編輯：關於android開發
相關文章
Android View體系（一）視圖坐標系
Android View體系（二）實現View滑動的六種方法
Android View體系（三）屬性動畫
Android View體系（四）從源碼解析Scroller
Android View體系（五）從源碼解析View的事件分發機制
Android View體系（六）從源碼解析Activity的構成
在上一篇我們了解了Activity的構成後，開始了解一下View的工作流程，就是measure、layout和draw。measure用來測量View的寬高，layout用來確定View的位置，draw則用來繪制View。這一講我們來看看measure流程，measure流程分為View的measure流程和ViewGroup的measure流程，只不過ViewGroup的measure流程除了要完成自己的測量還要遍歷去調用子元素的measure()方法。
先來看看onMeasure()方法（View.java）：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
看看setMeasuredDimension()方法：
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int opticalWidth = insets.left + insets.right;
int opticalHeight = insets.top + insets.bottom;
measuredWidth += optical ? opticalWidth : -opticalWidth;
measuredHeight += optical ? opticalHeight : -opticalHeight;
}
setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}
很顯然是用來設置View的寬高的，先來看看getDefaultSize()方法處理了什麼：
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
case MeasureSpec.UNSPECIFIED:
result = size;
break;
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
specMode是View的測量模式，而specSize是View的測量大小，看到這裡我們有必要先看看MeasureSpec類：
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
/**
* Measure specification mode: The parent has not imposed any constraint
* on the child. It can be whatever size it wants.
*/
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
/**
* 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.
*/
public static final int EXACTLY = 1 << MODE_SHIFT;
/**
* Measure specification mode: The child can be as large as it wants up
* to the specified size.
*/
public static final int AT_MOST = 2 << MODE_SHIFT;
...省略
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
...省略
}
MeasureSpec類幫助我們來測量View，它是一個32位的int值，高兩位為specMode （測量的模式），低30位為specSize （測量的大小），測量模式分為三種：
UNSPECIFIED：未指定模式，View想多大就多大，父容器不做限制，一般用於系統內部的測量。 AT_MOST：最大模式，對應於wrap_comtent屬性，只要尺寸不超過父控件允許的最大尺寸就行。 EXACTLY：精確模式，對應於match_parent屬性和具體的數值，父容器測量出View所需要的大小，也就是specSize的值。讓我們回頭看看getDefaultSize()方法，很顯然在AT_MOST和EXACTLY模式下，都返回specSize這個值，也就是View測量後的大小，而在UNSPECIFIED模式返回的是getDefaultSize()方法的第一次個參數的值，這第一個參數從onMeasure()方法來看是getSuggestedMinimumWidth()方法和getSuggestedMinimumHeight()得到的，那我們來看看getSuggestedMinimumWidth()方法做了什麼，我們只需要弄懂getSuggestedMinimumWidth()方法，因為這兩個方法原理是一樣的：
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
如果View沒有設置背景則取值為mMinWidth，mMinWidth是可以設置的，它對應於android:minWidth這個屬性設置的值或者View的setMinimumWidth的值，如果不指定的話則默認為0：
public void setMinimumWidth(int minWidth) {
mMinWidth = minWidth;
requestLayout();
}
如果View設置了背景在取值為max(mMinWidth, mBackground.getMinimumWidth())，取值mMinWidth和mBackground.getMinimumWidth()的最大值，上面我們說過了mMinWidth，那來看看mBackground.getMinimumWidth()，這個mBackground是Drawable類型的，看一下Drawable類的getMinimumWidth()方法（Drawable.java）：
public int getMinimumWidth() {
final int intrinsicWidth = getIntrinsicWidth();
return intrinsicWidth > 0 ? intrinsicWidth : 0;
}
intrinsicWidth得到的是這個Drawable的固有的寬度，如果固有寬度大於0則返回固有寬度，否則返回0。
總結一下getSuggestedMinimumWidth()方法就是：如果View沒有設置背景則返回mMinWidth ，如果設置了背景就返回mMinWidth 和Drawable最小寬度兩個值的最大值。
講完了View的measure流程，接下來看看ViewGroup的measure流程，對於ViewGroup，它不只要measure自己本身，還要遍歷的調用子元素的measure()方法，ViewGroup中沒有定義onMeasure()方法，但他定義了measureChildren()方法（ViewGroup.java）：
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) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}
就是遍歷子元素並調用measureChild()方法：
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
調用child.getLayoutParams()方法來獲得子元素的LayoutParams屬性，並獲取到子元素的MeasureSpec並調用子元素的measure()方法進行測量。getChildMeasureSpec()方法裡寫了什麼呢？
