unclear what you're asking Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking.
we have no way to display a boarder in MCPE. Do you want a fictional border? where players can't through? Because displaying a border like in MCPC 1.9+ (wait since when was it implemented) isn't possible in MCPE (yet)
It narrows as time passes. It is a square world and the boarders move in and close the players in. Think of it as a bit like this. Like the walls in that trash room (Sorry for the bit at the start, just skip to 00:39)
We are plugin developers, not those stupid server review YouTubers who nothing other than public relations.
To make a border: PHP: $borderX = 100;$borderZ = 100;$borderY = 50;$minBorderX = 0;$minBorderZ = 0;for($y = 0; $y <= $borderY; $y++){for($x = 0; $x <= $borderX; $x++){$level->setBlock(new Vector3($x, $y, $minBorderZ), Block::get(95));$level->setBlock(new Vector3($x, $y, $borderZ), Block::get(95));}for($z = 0; $z <= $borderX; $z++){$level->setBlock(new Vector3($minBorderX, $y, $z), Block::get(95));$level->setBlock(new Vector3($borderX, $y, $z), Block::get(95));}} As you can see I use the block 95, it's invisible bedrock, which you can't destroy and can't see, but it still blocks you. Now we want to shrink it, I am recommending to call a function. PHP: public function shrinkBorder($x, $y, $z, $minX, $minZ){// spawn the new shrinkborder$borderX = $x - $this->count;$borderZ = $y;$borderY = $z - $this->count;$minBorderX = $minX + $this->count;$minBorderZ = $minZ + $this->count;for($y = 0; $y <= $borderY; $y++){for($x = 0; $x <= $borderX; $x++){$level->setBlock(new Vector3($x, $y, $minBorderZ), Block::get(95));$level->setBlock(new Vector3($x, $y, $minBorderX), Block::get(95));}for($z = 0; $z <= $borderX; $z++){$level->setBlock(new Vector3($minBorderX, $y, $z), Block::get(95));$level->setBlock(new Vector3($borderX, $y, $z), Block::get(95));}}}// you can despawn the old border but I dont recommend you to do that, because it will only take unneccessary proccessing. To call the function: Make a public variable: $this->count = 0; Everytime the border shrinks, count $this->count up. PHP: $this->count++;$this->shrinkBorder($x, $y, $z, $minX, $minZ); $x, $y, $z, $minX, $minZ are free to choose. I hope I could help!
It's probably much more efficient to disallow player movement. It is very inefficient to send a large number of block changes in a short period to many players who would probably never run into the borders.