As for the calculator movie, we’ll only use one frame in our lander movie.

Make a new movie and use the properties inspector to resize it to 550 x 400 pixels. Use the properties inspector also to set the background colour to black and check that the frame rate is set to 12 frames per second. Make sure you can see the whole of the frame by choosing ‘show frame’ from the pull-down list in the top right hand corner of the frame window.

As we are concentrating on learning Flash scripting skills, we won’t bother initially with using a realistic photo of the lander. We’ll just use a red square instead – you can change the fill of the square later into a bitmap photo of a lander if you like. Select the rectangle tool in the toolbox, and set the fill colour to red. Think also about the stroke colour - we don't need Flash to draw four strokes (lines) around our rectangle so let's keep it simple by setting the stroke colour to 'none' .

As it stands, our red square cannot be given a name and therefore cannot be referred to by code. Click the red square to select it then convert it to a symbol by pressing F8. Give the name ‘lander’ and choose the ‘movie clip’ for the behaviour – this is the usual choice for any symbol that is not intended to act as a button. Also note that the 'registration point' of the movie clip is in the top left corner. Choose the bottom left corner instead so that we can more easily detect when the bottom of the lander hits the top of the pad. The registration point is the point to which the _x and _y coordinates refer. In other words, when you get the position of an object on the stage you are getting the position of that  object's registration point, which may have been placed in the top left corner or in the centre or in other places. You will notice that the symbol you have just created has been added to the library (press Ctrl-L if you can't see the library).

The red square on the form is now an instance of the lander symbol – use the properties inspector to change the instance name of the red square to ‘lander1’. This does not imply that there will be other lander instances, but it is a good reminder that an instance is not the same as a symbol and that we could make other instances of the same symbol if we wanted to.

We’ll build our program step by step, testing as we go, so let’s first see whether we can make our lander fall as if pulled down by the moon’s gravity. We did this in VB by using the timer control to run a block of code at frequent intervals (once every 100 milliseconds or 10 times every second). We had variables vx and vy which represented the horizontal and vertical velocities, respectively. 10 times every second, the downward velocity vy was increased by a constant amount, just as would be the case for a real lander. Since speed is just the distance moved in each unit of time, we had code that moved the lander horizontally and vertically by distances equal to vx and vy (we were assuming therefore that vx and vy were the velocities in ‘pixels per decisecond’).

Since Flash has nothing like VB's timer control we will use a different technique to animate the lander. You need to understand that:

Flash has an onEnterFrame event handler and code that is attached to this will run repeatedly at the frame rate of the movie since our movie keeps starting again from the beginning after playing the one and only frame. The frame rate of our movie is set to 12 frames per second and code that runs this often to move the lander will give smooth-looking animation, as we wish.

Make a new layer for this code then, as you did in the calculator movie, and name the new layer ‘Actions’. Right-click the empty keyframe in frame 1 of the Actions layer, open the Actions panel and paste a copy of the code below:

Study the above code carefully – it first creates a variable called vy and gives it the value zero - this variable will be used to store the vertical velocity of the lander. Please note that Flash is (unfortunately) case sensitive so vy is not the same as Vy, for example, and lander1 is not the same as Lander1. You can avoid case problems if you use lower case for everything. Next, the code defines the function ‘_root.onEnterFrame’. In the second line above, Flash needs to know where the frames are located, since a Flash movie can contain many ‘movie clips’ each with a different timeline. In this case we are referring to the main timeline, which is said to be at the ‘root level’ of the movie.

In the third line, we add a constant number to vy, so that the downward velocity will increase steadily as the movie runs.

In the fourth line, we add the value of vy to the ‘_y’ property of the lander (not forgetting to indicate that the lander is in the main or ‘root’ movie). As you have guessed, the ‘_y’ property is the equivalent of the ‘top’ property in VB – it gives the vertical position of a movie clip, measured from the top of the frame.

Test your movie (Ctrl-Enter) and if all goes well you will see the lander accelerate downwards and disappear off the frame. Well done! Note that it is also possible to test your movie by pressing Ctrl-F12 - this takes a little longer but shows you exactly what your movie will look like inside your browser.

Now let’s add a button to control the thruster rocket that pushes the lander upwards. Choose the line tool in toolbox then use it to make a sharp upward-pointing triangle in the lower left corner of the frame. Make sure the fill colour is set to blue, choose the paint bucket tool, and fill the triangle with blue colour. Delete the 3 strokes around the triangle and convert the triangle into a symbol with the name ‘thruster’ and with button behaviour (we usually use 'movie clip' behaviour' but 'button behaviour' allows us to use the simple 'on release' code below).

