParams {
5830 /**
5831 * Special value for the height or width requested by a View.
5832 * FILL_PARENT means that the view wants to be as big as its parent,
5833 * minus the parent's padding, if any. This value is deprecated
5834 * starting in API Level 8 and replaced by {@link #MATCH_PARENT}.
5835 */
5836 @SuppressWarnings({"UnusedDeclaration"})
5837 @Deprecated
5838 public static final int FILL_PARENT = -1;
..........................................
5918 public LayoutParams(int width, int height) {
5919 this.width = width;
5920 this.height = height;
5921 }
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分析總結:這裡的width與height,都被賦為LayoutParams.MATCH_PARENT,所以這裡的lp的寬與高,都為LayoutParams.MATCH_PARENT
/************************************分析PERFORMTRAVERSALS()點1 結束**********************************/
######################################################################################################################
/************************************分析PERFORMTRAVERSALS()點2 開始**********************************/
1563 if (mWidth != frame.width() || mHeight != frame.height()) {
1564 mWidth = frame.width();
1565 mHeight = frame.height();
1566 })
此時的mWidth為ViewRootImpl的變量,在這裡使它的值為frame.width()的值;
frame又是從哪裡來的呢?在performTraversals()方法中,1155行,原來它只是個局部變量,
1155 Rect frame = mWinFrame;
到了這裡,關鍵就是找出mWinFrame了,繼續找mWinFrame:
在ViewRootImpl的變量聲明中:
256 final Rect mWinFrame; // frame given by window manager.
在ViewRootImpl這個類的構造方法中:
360 mWinFrame = new Rect();
frame given by window manager?那大概就是說mWinFrame是由窗口管理類來賦值的了,那麼這麼裡mWinFrame應該就是屏幕的窗口大小了.我們這裡先這麼假設,後續文章再進行驗證.
/**********************************************分析PERFORMTRAVERSALS()點2 結束**************************************/
/**********************************************分析PERFORMTRAVERSALS()點3開始**************************************/
PERFORMTRAVERSALS()點3:performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
1,兩個參數:childWidthMeasureSpec,childHeightMeasureSpec分析
a)childWidthMeasureSpec:
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
先分析getRootMeasureSpec的兩個參數:
1)mWidth:
見分析點2(
1563 if (mWidth != frame.width() || mHeight != frame.height()) {
1564 mWidth = frame.width();
1565 mHeight = frame.height();
1566 })
所以猜想mWidth就是窗口的初始寬度(本文暫未驗證)
2)lp.width:這裡的lp就是分析點1中的 WindowManager.LayoutParams lp = mWindowAttributes;即:lp.width為LayoutParams.MATCH_PARENT;
由以上1)和2),我們先搞定了getRootMeasureSpec(mWidth,lp.width)這個方法的兩個參數的意義,接下來,我們進入getRootMeasureSpec(mWidth,lp.width)這個方法
b)childHeightMeasureSpec:
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
先分析getRootMeasureSpec的兩個參數:
1)mHeight:類似上述的猜想,這裡的mHeight就是窗口的初始高度
2)lp.height:這裡的lp就是分析點1中的 WindowManager.LayoutParams lp = mWindowAttributes;即:lp.height為LayoutParams.MATCH_PARENT;
因為上述a)與b)的調用過程類似,只不過a)是獲取寬度的規格,b)是獲取高度的規格,所以以下分析只以獲取寬度規格的過程來分析
*******************************************************************進入getRootMeasureSpec()方法的分析**********************************************
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1924 private static int getRootMeasureSpec(int windowSize, int rootDimension) {
1925 int measureSpec;
1926 switch (rootDimension) {
1927
1928 case ViewGroup.LayoutParams.MATCH_PARENT:
1929 // Window can't resize. Force root view to be windowSize.
1930 measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
1931 break;
1932 case ViewGroup.LayoutParams.WRAP_CONTENT:
1933 // Window can resize. Set max size for root view.
1934 measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
1935 break;
1936 default:
1937 // Window wants to be an exact size. Force root view to be that size.
1938 measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
1939 break;
1940 }
1941 return measureSpec;
1942 }
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此方法接收的第二個參數rootDimension,就是lp.width,通過上面的分析,lp.width=LayoutParams.MATCH_PARENT,所以,進入第一個switch分支
此方法的返回值measureSpec=MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
所以,分析此方法,我們也知道,當我們的自定義View的layout_width/layout_height設置成MATCH_PARENT時,MODE 為MeasureSpec.EXACTLY;當設置成WRAP_CONTENT時,MODE為MeasureSpec.AT_MOST;
接下來我們分析1938行:
1938 measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
***********************************************************************************************************************************************************
*******************************************************************進入MeasureSpec.makeMeasureSpec()方法的分析**********************************************
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17245 /**
17246 * Creates a measure specification based on the supplied size and mode.
