簡(jiǎn)介

組件在頁面上的展示要經(jīng)過測(cè)量、布局晴股、繪制三個(gè)過程城须，而測(cè)量就是把我們定義的組件寬和高轉(zhuǎn)換為具體的大小的這個(gè)過程。
我們常見的組件的寬高定義如下：

<TextView
    android:id="@+id/name1"
    android:layout_width="100dp"
    android:layout_height="100dp" />
<TextView
    android:id="@+id/name1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />
<TextView
    android:id="@+id/name1"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

寬高定義為100dp凹蜈，那么最后組件的寬高就是100dp限寞，這很好理解，那wrap_content和match_parent呢仰坦，我們都知道履植，wrap_content根據(jù)組件自身的大小變化而變化，match_parent指的要占滿父視圖悄晃。那么系統(tǒng)具體是如何解析的呢玫霎？

系統(tǒng)把組件的長(zhǎng)寬定義轉(zhuǎn)換相應(yīng)的測(cè)量規(guī)則

在頁面測(cè)量的時(shí)候都會(huì)調(diào)用measureChild或measureChildWithMargins方法對(duì)組件進(jìn)行測(cè)量凿滤，已measureChild為例：

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);}

關(guān)于MeasureSpec的定義就不再這里闡述了，大家可以查看其他文章庶近。
這段代碼很短翁脆，主要就是描述了通過getChildMeasureSpec把父視圖的測(cè)量規(guī)則和組件的布局參數(shù)轉(zhuǎn)換為子視圖的測(cè)量規(guī)則，進(jìn)行測(cè)量鼻种。再來看getChildMeasureSpec方法：

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:
       ... 
   }
   return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

1反番、如果childDimension >0，指的就是組件定義了具體的大胁嬖俊（MATCH_PARENT為-1罢缸，WRAP_CONTENT為-2），例如：

<TextView
    android:id="@+id/name1"
    android:layout_width="100dp"
    android:layout_height="100dp" />

組件的大小就是childDimension 投队，測(cè)量模式為MeasureSpec.EXACTLY枫疆。
2、如果childDimension == LayoutParams.MATCH_PARENT敷鸦，組件大小就是父視圖提供的大小size,而測(cè)量模式也是MeasureSpec.EXACTLY息楔。
3、如果childDimension == LayoutParams.WRAP_CONTENT扒披，組件的測(cè)量模式為MeasureSpec.AT_MOST值依，大小為父視圖提供的大小size,這里的大小實(shí)際上指的是組件的最大大小，組件的具體大小的測(cè)量還需要組件自己實(shí)現(xiàn)谎碍。
4、如果父視圖的測(cè)量模式為MeasureSpec.AT_MOST洞焙，及時(shí)組件childDimension == LayoutParams.MATCH_PARENT蟆淀，其測(cè)量模式也為MeasureSpec.AT_MOST。

組件如何根據(jù)測(cè)量模式測(cè)量出其大小的

child.measure(childWidthMeasureSpec, childHeightMeasureSpec)方法最終就會(huì)調(diào)用組件的onMeasure方法澡匪，在這里組件會(huì)根據(jù)測(cè)量規(guī)則計(jì)算出具體的寬高大小熔任。

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),            getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}

getDefaultSize方法就是計(jì)算出具體的寬高，setMeasuredDimension方法則是設(shè)置寬高唁情。
基類View的具體計(jì)算方法是getDefaultSize：

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;
}

無論是MeasureSpec.AT_MOST還是MeasureSpec.EXACTLY模式疑苔，最終得到的具體大小都為父視圖提供大小，這與我們對(duì)wrap_content的理解是有出入的甸鸟。其實(shí)具體的組件都需要重寫onMeasure方法的惦费。
例如TextView：

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int widthMode = MeasureSpec.getMode(widthMeasureSpec);
    int heightMode = MeasureSpec.getMode(heightMeasureSpec);
    int widthSize = MeasureSpec.getSize(widthMeasureSpec);
    int heightSize = MeasureSpec.getSize(heightMeasureSpec);
    int width;
    int height;
    if (widthMode == MeasureSpec.EXACTLY) {
        // Parent has told us how big to be. So be it.
        width = widthSize;
    } else {
      ...這里計(jì)算textView的大小width....       
        if (widthMode == MeasureSpec.AT_MOST) {
            width = Math.min(widthSize, width);
        }
    }
...
setMeasuredDimension(width, height);
}

1、如果模式為MeasureSpec.EXACTLY抢韭，大小就是父視圖提供的大小
2薪贫、如果模式是MeasureSpec.AT_MOST，會(huì)取textView的真實(shí)大小與與父視圖提供的大小中小的那個(gè)刻恭，這樣與我們理解的wrap_content就吻合了瞧省。