Customize ViewGroup container horizontalscrollviewex

        Here is a copy ViewGroup To increase your awareness of View Understanding of workflow . understand view The measurement of 、 Layout and drawing process .

        Code first , Then analyze it line by line .

public class HorizontalScrollViewEx extends ViewGroup {
	...
	
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int measuredWidth = 0;
        int measuredHeight = 0;
        final int childCount = getChildCount();
        measureChildren(widthMeasureSpec, heightMeasureSpec);

        int widthSpaceSize = MeasureSpec.getSize(widthMeasureSpec);
        int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
        int heightSpaceSize = MeasureSpec.getSize(heightMeasureSpec);
        int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
        if (childCount == 0) {
            setMeasuredDimension(0, 0);
        } else if (heightSpecMode == MeasureSpec.AT_MOST) {
            final View childView = getChildAt(0);
            measuredHeight = childView.getMeasuredHeight();
            setMeasuredDimension(widthSpaceSize, childView.getMeasuredHeight());
        } else if (widthSpecMode == MeasureSpec.AT_MOST) {
            final View childView = getChildAt(0);
            measuredWidth = childView.getMeasuredWidth() * childCount;
            setMeasuredDimension(measuredWidth, heightSpaceSize);
        } else {
            final View childView = getChildAt(0);
            measuredWidth = childView.getMeasuredWidth() * childCount;
            measuredHeight = childView.getMeasuredHeight();
            setMeasuredDimension(measuredWidth, measuredHeight);
        }
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        int childLeft = 0;
        final int childCount = getChildCount();
        mChildrenSize = childCount;

        for (int i = 0; i < childCount; i++) {
            final View childView = getChildAt(i);
            if (childView.getVisibility() != View.GONE) {
                final int childWidth = childView.getMeasuredWidth();
                mChildWidth = childWidth;
                childView.layout(childLeft, 0, childLeft + childWidth,
                        childView.getMeasuredHeight());
                childLeft += childWidth;
            }
        }
    }
    
    ...
}

        This is a Slide the container horizontally , It contains containers that slide up and down , So the page can turn left and right , Slide up and down to see more elements .

One 、 measurement onMeasure()

1.1 Why copy onMeasure() function

        First of all to see onMeasure() Measurement function , It is used to measure viewgroup And its internal sub view Size . We know that if we inherit directly View, Must copy onMeasure() Method . What we inherit here is View Subclasses of ViewGroup, View source discovery ViewGroup There is no duplication View Of onMeasure(), So direct inheritance ViewGroup Subclasses of must also inherit onMeasure() Method , And in this function, the layoutPrams==Wrap_content In this case, custom processing is implemented . If you don't deal with , Then even if the child view Of layoutPrams==Wrap_content, Then view The actual size is the remaining size of the parent container .

        Next up View To explain the above . space in between View Of layoutPrams==Wrap_content when , Its specMode=AT_MOST,getDefault() What's back is specSize, This value is the remaining space of the parent container .

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

1.2 Customize onMeasure() Internal logic

        This part is measurement viewgroup Oneself and all the children inside view Size .

1、 Because we only need to modify View Of onMeasure() in AT_MOST The logic of , Others don't need to be changed . therefore

First, execute super.onMeasure(widthMeasureSpec, heightMeasureSpec); Keep the original logic ;

2、 Next, for AT_MOST Special treatment for special cases .

        2.1 、 obtain viewgroup Inside view Number ;---viewgroup That's what we need

        2.2、 Measure all internal sub view size ;---viewgroup That's what we need

        2.3、 obtain viewgroup Wide and high specMode and specSize;

        2.4、 Yes viewgroup Of specMode=AT_MOST Special treatment

                2.4.1、childCount==0, Set up viewgroup The size is setMeasuredDimension(0, 0);

                2.4.2、widthSpecMode == MeasureSpec.AT_MOST

        HorizontalScrollViewEx Suppose all children view The width is the same as the height . So get the subscript 0 The son of view, Get its

Width ,” Son view Number * Son view Width “ Namely viewgroup Total width of . call setMeasuredDimension(measuredWidth, heightSpaceSize); Set up viewgroup The size of the .

                2.4.3、heightSpecMode == MeasureSpec.AT_MOST The logic is the same as above ;

                2.4.4、 Other situations .

Two 、 Layout onLayout()

        Its internal logic is ：

1、 obtain viewgroup All internal sub view Number ;

2、for Loop through all children view, Get each piece view Object for layout processing . If son view so , Get the son view The width value of , call

childView.layout(childLeft, 0, childLeft + childWidth,
        childView.getMeasuredHeight());

According to the given positions of the upper left and lower right points view. The point above x,y The size of the direction is in viewgroup The upper left corner is obtained from the origin of the coordinate system .