17247 *
17248 * The mode must always be one of the following:
17249 * <ul>
17250 * <li>{@link android.view.View.MeasureSpec#UNSPECIFIED}</li>
17251 * <li>{@link android.view.View.MeasureSpec#EXACTLY}</li>
17252 * <li>{@link android.view.View.MeasureSpec#AT_MOST}</li>
17253 * </ul>
17254 *
17255 * @param size the size of the measure specification
17256 * @param mode the mode of the measure specification
17257 * @return the measure specification based on size and mode
17258 */
17259 public static int makeMeasureSpec(int size, int mode) {
17260 return size + mode;
17261 }
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此方法在/frameworks/base/core/java/android/view/View.java中的內部類MeasureSpec中的方法,該方法返回兩個參數size+mode之和,參數size對應我們傳進來的windowSize,即:窗口的初始寬度(當傳進來的是mHeight時,為窗口的初始高度);
參數mode對應我們傳進來的MeasureSpec.EXACTLY
**************************************************************************************************************************************************************************
有了上面這些分析之後,我們可以進入performMeasure(childWidthMeasureSpec, childHeightMeasureSpec)的分析了:
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1913 private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
1914 Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
1915 try {
1916 mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
1917 } finally {
1918 Trace.traceEnd(Trace.TRACE_TAG_VIEW);
1919 }
1920 }
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*************************************mView.measure(childWidthMeasureSpec, childHeightMeasureSpec)的分析************************************************************
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16450 /**
16451 * <p>
16452 * This is called to find out how big a view should be. The parent
16453 * supplies constraint information in the width and height parameters.
16454 * </p>
16455 *
16456 * <p>
16457 * The actual measurement work of a view is performed in
16458 * {@link #onMeasure(int, int)}, called by this method. Therefore, only
16459 * {@link #onMeasure(int, int)} can and must be overridden by subclasses.
16460 * </p>
16461 *
16462 *
16463 * @param widthMeasureSpec Horizontal space requirements as imposed by the
16464 * parent
16465 * @param heightMeasureSpec Vertical space requirements as imposed by the
16466 * parent
16467 *
16468 * @see #onMeasure(int, int)
16469 */
16470 public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
.....................................................
16496 // measure ourselves, this should set the measured dimension flag back
16497 onMeasure(widthMeasureSpec, heightMeasureSpec);
16498 mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
16522 }
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這裡的measure()方法是個final方法,結合該方法的說明,
The actual measurement work of a view is performed in onMeasure()
並且measure的兩個參數同時傳入onMeasure()中,
所以,才有了文章開頭時引用的代碼,在自定義的View中,重寫onMeasure()方法,那麼,本文上部分遺留下來的兩個問題,至此就有了答案:
Q01:
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)是一個override的方法,它接收兩個參數,通過字面意思,我們知道,這兩個參數分別為寬度測量規格,高度測量規格,此時,我們會有一個疑問,這兩個參數是從哪裡來的?
通過:1639 int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);//詳見getRootMeasureSpec()方法的分析,onMeasure的第一個參數widthMeasureSpec就是這裡的childWidthMeasureSpec,heightMeasureSpec對應 childHeightMeasureSpec;
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Q02:
調試結果出來,此時傳入的measureSpec的值是-2147483648,到了這裡,我們又會產生一個疑問 ,為什麼是它?為什麼是這個值?
那麼這裡的measureSpec就是MeasureSpec.makeMeasureSpec()方法的分析中,返回的size+mode;size是手機顯示屏的像素寬或者高,文章上半部分中,我調試的手機像素寬是480,而且在自定義的View的布局文件中,layout_width設置成wrap_content,通過上面的分析,當設置成wrap_content時,模式為AT_MOST模式,通過文檔描述,它的十進制值是-2147483648,那麼size+mode就是480+(-2147483648)=-2147483168,也就是我們調試出來時,所得到的值-2147483648
)
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另外,我們或許還會有一個疑問 :為什麼MODE_MASK是1100 0000 0000 0000 0000 0000 0000 0000?EXACTLY為:0100 0000 0000 0000 0000 0000 0000 0000?
AT_MOST為:1000 0000 0000 0000 0000 0000 0000 0000?
其實對於這個問題,我們想,既然android規定了MODE必須是EXACTLY,AT_MOST,UNSPECIFIED這三種模式之一,那麼,就可以用32位二進制的最高兩位來表示,它有00,01,10,11這四種情況,那麼它的MODE_MASK取值為
1100 0000 0000 0000 0000 0000 0000 0000就能很方便地取到它的模式了,由getMode()的實現:
return (measureSpec & MODE_MASK);
我們就可以取到它的最高兩位,由此來確定它是哪種模式;同理對於getSize():
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
對於屏幕寬度,再大的屏幕也用不了32位二進制來表示其尺寸,所以才有measureSpec & ~MODE_MASK,這樣就能取到它的值了.
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