View的绘制流程

View的绘制流程主要包括measure，layout，draw三大流程，measure用来确定view的测量宽/高，layout用来确定view的最终宽/高和四个顶点的位置，而draw则将View绘制到屏幕上

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Measure

1. 如果只是一个原始的View，那么通过meaure方法就完成了其测量过程，如果是一个ViewGroup，除了完成自己的测量过程外，还会遍历去调用所有子View的measure方法，各个子元素再去递归调用这个流程

2. view的measure过程由measure方法来完成，measure方法是一个final方法，这意味着子类不能重写此方法，在View的measure方法中回去调用view的onMeasure方法

   

3. onMeasure方法如下(直接继承原始View)

   /**
    * <p>
    * 测量视图及其内容以确定测量的宽度和测量的高度,这个方法由measure方法调用，并且应该被子类重写，以
    * 准确和有效的测量其内容
    * Measure the view and its content to determine the measured width and the
    * measured height. This method is invoked by {@link #measure(int, int)} and
    * should be overridden by subclasses to provide accurate and efficient
    * measurement of their contents.
    * </p>
    *
    * <p>
    * 当子类重写了此方法，则必须调用setMeasuredDimension方法来存储view的测量宽/高，如果不这样做将会触发由
    *measure方法抛出的异常throw new IllegalStateException("View with id " + getId() + ": "
                           + getClass().getName() + "#onMeasure() did not set the"
                           + " measured dimension by calling"
                           + " setMeasuredDimension()");
    * <strong>CONTRACT:</strong> When overriding this method, you
    * <em>must</em> call {@link #setMeasuredDimension(int, int)} to store the
    * measured width and height of this view. Failure to do so will trigger an
    * <code>IllegalStateException</code>, thrown by
    * {@link #measure(int, int)}. Calling the superclass'
    * {@link #onMeasure(int, int)} is a valid use.
    * </p>
    *
    * <p>
    * The base class implementation of measure defaults to the background size,
    * unless a larger size is allowed by the MeasureSpec. Subclasses should
    * override {@link #onMeasure(int, int)} to provide better measurements of
    * their content.
    * </p>
    *
    * <p>
    * If this method is overridden, it is the subclass's responsibility to make
    * sure the measured height and width are at least the view's minimum height
    * and width ({@link #getSuggestedMinimumHeight()} and
    * {@link #getSuggestedMinimumWidth()}).
    * </p>
    *
    * @param widthMeasureSpec horizontal space requirements as imposed by the parent.
    *                         The requirements are encoded with
    *                         {@link android.view.View.MeasureSpec}.
    * @param heightMeasureSpec vertical space requirements as imposed by the parent.
    *                         The requirements are encoded with
    *                         {@link android.view.View.MeasureSpec}.
    *
    * @see #getMeasuredWidth()
    * @see #getMeasuredHeight()
    * @see #setMeasuredDimension(int, int)
    * @see #getSuggestedMinimumHeight()
    * @see #getSuggestedMinimumWidth() 取决于背景尺寸和minWidth的二者中的较大值minWidth默认为0
    * @see android.view.View.MeasureSpec#getMode(int)
    * @see android.view.View.MeasureSpec#getSize(int)
    */
   protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
       setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
               getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
   }
   

   onMeasure方法中必须调用setMeasuredDimension方法，否则将抛出异常，在这里我们只需关注getDefaultSize方法

    /**
     * 一个用来返回默认大小的实用方法，如果MeasureSpec没有提供任何限制则用系统提供的尺寸(背景尺寸或者minWidth
     * 的尺寸取二者最大);如果MeasureSpec允许，尺寸将会变大
     * Utility to return a default size. Uses the supplied size if the
     * MeasureSpec imposed no constraints. Will get larger if allowed
     * by the MeasureSpec.
     *
     * @param size Default size for this view
     * @param measureSpec Constraints imposed by the parent
     * @return The size this view should be.
     */
    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;
   

   可以看出getDefaultSize的逻辑很简单，我们只需要看AT_MOST和EXACTLY这两种情况，返回的大小就是MeasureSpec中的specSize，也就是View测量后的大小，直接继承view的自定义控件需要重写onMeasure方法并设置WRAP_CONTENT时自身的大小，否则在布局中使用WRAP_CONTENT就相当于MATCH_PARENT

4. ViewGoup的measure过程

   对于ViewGroup来说，除了完成自己的measure过程外，还会遍历去调用所有子元素的measure方法，各个子元素再去递归执行这个过程，ViewGroup是一个抽象类，没有重写onMeasure方法，但是提供了measureChildren，measureChild，measureChildWithMargins方法

   /**
    * Ask all of the children of this view to measure themselves, taking into
    * account both the MeasureSpec requirements for this view and its padding.
    * We skip children that are in the GONE state The heavy lifting is done in
    * getChildMeasureSpec.
    *
    * @param widthMeasureSpec The width requirements for this view
    * @param heightMeasureSpec The height requirements for this view
    */
   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);
           }
       }
   }
   
   /**
    * Ask one of the children of this view to measure itself, taking into
    * account both the MeasureSpec requirements for this view and its padding.
    * The heavy lifting is done in getChildMeasureSpec.
    *
    * @param child The child to measure
    * @param parentWidthMeasureSpec The width requirements for this view
    * @param parentHeightMeasureSpec The height requirements for this view
    */
   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);
   }
   

   measureChild的思想就是就是取出子元素的LayoutParams，然后再通过getChildMeasureSpec来创建子元素的MeasureSpec从而传递给子元素的measure方法。ViewGroup并没有onMeasure方法，这是因为ViewGroup是一个抽象类，其测量过程需要各个子类去具体实现，比如LinearLayout、RelativeLayout都有不同的布局特征，它们在onMeasure中都会调用measureChildWithMargins或者measureChild去传递测量过程，最终传递到单一View的onMeasure

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标签：size,MeasureSpec,int,流程,measure,View,绘制,view
来源： https://blog.csdn.net/parade0393/article/details/104483460