Android development study notes (21) ProgressBar ProgressBar

There are two types of progress bars: circular revolving type and lateral flow type. The
circular revolving type is relatively simple, just design a little style.
The horizontal flow type is more complicated. The difficulty is that the initial interface is gray and ugly, and further processing is required.
On the one hand, you need to open two more files to design its shape and gradient.
On the other hand, it is necessary to design its specific events in the java program to control the flow. (This part is the most complicated, some details have not been figured out yet)