Spinner can be used to show the loading state in your projects and express an unspecified wait time.
"Progress indicators have three main advantages: They reassure the user that the system has not crashed but is working on his or her problem; they indicate approximately how long the user can be expected to wait, thus allowing the user to do other activities during long waits; and they finally provide something for the user to look at, thus making the wait less painful." – Jacob Nielsen
"Be in a spin"
"Alice: How long is forever? White Rabbit: Sometimes, just one second."
Conventional wisdom tells us that when things are going to take a while, we should let people know. In most mobile applications that translates to adding progress bars or spinners when something is happening or loading. While the intentions behind these progress indicators are good, the end result can actually turn out to be bad.
It can be bad because progress indicators by definition call attention to the fact that someone needs to wait. It’s like watching the clock tick down - when you do, time seems to go slower. From the beginning, you might remember that our 3 seconds delay has been seen as a bug and the initial animation wasn’t good enough.