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Android視圖的繪制流程(上) View的測(cè)量

更新時(shí)間：2017年01月16日 11:52:55 作者：無(wú)嘴小呆子

這篇文章主要為大家詳細(xì)介紹了Android視圖的繪制流程上篇，View測(cè)量的相關(guān)資料，具有一定的參考價(jià)值，感興趣的小伙伴們可以參考一下

綜述

　　View的繪制流程可以分為三大步，它們分別是measure，layout和draw過(guò)程。measure表示View的測(cè)量過(guò)程，用于測(cè)量View的寬度和高度；layout用于確定View在父容器的位置；draw則是負(fù)責(zé)將View繪制到屏幕中。下面主要來(lái)看一下View的Measure過(guò)程。

測(cè)量過(guò)程

　　View的繪制流程是從ViewRoot的performTraversals方法開始的，ViewRoot對(duì)應(yīng)ViewRootImpl類。ViewRoot在performTraversals中會(huì)調(diào)用performMeasure方法來(lái)進(jìn)行對(duì)根View的測(cè)量過(guò)程。而在performMeasure方法中又會(huì)調(diào)用View的measure方法。對(duì)于View的measure方法它是一個(gè)final類型，也就是說(shuō)這個(gè)measure方法不能被子類重寫。但是在measure方法中調(diào)用了onMeasure方法。所以View的子類可以重寫onMeasure方法來(lái)實(shí)現(xiàn)各自的Measure過(guò)程。在這里也就是主要對(duì)onMeasure方法進(jìn)行分析。

MeasureSpec

　　MeasureSpec是View類中的一個(gè)靜態(tài)內(nèi)部類。一個(gè)MeasureSpec封裝了父布局傳遞給子布局的布局要求。每個(gè)MeasureSpec都代表著一個(gè)高度或?qū)挾鹊囊?。每個(gè)MesureSpec都是由specSize和specMode組成，它代表著一個(gè)32位的int值，其中高2位代表specSize，低30位代表specMode。
　　MeasureSpec的測(cè)量模式有三種，下面介紹一下這三種測(cè)量模式：

UNSPECIFIED
父容器對(duì)子View沒(méi)有任何的限制，子View可以是任何的大小。
EXACTLY
父容器為子View大小指定一個(gè)具體值，View的最終大小就是specSize。對(duì)應(yīng)View屬性match_parent和具體值。
AT_MOST
子View的大小最大只能是specSize，也就是所子View的大小不能超過(guò)specSize。對(duì)應(yīng)View屬性的wrap_content.

　　在MeasureSpec中可以通過(guò)specSize和specMode并使用makeMeasureSpec方法來(lái)創(chuàng)建一個(gè)MeasureSpec，還可以通過(guò)getMode和getSize來(lái)獲取MeasureSpec的specMode和specSize。

View的測(cè)量過(guò)程

　　在上面已經(jīng)說(shuō)到，View的Measure過(guò)程是由measure方法來(lái)完成的，而measure方法通過(guò)調(diào)用onMeasure方法來(lái)完成View的Measure過(guò)程。那么就來(lái)看一下onMeasure方法。

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

　　在View的onMeasure方法中只是調(diào)用了setMeasuredDimension方法，setMeasuredDimension方法的作用就是設(shè)置View的高和寬的測(cè)量值。對(duì)于View測(cè)量后寬和高的值是通過(guò)getDefaultSize方法來(lái)獲取的。下面就來(lái)一下這個(gè)getDefaultSize方法。

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;
}

　　對(duì)于MeasureSpec的AT_MOST和EXACTLY模式下，直接返回的就是MeasureSpec的specSize，也就是說(shuō)這個(gè)specSize就是View測(cè)量后的大小。而對(duì)于在UNSPECIFIED模式下，View的測(cè)量值則為getDefaultSize方法中的第一個(gè)參數(shù)size。這個(gè)size所對(duì)應(yīng)的寬和高是通過(guò)getSuggestedMinimumWidth和getSuggestedMinimumHeight兩個(gè)方法獲取的。下面就來(lái)看一下這兩個(gè)方法。

protected int getSuggestedMinimumHeight() {
  return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());

}

protected int getSuggestedMinimumWidth() {
  return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}

　　在這里可以看到對(duì)于View寬和高的取值是根據(jù)View是否存在背景進(jìn)行設(shè)置的。在這里以View的寬度來(lái)進(jìn)行說(shuō)明。若是View沒(méi)有背景則是View的寬度mMinWidth。對(duì)于mMinWidth值得設(shè)置可以在XML布局文件中設(shè)置minWidth屬性，它的默認(rèn)值為0。也可以通過(guò)調(diào)用View的setMinimumWidth()方法其賦值。若是View存在背景的話，則取View本身最小寬度mMinWidth和View背景的最小寬度它們中的最大值。

ViewGroup的測(cè)量過(guò)程

　　對(duì)于ViewGroup的Measure過(guò)程，ViewGroup處理Measure自己本身的大小，還需要遍歷子View，并調(diào)用它們的measure方法，然后各個(gè)子元素再去遞歸執(zhí)行Measure過(guò)程。在ViewGroup中并沒(méi)有重寫onMeasure方法，因?yàn)閂iewGroup它是一個(gè)抽象類，對(duì)于不同的具體ViewGroup它的onMeasure方法中所實(shí)現(xiàn)的過(guò)程不一樣。但是在ViewGroup中提供了一個(gè)measureChildren方法，對(duì)子View進(jìn)行測(cè)量。下面就來(lái)看一下這個(gè)measureChildren方法。

