3.3onMeasure流程设计

目录介绍

• 01.Measure流程分析
• 02.MeasureSpec的确定
• 03.View的测量流程
• 04.View的onMeasure源码
• 05.NestedScrollView测量过程
• 06.看几个思考题分析
• 07.View中onMeasure方法
• 08.ViewGroup中onMeasure方法

01.Measure流程分析

• 顾名思义，就是测量每个控件的大小。调用measure()方法，进行一些逻辑处理，然后调用onMeasure()方法，在其中调用setMeasuredDimension()设定View的宽高信息，完成View的测量操作。

  public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
  }

• measure()方法中，传入了两个参数 widthMeasureSpec, heightMeasureSpec 表示View的宽高的一些信息。

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

• 由上述流程来看Measure流程很简单，关键点是在于widthMeasureSpec, heightMeasureSpec这两个参数信息怎么获得？
  • 如果有了widthMeasureSpec, heightMeasureSpec，通过一定的处理(可以重写，自定义处理步骤)，从中获取View的宽/高，调用setMeasuredDimension()方法，指定View的宽高，完成测量工作。

  @Override
  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
      //super.onMeasure(widthMeasureSpec, heightMeasureSpec);
      //从中获取View的宽/高
      int width = getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec);
      int height = getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec);
      //指定View的宽高，完成测量工作
      setMeasuredDimension(width,height);
  }

02.MeasureSpec的确定

2.1 MeasureSpec是什么

• 先介绍下什么是MeasureSpec？
  • 
    img
• MeasureSpec由两部分组成，一部分是测量模式，另一部分是测量的尺寸大小。
  • 其中，Mode模式共分为三类
    • UNSPECIFIED ：不对View进行任何限制，要多大给多大，一般用于系统内部
    • EXACTLY：对应LayoutParams中的match_parent和具体数值这两种模式。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
    • AT_MOST ：对应LayoutParams中的wrap_content。View的大小不能大于父容器的大小。
• 那么MeasureSpec又是如何确定的？
  • 对于DecorView，其确定是通过屏幕的大小，和自身的布局参数LayoutParams。
  • 这部分很简单，根据LayoutParams的布局格式（match_parent，wrap_content或指定大小），将自身大小，和屏幕大小相比，设置一个不超过屏幕大小的宽高，以及对应模式。
  • 对于其他View（包括ViewGroup），其确定是通过父布局的MeasureSpec和自身的布局参数LayoutParams。
• 这部分比较复杂。以下列图表表示不同的情况：
  • 
    img
  • 当子View的LayoutParams的布局格式是wrap_content，可以看到子View的大小是父View的剩余尺寸，和设置成match_parent时，子View的大小没有区别。为了显示区别，一般在自定义View时，需要重写onMeasure方法，处理wrap_content时的情况，进行特别指定。
  • 从这里看出MeasureSpec的指定也是从顶层布局开始一层层往下去，父布局影响子布局。

2.2 MeasureSpec静态类

• 代码如下所示
  • MeasureSpec总结起来就是：
  • 它由2部分数据组成，分别为定义了View测量的模式和View的测量尺寸大小
  • 其中EXACTLY精确模式表示的是match_parent和具体值；AT_MOST最大模式表示的是wrap_content的情况

  //view.class
  public static class MeasureSpec {
      private static final int MODE_SHIFT = 30;
      private static final int MODE_MASK  = 0x3 << MODE_SHIFT;
  
      /**
       * 这种模式不用关心
       */
      public static final int UNSPECIFIED = 0 << MODE_SHIFT;
  
      /**
       * 精确模式，对应的是match_parent和具体值，比如100dp
      public static final int EXACTLY     = 1 << MODE_SHIFT;
  
      /**
       * 最大模式，对应的就是wrap_content
       */
      public static final int AT_MOST     = 2 << MODE_SHIFT;
  
     
      public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size,
                                        @MeasureSpecMode int mode) {
          if (sUseBrokenMakeMeasureSpec) {
              return size + mode;
          } else {
              return (size & ~MODE_MASK) | (mode & MODE_MASK);
          }
      }
  
      /**
       * 获取测量的模式
       */
      @MeasureSpecMode
      public static int getMode(int measureSpec) {
          //noinspection ResourceType
          return (measureSpec & MODE_MASK);
      }
  
      /**
       * 获取测量到的尺寸大小
       */
      public static int getSize(int measureSpec) {
          return (measureSpec & ~MODE_MASK);
      }
  }

03.View的测量流程

• View的测量流程如下所示
  • 
    img

04.View的onMeasure源码

• View中onMeasure方法已经默认为我们的控件测量了宽高，我们看看它做了什么工作：

  protected void onMeasure( int widthMeasureSpec, int heightMeasureSpec) {
      setMeasuredDimension( getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
              getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
  }
  /**
   * 为宽度获取一个建议最小值
   */
  protected int getSuggestedMinimumWidth () {
      return (mBackground == null) ? mMinWidth : max(mMinWidth , mBackground.getMinimumWidth());
  }
  /**
   * 获取默认的宽高值
   */
  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;
  }

