Android onMeasure & onLayout

基本概念

在android的自定义view中,onMeasure与onLayout这两个方法无疑是最重要的,只有深刻理解了这两个方法,才能写出我们自己想要的view/viewgroup

我们先看源码注释中对这两个方法的解释:
注：本文源码基于android-17

onMeasure

* <p>
* 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 overriden by subclasses to provide accurate and efficient
* measurement of their contents.
* </p>

如注释中所述: 通过测量当前view和它的contents,来决定当前所测量出的宽度和高度.这个方法被measure(int, int)调用,并且子类应当重写该方法,
用以提供精确高效的测量方法来测量它的contents.
上文提到onMeasure是在measure(int, int)中被调用的,下面就来看一下measure(int, int)方法的源码：
/**

 * <p>
 * This is called to find out how big a view should be. The parent
 * supplies constraint information in the width and height parameters.
 * </p>
 * <br>该方法被用于测量某个view所需大小，父视图提供宽度和高度的约束信息
 *
 * <p>
 * The actual measurement work of a view is performed in
 * {@link #onMeasure(int, int)}, called by this method. Therefore, only
 * {@link #onMeasure(int, int)} can and must be overridden by subclasses.
 * </p>
 *
 *
 * @param widthMeasureSpec Horizontal space requirements as imposed by the
 *        parent
 * @param heightMeasureSpec Vertical space requirements as imposed by the
 *        parent
 *
 * @see #onMeasure(int, int)
 */
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
    if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT || //mPrivateFlags是否为FORCE LAYOUT
            widthMeasureSpec != mOldWidthMeasureSpec ||
            heightMeasureSpec != mOldHeightMeasureSpec) {

        // first clears the measured dimension flag
        //清除MEASURED_DIMENSION_SET标记
        mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;

        //Resolve all RTL related properties.
        resolveRtlPropertiesIfNeeded();

        // measure ourselves, this should set the measured dimension flag back
        //测量view自身，并且重新设置PFLAG_MEASURED_DIMENSION_SET标记
        onMeasure(widthMeasureSpec, heightMeasureSpec);

        //如果未重新设置PFLAG_MEASURED_DIMENSION_SET标记将会抛出异常
        // flag not set, setMeasuredDimension() was not invoked, we raise
        // an exception to warn the developer
        if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
            throw new IllegalStateException("onMeasure() did not set the"
                    + " measured dimension by calling"
                    + " setMeasuredDimension()");
        }
        //设置标志，请求layout
        mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
    }

    mOldWidthMeasureSpec = widthMeasureSpec;
    mOldHeightMeasureSpec = heightMeasureSpec;
}

下面我们看下onMeasure方法源码，及该方法中调用的相关源码

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


/**
 * <p>This mehod must be called by {@link #onMeasure(int, int)} to store the
 * measured width and measured height. Failing to do so will trigger an
 * exception at measurement time.</p><br>设置当前视图的mMeasuredWidth与mMeasuredHeight
 *
 * @param measuredWidth The measured width of this view.  May be a complex
 * bit mask as defined by {@link #MEASURED_SIZE_MASK} and
 * {@link #MEASURED_STATE_TOO_SMALL}.
 * @param measuredHeight The measured height of this view.  May be a complex
 * bit mask as defined by {@link #MEASURED_SIZE_MASK} and
 * {@link #MEASURED_STATE_TOO_SMALL}.
 */
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
    mMeasuredWidth = measuredWidth;
    mMeasuredHeight = measuredHeight;

    mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}

/**
 * Utility to return a default size. Uses the supplied size if the
 * MeasureSpec imposed no constraints. Will get larger if allowed
 * by the MeasureSpec.<br>返回一个默认大小。如果MeasureSpec中不包含限制，将返回参数size的大小；
 *
 * @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;
}

/**
 * Returns the suggested minimum width that the view should use. This
 * returns the maximum of the view's minimum width)
 * and the background's minimum width<br>
 * 返回视图建议使用的最小宽度（该视图最小宽度与背景图最小宽度中的最大值）
 *  ({@link android.graphics.drawable.Drawable#getMinimumWidth()}).
 * <p>
 * When being used in {@link #onMeasure(int, int)}, the caller should still
 * ensure the returned width is within the requirements of the parent.
 *
 * @return The suggested minimum width of the view.
 */
protected int getSuggestedMinimumWidth() {
    return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}

protected int getSuggestedMinimumHeight() {
    return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());

}