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} 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;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent asked to see how big we want to be
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
// Child wants a specific size... let him have it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
resultSize = 0;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = 0;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
很顯然這是根據父容器的MeasureSpec的模式再結合子元素的LayoutParams屬性來得出子元素的MeasureSpec屬性，有一點需要注意的是如果父容器的MeasureSpec屬性為AT_MOST，子元素的LayoutParams屬性為WRAP_CONTENT，那根據代碼我們會發現子元素的MeasureSpec屬性也為AT_MOST，它的specSize值為父容器的specSize減去padding的值，也就是說跟這個子元素設置LayoutParams屬性為MATCH_PARENT效果是一樣的，為了解決這個問題需要在LayoutParams屬性為WRAP_CONTENT時指定一下默認的寬和高。
ViewGroup並沒有提供onMeasure()方法，而是讓其子類來各自實現測量的方法，究其原因就是ViewGroup有不同的布局的需要很難統一，接下來我們來簡單分析一下ViewGroup的子類LinearLayout的measure流程，先來看看它的onMeasure()方法（LinearLayout.java）：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}
來看看垂直measureVertical()方法的部分源碼：
void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
mTotalLength = 0;
mTotalLength = 0;
...省略
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null) {
mTotalLength += measureNullChild(i);
continue;
}
if (child.getVisibility() == View.GONE) {
i += getChildrenSkipCount(child, i);
continue;
}
if (hasDividerBeforeChildAt(i)) {
mTotalLength += mDividerHeight;
}
LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();
totalWeight += lp.weight;
if (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0) {
// Optimization: don't bother measuring children who are going to use
// leftover space. These views will get measured again down below if
// there is any leftover space.
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
skippedMeasure = true;
} else {
int oldHeight = Integer.MIN_VALUE;
if (lp.height == 0 && lp.weight > 0) {
// heightMode is either UNSPECIFIED or AT_MOST, and this
// child wanted to stretch to fill available space.
// Translate that to WRAP_CONTENT so that it does not end up
// with a height of 0
oldHeight = 0;
lp.height = LayoutParams.WRAP_CONTENT;
}
// Determine how big this child would like to be. If this or
// previous children have given a weight, then we allow it to
// use all available space (and we will shrink things later
// if needed).
measureChildBeforeLayout(
child, i, widthMeasureSpec, 0, heightMeasureSpec,
totalWeight == 0 ? mTotalLength : 0);
if (oldHeight != Integer.MIN_VALUE) {
lp.height = oldHeight;
}
final int childHeight = child.getMeasuredHeight();
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
lp.bottomMargin + getNextLocationOffset(child));
...省略
if (useLargestChild &&
(heightMode == MeasureSpec.AT_MOST || heightMode == MeasureSpec.UNSPECIFIED)) {
mTotalLength = 0;
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null) {
mTotalLength += measureNullChild(i);
continue;
}
if (child.getVisibility() == GONE) {
i += getChildrenSkipCount(child, i);
continue;
}
final LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams)
child.getLayoutParams();
// Account for negative margins
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + largestChildHeight +
lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
}
}
// Add in our padding
mTotalLength += mPaddingTop + mPaddingBottom;
int heightSize = mTotalLength;
// Check against our minimum height
定義了mTotalLength用來存儲LinearLayout在垂直方向的高度，然後遍歷子元素，根據子元素的MeasureSpec模式分別計算每個子元素的高度，如果是wrap_content則將每個子元素的高度和margin垂直高度等值相加並賦值給mTotalLength得出整個LinearLayout的高度。如果布局高度設置為match_parent者具體數值則和View的測量方法一樣。
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