The desired behavior in your video can easily be accomplished by
keeping your old code, and NOT setting the flickable of the PageHeader.
Actually I think that is the best solution for the users too because
otherwise the header may be showing/hiding while you swipe left/right.
About your proposal,
1) I think nobody disagrees with that
2) I think the use case you mention where the topMargin is maintained when the
header is hidden conflicts with this. In this case, I interpret the hidden
header as a header that goes away by scrolling, so its exposed property becomes
false. The header should *not* change the topMargin when exposed changes,
because changes in topMargin may change exposed again, and it will cause the
page contents to jump around when the header hides.
Another issue with 2) is the implementation. It means that when you
unset a header.flickable, it will restore the previous topMargin. So
what happens when you have this case (which happens in practice when
apps have multiple headers on a Page (for example an edit header and a
regular header, of which only one is visible).
Say, we have this QML code:
Page {
id: page
PageHeader { id: header1; visible: page.header==header1; flickable:
pageFlickable }
PageHeader { id: header2; visible: page.header==header2; flickable:
pageFlickable }
header: editMode ? header2 : header1
property bool editMode: false
Flickable { id: pageFlickable }
}
Internally, header1 and header2 have a property "previousHeaderHeight" that
they use to restore the previous header height when flickable is unset or the
header becomes invisible.
Let's assume that header1 is always updated first by the qml engine, and then
header2 (that happened in my case).
1. editMode = true
// header 1 first reverts the previous header height
// header1: visible = false; pageFlickable.topMargin = 0;
// header2: visible = true; previousHeaderHeight = 0; pageFlickable.topMargin =
header2.height;
2. editMode = false
// header1: visible = true; previousHeaderHeight = header2.height;
pageFlickable.topMargin = header1.height;
// header2: visible = false; pageFlickable.topMargin = 0 (previousHeaderHeight);
So now we have the wrong topMargin because we don't know in which order
the properties of the different headers will be updated. The solution to
this is to make the headers not store previousHeaderHeight, or to set
the topMargin to header.height but to add to the topMargin when a header
is enabled with a flickable, and subtract the header height from it when
the header is invisible or the flickable is unset.
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https://bugs.launchpad.net/bugs/1572525
Title:
[regression] Double header height is set as flickable topMargin
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