(a) District bonds issued may be secured by a pledge of all or part of the district's revenue, or by all or part of the revenue of one or more contracts previously or subsequently made or other revenue or income specified by board resolution or a trust indenture securing the bonds. The pledge may reserve the right, under conditions specified by the pledge, to issue additional bonds that will be on a parity with or subordinate to the bonds then being issued.
(b) The district may issue bonds secured by both taxes and revenue of the district described by Subsection (a).