How to show all the designators in Altium? I've done it by double-clicking a component one by one and uncheck hide column in Designator. Is there another way that possible to unhide all of the components at once?
If you right-click on any designator and select "Find Similar Objects", a window appears that allows you to select your filters. Next to "String Type" make sure it says "Designator" and the dropdown on the right is set to "same". Check the "Select Matched" and "Run Inspector" boxes at the bottom and click "OK":
This will select all designators on your board. At the same time it will start a tool called the "PCB Inspector", which allows you to change the properties of all selected components at the same time. Once in the inspector, simply uncheck the "Hide" box under the "Graphical" heading and press your enter key:
This will apply the change to all of your designators.
Alternatively, instead of right-clicking a designator, you could also right-click a component instead and select "Find Similar Objects". In the filter window just leave "Object Kind" as "Component", and again make sure the "Select Matching" and "Run Inspector" boxes are checked, and hit "OK". When the Inspector opens you can check the box labeled "Show Name" under the "Graphical" heading. This will also show all of the designators.
The second option may be your best bet in case for some reason "Find Similar Objects" doesn't select hidden items (I can't remember if it does or not).
Though what @DerStorm8 described is the proper way of hiding designators. I want to share my lazy way. I just used to hide all strings while doing the major work on the PCB design. Obvious is that all of the strings will be hidden, not just the designators but during the component placement and routing you probably won't have any other string on the board.
So to hide all string just click on the LS button at the bottom-left corner of the workspace to open the View Configuration window, an on the Show/Hide panel, at the Strings, select Hidden.
To restore them, select the Final option.