Android onMeasure(widthSpec, heightSpec)

每个View中都有一个测量绘制控件大小的方法onMeasure(int widthMeasureSpec, int heightMeasureSpec) ，该方法用于初始化控件所占的区域：

摘自SlidingDrawer.java，仅作参考说明onMeasure(int widthMeasureSpec, int heightMeasureSpec)方法

@Override

    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {

        //onMeasure方法传入的两个参数widthMeasureSpec, heightMeasureSpec，其单位为dp或dip

    

        int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);    //这里判断单位是dp或dip，有或者其他的单位类型

        int widthSpecSize =  MeasureSpec.getSize(widthMeasureSpec); //将单位转为px



        int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);

        int heightSpecSize =  MeasureSpec.getSize(heightMeasureSpec);



        if (widthSpecMode == MeasureSpec.UNSPECIFIED || heightSpecMode == MeasureSpec.UNSPECIFIED) {

            throw new RuntimeException("SlidingDrawer cannot have UNSPECIFIED dimensions");

        }



        //

        final View handle = mHandle;

        measureChild(handle, widthMeasureSpec, heightMeasureSpec);



        if (mVertical) {

            int height = heightSpecSize - handle.getMeasuredHeight() - mTopOffset;

            mContent.measure(MeasureSpec.makeMeasureSpec(widthSpecSize, MeasureSpec.EXACTLY),

                    MeasureSpec.makeMeasureSpec(height, MeasureSpec.EXACTLY));

        } else {

            int width = widthSpecSize - handle.getMeasuredWidth() - mTopOffset;

            mContent.measure(MeasureSpec.makeMeasureSpec(width, MeasureSpec.EXACTLY),

                    MeasureSpec.makeMeasureSpec(heightSpecSize, MeasureSpec.EXACTLY));

        }



        //最后绘制父控件的大小

        setMeasuredDimension(widthSpecSize, heightSpecSize);

        //super.onMeasure(widthMeasureSpec, heightMeasureSpec)原型则是下面这样写的:

        //setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),

        //        getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));

    }