Merge #205

205: Add FeatureWriter to stream writes r=urschrei a=michaelkirk

- [x] I agree to follow the project's [code of conduct](https://github.com/georust/geo/blob/master/CODE_OF_CONDUCT.md).
- [x] I added an entry to `CHANGES.md` if knowledge of this change could be valuable to users.
---

This allows the user to incrementally serialize a FeatureCollection, rather than requiring them to do it one-shot. This is a pretty logical counterpart to the FeatureReader introduced in #199.

Co-authored-by: Michael Kirk <michael.code@endoftheworl.de>

CHANGES.md

@@ -26,6 +26,9 @@
     geojson::ser::to_feature_collection_string(&my_structs).unwrap();
     ```
   * PR: <https://github.com/georust/geojson/pull/199>
+* Added `geojson::{FeatureReader, FeatureWriter}` to stream the reading/writing of your custom struct to and from GeoJSON.
+  * PR: <https://github.com/georust/geojson/pull/199> 
+  * PR: <https://github.com/georust/geojson/pull/205>
 * Added IntoIter implementation for FeatureCollection.
   * <https://github.com/georust/geojson/pull/196>
 * Added `geojson::Result<T>`.

examples/stream_reader_writer.rs

@@ -0,0 +1,45 @@
+use geojson::{de::deserialize_geometry, ser::serialize_geometry, FeatureReader, FeatureWriter};
+
+use serde::{Deserialize, Serialize};
+use std::error::Error;
+use std::fs::File;
+use std::io::{BufReader, BufWriter};
+
+#[cfg(not(feature = "geo-types"))]
+fn main() -> Result<(), Box<dyn Error>> {
+    panic!("this example requires geo-types")
+}
+
+#[cfg(feature = "geo-types")]
+fn main() -> Result<(), Box<dyn Error>> {
+    #[derive(Serialize, Deserialize)]
+    struct Country {
+        #[serde(
+            serialize_with = "serialize_geometry",
+            deserialize_with = "deserialize_geometry"
+        )]
+        geometry: geo_types::Geometry,
+        name: String,
+    }
+
+    let reader = {
+        let file_reader = BufReader::new(File::open("tests/fixtures/countries.geojson")?);
+        FeatureReader::from_reader(file_reader)
+    };
+
+    let mut writer = {
+        let file_writer = BufWriter::new(File::create("example-output-contries.geojson")?);
+        FeatureWriter::from_writer(file_writer)
+    };
+
+    let mut country_count = 0;
+    for country in reader.deserialize::<Country>()? {
+        let country = country?;
+        country_count += 1;
+
+        writer.serialize(&country)?;
+    }
+
+    assert_eq!(country_count, 180);
+    Ok(())
+}

src/de.rs

@@ -429,7 +429,6 @@ pub(crate) mod tests {
 
     use crate::JsonValue;
 
-    use serde::Deserialize;
     use serde_json::json;
 
     pub(crate) fn feature_collection() -> JsonValue {
@@ -491,6 +490,8 @@ pub(crate) mod tests {
     mod geo_types_tests {
         use super::*;
 
+        use serde::Deserialize;
+
         #[test]
         fn geometry_field() {
             #[derive(Deserialize)]
@@ -592,7 +593,7 @@ pub(crate) mod tests {
     #[test]
     fn roundtrip() {
         use crate::ser::serialize_geometry;
-        use serde::Serialize;
+        use serde::{Deserialize, Serialize};
 
         #[derive(Serialize, Deserialize)]
         struct MyStruct {

src/errors.rs

@@ -15,6 +15,8 @@ pub enum Error {
     /// This was previously `GeoJsonUnknownType`, but has been split for clarity
     #[error("Expected a Feature, FeatureCollection, or Geometry, but got an empty type")]
     EmptyType,
+    #[error("invalid writer state: {0}")]
+    InvalidWriterState(&'static str),
     #[error("IO Error: {0}")]
     Io(std::io::Error),
     /// This was previously `GeoJsonUnknownType`, but has been split for clarity