<p>Android中的任何一个布局、任何一个控件其实都是直接或间接继承自View实现的，当然也包括我们在平时开发中所写的各种炫酷的自定义控件了，所以学习View的工作原理对于我们来说显得格外重要，本篇博客，我们将一起深入学习Android中View的工作原理。</p>    <h2>ViewRoot和DecorView</h2>    <p>1.ViewRoot对应于ViewRootImpl类，是连接WindowManager和DecorView的纽带，View的三大流程均是通过ViewRoot来完成的。在ActivityThread中，当Activity对象被创建完毕后，会将DecorView添加到Window中，同时会创建ViewRootImpl对象，并将ViewRootImpl对象和DecorView建立关联。</p>    <p>2.View的绘制流程从ViewRoot的performTraversals开始，经过measure、layout和draw三个过程才可以把一个View绘制出来，其中measure用来测量View的宽高，layout用来确定View在父容器中的放置位置，而draw则负责将View绘制到屏幕上。</p>    <p>3.performTraversals会依次调用performMeasure、performLayout和performDraw三个方法，这三个方法分别完成顶级View的measure、layout和draw这三大流程。其中performMeasure中会调用measure方法，在measure方法中又会调用onMeasure方法，在onMeasure方法中则会对所有子元素进行measure过程，这样就完成了一次measure过程；子元素会重复父容器的measure过程，如此反复完成了整个View数的遍历。</p>    <p style="text-align:center"><img src="https://simg.open-open.com/show/188cbff7ac1167215ad01dde6e64f49c.png"></p>    <p>measure过程决定了View的宽/高，完成后可通过getMeasuredWidth/getMeasureHeight方法来获取View测量后的宽/高。Layout过程决定了View的四个顶点的坐标和实际View的宽高，完成后可通过getTop、getBotton、getLeft和getRight拿到View的四个定点坐标。Draw过程决定了View的显示，完成后View的内容才能呈现到屏幕上。</p>    <p>DecorView作为顶级View，一般情况下它内部包含了一个竖直方向的LinearLayout，里面分为两个部分（具体情况和Android版本和主题有关），上面是标题栏，下面是内容栏。在Activity通过setContextView所设置的布局文件其实就是被加载到内容栏之中的。</p>    <pre>  <code class="language-java">//获取内容栏  ViewGroup content = findViewById(R.android.id.content);  //获取我们设置的Viewcontext.getChildAt(0);  DecorView其实是一个FrameLayout，View层的事件都先经过DecorView，然后才传给我们的View。</code></pre>    <p style="text-align:center"><img src="https://simg.open-open.com/show/c445c1ea65eed8f947eb93fd79cc0e9e.png"></p>    <h2>MeasureSpec</h2>    <p>1.MeasureSpec很大程度上决定一个View的尺寸规格，测量过程中，系统会将View的layoutParams根据父容器所施加的规则转换成对应的MeasureSpec，再根据这个measureSpec来测量出View的宽/高。</p>    <p>2.MeasureSpec代表一个32位的int值，高2位为SpecMode，低30位为SpecSize，SpecMode是指测量模式，SpecSize是指在某种测量模式下的规格大小。</p>    <p>MpecMode有三类；</p>    <p>1.UNSPECIFIED 父容器不对View进行任何限制，要多大给多大，一般用于系统内部</p>    <p>2.EXACTLY 父容器检测到View所需要的精确大小，这时候View的最终大小就是SpecSize所指定的值，对应LayoutParams中的match_parent和具体数值这两种模式。</p>    <p>3.AT_MOST 父容器指定了一个可用大小即SpecSize，View的大小不能大于这个值，不同View实现不同，对应LayoutParams中的wrap_content。</p>    <p>当View采用固定宽/高的时候，不管父容器的MeasureSpec的是什么，View的MeasureSpec都是精确模式兵其大小遵循Layoutparams的大小。 当View的宽/高是match_parent时，如果他的父容器的模式是精确模式，那View也是精确模式并且大小是父容器的剩余空间；如果父容器是最大模式，那么View也是最大模式并且起大小不会超过父容器的剩余空间。 当View的宽/高是wrap_content时，不管父容器的模式是精确还是最大化，View的模式总是最大化并且不能超过父容器的剩余空间。</p>    <p>对于DecorView，它的MeasureSpec由Window的尺寸和其自身的LayoutParams来共同确定，对于普通的View，其MeasureSpec由父容器的MeasureSpec和自身的Layoutparams来共同确定。</p>    <p>对于 DecorView，在ViewRootImpl源码中的measureHierarchy有如下一段代码：</p>    <pre>  <code class="language-java">.........  if (baseSize != 0 && desiredWindowWidth > baseSize) {                  childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);                  childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);                  performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);                  if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured ("                          + host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")");                  if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) {                      goodMeasure = true;  .........</code></pre>    <p>我们查看一下getRootMeasureSpec方法的源码：</p>    <pre>  <code class="language-java">private static int getRootMeasureSpec(int windowSize, int rootDimension) {          int measureSpec;          switch (rootDimension) {            case ViewGroup.LayoutParams.MATCH_PARENT:              // Window can't resize. Force root view to be windowSize.              measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);              break;          case ViewGroup.LayoutParams.WRAP_CONTENT:              // Window can resize. Set max size for root view.              measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);              break;          default:              // Window wants to be an exact size. Force root view to be that size.              measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);              break;          }          return measureSpec;      }</code></pre>    <p>从上面的代码中就可以很容理解DecorView的MeasureSpec是如何产生的，rootDimension就是DecorView自身的LayoutParams，然后会根据这个值进行判断</p>    <p>LayoutParams.MATCH_PARENT：DecorView的MeasureSpec被赋值为精确模式，DecorView的大小就是Window的大小</p>    <p>ViewGroup.LayoutParams.WRAP_CONTENT：DecorView的MeasureSpec被赋值为最大模式，DecorView的大小不定，但是不能超过Window的大小</p>    <p>默认情况：DecorView的MeasureSpec被赋值为精确模式，DecorView的大小为自身LayoutParams设置的值，也就是rootDimension</p>    <p>接着是对于普通的View，也就是布局中的View，它的Measure过程由ViewGroup传递而来，其中有一个方法是measureChildWithMargins</p>    <pre>  <code class="language-java">protected void measureChildWithMargins(View child,              int parentWidthMeasureSpec, int widthUsed,              int parentHeightMeasureSpec, int heightUsed) {          final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();            final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,                  mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin                          + widthUsed, lp.width);          final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,                  mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin                          + heightUsed, lp.height);            child.measure(childWidthMeasureSpec, childHeightMeasureSpec);      }</code></pre>    <p>在对子view进行measure之前会先调用getChildMeasureSpec方法来获取子view的MeasureSpec，从这段代码就可以看出来子view的MeasureSpec的确定与父容器的MeasureSpec（parentWidthMeasureSpec）还有自身的LayoutParams（lp.height和lp.width），还有View自己的Margin和Padding有关</p>    <p>接下来查看getChildMeasureSpec方法源码：</p>    <pre>  <code class="language-java">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);      }</code></pre>    <p>这里参数中的padding是指父容器的padding，这里是父容器所占用的空间，所以子view能使用的空间要减去这个padding的值。同时这个方法内部其实就是根据父容器的MeasureSpec结合子view的LayoutParams来确定子view的MeasureSpec</p>    <h2>View的绘制流程</h2>    <p>measure的过程如果只是一个View，那么通过measure方法就完成了其测量的过程，如果是一个ViewGroup，除了测量自身外，还会调用子孩子的measure方法</p>    <p>1.View的measure过程View的measure过程由其measure方法完成，其中有下面一段内容</p>    <pre>  <code class="language-java">.........  int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :                      mMeasureCache.indexOfKey(key);              if (cacheIndex < 0 || sIgnoreMeasureCache) {                  // measure ourselves, this should set the measured dimension flag back                  onMeasure(widthMeasureSpec, heightMeasureSpec);                  mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;              } else {                  long value = mMeasureCache.valueAt(cacheIndex);                  // Casting a long to int drops the high 32 bits, no mask needed                  setMeasuredDimensionRaw((int) (value >> 32), (int) value);                  mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;              }  .........</code></pre>    <p>可以知道View的measure方法内，其实调用了自身的onMeasure方法</p>    <pre>  <code class="language-java">protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {          setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),                  getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));  }  //里面有一个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;      }</code></pre>    <p>一般我们只需要看MeasureSpec.AT_MOST和MeasureSpec.EXACTLY两种情况，这两种情况返回的result其实都是measureSpec中取得的specSize，这个specSize就是View测量后的大小，这里之所以是View测量后的大小，是因为View的最终大小是在layout阶段确定的，所以要加已区分，一般情况下View测量大小和最终大小是一样的。