Push Button
When a Push Button parameter is selected in the parameters list, the following user interface is displayed:
Push Button parameters can only be mapped to published boolean input ports in the composition. When the user presses the button, the input port is set to true for one frame only. The parameter can be configured to render the composition immediately, or to wait for the host to preview another frame.
The Button title text field lets you specify the control title.
The Apply option controls whether the composition is rendered immediately, or whether the parameter simply waits for the user to preview the next frame in the canvas.
The two apply modes can be explained in terms of user experience. In most cases the user adjusts effect parameters while a previewing area is visible, such as Final Cut Pro’s Canvas window. This implies that for every adjustment made to a parameter, a new frame is rendered in the preview area. That would mean that every time the user presses the button, a new frame must be rendered and thus your composition gets a chance to act upon the change in the input port value from false to true.
Unfortunately this is not true under all circumstances. The Canvas window may not be visible, or it may not be displaying the clip where the effect has been applied; the user may have disabled previews altogether, such as enabling the Caps Lock key in After Effects.
Your composition may want to perform an action immediately, whether the host is going to preview a frame or not. You should then choose Immediately for the Apply option, but you are not guaranteed that the composition will be rendered with valid image inputs.
On the other hand, if your composition requires all image inputs to be valid in response to the button being pressed, you should set the Apply mode to When the host renders the next frame. Doing so means that FxFactory will wait until the next frame (when valid image inputs are available) and only then the published input port is set to true.