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);
    }
  }
}

　　在這里獲取ViewGroup中所有的子View。然后遍歷ViewGroup中子View并調(diào)用measureChild方法來(lái)完成對(duì)子View的測(cè)量。下面看一下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);
}

　　在這段代碼中通過(guò)getChildMeasureSpec方法獲取子View寬和高的MeasureSpec。然后調(diào)用子View的measure方法開始對(duì)View進(jìn)行測(cè)量。下面就來(lái)看一下是如何通過(guò)getChildMeasureSpec方法來(lái)獲取View的MeasureSpec的。

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 = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
      resultMode = MeasureSpec.UNSPECIFIED;
    } else if (childDimension == LayoutParams.WRAP_CONTENT) {
      // Child wants to determine its own size.... find out how
      // big it should be
      resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
      resultMode = MeasureSpec.UNSPECIFIED;
    }
    break;
  }
  return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

　　在這段代碼對(duì)于MeasureSpec的獲取主要是根據(jù)父容器的MeasureSpec和View本身的LayoutParams。下面通過(guò)一張表格來(lái)看一下它們之間的對(duì)應(yīng)關(guān)系。

到這里通過(guò)getChildMeasureSpec方法獲取到子View的MeasureSpec以后，便調(diào)用View的Measure方法，開始對(duì)View進(jìn)行測(cè)量。
　　正如剛才說(shuō)的那樣對(duì)于ViewGroup它是一個(gè)抽象類，并沒(méi)有重寫View的onMeasure方法。但是到具體的ViewGroup時(shí)，例如FrameLayout，LinearLayout，RelativeLayout等，它們通過(guò)重寫onMeasure方法來(lái)來(lái)完成自身以及子View的Measure過(guò)程。下面以FrameLayout為例，看一下的Measure過(guò)程。在FrameLayout中，它的Measure過(guò)程也算是比較簡(jiǎn)單，下面就來(lái)看一下FrameLayout中的onMeasure方法。

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
  int count = getChildCount();

  final boolean measureMatchParentChildren =
      MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
      MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
  mMatchParentChildren.clear();

  int maxHeight = 0;
  int maxWidth = 0;
  int childState = 0;

  for (int i = 0; i < count; i++) {
    final View child = getChildAt(i);
    if (mMeasureAllChildren || child.getVisibility() != GONE) {
      measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
      final LayoutParams lp = (LayoutParams) child.getLayoutParams();
      maxWidth = Math.max(maxWidth,
          child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
      maxHeight = Math.max(maxHeight,
          child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
      childState = combineMeasuredStates(childState, child.getMeasuredState());
      if (measureMatchParentChildren) {
        if (lp.width == LayoutParams.MATCH_PARENT ||
            lp.height == LayoutParams.MATCH_PARENT) {
          mMatchParentChildren.add(child);
        }
      }
    }
  }

  // Account for padding too
  maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
  maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

  // Check against our minimum height and width
  maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
  maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

  // Check against our foreground's minimum height and width
  final Drawable drawable = getForeground();
  if (drawable != null) {
    maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
    maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
  }

  setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
      resolveSizeAndState(maxHeight, heightMeasureSpec,
          childState << MEASURED_HEIGHT_STATE_SHIFT));

  count = mMatchParentChildren.size();
  if (count > 1) {
    for (int i = 0; i < count; i++) {
      final View child = mMatchParentChildren.get(i);
      final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

      final int childWidthMeasureSpec;
      if (lp.width == LayoutParams.MATCH_PARENT) {
        final int width = Math.max(0, getMeasuredWidth()
            - getPaddingLeftWithForeground() - getPaddingRightWithForeground()
            - lp.leftMargin - lp.rightMargin);
        childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
            width, MeasureSpec.EXACTLY);
      } else {
        childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
            getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
            lp.leftMargin + lp.rightMargin,
            lp.width);
      }

      final int childHeightMeasureSpec;
      if (lp.height == LayoutParams.MATCH_PARENT) {
        final int height = Math.max(0, getMeasuredHeight()
            - getPaddingTopWithForeground() - getPaddingBottomWithForeground()
            - lp.topMargin - lp.bottomMargin);
        childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
            height, MeasureSpec.EXACTLY);
      } else {
        childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
            getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
            lp.topMargin + lp.bottomMargin,
            lp.height);
      }

      child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }
  }
}

　　在這部分代碼中邏輯也很簡(jiǎn)單，主要完成了兩件事。首先FrameLayout完成自身的測(cè)量過(guò)程，然后在遍歷子View，執(zhí)行View的measure方法，完成View的Measure過(guò)程。在這里代碼比較簡(jiǎn)單就不在進(jìn)行詳細(xì)描述。

總結(jié)

　　最后對(duì)View和ViewGroup的Measure過(guò)程做一下總結(jié)。對(duì)于View，它的Measure很簡(jiǎn)單，在獲取到View的高和寬的測(cè)量值之后，便為其設(shè)置高和寬。而對(duì)于ViewGroup來(lái)說(shuō)，除了完成自身的Measure過(guò)程以外，還需要遍歷子View，完成子View的測(cè)量過(guò)程。

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