Add RuntimeBuilder for customizing runtime

src/runtime.rs

@@ -24,8 +24,40 @@ pub struct RuntimeHandle {
     blocking: Executor,
 }
 
+pub struct RuntimeBuilder {
+    executor_count: usize,
+    blocking_count: usize,
+}
+
 struct RuntimeToken;
 
+impl RuntimeBuilder {
+    pub fn new() -> Self {
+        let executor_count = std::thread::available_parallelism()
+            .map(usize::from)
+            .unwrap_or(1);
+
+        Self {
+            executor_count,
+            blocking_count: executor_count * 5,
+        }
+    }
+
+    pub fn executor_count(mut self, count: usize) -> Self {
+        self.executor_count = count;
+        self
+    }
+
+    pub fn blocking_count(mut self, count: usize) -> Self {
+        self.blocking_count = count;
+        self
+    }
+
+    pub fn build(self) -> Runtime {
+        RuntimeState::create(self)
+    }
+}
+
 impl RuntimeHandle {
     pub fn current() -> Self {
         RUNTIME.with(|runtime| {
@@ -93,18 +125,12 @@ impl RuntimeState {
         RUNTIME.with(|runtime| runtime.borrow_mut().take())
     }
 
-    fn create() -> Runtime {
+    fn create(builder: RuntimeBuilder) -> Runtime {
         let executor = Executor::new();
         let blocking = Executor::new();
 
-        let base_threads = std::thread::available_parallelism()
-            .map(usize::from)
-            .unwrap_or(1);
-
-        let blocking_threads = base_threads * 5;
-
         let mut thread_handles = Vec::new();
-        for _ in 0..base_threads {
+        for _ in 0..builder.executor_count {
             let executor = executor.clone();
             let blocking = blocking.clone();
 
@@ -117,7 +143,7 @@ impl RuntimeState {
             }));
         }
 
-        for _ in 0..blocking_threads {
+        for _ in 0..builder.blocking_count {
             let executor = executor.clone();
             let blocking = blocking.clone();
 
@@ -139,7 +165,7 @@ impl RuntimeState {
 
 impl Runtime {
     pub fn new() -> Self {
-        RuntimeState::create()
+        RuntimeState::create(RuntimeBuilder::new())
     }
 
     pub fn handle(&self) -> &RuntimeHandle {
@@ -172,6 +198,12 @@ impl Runtime {
     }
 }
 
+impl Default for RuntimeBuilder {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl Default for Runtime {
     fn default() -> Self {
         Self::new()