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));
    }