Assigning a line plot object to a global property doesn't retain it after the object is deleted

Question

Jason 2024-6-14

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回答： Jason 2024-6-18

Is there a reason why when I delete a plot of a multiplot graph and assign it to a global (public) property it doesn't retain it.

ax=app.UIAxes;             % My plot is on a UIAxes
t = ax.Children(1)         % Get the last plot
app.myPlot=t               % myPlot is a Global property
delete(t);
app.myPlot

app.myPlot shows:

ans = 
handle to deleted Line

so instead I have to access all the properties of the line individually and assign to a struct.

s = struct;
s.X=t.XData;
s.Y=t.YData;
s.Cl=t.Color;
s.DN=t.DisplayName;
s.Ls=t.LineStyle;
s.Lw=t.LineWidth
s.M=t.Marker;      
app.myPlot=s;

Just seems more clunky!

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Answer 1

Steven Lord 2024-6-14

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All of the handles to the line object (which is a handle object) are references to the same thing on the screen. If you delete() the handle, you delete the thing on the screen. Trying to access it after it's been deleted would be like throwing away a piece of paper but hoping to be able to feel it using a picture you took on your phone. That's not going to work. A very few things do still work; asking for its class or asking if it isvalid would. But the line is gone; asking for its XData or YData won't work.

What were you hoping to have happen by deleting the line on the screen but then trying to reference it?

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Jason 2024-6-14

编辑：Jason 2024-6-14

在 MATLAB Online 中打开

Hi Steven, thanks for your reply.

Firstly the reason Im doing this is that Im writing my own graphing program which allows me to add multiple plots to the same graph. Occasionally, after adding (say 10 plots) and personalising these (i.e. colours, linestyles etc), when I come to add the next one I sometime make a mistake and want to delete it, hence i delete the 1st item in the ax.children list.

But to be prudent, I wanted an undo incase I accidentally deleted the last one by mistake.

Also to be clear, I assign the plot I want to delete BEFORE I actually delete it.

And actually, doing it this way does work (i.e. I can get the XData etc.

ax=app.UIAxes;             
t = ax.Children(1); 
s = struct;
s.X=t.XData;
s.Y=t.YData;
s.Cl=t.Color;
s.DN=t.DisplayName;
s.Ls=t.LineStyle;
s.Lw=t.LineWidth
s.M=t.Marker;      
app.myPlot=s;
delete(t);

Whilst this works, I was just wondering if there was a more elegant way where I could somehow save the whole object before I delete it and then call it back.

so what I can understand, "t" is the handle to the line object, but that means its just a reference. The actual data is for example got by t.XData.

Thanks

Steven Lord 2024-6-14

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If you want an "undo" capability, instead of deleting the lines consider setting their Visible property to 'off'. That will hide them from being displayed, but they will still be "alive". Undoing that "deletion" would mean changing that line's Visible property back to 'on'.

In fact, if you want LIFO undo capability (to undo multiple deletions) you could have two arrays of graphics handles (preallocate it with the gobjects function.) Any new lines you create get added to a VisibleLines array and any lines you delete get moved from that array into a DeletedLines array. When you undo you "undelete" the last entry in DeletedLines, moving it back to VisibleLines then removing it from DeletedLines.

I don't think you'll need the approach you described in your supplemental question any more, but one tool you could use to do this is the set function. It's no longer needed nearly as often with the new graphics object-based Handle Graphics system as it did with the numeric handle-based system, but it still has its uses. Let's create one line and get some of its properties into a struct.

h1 = plot(1:10, 1:10, 'r:s');

propertyValues.Color = h1.Color;

propertyValues.LineStyle = h1.LineStyle;

propertyValues.Marker = h1.Marker

propertyValues = struct with fields:

Color: [1 0 0] LineStyle: ':' Marker: 'square'

Now create another line with different color, line style, and marker.

figure

h2 = plot(1:10, 10:-1:1, 'b--^');

If we create a third line using that exact same command as we did for the second line then set its properties using the propertyValues struct, you can see that it looks like the first line. [I'll change the name of the variable so if you want to experiment you can easily play with h1, h2, and h3.]

figure

h3 = plot(1:10, 10:-1:1, 'g--^');

set(h3, propertyValues)

h3 now has the same color, line style, and marker as the h1 line but the same data as the h2 line.

Steven Lord 2024-6-14

在 MATLAB Online 中打开

You can control what gets displayed in the legend by passing a vector of handles into legend.

x = 0:360;

axis([0 360 -1 1])

hold on

s = plot(x, sind(x), '-', DisplayName = "sine");

c = plot(x, cosd(x), ':', DisplayName = "cosine");

legend(s) % Show only the sine curve in the legend

Jason 2024-6-14

编辑：Jason 2024-6-14

Thanks. I know this and thats how I create my legend (thru the parameter displayName in the plot function), but when I turn off the last line visibility, the legend item reamins, but dims out.

So the purple curve is the last plot (hence is accesssed thru ax.Children(1)