We have learned that components can accept props, which can be JavaScript values of any type. But how about template content? In some cases, we may want to pass a template fragment to a child component, and let the child component render the fragment within its own template.
Slot content does not have access to the child component's data. Expressions in Vue templates can only access the scope it is defined in, consistent with JavaScript's lexical scoping. In other words:
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In the example below we define a Card component with three conditional slots: header, footer and the default one. When the header / footer / default is present we want to wrap them to provide additional styling:
However, there are cases where it could be useful if a slot's content can make use of data from both the parent scope and the child scope. To achieve that, we need a way for the child to pass data to a slot when rendering it.
Receiving the slot props is a bit different when using a single default slot vs. using named slots. We are going to show how to receive props using a single default slot first, by using v-slot directly on the child component tag:
If you are mixing named slots with the default scoped slot, you need to use an explicit tag for the default slot. Attempting to place the v-slot directive directly on the component will result in a compilation error. This is to avoid any ambiguity about the scope of the props of the default slot. For example:
You may be wondering what would be a good use case for scoped slots. Here's an example: imagine a component that renders a list of items - it may encapsulate the logic for loading remote data, using the data to display a list, or even advanced features like pagination or infinite scrolling. However, we want it to be flexible with how each item looks and leave the styling of each item to the parent component consuming it. So the desired usage may look like this:
The use case we discussed above encapsulates both reusable logic (data fetching, pagination etc.) and visual output, while delegating part of the visual output to the consumer component via scoped slots.
If we push this concept a bit further, we can come up with components that only encapsulate logic and do not render anything by themselves - visual output is fully delegated to the consumer component with scoped slots. We call this type of component a Renderless Component.
While an interesting pattern, most of what can be achieved with Renderless Components can be achieved in a more efficient fashion with Composition API, without incurring the overhead of extra component nesting. Later, we will see how we can implement the same mouse tracking functionality as a Composable.
Back in the olden days, you had no choice but to use slots if you want to connect to signals. This is no longer the case in Qt 5, where connections can be made to regular member functions or even free functions or lambdas.
Declaring a slot registers that function into that particular object's meta data, making it readily available to all of Qt's functionality which relies on the meta object. Other than that, it is a regular member function as the documentation states. There is nothing special in the slot function itself, the difference is in the generation of meta data for it in the meta object.
This means that declaring slots comes at some cost, albeit a small one, both in compilation time and executable size. I'd say it is overkill to have all your public functions as slots. It would be more efficient to only use slots when you actually need slots, if it can't work with a regular function, make it a slot.
Also, note that in almost all of the cases, signals are declared with a return type void. This has to do with the typical usage case of signals - they can often pass a parameter, but rarely return anything. Even though it is possible to return a value through a signal connected to a slot, this is used extremely rarely. So it doesn't make a lot of sense to declare a function that returns something as a slot you intend to connect to, as the very fact it returns a value means it will most likely not be used in the typical signal/slot context. That's why the getter is not a slot in that example. The setter as a slot is redundant in Qt 5, and is probably the product of this example code dating back to Qt 4.
Lastly, separating slots from regular public functions is a good way to illustrate intent, or the "API" of that class if you will. For example, I mostly use slots when extending QML, so I don't have to mark every function explicitly as invokable - unlike the scenario mentioned in the above paragraph, such slots often return stuff, but they aren't really used in connections. This way I get a clear overview of the design of the interface the class provides.
In addition to ddriver's answer, which is the best / correct answer (+1 there), I would also argue that it is confusing to define all member functions as public slots. They way you define functions (private / public / slots etc...) has an effect on the perceived usage of the class.
What I mean by that is.... you could argue that all functions should just be public (or public slots) and then this covers all cases. However this could be confusing to a future user of your class. Consider that int value() is a public slot (its not the best example) someone could try to use it as one, but the function itself has a return value which does not really make sense for a slot. It does make sense for a normal member function where (as a normal function call) the return value can be accessed.
One rule to go by is to always keep your functions and variables as local scoped and private as you can (by default) and only open them up for other use (public-ness, slots, global, etc...) when you really need them. This keeps your class interface much easier to understand and avoid confusion for later users.
Another minor example is the auto complete... if all your functions are slots, then when you are doing your connect(this, myClass::mySignal, &someOtherClass, SomeOtherClass:: ); your list of auto complete options could be long and unclear what is what. If you just have a few specific members that are slots then its easier to know which to choose.
Another use case where you possibly don't want all methods to be slots is when you are exposing objects to JavaScript through Qt's WebKit Bridge:all public slots are invocable from the JavaScript and that might open up security issues, depending on whether the JavaScript can be trusted or not.
For certain functions, you need a return value. This will not work as easily in slots. In slots you cant use the return value of a function or give a reference parameter to them. Yes you can do it, but you have a timing Problem. Whether you're using a slot or a normal member function dependents on your software architecture.
A BigQuery slot is a virtual CPU used by BigQueryto execute SQL queries. During the query execution, BigQueryautomatically calculates how many slots a query requires, depending on the querysize and complexity.
You have a choice of using an on-demand pricing modelor a capacity-based pricing model. Bothmodels useslots for data processing. With a capacity-based model, you can pay for dedicatedor autoscaled query processing capacity. The capacity-based model gives youexplicit control over slots and analytics capacity, whereas the on-demand modeldoes not.
Customers on the capacity-based pricing model explicitly choose how many slots toreserve. Your queries run within that capacity, and you pay for that capacitycontinuously every second it's deployed. For example, if you purchase 2,000BigQuery slots, your queries in aggregate are limited to using2,000 virtual CPUs at any given time. You have this capacity until you deleteit, and you pay for 2,000 slots until you delete them.
Projects on the BigQuery on-demand pricing model are subject toper-project slot quota with transient burstcapability. Most users on the on-demand model find the default slot capacitymore than sufficient. Depending on the workload, access to more slots improvesquery performance. To check how many slots your account uses, seeBigQuery monitoring.
BigQuery is architected to scale efficiently with increasedresources. Depending on the workload, incremental capacity is likely to give youincremental benefits. Therefore, choosing the optimal number of slots topurchase depends on your requirements for performance, throughput, and utility.
You can experiment with baseline and autoscaling slots to determine the bestconfiguration of slots. For example, you can test your workload with 500baseline slots, then 1,000, then 1,500, and 2,000, and observe the impact onperformance.
You can also examine the current slot usage of your projects, along with thechosen monthly price that you want to pay. On-demand workloads have asoft slot cap of 2,000 slots, but it is important to check how many slots areactually being used by your projects by usingINFORMATION_SCHEMA.JOBS* views,Cloud Logging, the Jobs API, or BigQuery Auditlogs. For more information, seeVisualizing slots available and slots allocated.
After you purchase slots and run your workloads for at least seven days, you canuse the slot estimator to analyze performance and model the effect of adding orreducing slots. For more information, seeEstimate slot capacity requirements.
When BigQuery executes a query job, it converts the declarativeSQL statement into a graph of execution, broken up into a series of querystages, which themselves are composed of more granular sets of executionsteps. BigQuery uses a heavily distributed parallelarchitecture to run these queries, and the stages model the units of workthat many potential workers may execute in parallel. Stages communicate withone another by using a fastdistributed shuffle architecture,which is discussed in more detail on theGoogle Cloud blog.
BigQuery query execution is dynamic, which means that thequery plan can be modified while a query is in flight. Stages that areintroduced while a query is running are often used to improve datadistribution throughout query workers. 152ee80cbc
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