</p>    <p>UNSPECIFIED情况下，result的值就是getSuggestedMinimumWidth()方法和getSuggestedMinimumHeight()返回的值，查看这两个方法</p>    <pre>  <code class="language-java">protected int getSuggestedMinimumWidth() {          return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());      }      protected int getSuggestedMinimumHeight() {          return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());    }</code></pre>    <p>从getSuggestedMinimumWidth代码可以看出，如果View没有设置背景，那么宽度就为mMinWidth，这个值对应android：minWidth这个属性所设定的值，如果View设置了背景，则为max(mMinWidth, mBackground.getMinimumWidth())</p>    <pre>  <code class="language-java">public int getMinimumWidth() {          final int intrinsicWidth = getIntrinsicWidth();          return intrinsicWidth > 0 ? intrinsicWidth : 0;      }</code></pre>    <p>查看mBackground.getMinimumWidth()方法，它其实是Drawable的方法，如果intrinsicHeight也就是原始的宽度不为0，就返回它，如果为0，就返回0。</p>    <p>从View的getDefaultSize方法可以得出结论：View的宽高由specSize决定，如果我们通过继承View来自定义控件需要重写onMeasure方法，并设置WRAP_CONTENT时的大小，否则在布局中使用WRAP_CONTENT相当于使用MATCH_PARENT</p>    <p>原因：因为View在布局中使用WRAP_CONTENT就相当于specMode为AT_MOST，而这种情况下，result = specSize，这个specSize的大小为parentSize, parentSize就是父容器目前可用的大小，也就是父容器当前剩余空间的大小，那这时候和在布局中使用MATCH_PARENT效果是一样的</p>    <p>所以在AT_MOST模式下，我们一般都会给View设定默认的内部宽高，并在WRAP_CONTENT时设置此宽高即可。</p>    <p>可以通过查看TextView、ImageView的源码，可以得知在WRAP_CONTENT下，onMeasure方法均做了特殊的处理，下面是TextView的onMeasure中的一段内容</p>    <pre>  <code class="language-java">if (widthMode == MeasureSpec.AT_MOST) {                  width = Math.min(widthSize, width);              }</code></pre>    <p>2.ViewGroup的measure流程ViewGroup是一个抽象类，它没有重写View的onMeasure方法，而是自己提供了一个measureChildren方法</p>    <pre>  <code class="language-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);              }          }      }</code></pre>    <p>里面会对子元素进行遍历，然后调用measureChild方法去测量每一个子元素的宽高</p>    <pre>  <code class="language-java">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);  }</code></pre>    <p>在对子view进行measure之前会先调用getChildMeasureSpec方法来获取子孩子的MeasureSpec，从这段代码就可以看出来子view的MeasureSpec的确定与父容器的MeasureSpec（parentWidthMeasureSpec和parentHeightMeasureSpec）还有自身的LayoutParams（lp.height和lp.width），还有View自己的Margin和Padding有关，最后就是调用子view的measure方法</p>    <p>ViewGroup并没有去定义测量的具体过程，这是因为ViewGroup是一个抽象类，其onMeasure方法需要各个子类去实现，因为每个ViewGroup的实现类，例如LinearLayout,RelativeLayout等的布局方式都是不同的，所以不可能一概而论的来写onMeasure方法。</p>    <p>接下来分析LinearLayout的onMeasure方法：</p>    <pre>  <code class="language-java">protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {          if (mOrientation == VERTICAL) {              measureVertical(widthMeasureSpec, heightMeasureSpec);          } else {              measureHorizontal(widthMeasureSpec, heightMeasureSpec);          }  }</code></pre>    <p>查看measureVertical方法</p>    <pre>  <code class="language-java">// See how tall everyone is. Also remember max width.          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));                    if (useLargestChild) {                      largestChildHeight = Math.max(childHeight, largestChildHeight);                  }              }                /**               * If applicable, compute the additional offset to the child's baseline               * we'll need later when asked {@link #getBaseline}.               */              if ((baselineChildIndex >= 0) && (baselineChildIndex == i + 1)) {                 mBaselineChildTop = mTotalLength;              }                // if we are trying to use a child index for our baseline, the above              // book keeping only works if there are no children above it with              // weight.  fail fast to aid the developer.              if (i < baselineChildIndex && lp.weight > 0) {                  throw new RuntimeException("A child of LinearLayout with index "                          + "less than mBaselineAlignedChildIndex has weight > 0, which "                          + "won't work.  Either remove the weight, or don't set "                          + "mBaselineAlignedChildIndex.");              }                boolean matchWidthLocally = false;              if (widthMode != MeasureSpec.EXACTLY && lp.width == LayoutParams.MATCH_PARENT) {                  // The width of the linear layout will scale, and at least one                  // child said it wanted to match our width. Set a flag                  // indicating that we need to remeasure at least that view when                  // we know our width.                  matchWidth = true;                  matchWidthLocally = true;              }                final int margin = lp.leftMargin + lp.rightMargin;              final int measuredWidth = child.getMeasuredWidth() + margin;              maxWidth = Math.max(maxWidth, measuredWidth);              childState = combineMeasuredStates(childState, child.getMeasuredState());                allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;              if (lp.weight > 0) {                  /*                   * Widths of weighted Views are bogus if we end up                   * remeasuring, so keep them separate.                   */                  weightedMaxWidth = Math.max(weightedMaxWidth,                          matchWidthLocally ? margin : measuredWidth);              } else {                  alternativeMaxWidth = Math.max(alternativeMaxWidth,                          matchWidthLocally ? margin : measuredWidth);              }                i += getChildrenSkipCount(child, i);          }</code></pre>    <p>遍历子元素，调用他们的measureChildBeforeLayout方法，这个方法内会测量子孩子的宽高，并且有一个mTotalLength来记录LinearLayout 在竖直方向的初步高度，每测量一次子元素，mTotalLength都会增加，增加部分包括子元素的高度以及子元素竖直方向的margin</p>    <pre>  <code class="language-java">void measureChildBeforeLayout(View child, int childIndex,              int widthMeasureSpec, int totalWidth, int heightMeasureSpec,              int totalHeight) {          measureChildWithMargins(child, widthMeasureSpec, totalWidth,                  heightMeasureSpec, totalHeight);      }  里面调用了child.measure方法，也就是子孩子的measure方法  protected void measureChildWithMargins(View child,              int parentWidthMeasureSpec, int widthUsed,              int parentHeightMeasureSpec, int heightUsed) {          final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();            final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,                  mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin                          + widthUsed, lp.width);          final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,                  mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin                          + heightUsed, lp.height);            child.measure(childWidthMeasureSpec, childHeightMeasureSpec);  }</code></pre>    <p>当子元素测量完毕后，LinearLayout会测量自身的大小，对于竖直的LinearLayout，它在水平方向上的测量过程，遵循View的测量过程，在竖直方向上，如果采用的是match_parent或者具体的数值，那么它的测量过程和View的一致，即高度为specSize；如果它的布局中高度采用wrap_content，那么高度是子元素所占用的高度总和，但这个和不能超过父容器的剩余空间，当然还要考虑padding，竖直方向的结论可以从下面代码得知：</p>    <pre>  <code class="language-java">public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {          final int specMode = MeasureSpec.getMode(measureSpec);          final int specSize = MeasureSpec.getSize(measureSpec);          final int result;          switch (specMode) {              case MeasureSpec.AT_MOST:                  if (specSize < size) {                      result = specSize | MEASURED_STATE_TOO_SMALL;                  } else {                      result = size;                  }                  break;              case MeasureSpec.EXACTLY:                  result = specSize;                  break;              case MeasureSpec.UNSPECIFIED:              default:                  result = size;          }          return result | (childMeasuredState & MEASURED_STATE_MASK);  }</code></pre>    <p>有时候onMeasure中拿到的测量宽高可能是不准确的，比较好的习惯是在onLayout中去获取View的测量宽高和最终宽高</p>    <p>在Activity中，在onCreate,onStart,onResume中均无法正确获得View的宽高信息，这是因为measure和Activity的生命周期是不同步的，所以很可能View没有测量完毕，获得的宽高是0.</p>    <p>measure总结1.measure过程主要就是从顶层父View向子View递归调用view.measure方法（measure中又回调onMeasure方法）的过程。具体measure核心主要有如下几点：</p>    <p>2.MeasureSpec（View的内部类）测量规格为int型，值由高2位规格模式specMode和低30位具体尺寸specSize组成。其中specMode只有三种值：</p>    <pre>  <code class="language-java">MeasureSpec.EXACTLY //确定模式，父View希望子View的大小是确定的，由specSize决定；  MeasureSpec.AT_MOST //最多模式，父View希望子View的大小最多是specSize指定的值；  MeasureSpec.UNSPECIFIED //未指定模式，父View完全依据子View的设计值来决定；</code></pre>    <p>3.View的measure方法是final的，不允许重载，View子类只能重载onMeasure来完成自己的测量逻辑。</p>    <p>4.最顶层DecorView测量时的MeasureSpec是由ViewRootImpl中getRootMeasureSpec方法确定的（LayoutParams宽高参数均为MATCH_PARENT，specMode是EXACTLY，specSize为物理屏幕大小）。</p>    <p>5.ViewGroup类提供了measureChild，measureChild和measureChildWithMargins方法，简化了父子View的尺寸计算。</p>    <p>6.只要是ViewGroup的子类就必须要求LayoutParams继承子MarginLayoutParams，否则无法使用layout_margin参数。</p>    <p>7.View的布局大小由父View和子View共同决定。</p>    <p>8.使用View的getMeasuredWidth()和getMeasuredHeight()方法来获取View测量的宽高，必须保证这两个方法在onMeasure流程之后被调用才能返回有效值。</p>    <p>layout的过程ViewGroup的位置确定后，它在onLayout中会遍历所有的子元素并调用子元素layout方法，子元素layout方法中又会调用onLayout方法，View的layout方法确定自身的位置，而onLayout方法方法确定子孩子的位置</p>    <pre>  <code class="language-java">public void layout(int l, int t, int r, int b) {          if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {              onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);              mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;          }            int oldL = mLeft;          int oldT = mTop;          int oldB = mBottom;          int oldR = mRight;            boolean changed = isLayoutModeOptical(mParent) ?                  setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);            if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {              onLayout(changed, l, t, r, b);              mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;                ListenerInfo li = mListenerInfo;              if (li != null && li.mOnLayoutChangeListeners != null) {                  ArrayList<OnLayoutChangeListener> listenersCopy =                          (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();                  int numListeners = listenersCopy.size();                  for (int i = 0; i < numListeners; ++i) {                      listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);                  }              }          }            mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;          mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;  }</code></pre>    <p>layout方法的大致流程如下：首先会通过setFrame方法来确定mLeft;mTop;mBottom;</p>    <p>mRight;只要这四个点一旦确定，那么View在父容器中的位置就确定了，接着会调用onLayout方法，该方法目的是父容器来确定子元素的位置，无论是View还是ViewGroup都没有实现onLayout方法，我们查看LinearLayout的onLayout方法</p>    <pre>  <code class="language-java">@Override      protected void onLayout(boolean changed, int l, int t, int r, int b) {          if (mOrientation == VERTICAL) {              layoutVertical(l, t, r, b);          } else {              layoutHorizontal(l, t, r, b);          }  }</code></pre>    <p>查看layoutVertical中关键代码</p>    <pre>  <code class="language-java">for (int i = 0; i < count; i++) {              final View child = getVirtualChildAt(i);              if (child == null) {                  childTop += measureNullChild(i);              } else if (child.getVisibility() != GONE) {                  final int childWidth = child.getMeasuredWidth();                  final int childHeight = child.getMeasuredHeight();                    final LinearLayout.LayoutParams lp =                          (LinearLayout.LayoutParams) child.getLayoutParams();                    int gravity = lp.gravity;                  