View的绘制流程

2020-02-25 09:42:59 阅读：272 来源： 互联网

标签：size MeasureSpec int 流程 measure View 绘制 view

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

Measure

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
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

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

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

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View的绘制流程

Measure