mat-select-trigger is invisible if value is undefined or null

[hsinyu-chen]

Bug, feature request, or proposal:

md-select-trigger is invisible if value is undefined or null
when MD_PLACEHOLDER_GLOBAL_OPTIONS set to { float: 'always' }(use to show field name)
i cant show 'All' or 'Please Select ... ' on the md-select

What is the expected behavior?

need a way to show hint text / empty option alternative text

What is the current behavior?

mat-select-trigger is invisible if value is undefined or null

What are the steps to reproduce?

https://stackblitz.com/edit/angular-material2-issue-vyqfqf

What is the use-case or motivation for changing an existing behavior?

use select as a list filter, or form control with a hint text

[crisbeto]

This is the intended behavior. Options with a value of undefined or null act as blank options which reset the value.

[willshowell]

@crisbeto while this is currently intended behavior, is there any wisdom in always rendering the trigger value if mat-select-trigger is defined?

[mmalerba]

@crisbeto But you should be able to put your own display value for when nothing is selected like "-- Please select --" or something.

[crisbeto]

@mmalerba the problem is knowing when to show it. We probably don't want to show it and float the label on a blank select that the user hasn't interacted with, because it makes it stand out next to other blank fields and we can't know whether a value of undefined came from the consumer passing in undefined or the control not having a value at all. I see a couple of solutions:

1. Making it so only the undefined options act as resets, while the null ones could be used as the -- Please select -- placeholder. I can see this approach being a little confusing for users.
2. Instead of floating the label for undefined and null, we replace it with the option's label. This could be a little confusing API-wise, because it makes the options act as placeholder text, but it seems like it might be what we want.

[mmalerba]

I see. I have an issue filed for input to separate the concept of label vs placeholder (#6194) to make it match the latest mocks. Maybe a similar sort of thing could work for the select, when the label floats we reveal the null/undefined display value.

[hsinyu-chen]

Why don't we show the trigger whenever the label floats?
Would it be better if nothing is selected, the text colors are lighter.

[jandsu]

I believe that there is similar issue when the select is multiple and no value is selected: the value is then an empty array, evaluating to a falsy value just as null or undefined....

Please let me know if this deserves another issue or not. I wanted to remain on the less-noisy option by default.

[mmalerba]

@crisbeto assigning to you since its related to the label / placeholder split