• 从源码可以知道：
  • 如果View的宽高模式为未指定，他的宽高将设置为android:minWidth/Height =”“值与背景宽高值中较大的一个；
  • 如果View的宽高 模式为 EXACTLY （具体的size ），最终宽高就是这个size值；
  • 如果View的宽高模式为EXACTLY （填充父控件 ），最终宽高将为填充父控件；
  • 如果View的宽高模式为AT_MOST （包裹内容），最终宽高也是填充父控件。
• 也就是说如果我们的自定义控件在布局文件中，只需要设置指定的具体宽高，或者MATCH_PARENT 的情况，我们可以不用重写onMeasure方法。但如果自定义控件需要设置包裹内容WRAP_CONTENT ，我们需要重写onMeasure方法，为控件设置需要的尺寸；默认情况下WRAP_CONTENT 的处理也将填充整个父控件。

05.NestedScrollView测量过程

• NestedScrollView是继承FrameLayout的控件，然后先看看onMeasure方法中的源码

  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
      //通过super方法先测量父布局
      super.onMeasure(widthMeasureSpec, heightMeasureSpec);
      if (this.mFillViewport) {
          int heightMode = MeasureSpec.getMode(heightMeasureSpec);
          if (heightMode != 0) {
              if (this.getChildCount() > 0) {
                  //获取自布局，NestedScrollView下面只能嵌套一个线性布局或者相对布局。
                  View child = this.getChildAt(0);
                  LayoutParams lp = (LayoutParams)child.getLayoutParams();
                  int childSize = child.getMeasuredHeight();
                  int parentSpace = this.getMeasuredHeight() - this.getPaddingTop() - this.getPaddingBottom() - lp.topMargin - lp.bottomMargin;
                  if (childSize < parentSpace) {
                      int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec, this.getPaddingLeft() + this.getPaddingRight() + lp.leftMargin + lp.rightMargin, lp.width);
                      int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(parentSpace, 1073741824);
                      //测量子类child的控件
                      child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
                  }
              }
  
          }
      }
  }

06.看几个思考题分析

6.1 getWidth()方法和getMeasureWidth()

• getWidth()方法和getMeasureWidth()区别是什么？
  • getMeasureWidth()
    • getMeasureWidth()方法在measure()过程结束后就可以获取到了，另外，getMeasureWidth()方法中的值是通过setMeasuredDimension()方法来进行设置的
    • 这里mMeasuredWidth & MEASURED_SIZE_MASK表示的是测量阶段结束之后，view真实的值。

    public final int getMeasuredWidth() {
        return mMeasuredWidth & MEASURED_SIZE_MASK;
    }

  • getWidth()
    • getWidth()方法要在layout()过程结束后才能获取到，getWidth()方法中的值则是通过视图右边的坐标减去左边的坐标计算出来的。
    • mRight和mLeft是什么值，是在什么时候被设置的。具体看layout()过程中源码

    @ViewDebug.ExportedProperty(category = "layout")
    public final int getWidth() {
        return mRight - mLeft;
    }

6.2 getWidth()或者getMeasureWidth()得到0

• 为什么有时候用getWidth()或者getMeasureWidth()得到0
  • 问题描述：使用getMeasuredWidth()和getMeasuredHeight()方法，无论是在onCreate()、onStart()、onResume()中调用，都无法得到控件的长度、和宽度。如下图，测量的结果为０。
  • 解释：技术博客大总结
    • 因为View的Measure过程和Activity的生命周期方法不是同步执行的，所以无法保证Activity执行了onCreate()、onStart()、onResume()时某个View已经测量完毕了，如果View还没有测量完毕，那么获得宽／高就是０。
• 解决控件测量宽高问题
  • 如下所示

  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      test();
  }
  
  @Override
  public void onWindowFocusChanged(boolean hasFocus) {
      super.onWindowFocusChanged(hasFocus);
      getWidth(4);
  }
  
  private void test(){
      getWidth(1);
      
      marqueeView.measure(0, 0);
      getWidth(2);
      
      marqueeView.post(new Runnable() {
          @Override
          public void run() {
              getWidth(3);
          }
      });
  }
  
  private void getWidth(int a){
      int width2 = marqueeView.getWidth();
      int measuredWidth2 = marqueeView.getMeasuredWidth();
      Log.e(a+"MainActivity-----",width2+"-----"+measuredWidth2);
  }
  
