Effects are rarely needed in most application code, but may be useful in specific circumstances. Here are some examples of situations where an effect might be a good solution:

When you read conditionalCount, if showCount is false the "Nothing to see here!" message is returned without reading the count signal. This means that if you later update count it will not result in a recomputation of conditionalCount.

For example, suppose that when currentUser changes, the value of a counter should be logged. you could create an effect which reads both signals:

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As a result, you can safely perform computationally expensive derivations in computed signals, such as filtering arrays.

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When you create an effect, it is automatically destroyed when its enclosing context is destroyed. This means that effects created within components are destroyed when the component is destroyed. The same goes for effects within directives, services, etc.

Effects always run at least once. When an effect runs, it tracks any signal value reads. Whenever any of these signal values change, the effect runs again. Similar to computed signals, effects keep track of their dependencies dynamically, and only track signals which were read in the most recent execution.

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Only the signals actually read during the derivation are tracked. For example, in this computed the count signal is only read if the showCount signal is true:

The doubleCount signal depends on the count signal. Whenever count updates, Angular knows that doubleCount needs to update as well.

Effects might start long-running operations, which you should cancel if the effect is destroyed or runs again before the first operation finished. When you create an effect, your function can optionally accept an onCleanup function as its first parameter. This onCleanup function lets you register a callback that is invoked before the next run of the effect begins, or when the effect is destroyed.

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Note that dependencies can be removed during a derivation as well as added. If you later set showCount back to false, then count will no longer be considered a dependency of conditionalCount.

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Effects return an EffectRef that you can use to destroy them manually, by calling the .destroy() method. You can combine this with the manualCleanup option to create an effect that lasts until it is manually destroyed. Be careful to actually clean up such effects when they're no longer required.

If you then change count, Angular knows that doubleCount's cached value is no longer valid, and the next time you read doubleCount its new value will be calculated.

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When you read a signal within an OnPush component's template, Angular tracks the signal as a dependency of that component. When the value of that signal changes, Angular automatically marks the component to ensure it gets updated the next time change detection runs. Refer to the Skipping component subtrees guide for more information about OnPush components.

A signal is a wrapper around a value that notifies interested consumers when that value changes. Signals can contain any value, from primitives to complex data structures.

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Angular Signals is a system that granularly tracks how and where your state is used throughout an application, allowing the framework to optimize rendering updates.

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Writable signals provide an API for updating their values directly. You create writable signals by calling the signal function with the signal's initial value:

By default, you can only create an effect() within an injection context (where you have access to the inject function). The easiest way to satisfy this requirement is to call effect within a component, directive, or service constructor:

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Signals are useful because they notify interested consumers when they change. An effect is an operation that runs whenever one or more signal values change. You can create an effect with the effect function:

doubleCount's derivation function does not run to calculate its value until the first time you read doubleCount. The calculated value is then cached, and if you read doubleCount again, it will return the cached value without recalculating.

This example will log a message when either currentUser or counter changes. However, if the effect should only run when currentUser changes, then the read of counter is only incidental and changes to counter shouldn't log a new message.

If you set showCount to true and then read conditionalCount again, the derivation will re-execute and take the branch where showCount is true, returning the message which shows the value of count. Changing count will then invalidate conditionalCount's cached value.

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When creating a signal, you can optionally provide an equality function, which will be used to check whether the new value is actually different than the previous one.

Avoid using effects for propagation of state changes. This can result in ExpressionChangedAfterItHasBeenChecked errors, infinite circular updates, or unnecessary change detection cycles.

Computed signal are read-only signals that derive their value from other signals. You define computed signals using the computed function and specifying a derivation:

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Rarely, you may want to execute code which may read signals within a reactive function such as computed or effect without creating a dependency.