if (gravity < 0) {                      gravity = minorGravity;                  }                  final int layoutDirection = getLayoutDirection();                  final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);                  switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {                      case Gravity.CENTER_HORIZONTAL:                          childLeft = paddingLeft + ((childSpace - childWidth) / 2)                                  + lp.leftMargin - lp.rightMargin;                          break;                        case Gravity.RIGHT:                          childLeft = childRight - childWidth - lp.rightMargin;                          break;                        case Gravity.LEFT:                      default:                          childLeft = paddingLeft + lp.leftMargin;                          break;                  }                    if (hasDividerBeforeChildAt(i)) {                      childTop += mDividerHeight;                  }                    childTop += lp.topMargin;                  setChildFrame(child, childLeft, childTop + getLocationOffset(child),                          childWidth, childHeight);                  childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);                    i += getChildrenSkipCount(child, i);              }          }</code></pre>    <p>这个方法会遍历所有的子元素并调用setChildFrame方法来为子元素指定对应的位置，其中childTop的数值会不断的增大，这意味着后面的子元素还位于靠下的位置，刚好符合竖直的LinearLayout的特性，setChildFrame方法中不过是调用了子元素的Layout方法而已</p>    <pre>  <code class="language-java">private void setChildFrame(View child, int left, int top, int width, int height) {                  child.layout(left, top, left + width, top + height);  }</code></pre>    <p>同时，会发现setChildFrame中的width和height实际上就是子元素的测量宽高</p>    <pre>  <code class="language-java">final int childWidth = child.getMeasuredWidth();               final int childHeight = child.getMeasuredHeight();</code></pre>    <p>View的layout方法中会通过setFrame方法去设置子元素四个顶点的位置，这样子元素的位置就可以确定</p>    <pre>  <code class="language-java">int oldWidth = mRight - mLeft;              int oldHeight = mBottom - mTop;              int newWidth = right - left;              int newHeight = bottom - top;              boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);                // Invalidate our old position              invalidate(sizeChanged);                mLeft = left;              mTop = top;              mRight = right;              mBottom = bottom;              mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);</code></pre>    <p>接下来是View的getWidth和getHeight方法，结合里面的实现，可以发现他们分别返回的就是View测量的宽度和高度</p>    <pre>  <code class="language-java">@ViewDebug.ExportedProperty(category = "layout")      public final int getWidth() {          return mRight - mLeft;      }        /**       * Return the height of your view.       *       * @return The height of your view, in pixels.       */      @ViewDebug.ExportedProperty(category = "layout")      public final int getHeight() {          return mBottom - mTop;  }</code></pre>    <p>layout总结1.layout也是从顶层父View向子View的递归调用view.layout方法的过程，即父View根据上一步measure子View所得到的布局大小和布局参数，将子View放在合适的位置上。</p>    <p>2.View.layout方法可被重载，ViewGroup.layout为final的不可重载，ViewGroup.onLayout为abstract的，子类必须重载实现自己的位置逻辑。</p>    <p>3.measure操作完成后得到的是对每个View经测量过的measuredWidth和measuredHeight，layout操作完成之后得到的是对每个View进行位置分配后的mLeft、mTop、mRight、mBottom，这些值都是相对于父View来说的。</p>    <p>4.凡是layout_XXX的布局属性基本都针对的是包含子View的ViewGroup的，当对一个没有父容器的View设置相关layout_XXX属性是没有任何意义的。</p>    <p>5.使用View的getWidth()和getHeight()方法来获取View测量的宽高，必须保证这两个方法在onLayout流程之后被调用才能返回有效值。