  //11-28 17:03:17.559 15990-15990/com.yc.cn.ycbanner E/1MainActivity-----: 0-----0
  //11-28 17:03:17.567 15990-15990/com.yc.cn.ycbanner E/2MainActivity-----: 0-----760
  //11-28 17:03:17.684 15990-15990/com.yc.cn.ycbanner E/3MainActivity-----: 960-----960
  //11-28 17:03:17.685 15990-15990/com.yc.cn.ycbanner E/4MainActivity-----: 960-----960

• 什么时候测量宽高不等于实际宽高？
  • MeasuredWidth/height!=getWidth/Height()的场景：更改View的布局参数并进行重新布局后，就会导致测量宽高!=实际宽高

07.View中onMeasure方法

• 下面是真是开发案例中的代码，如下所示

  //Android7.0以后，优化了View的绘制，onMeasure和onSizeChanged调用顺序有所变化
  //Android7.0以下：onMeasure--->onSizeChanged--->onMeasure
  //Android7.0以上：onMeasure--->onSizeChanged
  @Override
  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
      int heightSize = MeasureSpec.getSize(heightMeasureSpec);
      int heightMode = MeasureSpec.getMode(heightMeasureSpec);
      /*
       * onMeasure传入的widthMeasureSpec和heightMeasureSpec不是一般的尺寸数值，而是将模式和尺寸组合在一起的数值
       * MeasureSpec.EXACTLY 是精确尺寸
       * MeasureSpec.AT_MOST 是最大尺寸
       * MeasureSpec.UNSPECIFIED 是未指定尺寸
       */
  
      if (heightMode == MeasureSpec.EXACTLY) {
          heightSize = MeasureSpec.makeMeasureSpec(heightSize, MeasureSpec.EXACTLY);
      } else if (heightMode == MeasureSpec.AT_MOST && getParent() instanceof ViewGroup
              && heightSize == ViewGroup.LayoutParams.MATCH_PARENT) {
          heightSize = MeasureSpec.makeMeasureSpec(((ViewGroup) getParent()).getMeasuredHeight(), MeasureSpec.AT_MOST);
      } else {
          int heightNeeded;
          if (gravity == Gravity.CENTER) {
              if (tickMarkTextArray != null && tickMarkLayoutGravity == Gravity.BOTTOM) {
                  heightNeeded = (int) (2 * (getRawHeight() - getTickMarkRawHeight()));
              } else {
                  heightNeeded = (int) (2 * (getRawHeight() - Math.max(leftSB.getThumbScaleHeight(), rightSB.getThumbScaleHeight()) / 2));
              }
          } else {
              heightNeeded = (int) getRawHeight();
          }
          heightSize = MeasureSpec.makeMeasureSpec(heightNeeded, MeasureSpec.EXACTLY);
      }
      super.onMeasure(widthMeasureSpec, heightSize);
  }

• 下面是ImageView的源代码

  @Override
  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
      resolveUri();
      int w;
      int h;
  
      // Desired aspect ratio of the view's contents (not including padding)
      float desiredAspect = 0.0f;
  
      // We are allowed to change the view's width
      boolean resizeWidth = false;
  
      // We are allowed to change the view's height
      boolean resizeHeight = false;
  
      final int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
      final int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
  
      if (mDrawable == null) {
          // If no drawable, its intrinsic size is 0.
          mDrawableWidth = -1;
          mDrawableHeight = -1;
          w = h = 0;
      } else {
          w = mDrawableWidth;
          h = mDrawableHeight;
          if (w <= 0) w = 1;
          if (h <= 0) h = 1;
  
          // We are supposed to adjust view bounds to match the aspect
          // ratio of our drawable. See if that is possible.
          if (mAdjustViewBounds) {
              resizeWidth = widthSpecMode != MeasureSpec.EXACTLY;
              resizeHeight = heightSpecMode != MeasureSpec.EXACTLY;
  
              desiredAspect = (float) w / (float) h;
          }
      }
  
      final int pleft = mPaddingLeft;
      final int pright = mPaddingRight;
      final int ptop = mPaddingTop;
      final int pbottom = mPaddingBottom;
  
      int widthSize;
      int heightSize;
  
      if (resizeWidth || resizeHeight) {
          /* If we get here, it means we want to resize to match the
              drawables aspect ratio, and we have the freedom to change at
              least one dimension.
          */
  
          // Get the max possible width given our constraints
          widthSize = resolveAdjustedSize(w + pleft + pright, mMaxWidth, widthMeasureSpec);
  
          // Get the max possible height given our constraints
          heightSize = resolveAdjustedSize(h + ptop + pbottom, mMaxHeight, heightMeasureSpec);
  
          if (desiredAspect != 0.0f) {
              // See what our actual aspect ratio is
              final float actualAspect = (float)(widthSize - pleft - pright) /
                                      (heightSize - ptop - pbottom);
  
              if (Math.abs(actualAspect - desiredAspect) > 0.0000001) {
  
                  boolean done = false;
  
