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

The above method is to call setMeasuredDismension() to set mMeasuredWidth and mMeasuredHeight. These two variables will be used in the layout and draw ing processes of View. But when setMeasuredDismension is called, its parameters are obtained by getDefaultSize(). The source code is as follows:

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;
}

protected int getSuggestedMinimumWidth() {
	return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}

Where getDefaultSize is based on View's MesureSpec and another parameter size to get the return value.

Among them, we need to pay attention to the two modes of MeasureSpec.AT_MOST and MesureSpec. EXACTLY, that is, both return specSize. This specSize is the size of getsize called by MesureSpec of View generated by MesureSpec of parent container and LayoutParams of View itself, which can be understood as View. The width and height obtained after measurement.

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) {//Visible
			measureChild(child, widthMeasureSpec, heightMeasureSpec);
		}
	}
}
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);
	
}

The above code calls the measure() method of child view iteratively, and if child view has children, the measureChildren() will be executed recursively.