Android Study Plan V

Android学习计划

话题：关于View的知识

1、View的getWidth()和getMeasuredWidth()有什么区别吗？
2、如何在onCreate中拿到View的宽度和高度？

答案

1. View的getWidth()和getMeasuredWidth()有什么区别吗？

• getWidth()通过setFrame方法来决定四个顶点位置，初始化mLeft,mTop,mRight,mBottom四个参数，这四个值固定则位置确定。必须在layout过程结束才有值。

   /**
       * Return the width of the your view.
       *
       * @return The width of your view, in pixels.
       * 利用屏幕上的右坐标减去左边的坐标 
       */
      @ViewDebug.ExportedProperty(category = "layout")
      public final int getWidth() {
          return mRight - mLeft;
      }
   
     protected boolean setFrame(int left, int top, int right, int bottom) {
   ...
              mLeft = left;
              mTop = top;
              mRight = right;
              mBottom = bottom;
  ...
    }

• getMeasuredWidth是在view的绘制流程中的measure结束后有值，获取的是view的测量宽高。mMeasuredWidth是在setMeasuredDimensionRaw方法中赋值的

  public static final int MEASURED\_SIZE\_MASK = 0x00ffffff;  
  /**
       * Like {@link #getMeasuredWidthAndState()}, but only returns the
       * raw width component (that is the result is masked by
       * {@link #MEASURED_SIZE_MASK}).
       *
       * @return The raw measured width of this view.
       */
      public final int getMeasuredWidth() {
          return mMeasuredWidth & MEASURED_SIZE_MASK;//与任何数字的运算得到的结果是mMeasuredWidth
      }
   
  private void setMeasuredDimensionRaw(int measuredWidth, measuredHeight) {
          mMeasuredWidth = measuredWidth;
          mMeasuredHeight = measuredHeight;
   
          mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
      }

• 一般情况下 getMeasuredWidth和getWidth的值是相同的，从源码中可以看出setMeasuredDimensionRaw会对mMeasuredWidth进行赋值，当调用了onMeasure，会调用到setMeasuredDimensionRaw则获取的结果将会不相同

  public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
    boolean optical = isLayoutModeOptical(this);
  2if (optical != isLayoutModeOptical(mParent)) {
  22Insets insets = getOpticalInsets();
  2   int oWidth  = insets.left + insets.right;
  22int oHeight = insets.top  + insets.bottom;
  22widthMeasureSpec  = MeasureSpec.adjust(widthMeasureSpec,  optical ? -oWidth  : oWidth);
  22heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
  2}
   ...
    if (forceLayout || needsLayout) {
  22// first clears the measured dimension flag
  22mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
  
  2   resolveRtlPropertiesIfNeeded();
  
  22int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
  22if (cacheIndex < 0 || sIgnoreMeasureCache) {
  222// measure ourselves, this should set the measured dimension flag back
  222onMeasure(widthMeasureSpec, heightMeasureSpec);
  22   mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
  22} else {
  222long value = mMeasureCache.valueAt(cacheIndex);
  222// Casting a long to int drops the high 32 bits, no mask needed
  22   setMeasuredDimensionRaw((int) (value >> 32), (int) value);
  222mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
  22}
  
  22// flag not set, setMeasuredDimension() was not invoked, we raise
  22// an exception to warn the developer
  22if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
  222throw new IllegalStateException("View with id " + getId() + ": "
  2222   + getClass().getName() + "#onMeasure() did not set the"
  22222+ " measured dimension by calling"
  22222+ " setMeasuredDimension()");
  22}
  
  22mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
    }
    ...
    }

• view的绘制流程measure layout draw

  {% fullimage /images/study_plan/view_process.png, alt,流程图 %}
  • measure为了计算出控件树中的各个控件要显示的内容以及大小，起点为ViewRootImpl 的 measureHierarchy ()。
    • SpecMode : EXACTLY(确切大小) AT_MOST(子view的大小不得超过SpecSize) UNSPECIFIED(对子view尺寸不做限制)

      /**
               * 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;

• layout 从根view开始，递归的完成控件树的布局工作，确定view的位置。先递归的对子view进行布局，在完成父布局的位置设置

  • draw 从根view开始进行绘制，利用Viwe.draw()

2.如何在onCreate中拿到View的宽度和高度？

• 在 Activity#onWindowFocusChanged 回调中获取宽高。

• view.post(runnable)，在 runnable 中获取宽高。
  利用Handler通信机制，发送一个Runnable在MessageQuene中，当layout处理结束时则会发送一个消息通知UI线程，可以获取到实际宽高。

• ViewTreeObserver 添加 OnGlobalLayoutListener，在 onGlobalLayout 回调中获取宽高。
  监听全局View的变化事件，使用后需要注意移除OnGlobalLayoutListener 监听，以免造成内存泄露
• 调用 view.measure()，再通过 getMeasuredWidth 和 getMeasuredHeight 获取宽高。

补充知识点

• matchParent无法measure(在view的measure过程中，需要知道parentSize即父容器的剩余空间，所以无法得出measure的大小)
• 深入理解View绘制流程