This example is included to show-off something that I thought was mind-blowing when I first discovered it - the ability to call native methods from javascript seamlessly using the excellent node-ffi package.
The app does a couple of things - it sets up a way to call some native Windows APIs, and it provides a simple command-line interface that allows us to disable or enable repainting on a target window (specified using its title).
In order to do this exercise you'll need to set your system up so node-gyp works correctly, since this involves interfacing with native code (node-gyp is a tool which allows compiling C++ sources on a variety of platforms).
In order to make sure the example will work on your system you need to follow the steps for installation in the node-gyp README (see link above) - this mainly involves installing python and the VC++ compiler tools if you don't have them.
Once your machine is ready, you can run (from a prompt in the 2-ffi directory)
npm installThis will read the package.json file present in the directory and install any necessary (non built-in) dependencies listed in the file - in this case node-ffi.
The built-in API for reading input from the command-line is a tad clunky, but here is how we set up a looping prompt with it.
// We use the built-in 'readline' module for reading console input
var rl = require('readline'),
prompt = rl.createInterface(process.stdin, process.stdout, null);
// In order to loop calls to the async 'question' method, we use recursion as
// a naive approach. This is not a sustainable strategy as it eventually stack
// overflows, but is used here for code clarity.
function sendPrompt() {
// We ask a question and receive the next entered line in a callback
prompt.question("Type 'freeze <window title>', 'defrost', or 'exit':\n", function(answer) {
switch(answer) {
// We will add cases to deal with the various answers later
default:
// Keep prompting til exit
sendPrompt();
break;
}
});
}
// Start the loop by calling sendPrompt
sendPrompt();Before we move onto dealing with the answers to the prompt, we'll bring in node-ffi and set-up our WinAPI method calls, ready to perform them in response to prompt answers.
The two methods we'll be needing are:
- FindWindow - which allows us to get a window's handle using it's title.
- SendMessage - Which allows us to use Windows' messaging system to send messages to a window, using the window's handle. Two messages we can send to a window are disable painting and enable painting - the effects of which can be observed by the window becoming 'un-interactable' when painting is disabled.
// 'require' in ffi
var ffi = require('ffi');
// WinAPI methods require various constants, so store them in named variables
// Here we need the message id for the SETDRAW message along with the integer
// values for FALSE and TRUE
var WM_SETREDRAW = 11,
FALSE = 0,
TRUE = 1;
// Get proxy functions for native WINAPI FindWindow and SendMessage functions
// ffi requires the name of a dll to call methods from, along with an object
// describing the methods we want access to - their names and an array which
// contains their return type and an array of input types
var winapi = new ffi.Library("User32", {
"FindWindowA": ["int32", ["string", "string"]],
"SendMessageA": ["int64", ["int32", "uint32", "int32", "int32"]]
});
/*prompt code omitted for brevity*/We'll now add some basic code for handling prompt answers.
// We use the built-in 'readline' module for reading console input
var rl = require('readline'),
prompt = rl.createInterface(process.stdin, process.stdout, null),
freezeRegex = /^freeze (\w+)/;
// In order to loop calls to the async 'question' method, we use recursion as
// a naive approach. This is not a sustainable strategy as it eventually stack
// overflows, but is used here for code clarity.
function sendPrompt() {
// We ask a question and receive the next entered line in a callback
prompt.question("Type 'freeze <window title>', 'defrost', or 'exit':\n", function(answer) {
var playOn = true;
switch(answer) {
case 'defrost':
// TODO: Re-enable paints on window so it goes back to normal
break;
case 'exit':
// These lines allow the program to terminate
// Without them, it would run forever
prompt.close();
process.stdin.destroy();
playOn = false;
break;
default:
freezing = freezeRegex.exec(answer);
if(freezing) {
// TODO: Freeze target window based on match[1]
}
break;
}
if(playOn) {
// Keep prompting til exit
sendPrompt();
}
});
}
// Start the loop by calling sendPrompt
sendPrompt();Combining the two above pieces of code, we're able to easily invoke the methods we registered with node-ffi to complete the app.
// We use the built-in 'readline' module for reading console input
var rl = require('readline'),
ffi = require('ffi'),
prompt = rl.createInterface(process.stdin, process.stdout, null),
// WinAPI methods require various constants, so store them in named variables
// Here we need the message id for the SETDRAW message along with the integer
// values for FALSE and TRUE
WM_SETREDRAW = 11,
FALSE = 0,
TRUE = 1,
freezeRegex = /^freeze (\w+)/,
handle;
// Get proxy functions for native WINAPI FindWindow and SendMessage functions
// ffi requires the name of a dll to call methods from, along with an object
// describing the methods we want access to - their names and an array which
// contains their return type and an array of input types
var winapi = new ffi.Library("User32", {
"FindWindowA": ["int32", ["string", "string"]],
"SendMessageA": ["int64", ["int32", "uint32", "int32", "int32"]]
});
// In order to loop calls to the async 'question' method, we use recursion as
// a naive approach. This is not a sustainable strategy as it eventually stack
// overflows, but is used here for code clarity.
function sendPrompt() {
// We ask a question and receive the next entered line in a callback
prompt.question("Type 'freeze <window title>', 'defrost', or 'exit':\n", function(answer) {
var playOn = true;
switch(answer) {
case 'defrost':
if(handle) {
// Re-enable drawing by sending SETDRAW with TRUE
winapi.SendMessageA(handle, WM_SETREDRAW, TRUE, 0);
handle = null;
} else {
console.log("Please use 'freeze <window title>' before 'defrost'");
}
break;
case 'exit':
// These lines allow the program to terminate
// Without them, it would run forever
prompt.close();
process.stdin.destroy();
playOn = false;
break;
default:
match = freezeRegex.exec(answer);
if(match) {
if(handle) {
console.log("Already frozen, use 'defrost' to unfreeze target.");
break;
}
// Call find window to get the handle based on title
// Store in 'handle' for use in future answers
// Invoking the methods registered wif ffi is as simple as calling
// them like any other javascript method
handle = winapi.FindWindowA(null, match[1]);
// Use SendMessage to stop all paints for the window,
// effectively rendering it non-interactable
// Disable drawing by sending SETDRAW message with FALSE
winapi.SendMessageA(handle, WM_SETREDRAW, FALSE, 0);
}
break;
}
if(playOn) {
// Keep prompting til exit
sendPrompt();
}
});
}
// Start the loop by calling sendPrompt
sendPrompt();Use
node index.jsto start the app, then enter commands to see it in action!