Make sure the blue triangle (now an unnamed instance of the ‘thruster’ symbol) is selected, then open the Actions panel and paste or type the following code:

It should be obvious what this code does – when the mouse button is pressed down and then released over this button, 2 will be subtracted from the downwards velocity (vy) of the lander. Test your movie and make sure you can now control the vertical velocity of the lander.

Let’s add the other two buttons to control the horizontal thrusters. Make sure the movie’s library panel is visible (choose Window > Library otherwise) then drag a second instance of the thruster symbol from the library onto the frame, placing it to the left of the existing thruster button. Choose the free transform tool in the toolbox and use it to rotate the new instance until it points to the left. Add a third instance and make it point to the right. Add the code for the two new buttons – the one on the left should subtract 2 from vx each time it is clicked and the one on the right should add 2.

The 2 new buttons won’t affect the lander’s motion until we modify the ‘onEnterFrame’ function definition to take into account the vx variable. The code below also contains a line which creates and initializes the variable vx.

Test your movie and check that you can control the horizontal as well as the vertical velocity of the lander.

Now you can add a landing pad – just a green rectangle, to the lower right corner of the frame. Remove the 4 strokes around it then convert it to a movie clip symbol called ‘pad’. Name the instance of the pad symbol now on the frame ‘pad1’ so that we can refer to it in the code. Your frame should now look something like this (but not yet including the text box in the top right corner):

Now we need to modify the ‘onEnterFrame’ function definition to test whether the lander has fallen far enough to be touching the pad. In VB there is no property called ‘bottom’ that represents the location of the bottom edge of an object, so we had to add ‘top’ and ‘height’ to get the position of the bottom of the lander. In Flash we can avoid this problem because the ‘_y’ property gives the position of the lander’s ‘registration point’ and this is something that we can move by editing the lander symbol. To edit the lander symbol, double-click the instance (lander1) of the symbol that we have on the frame. You are now in symbol-editing mode – you can’t drag the registration point (the circle at the centre of the shape) but you can drag the shape - move the shape up until the registration point coincides with the bottom edge of the lander (we could have avoided this adjustment if we had specified the location of the registration point when the lander symbol was first created). Exit symbol-editing mode by clicking the ‘Scene 1’ label just above the frame window.

Now edit the pad symbol so that its registration point coincides with the upper edge of the pad.

Having made these changes, we can make a rather simple modification to the onEnterFrame code:

The new line checks whether the registration point of the lander is less far down the frame than the registration point of the pad, and if so then the lander is moved as previously. If the test gives a false result then the following lines are not executed so the lander stops moving.

Test your movie – you should see the message appear whenever the lander reaches the level of the pad, WHETHER OR NOT THE LANDER IS ABOVE THE PAD. Clearly the program is not yet finished – I’ll leave you the rather difficult challenge of adding the code to check whether the lander actually lands correctly, entirely within the width of the pad.

Before you do that, however, make a message appear when you land. In our VB lander program a message box appeared at the moment of the landing and announced ‘You have landed’. There is no messagebox statement in Flash but a similar effect can be achieved by changing the contents of an initially-empty dynamic text box.

Use the text tool to make a text box in the top right corner. Set the text type to dynamic text, the ‘show border around text’ option to off, and the text colour to bright yellow. Set also the var property of the text box to ‘message’. Now the following modification to the code will make the message appear when you land:

Note that there are other ways of making a message appear, e.g. a previously-hidden text box could be made visible (Flash movie clips have a ‘_visible’ property which corresponds to the ‘visible’ property of VB controls), or the alpha value (opacity) of the text box could have been set to 0 initially (using the properties inspector) and then (using code) to 100% when the lander landed. (A transparent text box is not the same as an invisible text box, for the transparent box is still enabled – it can still be clicked).

As mentioned earlier, you are now left with the challenge of modifying the code so that the program can detect a good landing, and react accordingly. As in VB, you will need to do this with a ‘nested’ if structure, and you will probably need to use the ‘_width’ property of the lander and the pad. Note that you should use && in place of AND in Flash - I warned you that Flash is not intuitive! The result could look something like this (refresh the page by pressing F5 to restart the movie)...