                  // Try adjusting width to be proportional to height
                  if (resizeWidth) {
                      int newWidth = (int)(desiredAspect * (heightSize - ptop - pbottom)) +
                              pleft + pright;
  
                      // Allow the width to outgrow its original estimate if height is fixed.
                      if (!resizeHeight && !sCompatAdjustViewBounds) {
                          widthSize = resolveAdjustedSize(newWidth, mMaxWidth, widthMeasureSpec);
                      }
  
                      if (newWidth <= widthSize) {
                          widthSize = newWidth;
                          done = true;
                      }
                  }
  
                  // Try adjusting height to be proportional to width
                  if (!done && resizeHeight) {
                      int newHeight = (int)((widthSize - pleft - pright) / desiredAspect) +
                              ptop + pbottom;
  
                      // Allow the height to outgrow its original estimate if width is fixed.
                      if (!resizeWidth && !sCompatAdjustViewBounds) {
                          heightSize = resolveAdjustedSize(newHeight, mMaxHeight,
                                  heightMeasureSpec);
                      }
  
                      if (newHeight <= heightSize) {
                          heightSize = newHeight;
                      }
                  }
              }
          }
      } else {
          /* We are either don't want to preserve the drawables aspect ratio,
             or we are not allowed to change view dimensions. Just measure in
             the normal way.
          */
          w += pleft + pright;
          h += ptop + pbottom;
  
          w = Math.max(w, getSuggestedMinimumWidth());
          h = Math.max(h, getSuggestedMinimumHeight());
  
          widthSize = resolveSizeAndState(w, widthMeasureSpec, 0);
          heightSize = resolveSizeAndState(h, heightMeasureSpec, 0);
      }
  
      setMeasuredDimension(widthSize, heightSize);
  }

08.ViewGroup中onMeasure方法

• 下面是真是开发案例中LinearLayout的代码，如下所示

  @Override
  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
      int myWidth = -1;
      int myHeight = -1;
  
      int width = 0;
      int height = 0;
  
      final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
      final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
      final int widthSize = MeasureSpec.getSize(widthMeasureSpec);
      final int heightSize = MeasureSpec.getSize(heightMeasureSpec);
  
      // Record our dimensions if they are known;
      if (widthMode != MeasureSpec.UNSPECIFIED) {
          myWidth = widthSize;
      }
  
      if (heightMode != MeasureSpec.UNSPECIFIED) {
          myHeight = heightSize;
      }
  
      if (widthMode == MeasureSpec.EXACTLY) {
          width = myWidth;
      }
  
      if (heightMode == MeasureSpec.EXACTLY) {
          height = myHeight;
      }
  
      View ignore = null;
      int gravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
      final boolean horizontalGravity = gravity != Gravity.START && gravity != 0;
      gravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
      final boolean verticalGravity = gravity != Gravity.TOP && gravity != 0;
  
      int left = Integer.MAX_VALUE;
      int top = Integer.MAX_VALUE;
      int right = Integer.MIN_VALUE;
      int bottom = Integer.MIN_VALUE;
  
      boolean offsetHorizontalAxis = false;
      boolean offsetVerticalAxis = false;
  
      if ((horizontalGravity || verticalGravity) && mIgnoreGravity != View.NO_ID) {
          ignore = findViewById(mIgnoreGravity);
      }
  
      final boolean isWrapContentWidth = widthMode != MeasureSpec.EXACTLY;
      final boolean isWrapContentHeight = heightMode != MeasureSpec.EXACTLY;
  
      // We need to know our size for doing the correct computation of children positioning in RTL
      // mode but there is no practical way to get it instead of running the code below.
      // So, instead of running the code twice, we just set the width to a "default display width"
      // before the computation and then, as a last pass, we will update their real position with
      // an offset equals to "DEFAULT_WIDTH - width".
      final int layoutDirection = getLayoutDirection();
      if (isLayoutRtl() && myWidth == -1) {
          myWidth = DEFAULT_WIDTH;
      }
  
      View[] views = mSortedHorizontalChildren;
      int count = views.length;
  
      for (int i = 0; i < count; i++) {
          View child = views[i];
          if (child.getVisibility() != GONE) {
              LayoutParams params = (LayoutParams) child.getLayoutParams();
              int[] rules = params.getRules(layoutDirection);
  
              applyHorizontalSizeRules(params, myWidth, rules);
              measureChildHorizontal(child, params, myWidth, myHeight);
  
              if (positionChildHorizontal(child, params, myWidth, isWrapContentWidth)) {
                  offsetHorizontalAxis = true;
              }
          }
      }
  
      views = mSortedVerticalChildren;
      count = views.length;
      final int targetSdkVersion = getContext().getApplicationInfo().targetSdkVersion;
      
      //省略部分代码
  
      setMeasuredDimension(width, height);
  }

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