Add log level and ignore missing frame log

Source/APNGKit/APNGImageView.swift

@@ -348,7 +348,7 @@ open class APNGImageView: PlatformView {
         guard let renderer = renderer, let _ = renderer.output /* the frame image is rendered */ else {
             // but unfortunately the decoding missed the target.
             // we can just wait for the next `step`.
-            printLog("Missed frame for image \(image): target index: \(nextFrameIndex), while displaying the current frame index: \(displayingFrameIndex).")
+            printLog("Missed frame for image \(image): target index: \(nextFrameIndex), while displaying the current frame index: \(displayingFrameIndex).", logLevel: .verbose)
             onFrameMissed(nextFrameIndex)
             frameMissed = true
             return

Source/APNGKit/Extensions.swift

@@ -63,6 +63,11 @@ extension UnsignedInteger {
     }
 }
 
-func printLog(_ item: Any) {
-    print("[APNGKit] \(item)")
+func printLog(_ item: Any, logLevel: LogLevel = .default) {
+    if logLevel == .off || LogLevel.current == .off {
+        return
+    }
+    if logLevel <= LogLevel.current {
+        print("[APNGKit] \(item)")
+    }
 }

Source/APNGKit/LogLevel.swift

@@ -0,0 +1,27 @@
+//
+//  LogLevel.swift
+//  
+// 
+//  Created by: onevcat (Wei Wang) on 2022/05/29
+//
+
+import Foundation
+
+/// Log level when APNGKit to determine if the log should be printed into console.
+public enum LogLevel: Int32, Comparable {
+    public static func < (lhs: LogLevel, rhs: LogLevel) -> Bool {
+        lhs.rawValue < rhs.rawValue
+    }
+
+    /// The log should be turned off.
+    case off       = 0
+    /// The default log level. Only critical issues or errors will be printed
+    case `default` = 0x00000001
+    /// Also log some ignorable logs for diagnose purpose.
+    case info      = 0x00001000
+    /// Verbose log can be used to show all information. Not in use now.
+    case verbose   = 0x10000000
+
+    /// The global setting of the log printed by APNGKit.
+    public static var current: LogLevel = .default
+}