This code works but I would like to know whether this is good solution? Would this solution using Expression Tree be considered better than Emit and OpCodes?
var target = Expression.Lambda(
Expression.Block(
new ParameterExpression[] { },
Expression.Call(
typeof(MessageBox).GetMethod("Show", new[] { typeof(string) }),
Expression.Constant(1.ToString(), typeof(string))
)
),
new ParameterExpression[] { }
);
AssemblyName aName = new AssemblyName("DynamicAssemblyExample");
AssemblyBuilder ab =
AppDomain.CurrentDomain.DefineDynamicAssembly(
aName,
AssemblyBuilderAccess.RunAndSave);
ModuleBuilder mb =
ab.DefineDynamicModule(aName.Name, aName.Name + ".dll");
TypeBuilder tb = mb.DefineType("MyDynamicType", TypeAttributes.Public);
var method = tb.DefineMethod("dynamicMethod", MethodAttributes.Public | MethodAttributes.Static);
target.CompileToMethod(method);
The code in your example seems like just "example" code. I don't see it as requiring expression trees. If I read it correctly it is merely creating the constant expression () => MessageBox.Show(1.ToString());.)
Expression trees are great, among others, when actual dynamic code needs to be created i.e. when interpreting some other language within a program or for operations that would otherwise require reflection (e.g. in programs such as Automapper.)
Are they better than emitting the op-codes directly. It really depends: can do a better job than the compiler? Emitting op-codes is like writing assembly, it is time consuming and you really need to know what you are doing to be able to write optimal code. Is the performance difference between compiler and hand optimized code worth the extra effort?
Related
I have a list of valid values that I am storing in a data store. This list is about 20 items long now and will likely grow to around 100, maybe more.
I feel there are a variety of reasons it makes sense to store this in a data store rather than just storing in code. I want to be able to maintain the list and its metadata and make it accessible to other services, so it seems like a micro-service data store.
But in code, we want to make sure only values from the list are passed, and they can typically be hardcoded. So we would like to create an enum that can be used in code to ensure that valid values are passed.
I have created a simple node.js that can generate a JS file with the enum right from the data store. This could be regenerated anytime the file changes or maybe on a schedule. But sharing the enum file with any node.js applications that use it would not be trivial.
Has anyone done anything like this? Any reason why this would be a bad approach? Any feedback is welcome.
Piggy-backing off of this answer, which describes a way of creating an "enum" in JavaScript: you can grab the list of constants from your server (via an HTTP call) and then generate the enum in code, without the need for creating and loading a JavaScript source file.
Given that you have loaded your enumConstants from the back-end (here I hard-coded them):
const enumConstants = [
'FIRST',
'SECOND',
'THIRD'
];
const temp = {};
for (const constant of enumConstants) {
temp[constant] = constant;
}
const PlaceEnum = Object.freeze(temp);
console.log(PlaceEnum.FIRST);
// Or, in one line
const PlaceEnum2 = Object.freeze(enumConstants.reduce((o, c) => { o[c] = c; return o; }, {}));
console.log(PlaceEnum2.FIRST);
It is not ideal for code analysis or when using a smart editor, because the object is not explicitly defined and the editor will complain, but it will work.
Another approach is just to use an array and look for its members.
const members = ['first', 'second', 'third'...]
// then test for the members
members.indexOf('first') // 0
members.indexOf('third') // 2
members.indexOf('zero') // -1
members.indexOf('your_variable_to_test') // does it exist in the "enum"?
Any value that is >=0 will be a member of the list. -1 will not be a member. This doesn't "lock" the object like freeze (above) but I find it suffices for most of my similar scenarios.
I am trying to use a string ('npcName') as a variable name. So far I have tried casting dialogMap into a DynamicAccess object, but it gives me the error 'Invalid array access' when I try this:
var npcName:String = 'TestNPC';
var casted = (cast Registry.dialogMap:haxe.DynamicAccess<Dynamic>);
var tempname = casted[root.npcName[0].message];
trace(tempname);
'dialogMap' is an empty map which I want to fill like so:
Registry.dialogMap['message'] = root.npcName[0].message;
How can I use npcName, a string, in the above line of code? Is there a way to transform the string into something usable? Any help would be appreciated.
The haxe.DynamicAccess doesn't have array access (like map[key]), but is an abstract type for working with anonymous structures that are intended to hold collections of objects by the string key. It is designed to work with map.get(key) and map.set(key). It is basically a nicer wrapper around Reflect.field and Reflect.setField and does some safety checks with Reflect.hasField.
var variable = "my_key";
var value = 123;
var dynamicMap = new haxe.DynamicAccess<Dynamic>();
dynamicMap.set(variable, value);
I'm noticing you are doing very much cast and dynamic, so untyped code, which is a bit of contradiction in a typed language. What is the actual type of dialogMap?
Not sure you are aware of it but, Haxe has its own maps, which are fully typed, so you don't need casts.
var map = new Map<String, Int>();
map[variable] = value;
I think this article helps understanding how to work with dynamic (untyped) objects.
Tip; for testing such small functionalities you can doodle around on the try.haxe site : http://try.haxe.org/#4B84E
Hope this helps, otherwise here is some relevant documentation:
http://api.haxe.org/haxe/DynamicAccess.html
https://haxe.org/manual/std-reflection.html
https://haxe.org/manual/types-dynamic.html
http://code.haxe.org/category/beginner/string-variable-reflection.html
UDATED
How do I go about this?
