76 lines
2.2 KiB
Markdown
76 lines
2.2 KiB
Markdown
---
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title: "Obscure Qt: Default choices for DelegateChooser"
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date: 2023-04-19
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draft: false
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tags:
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- Qt
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series:
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- Obscure Qt
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---
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I decided to start a short series of describing weird, obscure Qt behavior! Because of my new job, I'm working with Qt even more than I did before. As I'm finding new and undocumented behavior, I feel that it's too useful to be posted on something like Mastodon and then forgotten.
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A really useful component is the [DelegateChooser](https://doc.qt.io/qt-6/qml-qt-labs-qmlmodels-delegatechooser.html). Say you are building a chat application, and you need to support different types of messages (a regular text message, an image, a location and so on). In order to accomplish this in Qt Quick is to do something like this:
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```qml
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DelegateChooser {
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role: "type"
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DelegateChoice {
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roleValue: "message"
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Label {
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required property string message
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text: message
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}
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}
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DelegateChoice {
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roleValue: "image"
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Image {
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required property string url
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source: url
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}
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}
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...
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}
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```
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At runtime, Qt Quick will use `role` and check it against every one of the [DelegateChoice](https://doc.qt.io/qt-6/qml-qt-labs-qmlmodels-delegatechoice.html)'s `roleValue`. When it finds a match, it'll choose that component. Really useful stuff!
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But today I encountered a different case, where I had a bunch of values in my role but not all of them had to be specialized. In most cases I had one component that's generalized for most of them, but only one or two that needed a different component. Curiously, the Qt documentation does not say how to accomplish something like this:
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```cpp
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switch(type) {
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case SpecialType:
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return SomeComponent{};
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default:
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return DefaultComponent();
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}
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```
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After doing some digging I found a Qt mailing list from a couple years ago, with some developers discussing what to do about "default" cases for `DelegateChooser`. Someone suggested leaving the `roleValue` empty as a way to support that. And it seems to work in Qt today, albeit undocumented:
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```qml
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DelegateChooser {
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role: "type"
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DelegateChoice {
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roleValue: "specialType"
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Label {
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text: "I'm a special type!"
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}
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}
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// This is the default choice!
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DelegateChoice {
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Label {
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text: "I'm everyone else..."
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}
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}
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}
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```
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