</p>    <p><strong>draw的过程</strong></p>    <p>View的绘制过程遵循以下几步：</p>    <p>1)绘制背景background.draw(canvas)</p>    <p>2)绘制自己（onDraw）</p>    <p>3)绘制 children（dispatchDraw）</p>    <p>4)绘制装饰（onDrawScrollBars）</p>    <pre>  <code class="language-java">public void draw(Canvas canvas) {          final int privateFlags = mPrivateFlags;          final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&                  (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);          mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;            /*           * Draw traversal performs several drawing steps which must be executed           * in the appropriate order:           *           *      1. Draw the background           *      2. If necessary, save the canvas' layers to prepare for fading           *      3. Draw view's content           *      4. Draw children           *      5. If necessary, draw the fading edges and restore layers           *      6. Draw decorations (scrollbars for instance)           */            // Step 1, draw the background, if needed          int saveCount;            if (!dirtyOpaque) {              drawBackground(canvas);          }            // skip step 2 & 5 if possible (common case)          final int viewFlags = mViewFlags;          boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;          boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;          if (!verticalEdges && !horizontalEdges) {              // Step 3, draw the content              if (!dirtyOpaque) onDraw(canvas);                // Step 4, draw the children              dispatchDraw(canvas);                // Overlay is part of the content and draws beneath Foreground              if (mOverlay != null && !mOverlay.isEmpty()) {                  mOverlay.getOverlayView().dispatchDraw(canvas);              }                // Step 6, draw decorations (foreground, scrollbars)              onDrawForeground(canvas);                // we're done...              return;          }            /*           * Here we do the full fledged routine...           * (this is an uncommon case where speed matters less,           * this is why we repeat some of the tests that have been           * done above)           */            boolean drawTop = false;          boolean drawBottom = false;          boolean drawLeft = false;          boolean drawRight = false;            float topFadeStrength = 0.0f;          float bottomFadeStrength = 0.0f;          float leftFadeStrength = 0.0f;          float rightFadeStrength = 0.0f;            // Step 2, save the canvas' layers          int paddingLeft = mPaddingLeft;            final boolean offsetRequired = isPaddingOffsetRequired();          if (offsetRequired) {              paddingLeft += getLeftPaddingOffset();          }            int left = mScrollX + paddingLeft;          int right = left + mRight - mLeft - mPaddingRight - paddingLeft;          int top = mScrollY + getFadeTop(offsetRequired);          int bottom = top + getFadeHeight(offsetRequired);            if (offsetRequired) {              right += getRightPaddingOffset();              bottom += getBottomPaddingOffset();          }            final ScrollabilityCache scrollabilityCache = mScrollCache;          final float fadeHeight = scrollabilityCache.fadingEdgeLength;          int length = (int) fadeHeight;            // clip the fade length if top and bottom fades overlap          // overlapping fades produce odd-looking artifacts          if (verticalEdges && (top + length > bottom - length)) {              length = (bottom - top) / 2;          }            // also clip horizontal fades if necessary          if (horizontalEdges && (left + length > right - length)) {              length = (right - left) / 2;          }            if (verticalEdges) {              topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));              drawTop = topFadeStrength * fadeHeight > 1.0f;              bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));              drawBottom = bottomFadeStrength * fadeHeight > 1.0f;          }            if (horizontalEdges) {              leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));              drawLeft = leftFadeStrength * fadeHeight > 1.0f;              rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));              drawRight = rightFadeStrength * fadeHeight > 1.0f;          }            saveCount = canvas.getSaveCount();            int solidColor = getSolidColor();          if (solidColor == 0) {              final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;                if (drawTop) {                  canvas.saveLayer(left, top, right, top + length, null, flags);              }                if (drawBottom) {                  canvas.saveLayer(left, bottom - length, right, bottom, null, flags);              }                if (drawLeft) {                  canvas.saveLayer(left, top, left + length, bottom, null, flags);              }                if (drawRight) {                  canvas.saveLayer(right - length, top, right, bottom, null, flags);              }          } else {              scrollabilityCache.setFadeColor(solidColor);          }            // Step 3, draw the content          if (!dirtyOpaque) onDraw(canvas);            // Step 4, draw the children          dispatchDraw(canvas);            // Step 5, draw the fade effect and restore layers          final Paint p = scrollabilityCache.paint;          final Matrix matrix = scrollabilityCache.matrix;          final Shader fade = scrollabilityCache.shader;            if (drawTop) {              matrix.setScale(1, fadeHeight * topFadeStrength);              matrix.postTranslate(left, top);              fade.setLocalMatrix(matrix);              p.setShader(fade);              canvas.drawRect(left, top, right, top + length, p);          }            if (drawBottom) {              matrix.setScale(1, fadeHeight * bottomFadeStrength);              matrix.postRotate(180);              matrix.postTranslate(left, bottom);              fade.setLocalMatrix(matrix);              p.setShader(fade);              canvas.drawRect(left, bottom - length, right, bottom, p);          }            if (drawLeft) {              matrix.setScale(1, fadeHeight * leftFadeStrength);              matrix.postRotate(-90);              matrix.postTranslate(left, top);              fade.setLocalMatrix(matrix);              p.setShader(fade);              canvas.drawRect(left, top, left + length, bottom, p);          }            if (drawRight) {              matrix.setScale(1, fadeHeight * rightFadeStrength);              matrix.postRotate(90);              matrix.postTranslate(right, top);              fade.setLocalMatrix(matrix);              p.setShader(fade);              canvas.drawRect(right - length, top, right, bottom, p);          }            canvas.restoreToCount(saveCount);            // Overlay is part of the content and draws beneath Foreground          if (mOverlay != null && !mOverlay.isEmpty()) {              mOverlay.getOverlayView().dispatchDraw(canvas);          }            // Step 6, draw decorations (foreground, scrollbars)          onDrawForeground(canvas);  }</code></pre>    <p>View的绘制过程的传递是通过dispatchDraw实现的，dispatchdraw会遍历调用所有子元素的draw方法。如此draw事件就一层一层的传递下去。</p>    <p>draw总结1.如果该View是一个ViewGroup，则需要递归绘制其所包含的所有子View。</p>    <p>2.View默认不会绘制任何内容，真正的绘制都需要自己在子类中实现。</p>    <p>3.View的绘制是借助onDraw方法传入的Canvas类来进行的。</p>    <p>4.在获取画布剪切区（每个View的draw中传入的Canvas）时会自动处理掉padding，子View获取Canvas不用关注这些逻辑，只用关心如何绘制即可。</p>    <p>5.默认情况下子View的ViewGroup.drawChild绘制顺序和子View被添加的顺序一致，但是你也可以重载ViewGroup.getChildDrawingOrder()方法提供不同顺序。</p>    <p>参考资料</p>    <p>《Android开发艺术探索》</p>    <p> </p>    <p>来自：http://blog.csdn.net/u012124438/article/details/71435787</p>    <p> </p>