The compile function is a powerful tool that allows you to modify the DOM before binding the resulting template function to the scope.
For example, consider the code snippet below:
<div my-directive></div>
You can utilize the compile function to alter the template DOM like so:
app.directive('myDirective', function(){
return {
// The compile function modifies the template DOM
// It operates before being bound to the scope, hence no injected scope here
compile: function(tElem, tAttrs){
// Modifying the markup before reaching the link function
// Angular will also process the "another-directive" directive accordingly
tElem.append('<div another-directive></div>');
// The link function works on instances and receives the scope
// for generating a dynamic view
return function(scope, iElem, iAttrs){
// Adding similar markup here won't work as Angular has already processed it during compilation
// We're simply linking with the scope at this stage
iElem.append('<div another-directive></div>');
}
}
}
});
The compile function allows you to customize the template DOM according to your requirements.
In most scenarios, tElem and iElem refer to the same DOM element, but there might be differences when a directive clones the template for multiple copies (e.g., ngRepeat).
Angular employs a two-way rendering process (compile + link) behind the scenes to efficiently duplicate compiled pieces of DOM. This prevents redundant processing of the same DOM structure for each instance and leads to improved performance.
I hope this clarifies things!
ADDED AFTER COMMENT:
Distinguishing between a template and compile function:
Template Function
{
template: function(tElem, tAttrs){
// Generate string content for template replacement
return 'string to use as template';
}
}
Compile Function
{
compile: function(tElem, tAttrs){
// Manually manipulate the element's DOM
// Return the linking function
return linkFn(){};
}
}
The template function executes before the compile function.
While they serve similar purposes and have the same structure, the key distinction lies in their output - the template function replaces directive content or markup (with replace: true), while the compile function programmatically alters the DOM and returns a link function (or object with pre and post link functions).
Think of the template function as a shortcut if you just need to replace content with a string value, without using the compile function.
I trust this explanation helps!