This appendix is intended to provide more background on Ivy changes. Many of these examples list error messages you may see, so searching by error message might be a good idea if you are debugging.
<divclass="alert is-critical">
NOTE: Most of these issues affect a small percentage of applications encountering unusual or rare edge cases.
An element could be a match as long as there were no other directives between the element and the requesting directive.
This made sense for directives with nesting like tabs, where nested tab directives might not be desirable to match.
However, this caused surprising behavior for users because adding an unrelated directive like `ngClass` to a wrapper element could invalidate query results.
For example, with the content query and template below, the last two `Tab` directives would not be matches:
In the previous rendering engine, the following would work:
```
export class DataService {
constructor(@Inject('CONFIG') public config: DataConfig) {}
}
@Injectable()
export class AppService extends DataService {...}
```
In Ivy, it will throw an error because `DataService` is using Angular dependency injection, but is missing an `@Injectable` decorator.
The following would also work in the previous rendering engine, but in Ivy would require a `@Directive` decorator because it uses DI:
```
export class BaseMenu {
constructor(private vcr: ViewContainerRef) {}
}
@Directive({selector: '[settingsMenu]'})
export class SettingsMenu extends BaseMenu {}
```
The same is true if your directive class extends a decorated directive, but does not have a decorator of its own.
If you're using the CLI, there are two automated migrations that should transition your code for you ([this one](guide/migration-injectable) and [this one](guide/migration-undecorated-classes)).
However, as you're adding new code in version 9, you may run into this difference.
When a class has an Angular decorator like `@Injectable` or `@Directive`, the Angular compiler generates extra code to support injecting dependencies into the constructor of your class.
When using inheritance, Ivy needs both the parent class and the child class to apply a decorator to generate the correct code.
Otherwise, when the decorator is missing from the parent class, the subclass will inherit a constructor from a class for which the compiler did not generate special constructor info, and Angular won't have the dependency info it needs to create it properly.
In the previous rendering engine, the compiler had global knowledge, so in some cases (such as AOT mode or the presence of certain injection flags), it could look up the missing data.
However, the Ivy compiler only processes each class in isolation.
This means that compilation has the potential to be faster (and opens the framework up for optimizations and features going forward), but the compiler can't automatically infer the same information as before.
Please check that 1) the type for the parameter at index X is correct and 2) the correct Angular decorators are defined for this class and its ancestors.
Uncaught TypeError: Cannot read property 'bar' of undefined
```
If you see an error like this, and the `undefined` value refers to something that should have been injected, it may be this change.
### Recommended fix
- Add an `@Injectable` decorator to anything you plan to provide or inject.
```
@Injectable()
export class DataService {
constructor(@Inject('CONFIG') public config: DataConfig) {}
}
@Injectable()
export class AppService extends DataService {...}
```
- Add a [selectorless `@Directive` decorator](guide/migration-undecorated-classes#what-does-it-mean-to-have-a-directive-decorator-with-no-metadata-inside-of-it) to any class that extends a directive or any class from which a directive inherits.
```
@Directive() // selectorless, so it's not usable directly
Then the DOM binding pass would run, which would check the `value` binding.
At this time, it would be able to match the value against one of the existing options, and set the value of the `<select>` element in the DOM to display that option.
In Ivy, bindings are checked in the order they are defined in the template, regardless of whether they are directive input bindings or DOM bindings.
This change makes change detection easier to reason about for debugging purposes, since bindings will be checked in depth-first order as declared in the template.
Consequently, the value of the `<select>` element will be set before any options are created, and it won't be able to match and display the correct option in the DOM.
### Example of error
There is no error thrown, but the `<select>` in question will not have the correct initial value displayed in the DOM.
### Recommended fix
To fix this problem, we recommend binding to the `selected` property on the `<option>` instead of the `value` on the `<select>`.
*Before*
```html
<select[value]="someValue">
<option*ngFor="let option of options"[value]="option"> {{ option }} <option>
</select>
```
*After*
```html
<select>
<option*ngFor="let option of options"[value]="option"[selected]="someValue ==option">
## Forward references to directive inputs accessed through local refs are no longer supported.
### Basic example of change
```ts
@Directive({
selector: '[myDir]',
exportAs: 'myDir'
})
export class MyDir {
@Input() message: string;
}
```
```html
{{ myDir.name }}
<divmyDir #myDir="myDir"[name]="myName"></div>
```
In the View Engine runtime, the above code would print out the name without any errors.
In Ivy, the `myDir.name` binding will throw an `ExpressionChangedAfterItHasBeenCheckedError`.
### Background
In the ViewEngine runtime, directive input bindings and element bindings were executed in different stages. Angular would process the template one full time to check directive inputs only (e.g. `[name]`), then process the whole template again to check element and text bindings only (e.g.`{{ myDir.name }}`). This meant that the `name` directive input would be checked before the `myDir.name` text binding despite their relative order in the template, which some users felt to be counterintuitive.
In contrast, Ivy processes the template in just one pass, so that bindings are checked in the same order that they are written in the template. In this case, it means that the `myDir.name` binding will be checked before the `name` input sets the property on the directive (and thus it will be `undefined`). Since the `myDir.name` property will be set by the time the next change detection pass runs, a change detection error is thrown.
### Example of error
Assuming that the value for `myName` is `Angular`, you should see an error that looks like
```
Error: ExpressionChangedAfterItHasBeenCheckedError: Expression has changed after it was checked. Previous value: 'undefined'. Current value: 'Angular'.
```
### Recommended fix
To fix this problem, we recommend either getting the information for the binding directly from the host component (e.g. the `myName` property from our example) or to move the data binding after the directive has been declared so that the initial value is available on the first pass.
## Foreign functions and foreign values aren't statically resolvable
### Basic example of change
Consider a library that defines and exports some selector string to be used in other libraries:
```
export let mySelector = '[my-selector]';
```
This selector is then imported in another library or an application:
```
import {mySelector} from 'my-library';
@Directive({selector: mySelector})
export class MyDirective {}
```
Because the `mySelector` value is imported from an external library, it is part of a different compilation unit and therefore considered _foreign_.
While this code would work correctly in the View Engine compiler, it would fail to compile in Ivy in AOT mode.
### Background
In View Engine, the compiler would capture the source code of a library in `metadata.json` files when bundling the library, so that external consumers could "look inside" the source code of an external library.
When AOT-compiling the application, the `metadata.json` files would be used to determine the value of `mySelector`.
In Ivy, the `metadata.json` files are no longer used. Instead, the compiler extracts metadata for external libraries from the `.d.ts` files that TypeScript creates.
This has several benefits such as better performance, much improved error reporting, and enables more build caching opportunities as there is no longer a dependency on library internals.
Looking back at the previous example, the `mySelector` value would be represented in the `.d.ts` as follows:
```
export declare let mySelector: string;
```
Notice that the actual value of the selector is no longer present, so that the Ivy compiler is unable to use it during AOT compilations.
### Example of error
In the above example, a compilation error would occur when compiling `MyDirective`:
```
error NG1010: selector must be a string
Value is a reference to 'mySelector'.
1 export declare let mySelector: string;
~~~~~~~~~~
Reference is declared here.
```
### Recommended fix
When exporting values from a library, ensure the actual value is present in the `.d.ts` file. This typically requires that the variable be declared as a constant:
```
export const mySelector = '[my-selector]';
```
In classes, a field should be declared using the `static readonly` modifiers:
