Tetris for Android in the Unity Engine – Part 2 (Player Input)

Hello all – this is the second part of the series of posts in which I have been developing the video game ‘Tetris’ for the Android platform using the ‘Unity’ engine. In this post, we will enable the player to move the falling blocks from left to right using buttons on the HUD (head-up display). Additionally, we will also implement a feature which enables the player to increase the rate of time and cause the falling blocks to fall faster – this feature is present in many other versions of the game.

The final product of this post of this post can be downloaded here.

In order to allow the player to give input, we need to add three buttons to the user interface – Unity has a UI development toolkit which allows us to do this relatively easily. In Unity, UI objects can be treated as actual GameObjects which are children of a ‘Canvas’ object – these UI objects contain certain components which allow them to function as various different interface elements (such as buttons, sliders and toggles). We need to add a ‘move block left’ button, a ‘move block right’ button and a ‘speed up time’ button – this can be done within the editor. Since the buttons are treated as GameObjects, they can be moved and shaped using the engine’s level editing tools.



Now, we can write the functions which will be called by the engine when the user presses the buttons. The ‘move left’ and ‘move right’ buttons only need to provide a response to tap actions – on the other hand, the ‘speed up time’ button must react to both being pressed and being released. First, we will write the functions called when the left/right movement buttons are pressed. The code which moves the tetrominoes downwards can be copied and edited slightly in order to provide functionality for moving them sideways – instead of looping through each block in the current tetromino and moving it down one row, we can loop through each block and move it either one column to the left or one column to the right. However, before moving any blocks, we need to loop through the entire tetromino and ensure that it is not already positioned on the very edge of the screen (we can do this by checking if any of its blocks are on the very left column or the very right column). If this check is passed, each block in the current tetromino can then be translated by one unit – the grid array which we created in the previous post must be updated in order to reflect this chance. The functions are have been made public in order for them to be called through UI input events.


In order to hook these public functions to their buttons, we set the ‘On Click ()’ option on the ‘Button’ components of the UI button objects which we added to the scene earlier.


Now, if the user presses the ‘move left’ button, the tetromino which is currently falling gets translated one column to the left, and if the user presses the ‘move right’ button, the tetromino which is currently falling gets translated one column to the right.

To speed up the falling block, the player must hold down the ‘speed up time’ button. Therefore, we need to implement two separate functions – one which reacts to the button being initially pressed down (this is where the button starts getting held down) and another which reacts to the button being released (where the button is no longer being held down). By default, the ‘Tick ()’ function which controls the falling tetrominoes is called once per second – in order to speed up the falling blocks, we cancel this pattern using the function ‘CancelInvoke ()’ and then use ‘InvokeRepeating ()’ in order to create a new calling pattern for the ‘Tick ()’ function with less time in between each call. The second argument to the new ‘InvokeRepeating ()’ call is set to 0.0f – this ensures that the ‘Tick ()’ function is instantly called when the ‘speed up time’ button is pressed (if this argument was not 0.0f, it would theoretically be possible for the player to completely freeze the block in mid-air by repeatedly pressing and releasing the ‘speed up time’ button). The same technique for speeding up the falling blocks is used to slow them down when the button is released – in the second function, we use ‘CancelInvoke ()’ to cancel the faster calling pattern before using ‘InvokeRepeating ()’ to replaced the cancelled pattern with the default calling pattern (which has 1-second intervals in between each tick).


We can then add ‘Event Trigger’ components to the ‘speed up time’ button, in order to hook these two public functions to their corresponding button events.


Now, the player has significantly control over the falling blocks in our version of Tetris – the blocks can be moved to the left, moved to the right, or pushed downwards at a higher speed. In the next post, we will start using the Unity editor to construct actual Tetrominoes (coloured groups of blocks) similar to those found in the original game.