WEBVTT 00:00.080 --> 00:01.000 Welcome back. 00:01.040 --> 00:07.480 We talked about flow and how important it is for fetching real time updates. 00:07.520 --> 00:13.240 We said that flow is a called asynchronous stream that emits values over time. 00:13.280 --> 00:20.320 We need to put real time data in this flow in order to return this function. 00:20.360 --> 00:24.000 A flow of list of destination markers. 00:24.040 --> 00:34.720 So from where we need to get the real time updates, this code gets the data once when the app launches. 00:34.720 --> 00:44.600 And this is a problem if there is a new marker inserted into database during the app run. 00:44.640 --> 00:48.680 This new marker will not be represented on the map. 00:48.680 --> 00:52.880 And this is what we've seen in the previous videos. 00:53.000 --> 01:01.540 Here we need to listen for real time updates from Firestore collection, which is the marker for this. 01:01.820 --> 01:03.340 Remove this code. 01:03.420 --> 01:05.380 This code the previous code. 01:06.140 --> 01:10.900 Get the data once during the runtime. 01:10.900 --> 01:14.820 We don't need to get only one time. 01:14.820 --> 01:19.540 We need to fetch and get real time data changes. 01:19.540 --> 01:26.700 For this use val listener equals to firebase dot collection. 01:26.940 --> 01:29.820 Set the collection the same as before. 01:30.020 --> 01:36.940 And here the newly used code is add snapshot listener. 01:37.100 --> 01:44.300 Here we are listening for real time updates from Firestore collection named markers. 01:44.460 --> 01:54.820 Whenever the data in markers changes added, modified or removed, the snapshot callback is triggered. 01:55.100 --> 01:59.540 If an error occurs, new network issues permission denied. 01:59.780 --> 02:02.990 The error callback is executed. 02:03.030 --> 02:09.390 Okay, so this is the idea behind getting the real time updates. 02:09.670 --> 02:13.470 Okay, so whenever data changes, snapshot is triggered. 02:13.470 --> 02:19.030 Whenever an error occurs, the error callback is executed. 02:19.070 --> 02:27.230 As we said before, the collection of Firebase Firestore is named markers that add the snapshot listener 02:27.550 --> 02:32.270 attaches a real time listener to the collection. 02:32.270 --> 02:38.550 So here we are attaching a real time listener for the collection. 02:38.550 --> 02:44.270 I want from you to focus with me and write the notes down. 02:44.270 --> 02:52.110 Because these notes are very important and help you in memorizing every code. 02:52.270 --> 02:58.030 Okay, here we are attaching a real time listener to the collection. 02:58.070 --> 03:02.890 The snapshot a query snapshot containing the latest data. 03:02.930 --> 03:07.090 Error an exception if something went wrong. 03:07.290 --> 03:10.370 Let me continue with this code here. 03:10.570 --> 03:17.170 Let me start with if start with if error is not null. 03:17.410 --> 03:23.490 So if there is an error what we need to do return add snapshot listener. 03:23.490 --> 03:31.090 And this is error dot print stack trace and return and snapshot listener. 03:31.130 --> 03:37.490 It exits the listener callback early to avoid processing invalid data. 03:37.530 --> 03:40.250 This is in case of an error. 03:40.410 --> 03:44.290 Now let's handle if there is no error. 03:44.330 --> 03:46.170 Now use. 03:46.210 --> 03:54.130 If snapshot does not equal to null and snapshot is not empty. 03:54.130 --> 03:58.930 So there is no null values and it's not empty. 03:59.290 --> 04:06.620 Let me use vowel markers equals to snapshot dot map. 04:06.740 --> 04:12.180 We used map in the previous videos, but I'll show you here. 04:12.220 --> 04:15.660 Fetched marker or my marker. 04:15.660 --> 04:17.420 You can use my marker. 04:17.420 --> 04:22.660 And here let me customize it as a destination marker okay. 04:22.700 --> 04:24.540 Here like this. 04:24.540 --> 04:28.620 And let me explain what we've done here. 04:28.620 --> 04:32.660 Here we are checking if snapshot is valid. 04:32.780 --> 04:41.300 It's not null ensures that query snapshot Firebase response is not null and snapshot is not empty. 04:41.580 --> 04:44.860 Checks if the snapshot contains documents. 04:45.060 --> 04:54.620 Avoids processing empty results if both conditions pass, the snapshot has valid data to process and 04:54.620 --> 04:59.820 convert Firestore documents to destination marker objects. 04:59.880 --> 05:10.040 Snapshot dot map iterates iterates over over each document query document snapshot in the snapshot and 05:10.040 --> 05:13.760 maps it to a destination marker. 05:13.760 --> 05:23.160 So every document, every marker in the markers list would be customized into a destination marker object. 05:23.320 --> 05:30.560 The fields marker ID marker that got double latitude longitude markers dot get double longitude and 05:30.560 --> 05:32.840 marker dot get string that title. 05:33.040 --> 05:39.000 So here we are getting every document in the snapshot. 05:39.160 --> 05:48.080 Go and create a destination marker and fetch the IDs, the latitude, longitude and title variables 05:48.080 --> 05:50.120 from this document. 05:50.160 --> 05:59.480 Now we need to emit map data via try sent here outside the if block. 05:59.730 --> 06:09.530 Use dry sand markers, sending the mapped list destination marker to the flow consumers, for example, 06:09.530 --> 06:17.370 Jetpack Compose UI and part of the callback flow or mutable shared flow API, which is used to bridge 06:17.370 --> 06:20.850 callback based APIs like Firebase Firestore. 06:20.890 --> 06:32.090 Listeners with Kotlin Flow and we can handle empty or invalid snapshots using try or else try send an 06:32.090 --> 06:45.690 empty list, then remove this return and here use await close and set the listener remove in the preview. 06:45.770 --> 06:52.370 In the next videos, we're going to learn about the callback and how to solve those errors and how to 06:52.410 --> 06:54.010 import the correct package.