I got this from Main.hx:
function onMouseOver(e:MouseEvent){
if(Std.is(e.currentTarget, MovieClip)){
initializer (cast e.currentTarget,["scaleX",1.5,"scaleY",1.5])
}
}
Then this is the pointed function in my Animation Class
//here if i set mc:Dynamic everything goes great! but when this one
function initializer(mc:MovieClip, vars:Array<Dynamic>){
var varsLength:Int = Math.round(vars.length/2);
for(m in 0...varsLength){
ini[m] = mc[vars[2*m]];
}
}
then when i compile it, an error appears:
Error: Array access is not allowed in flash.display.MovieClip
How do I resolve this?
EDIT:
vars: are properties of the MovieClip, for example when I pass these parameters:
initializer (mcClip1,["scaleX",1.5,"scaleY",1.5])
so:
vars = ["scaleX",1.5,"scaleY",1.5]
and:
ini[m] will store "scaleX" and "scaleY"`
X-Ref: https://groups.google.com/forum/#!topic/haxelang/_hkyt__Rrzw
In AS3, you can access fields of an object via their String name using [] (array access). This is called Reflection.
In Haxe, Reflection works differently - you need to make use of the Reflect API.
It's considered bad practice - it's not type-safe, which means the compiler can do very little to help you with error messages, and it's quite slow as well. This is why the usage makes it very explicit that Reflection is actually going on (while in AS3, this fact is somewhat hidden). Consider if there are other ways of solving this problem that don't require Reflection.
Now, to get back to your example, here's what it would look like in Haxe:
function onMouseOver(e:MouseEvent){
if (Std.is(e.currentTarget, MovieClip)) {
initializer(cast e.currentTarget, ["scaleX", 1.5, "scaleY", 1.5])
}
}
function initializer(mc:MovieClip, vars:Array<Dynamic>) {
for (m in 0...Std.int(vars.length / 2)) {
ini[m] = Reflect.getProperty(mc, vars[2*m]);
}
}
Btw, your loop was running for too long since you only use half of the values in the array - if you don't divide it by two like I did, you'll end up with [scaleX, scaleY, null, null] instead of the desired [scaleX, scaleY].
I need to grab a large amount of data from one set of tables and SQLBulkInsert into another set...unfortunately the source tables are ALL varchar(max) and I would like the destination to be the correct type. Some tables are in the millions of rows...and (for far too pointless policital reasons to go into) we can't use SSIS.
On top of that, some "bool" values are stored as "Y/N", some "0/1", some "T/F" some "true/false" and finally some "on/off".
Is there a way to overload IDataReader to perform type conversion? Would need to be on a per-column basis I guess?
An alternative (and might be the best solution) is to put a mapper in place (perhaps AutoMapper or custom) and use EF to load from one object and map into the other? This would provoide a lot of control but also require a lot of boilerplate code for every property :(
In the end I wrote a base wrapper class to hold the SQLDataReader, and implementing the IDataReader methods just to call the SQLDataReader method.
Then inherit from the base class and override GetValue on a per-case basis, looking for the column names that need translating:
public override object GetValue(int i)
{
var landingColumn = GetName(i);
string landingValue = base.GetValue(i).ToString();
object stagingValue = null;
switch (landingColumn)
{
case "D4DTE": stagingValue = landingValue.FromStringDate(); break;
case "D4BRAR": stagingValue = landingValue.ToDecimal(); break;
default:
stagingValue = landingValue;
break;
}
return stagingValue;
}
Works well, is extensible, and very fast thanks to SQLBulkUpload. OK, so there's a small maintenance overhead, but since the source columns will very rarely change, this doesn't really affect anything.
I need to adapt my code to the stricter Java 7 verifier and have to add visitFrame calls in my MethodNode (I'm using the tree api). I could not find any information on how Type maps to the Object[]s used in visitFrame, so please help me out here...
This is what I have so far:
private Object getFrameType(Type type) {
switch (type.getSort()) {
case Type.BOOLEAN:
case Type.CHAR:
case Type.BYTE:
case Type.SHORT:
case Type.INT:
return Opcodes.INTEGER;
case Type.LONG:
return Opcodes.LONG;
case Type.FLOAT:
return Opcodes.FLOAT;
case Type.DOUBLE:
return Opcodes.DOUBLE;
case Type.OBJECT:
case Type.ARRAY:
return type.getInternalName();
}
throw new RuntimeException(type + " can not be converted to frame type");
}
What I'd like to know is: what are Type.VOID and Type.METHOD?
When do I need Opcodes.TOP, Opcodes.NULL and Opcodes.UNINITIALIZED_THIS?
I'm guessing UNINITIALIZED_THIS is only used in the constructor and I can probably ignore VOID and METHOD, but I'm not sure and I don't have the slightest idea what TOP is.
If I understood your need correctly, you could just let ASM calculate the frames for you. This will probably slow down the class generation a bit, but certainly worth a try.
When creating the ClassWriter, just add COMPUTE_FRAMES to the flags argument of the constructor, e.g.
new ClassWriter(ClassWriter.COMPUTE_FRAMES);
Similarly, if you are transforming a class, the ClassReader can be asked to expand the frames like:
ClassReader cr = ...;
ClassNode cn = new ClassNode(ASM4);
cr.accept(cn, ClassReader.EXPAND_FRAMES);
The former option has the benefit that you can forget about the frames (and "maxs") altogether while the latter option might require you to patch the frames yourself depending on what kind of transformation you do.
The examples are for ASM version 4, but these features have been supported at least since version 3 - the parameters are just passed a bit differently.